Clinical Science (1991) 81, 129-139
129
Editorial Review
Mechanisms of vasodilatation induced by potassium-channel
activators
SHUNICHI KAJIOKA, MIKIO NAKASHIMA, KENJI KITAMURA
AND
HIROSI KURIYAMA
Department of Pharmacology, Faculty of Medicine, Kyushu University, Fukuoka, Japan
INTRODUCTION
One of the main mechanisms underlying the induction of
vasodilatation is thought to be related to a reduction in
the cytosolic concentration of free Ca2+.This is because
vascular smooth muscle relaxes in the presence of 0.1
pmolll Ca2+, whereas in the presence of 5-10 pmol/l
Ca2+ these cells produce a maximum contraction as
assessed by using a Ca2+-sensitive electrode [l], by
measuring Ca2+ transients using fluorescent dyes [2, 31
or in studies of contraction evoked in skinned muscle
[4-71. Vascular smooth muscle displays an intrinsic tone
(low amplitude of sustained resting tension development),
and this implies that the cytosolic free Ca2+ concentrations are higher than 0.1 pmolll (under physiological conditions) [3]. However, the cytosolic free CaZ+
concentration needed to maintain tone may differ in
different vascular tissues, regions and species.
Vasodilatation may be the result of various processes:
for example, reduced influx of Ca2+ due to inhibition of
the voltage-dependent Ca2+ channels or from activation
of receptor-operated cation channels [8, 91. The latter
mechanism has a close relationship with the actions of
neurotransmitters, such as noradrenaline, ATP and
calcitonin-gene related peptide [lo, 111, and endothelium-derived factors such as endothelium-derived
relaxing and hyperpolarizing factors, as well as prostaglandin I, (prostacyclin), endothelin, thromboxane A,, etc.
[12-201. Other mechanisms, such as acceleration of Ca2+
efflux by activation of the Ca2+-MgZCATPase pump or
Na+-Ca2+ exchange mechanisms, may also induce vasodilatation. In addition, inhibition of the inositol 1,4,5trisphosphate (IP,)-coupled o r ryanodine-coupled
Correspondence: Professor H. Kuriyama, Department of
Pharmacology, Faculty of Medicine, Kyushu University,
Fukuoka 812, Japan.
(caffeine-sensitive) Ca2+ channels [21, 221 in the sarcoplasmic reticulum (SR) may reduce the cytosolic Ca2+
concentration. Finally, acceleration of the CaZ -active
pump in the SR may also reduce the cytosolic concentration of CaZ .
In contrast to the Ca2+-channel blockers which have
direct actions on voltage-dependent Ca2+channels, a new
series of drugs, namely K+-channel activators, has been
introduced recently, each of which inhibit the voltagedependent Ca2+ channels and receptor-operated cation
channels indirectly by activating the K + channel. Furthermore, the target K + channel for K+-channel activators
appears to be ATP- and glibenclamide-sensitive, as judged
by recordings from @-cells of the pancreas and from
cardiac and smooth-muscle cells. These drugs not only
have the ability to act as K+-channel activators but also
possess additional actions which augment their vasodilatory properties. In this article, we review the actions of
these agents on vascular smooth muscle.
+
+
WHAT ARE THE K+-CHANNELACTIVATORS?
Recently, Edwards & Weston [23] classified K+-channel
activators into several groups based on their chemical
structures: (i) benzopyran-nucleus-containing compounds
[e.g. BRL34915 or cromakalim; the trans-( - )-enantiomer
of cromakalim, lemakalim, and EMD 526921, (ii) cyanoguanidine-containing compounds (e.g. pinacidil), (iii)
pyridine derivatives (e.g. nicorandil and KRN 2391), (iv)
pyrimidine- or triazine-nucleus-containingcompounds
(e.g. minoxidil), (v) benzothiadiazine-nucleus-containing
compounds (e.g. diazoxide, chlorothiazide and quinethazone), (vi) dihydropyridine derivatives (e.g. nigludipine)
and (vii) thioformamide-nucleus-containingcompounds
(e.g. Rp 52891, EP 49356 and EP-A-0326297). However,
this classification may not cover the characterization of
130
S. Kajioka et al.
K +-channel activators adequately, because new drugs that
are being synthesized with the properties of K+-channel
activators display different chemical structures from those
outlined above.
Historical background
In 1978 and 1989, Uchida and co-workers [24, 251
introduced a new vasodilator, nicorandil, and reported
that this agent had a potency comparable with that of
papaverine in the dog and could ameliorate cyclic elevations of the ST segment of the ECG caused by subtotal
occlusion of the left anterior descending coronary artery.
Taira el al. [26] confirmed this observation in anaesthetized open-chest dogs and concluded that nicorandil is a
potent anti-anginal agent that has no cardiodepressant
activity. When the effect of nicorandil on excised strips of
porcine coronary artery was studied, the agent hyperpolarized the membrane with increased ionic conductance in a concentration-dependent manner [27, 281.
Hyperpolarization occurred only in the presence of a K +
concentration of less than 30 mmol/l, and with increasing
K + concentration nicorandil caused no further hyperpolarization. When the effect of nicorandil on the membrane current-voltage relationship was studied, the
curves obtained before and after application of nicorandil
crossed at about the K+ equilibrium potential ( - 8 0 to 85
mV in coronary, mesenteric and basilar arteries) and the
increase in the K + conductance of the membrane caused
by nicorandil was confirmed by 86Rb-efflux experiments
[29, 301. These results suggest that nicorandil hyperpolarizes the vascular smooth-muscle cell membrane by
increasing K + permeability [27, 29-39]. However, nicorandil is not a selective K+-channel activator; it has
additional nitroglycerine-like actions on vascular smooth
muscle [37,40-431. Although some reduction in systemic
blood pressure is seen, this effect is transient and the
predominant action of nicorandil is to increase coronary
blood flow under a variety of conditions [24-26, 44-53].
In clinical practice, nicorandil is used for the treatment of
angina pectoris by increasing coronary flow and reducing
pre- and after-load. Recently, the more selective and
potent K+-channel activator, KRN 2391, a derivative of
nicorandil, has been introduced. This drug is more potent
than nicorandil and has no nitroglycerine-like action [54].
In 1984, Ashwood e t a / . [ 5 5 , 561 reported that cromakalim [4-(cyclo amido-3,4-dihydropyran, BRL 349 151
lowered blood pressure in experimental animals, and this
action was thought to result from hyperpolarization of the
membrane by increased K + permeability from estimations of the mechanical responses, membrane potential
and XhRband J2Keffluxes [29,30,58-661. Buckingham et
01. [57] reported that cromakalim is more potent than
nifedipine (a dihydropyridine derivative) as an anti-hypertensive agent and produced less reflex tachycardia [39].
Cromakalim acts as an anti-hypertensive agent by increasing peripheral blood flow and reducing after-load. In
clinical trials for treatment of mild essential hypertension,
cromakalim lowered blood pressure but there was some
incidence of headache [67].
Recently, pinacidil ( N-alkyl-N"-c yano-N'-p yrid ylguanidine, P1060) has been introduced as an anti-hypertensive
agent in some countries [68]. Cohen [69] reported that
this agent is at least three times as potent as hydralazine,
and has a similar action to nifedipine in animal experiments in viva Following a re-examination of the actions of
this drug after the description of K+-channel activators
such as cromakalim and nicorandil, it was realized that
pinacidil also acts as a K+-channel activator by causing
hyperpolarization of the smooth-muscle cell membrane
[64, 65, 701. Cromakalim and pinacidil hyperpolarized
the membrane, but in high-K+ solutions, the hyperpolarization was no longer observed, as was reported previously
for nicorandil[28]. The clinical trials of pinacidil revealed
that this agent reduces blood pressure and subsequently
induces a tachycardia, slight headache and oedema [7 11.
Minoxidil [72, 731 is also thought to act as a K+channel activator. However, the actions of minoxidil may
differ from those of cromakalim and pinacidil, because
this agent has no effect on 86Rb efflux [74]. This agent
produces marked hypertrichosis as a side effect and the
mechanism underlying this effect is being more energetically investigated currently than is that of its anti-hypertensive action.
RP 49356 [N-methyl-2-(s-pyridil)-tetrahydrothiopyran-2-carbothio-arnide-l-oxide] has also been reported to
have K+-channel-activating properties [75]. In cardiac
muscle, this drug acts on the ATP-dependent K + channel
from the inside of the cell membrane rather than from the
extracellular side [76]. However, detailed experiments on
vascular smooth-muscle cells have not yet been performed.
EMD 52692 [4-(1,2-dihydro-2-oxo-l-pyridil)-2,2-dimethyl-2H-1-benzopyran-6-carbonitrile]has actions as a
K+-channel activator in arterial smooth-muscle cells and
this agent hyperpolarizes the membrane through an
increase in ionic conductance, as measured by the partition-stimulating method [77]. However, more detailed
studies are awaited.
Recently, glibenclamide has been used to explore K+channel function. Glibenclamide is a sulphonylurea
derivative that induces hypoglycaemia through increased
release of insulin from the P-cells of the pancreas [78].
The K + channels in cardiac and P-pancreatic cells are
also sensitive to these K+-channel activators (except for
low concentrations of cromakalim) and, in addition, some
inhibitors (derivatives of sulphonylurea) of P-cell activation, which effect insulin release, are also antagonists of
the agents classified as K + -channel activators. In this area,
newly categorized drugs such as diazoxide have been
introduced. This agent also acts as a K+-channel activator
and its hyperpolarizing action can be antagonized by the
application of glibenclamide [78-SO]. These ATP-sensitive K + channels will be discussed in more detail in
relation to other excitable cells below. Hereafter, we will
mainly discuss the actions of nicorandil, cromakalim and
pinacidil on the ion channel, and only briefly discuss the
action of glibenclamide in relation to the ATP-sensitive
K + channel in smooth-muscle, cardiac and pancreatic
cells in a comparative manner.
K+-channeI activators in vascular tissues
WHICH K + CHANNEL IS THE TARGET CHANNEL
FOR K+-CHANNELACTIVATORS?
Classification of K + channels distributed on smoothmuscle cell membranes
Macroscopic K + current. In dispersed smooth-muscle
cells, at least 10 different K + channels have been
described by investigators using the voltage- and patchclamp procedures. However, there may be overlap
between channel classifications owing to the complex
natures of these channels and to measurements being
made using different protocols. Here, we will introduce
briefly the general properties of K + channels. Tables 1
and 2 summarize the K+-current classification (Table 1:
macroscopic current; Table 2: unitary current) made in
smooth-muscle cell membranes.
Using the whole-cell voltage-clamp procedure, the
macroscopic current can be recorded after the application
of depolarizing pulses. With this method (e.g. holding
potential of -60 to -80 mV and command pulses of
above - 40 mV), the K + currents generated can be classified first into a transient outward current (I,,; divided into
Ca2+-sensitive and Ca2+-insensitive I,,), which occurs just
after the generation of the transient inward current (a
voltage-dependent Ca2+ current, Ica). This transient
outward current attenuates with time due to its inactivation and subsequently a sustained outward current
occurs, which shows less attenuation in amplitude even
131
during a long depolarization [delayed outward current, I ,
and divided into tetraethylammonium (TEA)-sensitive
and TEA-insensitive I,]. In addition, when the membrane
is depolarized by more than - 40 mV, oscillations of the
outward-going current occur with different amplitudes
and frequencies (spontaneous transient outward current,
STOC, or oscillatory outward currents, I,,) [81-911. Most
of I,, and all of I,, are Ca2+-dependentand the I, is Ca2+independent, as determined using different ionic environments. The I,, has a causal, but indirect, relation to the
influx of Ca2+ induced by the I,, (except in the ureter),
because depletion of Ca2 in the SR diminishes the generation of the I,, even after generation of the Ica. Therefore,
influx of Ca2+ activates a Ca2+-induced Ca2+ release
mechanism and the resulting increase in the concentration
of Ca2+ in the cytosol may activate the Ca2+-dependent
K + channel. This channel is apamin-insensitive, but is
sensitive to T E A and has a close relationship with the
activity of a large-conductance Ca2+-dependentK + channel [82-841. The I,, also has a close relationship with
extra- and intra-cellular Ca2+ concentrations, and after
depletion of Ca2+in the SR, this current ceases. Thus, IP,
lowers the threshold potential level for the triggering of
the I<,,.In the SR, there are IP,-sensitive [22, 92, 931 and
ryanodine (caffeine)-sensitive Ca2+channels [21, 94-96],
and the latter correlate with the release of Ca2+ through
the activation of the Ca2+-inducedCa2+ release mechanism in vascular smooth-muscle cells [97]. The Ca2+
+
Table 1. Classification of the K + channels in various smooth-muscle cells from macroscopic currents
Abbreviations: [Ca2+Ii,intracellular free Ca2+concentration; CTX, charybdotoxin; ACh, acetylcholine; SP, substance P.
Classification
Characteristics
Species and tissues
(A)Voltage-dependent K + current
(1) Transient outward current (
(a) Ca*+-dependent
CTX-sensitive, submillimolar
Rabbit ear artery
TEA-sensitive, 4-AP-insensitive, Rabbit portal vein
apamin-insensitive
(b) Ca2+-independentPI,)
4-AP-sensitive
Guinea-pig ureter
(2) Oscillatory outward current (I,,, STOC) The same as Ca2+-dependentI,,,
Rabbit poral vein
[Ca2+Ii-sensitive
(3) Delayed outward current ( I , )
(a) TEA-insensitive
Ca2+-insensitive,4-AP-sensitive, Rabbit pulmonary artery
apamin-insensitive
Ca2+-insensitive
Rabbit portal vein
(17) TEA-sensitive
Ba2+-sensitive,Caz+-insensitive Rabbit portal vein
(4) Background K + current
(B) Voltage-independent K + current
(1) ATP-sensitive K + current (KiT,.)
(a) Ca2+-dependent
(b) Ca?+-independent
(C) Receptor-operatedK + current ( ' M )
( I ) Muscarinic K + current
References
~ 9 1
[83,861
[901
~ 3 1
~371
[861
[861
CTX-insensitive, millimolar
Rabbit mesenteric, rat portal
TEA-sensitive
vein
4-AP-sensitive, apamin-insensitive Porcine coronary artery
[loo]
Blocked by ACh and SP,
activated by cyclic AMP
and P-agonists
Toad stomach cells
[104,1SO]
Rabbit jejunal artery and
rabbit portal vein
[88,151]
K. Okabe et al.
unpublished
work
[loo-1031
[1141
(D) Non-selective cation channel
(1) Hyperpolarization-activated current ( I , )
(anomalous rectifying)
S. Kajioka et al.
132
Table 2. Classification of the K + channel in vascular smooth-muscle cells from unitary currents
Abbreviations: CTX, charybdotoxin; [Caz+]i,intracellular free Ca2+ concentration; [Ca"],, extracellular free Ca2+
concentration.
ATP-sensitive K + channel
Ca2+-dependentK + channel
[Ca2+Ii-dependent
[Ca*+],-dependent [Ca2+],,-dependent[Ca2+Ii-depcndentCa2+-insensitive
200 ps
50 ps
Submillimolar Millimolar
Millimolar
_ _ _ _ _ _ ~
Conductance
TEA
4-AP
CTX
Apamin
Cyclic AMP and
Insensitive
Sensitive
Insensitive
Activated
?
?
Sensitive
?
100 ps
?
9
?
?
41-135 PS
Millimolar
30 ps
Millimolar
Sensitive
20 ps
?
?
?
Insensitive
Insensitive
?
Insensitive
Millimolar
Sensitive
A kinase
?
Insensitive
9
GMP
Tissue
?
?
Activated
?
Many vascular Rabbit portal vein Rabbit portal vein Pig coronary
artery
tissues
?
'>
Rat portal vein
Rabbit mesentary
artery and
portal vein
References
[81,91,99,100][98]
[loll
[loo1
released by the above mechanisms may contribute to the
generation of I,,,, [82-841.
Unitary K + currents. Using the patch-clamp
procedure, the K + channel can be classified into Ca'+dependent and Ca?+-independent K' channels. From
channel conductances, the unitary currents recorded from
the Ca?+-dependent K+ channel can be divided into
large- (Maxi-K or BK), middle- and small (SK)-conductance unitary currents [98, 991. The middle-conductance Ca'+-dependent K + channel is sensitive to
extracellular Ca2+ rather than to cytosolic C a l f . The
Ca2+-independent K + channel generates a smallconductance unitary current [%]. The Ca2+-dependent
unitary current can be classified according to factors
other than the unitary current conductance, namely as
apamin-sensitive small-conductance of K + or apamininsensitive large-conductance Ca2+-dependent K +
channels, charybdotoxin-sensitive and charybdotoxininsensitive large-conductance Ca*+-dependent K + channels [loo-1021, or as TEA- and 4-aminopyridine
(4-AP)-sensitive channels [ 101, 1031. In addition, activation of receptors by individual agonists or synthesizcd
second messengers can activate or inhibit the K + channel
in many excitable cells ('M' channel [104]).
ATP-sensitive K + channel. Subdivision of the K +
channels into ATP-sensitive and ATP-insensitive channels
is also a means of classification. However, we will review
the features of the ATP-sensitive K + channel in a separate
section because this channel has a close relation with K + channel activators and indeed it is generally thought that
this channel is the target protein for K+-channel activators.
Since Noma's [lo51 first report, in 1983, the regulation of. the ATP-sensitive K+ channels has been extensively investigated in cardiac, nerve, pancreatic and
smooth-muscle cells [78, 791. This channel is dependent
on cytosolic ATP, and is inhibited by millimolar concentrations of cytosolic ATP. The activities of this channel are
related to the inhibition of insulin secretion (pancreatic
p-cells [76]), the hyperpolarization of ischaemic heart
~141
cells [78, 1061 and the relaxation of vascular tissues
[ 100-1O3]. However, this ATP-sensitive K + channel
possesses different unitary conductances in cells excised
from different tissues and also has different sensitivities to
ATP, although all these ATP-sensitive K+ channels are
blocked by glibenclamide. In pancreatic p-cells, the values
of IC,, (concentration producing half-maximal inhibition)
for 86Rb efflux, measured using various sulphonylurea
derivatives, varied from 0.06 pmol/l for glibenclamide to
40 pmol/l for carbutamide [107, 1081. These differences
suggest that the ATP- and glibenclamide-sensitive K +
channels recorded from different cells possess different
natures. Recent investigations have clearly demonstrated
the fact that this ATP- and glibenclamide-sensitive K +
channel is a target channel for K+-channel activators,
with some tissue differences and even regional differences
in the same tissue. Here, we will mainly describe the
effects of these agents on vascular smooth-muscle cells.
Actions of K+-channel activators on the ATP-sensitive
K + channel
Using the whole-cell voltage-clamp technique, the
effects of K+-channel activators have been studied by
several investigators. When cromakalim or nicorandil was
added to the bath while the holding potential was kept at
- GO mV, an outward current was recorded. The outward
current was of greater amplitude in the depolarized
membrane. When ramp currents ( - 120 mV to + 40 mV,
300 ms duration) were applied during the generation of
the cromakalim-, nicorandil- or pinacidil-induced
outward current, the ionic conductance was increased
(with 5.9 mmol/l external K + , the reversal potential level
was - 80 mV [101-1031). The measured current-voltage
relationships before and after application of nicorandil or
cromakalim crossed at around the K + equilibrium
potential level at any given concentration of K + in the
solution [96-981. Beech & Bolton [lo91 reported that in
isolated cells of the rabbit portal vein, cromakalim evoked
a delayed outward K + current that was essentially Ca2+-
K+-channel activators in vascular tissues
independent, and that the current evoked by cromakalim
was inhibited by various K+-channel blockers (phenycyclidine > quinidine > 4-AP> TEA).
Table 3 summarizes the target unitary K+-channel
current for K+-channel activators recorded from vascular
smooth-muscle cells. T h e [cytosolic Ca2+]-dependent
unitary K + current recorded from the rat portal vein can
be classified into the BK and SK [98, 99, 1101. Differences between these channels have been observed in
the smooth-muscle cells of the rat portal vein. Thus, to
activate the BK, much higher cytosolic concentrations of
Ca2+ (0.3 pmol/l) were required than to activate the SK
(0.1 pmol/l). Other differences between the activities of
these channels were that 0.1 mmol/l TEA inhibited, and
3 mmol/l T E A completely blocked, the channel activity of
the BK. On the other hand, the SK was not modified by
1 mmol/l T E A in the open probability of BK and the
current amplitude was unchanged. However, when 3
mmol/l T E A was applied, the open probability was
markedly inhibited [loll. When 4-AP was applied to the
same cells, 10 mmol/l 4-AP did not modify the open
probability or amplitude of the unitary current of the BK,
whereas this concentration of 4-AP completely blocked
the opening of the SK channel [ l o l l . The BK but not the
SK, was blocked by charybdotoxin, as also observed in
many other excitable cells [78, 101, 1021. However, in the
mesenteric artery, Standen et al. [loo] found a
charybdotoxin-insensitive BK.
From the actions of Kf -channel activators on vascular
smooth-muscle cells, the target channel for K+-channel
activators is postulated to be the glibenclamide- and ATPsensitive K + channel. Furthermore, nicorandil, cromakalim and pinacidil act on the same K + channel, although
the channel properties differ in different regions of
133
vascular tissue [ 100-1031. Moreover, further detailed
analysis indicates that channel identification is not straight
forward. Thus, in the rabbit and bovine aortae, using the
cell-attached patch-clamp procedure, a Ca2+-dependent,
TEA-resistant, cromakalim-sensitive 200 pS K + channel
was found (150 mmol/l K + in both the pipette and bath
[ 1111. In the rat azygous vein (primary cultured cells), a
Ca2 -dependent, cromakalim-sensitive 200 pS K
channel was reported (4.7 mmol/l K+ in the pipette and
140 mmol/l K + in the bath [112]). Using the microsomal
fraction of the rabbit thoracic aorta, Gelband et al. [ 1131
reported the presence of a Ca2+-dependent,cromakalimsensitive 337 pS K + channel (250 mmol/l K + in both the
pipette and bath). In the rabbit mesenteric artery, a largeconductance, Ca2+-independent 135 pS K + channel
(Ca*+-dependent BK; 60 mmol/l K + in the pipette and
104 mmol/l K + in the bath) was reported to be responsible for the actions of K+-channel activators [loo]. In the
rat portal vein, K+-channel activators are reported to act
via a small-amplitude Ca2+-dependent, TEA-resistant,
4-AP-sensitive, glibenclamide-sensitive 10 pS K + channel
(SK; 6 mmol/l K + in the pipette and 140 mmol/l K+ in
the bath; with 140 mmol/l K + in the pipette, this value
shifted to 20 pS [101, 1021). Inoue et al. [114] reported
that in smooth-muscle cells of the pig coronary artery, the
activities of the [extracellular Ca2+]-sensitiveK + channel
was accelerated by nicorandil (unitary conductance 30 pS
with 140 mmol/l K + in both the pipette and bath and 0.1
mmol/l Ca2+ in the pipette and 1.4 mmol/l Ca2+ in the
bath).
In rat and rabbit portal veins, the bee venom, apamin,
inhibited one type of the Ca2+-dependentSK (not all the
Ca2+-dependent SK), but did not inhibit the ATP-sensitive SK. Charybdotoxin did inhibit the BK but did not
+
+
Table 3. Target channels for K+-channel activators in smooth-muscle cells
Abbreviations: ScTX, scorpion toxin; [Ca2+],, extracellular free Ca2+concentration.
Species and tissues
Conductance
Channel properties
Ionic condition
Pipette (cis)
Bath (trans)
Procedure
Rabbit and bovine aortae
[I491
200 p s
Ca2+-dependent
TEA-resistant ( < 1 mmol/l),
ScTX-sensitive
150 mmol/l KCI
150 mmol/l KCI
Patch-clamp
(cromakalim)
Rat azygous vein (primary
culture cell) [ 1121
200 ps
Ca2+-dependent
4.7 mmol/l KCI,
1 mmol/l EGTA,
1.8 mmol/l CaCl,
140 mmol/l KCI,
0.77 mmol/l EGTA,
0.7 mmol/l CaCI,
Patch-clamp
(pinacidil)
Rabbit thoracic aorta
(microsome) [ 1131
337 p s
Ca2+ -dependent
(250 mmol/l KCI),
1 mmol/l CaCI,
Rabbit mesenteric artery*
[ 1001
135 pS
Ca2+ -insensitive,
ATP-sensitive,
TEA-resistant
60 mmol/l KCI,
60 mmol/l NaCI,
1.8 mol/l CaCI,
104 mmol/l KCI,
16 mmol/l KOH,
5 mmol/l EGTA
Patch-clamp
(cromakalim)
Rat portal vein* [I011
10 p s
Ca2+-sensitive,
ATP-sensitive
TEA-resistant ( < 1 mmol/l)
4AP-sensitive ( > 1 mmol/l)
6 mmol/l KCI,
2.5 mmol/l CaCI,,
140 mmol/l KCI
140 mmol/I KCI,
4 mmol/l EGTA
140 mmol/l KCI
Patch-clamp
(nicorandil)
Pig coronary artery* [ 1131
30 pS
[Ca2+],-dependent
4AP-sensitive ( 5 mmol/l)
140 mmol/l KCI,
10-100 pmol/l CaCI,
140 mmol/l KCI,
1.4 mmol/l CaCI,
Patch-clamp
(cromakalim)
20 pS
*Defined as an ATP- and glibenclamide-sensitiveK+ channel.
(250 mmol/l KCI), Planar lipid bilayer
2 mmol/l EGTA
(cromakalim)
134
S. Kajioka et al.
inhibit the SK [29, 101-103, 1151. Therefore, the
apamin-sensitive and charybdotoxin-sensitive channels
may differ from the ATP-sensitive K + channel.
In studies on smooth-muscle cells using microelectrodes, nicorandil hyperpolarized the membrane
transiently in guinea-pig mesenteric artery; the hyperpolarization ceased within a few minutes. Subsequent application of the maximum concentration of cromakalim (10
pniol/l) produced no more hyperpolarization. A similar
result was observed when the drugs were applied in the
reverse sequence [115], i.e. the second agent applied
failed to act in synergy with the first. This observation also
suggests that nicorandil and cromakalim act on the same
K+ channel. Desensitization of the unitary K + current
activity has also been observed, using the cell-attached
patch-clamp procedure. In addition, Yamanaka et al.
[116] compared the effects of exogenously applied ATP
and nicorandil on smooth-muscle cells of the guinea-pig
small-intestine by using the microelectrode technique and
reported that nicorandil may act on the Ca’ +-independent K + channel, which differs from the channel activated
by exogenously applied ATP (purinergic I1 receptor activation). Using the patch-clamp procedure, the ATP-sensitive K+ channel in the portal vein has proved sensitive to
very low cytosolic concentrations of Ca2+. Therefore,
even with 4-10 mmol/l EGTA in the pipette, this channel
activity was not completely blocked, and increasing the
Ca’+ concentration increased the channel opening in a
concentration-dependent manner. In fact, we could not
determine the minimum requirement for Ca2+in the cytosol using the microelectrode or the patch- and voltageclampprocedures[111-113,1161.
Wide variations in the ATP-sensitive K+ channel, as
observed from unitary current conductances, have been
reported in excitable cclls other than vascular smoothmuscle cells. In pancreatic p-cells, the unitary current was
reported to bc 50-70 pS [78, 117-1231. In guinea-pig
atrial and ventricular muscle cells, it was 70-80 pS [78,
105, 123-1271 with 140 mmol/l K+ in both the pipette
and bath (45 pS at + 40 mV potential level and 71 pS at
- 40 mV potential level [ 1281). For the ATP-sensitive, RP
49356-sensitive K + channel the unitary current was 66
pS [76]; for the nicorandil-, cromakalim- and pinacidilsensitive and ATP-sensitive K + channel it was 60-70 pS
[129-13 11. In frog skeletal muscle, the unitary current
was 42 pS [ 13 11. Finally, in rat cerebral cortical neurons,
a 53 pS ATP-sensitive K+ channel has been reported [7S].
in addition, marked differences in the sensitivity to K+channel activators have been reported bctwcen K
channels in smooth muscles and thosc in other excitable
tissues.
Kajioka eta/. 1101, 1021 and Okabe eta/. [103] studied
the action of K+-channel activators on the macroscopic
and unitary currents, and noted that cromakalim, nicorandil and pinacidil increase the open probability of the
same channel (SK)without any change in the current conductance. These effects of K+-channel activators were
observed, using the cell-attached patch-clamp procedure.
However, long recordings of SK channel activities, using
the inside-out patch-clamp procedure, were difficult due
+
to the occurrence of a ‘run-down phenomenon’, as
commonly observed with the voltage-dependent Ca”
channel.
So-called ATP- and glibenclamide-sensitive K+
channels have been observed in pancreatic p-cells,
cardiac muscle, smooth muscle, skeletal muscle and cerebral cortical neurons, but individual channels have different sensitivities to ATP and glibenclamide [132-1361. As
estimated from the IC,, values for ATP, the P-cell was the
most sensitive. The IC,, value for the neonatal rat
pancreas was 15 pmol/l[ 1171, and for the adult rat it was
20-200 pmol/l[121]. In the adult mouse it was 18 prnol/l
[ 1341. The corresponding values were much higher in
cerebral cortical neurons (in the order of mmol/l). In
skeletal muscle, the IC,, value for ATP was reported to be
135 pmol/l 11351, whereas in cardiac muscle cells it was
100-500 pmol/l [105, 125, 1261. In vascular smooth
muscle, Standen et al. [loo] reported that the equivalent
IC5,,value was about 50 pmol/l. Thus marked differences
in the IC,, values for ATP are apparent in different
tissues. When the IC,, values for glibenclamide were
compared in various tissues, the values were also
markedly different [79] and again the rat pancreatic pcells showed the highest sensitivity (4 nmol/l [ 1361).
When the effects of K+-channel activators were compared in various tissues, much higher concentrations of
cromakalim were required in the p-cells of the rat
pancreas than in the guinea-pig mesenteric artery and
guinea-pig, rat and rabbit portal veins [101-103, 115,
1371. In the case of nicorandil, the concentration required
for activation of the ATP-sensitive K + channel was higher
in cardiac muscles than in the rat portal vein [102, 1301.
Again, the above results indicate that the ATP-sensitive
K + channel differs in nature not only in different regions
of vascular tissue but also in different organs.
EFFECTS OF K+-CHANNEL ACTIVATORS ON
VASCULAR SMOOTH-MUSCLE CELLS OTHER
THAN K + CHANNELS
Nicorandil possesses not only a K+-channel activating
action, but also a nitroglycerine-like action, and, as a
consequence, in the presence of high-K+ buffers this
agent is able to relax vascular tissues without membrane
hyperpolarization. In fact, this agent increases the synthesis of cyclic GMP, although its action is less potent than
that of nitroglycerine when equimolar concentrations of
these drugs were tested [27,28, 371. However, nicorandil
has no effect on the synthesis of cyclic AMP in normaland high-K+ solutions. Synthesis of cyclic GMP and its
roles in vasodilatation have been reviewed in relation to
endothelium-derived relaxing factor by Murad et al. [42]
and Ignarro [43].
Until recently, cromakalim and pinacidil were thought
to be rather selective K +-channel activators. However,
accumulated results indicate that these agents have
multiple actions on vascular tissues. For instance, in the
rat portal vein, Okabe et a/. [lo31 reported that cromakalim shows much the same actions as dihydropyridine
Ca2+-antagonists. Using the whole-cell voltage-clamp
K+-channel activators in vascular tissues
procedure, they found that the application of cromakalim
in concentrations similar to those that produced hyperpolarization of the membrane and increased the opening
of the ATP-sensitive K + channel also inhibited the
voltage-dependent Ca2+ current, as estimated from the
current-voltage relationship. In addition, when the
steady-state inactivation curve (using a double-pulse
procedure) was recorded before and after application of
cromakalim, the inactivation curve shifted to a more
hyperpolarized level than in the control. These two
phenomena are similar to those observed on application
of dihydropyridine derivatives such as nicardipine or
nifedipine.
Pinacidil or nicorandil inhibit the contraction evoked
by agonists due to a reduction in the free Ca2+concentration as estimated using fura-2 or quin-2 [37, 1141. These
agents inhibit the synthesis of IP3 [138] and, as a consequence, a reduction in the free Ca2+ concentration may
occur. In skinned muscle, the IP3-induced contraction is
not directly modified by pinacidil [138]. In addition,
inhibitory actions of pinacidil and cromakalim were
reported on Ca2+-refilling into and on Ca2+-releasing
mechanisms from the intracellular Ca2+ storage sites
[139-1431. However, these inhibitory actions on the
refilling and releasing mechanisms of the intracellular
organelles, mainly the SR, are not fully supported by
other investigators [ 144-1461.
Pinacidil seems to act on the K + channel in perivascular sympathetic nerve terminals. This agent
increased the release of noradrenaline from nerve
terminals in the rabbit mesenteric artery [147]. These
observations confirm results obtained in the rabbit
pulmonary artery by Nedergaard [148]. If the membranes
of perivascular nerve terminals (including varicosities) are
hyperpolarized by pinacidil, subsequently applied strong
nerve stimulation may produce a larger potential change
at the nerve terminals than before application of pinacidil,
thus causing an increased release of neurotransmitter.
However, this increased release of transmitter may not
directly facilitate the activation of the post-junctional
muscle membrane because the muscle membrane will
have been hyperpolarized due to the increase in the ionic
conductance of the membrane caused by pinacidil.
135
K+-channel activators act on various visceral smoothmuscle cells (especially the trachea and bladder), clinical
applications of these agents to smooth-muscle tissues
other than the cardiovascular system are also expected.
In addition, individual K+-channel activators do not
only act on the K + channel, but have additional actions,
such as nitroglycerine-like actions (nicorandil), dihydropyridine-derivative-like actions (cromakalim) or Ip,synthesis-inhibitor actions (pinacidil). Most of these
additional actions may partly produce vasodilatation.
Minoxidil exhibits the side-effect of hypertrichosis. This is
an unexpected but interesting side-effect, and whether or
not it has a causal relation with a K+-channel activating
property should be clarified because pinacidil and
diazoxide also produce similar problems. For clinical
purposes, pinacidil and nicorandil have been demonstrated to be anti-hypertensive and anti-anginal agents,
respectively. Lemakalim, an enantiomer of cromakalim
(but not cromakalim), is now under trial for clinical application as an anti-hypertensive agent. Diazoxide has an
anti-hypertensive action which also induces hyperglycaemia, and minoxidil sulphate is an established antihypertensive agent [73,149].
At present, the mechanism underlying the maintenance
of vascular smooth-muscle tone cannot yet be completely
explained by activation of the voltage-dependent Ca2+
channel. With regard to the action of K+-channel activators, the hyperpolarization of quiescent smooth-muscle
cell membranes induced by these agents may not have a
crucial role in the inhibition of the voltage-dependent
Ca2+ channel. In electrically quiescent smooth-muscle
tissues, if K+-channel activators induce hyperpolarization
and reduce the muscle tone in physiological and pathophysiological (eg. ischaemic) conditions, we would expect
a reduction in the cytosolic free Ca2+concentration to be
induced by the hyperpolarization of the smooth-muscle
membrane. However, data in support of the above
hypothesis have not yet been reported.
ACKNOWLEDGMENT
This work is supported by a grant from the Ministry of
Education, Culture and Welfare.
CONCLUSION
In this article we have described the actions of K+channel activators on vascular smooth-muscle
membranes, and discussed the possibility that ATP-sensitive, glibenclamide-sensitive K + channels in excitable
cells act as the target channels. In vascular smooth-muscle
cells, these drugs act on the ATP- and glibenclamide-sensitive and 86Rb-effluxchannels and induce vasodilatation.
However, individual investigators have reported different
ATP-sensitive K + channels in different vascular tissues, as
evidenced by the unitary current conductance and the
Ca2+and Mg2+-sensitivities. Further detailed investigation
is required to clarify whether or not the so-called ATPand glibenclamide-sensitive K + channel in vascular
smooth-muscle cells has a heterogenous nature. Since
REFERENCES
1. Yamaguchi, H. Recordings of intracellular Ca2+ from
smooth muscle cells by submicron tip, double barrelled
Ca2+ selective microelectrodes. Cell Calcium 1986; 7,
203-19.
2. Fay, FS., Shleven, M.H., Granger, W.C. & Taylor, S.R.
Aequorin luminescence during activation of single smooth
muscle cells. Nature (London) 1979; 180,506-8.
3. Morgan, J.P. & Morgan, K.G. Vascular smooth muscle: the
first recorded Ca2+transients. Pfliigers Arch. 1982; 395,
75-7.
4. Endo, M. Calcium release from the sarcoplasmic reticulum. Physiol. Rev. 1977; 57,71-108.
5. Saida, K. & Nonomura, Y. Characteristics of Ca2+-and
Mg*+-inducedtension development in chemically skinned
smooth muscle fibers. J. Gen. Physiol. 1978; 72,l-14.
136
S . Kajioka et al.
6. Itoh, T., Kuriyama, H. & Suzuki, H. Excitation-contraction
coupling in smooth muscle cells of the guinea-pig mesenteric artery. J. Physiol. (London) 1981; 321,515-35.
7. Ruegg, J.C. Calcium in muscle activation-a comparative
approach. Berlin: Springer-Verlag, 1986.
8. Bolton, T.B. Mechanisms of action of transmitters and
other substances on smooth muscle. Physiol. Rev. 1979;
59,606-718.
9. Kuriyama, H., Ito, Y., Suzuki, H., Kitamura, K. & Itoh, T.
Factors modifying contraction-relaxation cycle in vascular
smooth muscles. Am. J. Physiol. 1982; 246, H641-62.
10. Starke, K., Gotert, M. & Kilbinger, H. Modulation of
neurotransmitter release by presynaptic autoreceptor.
Physiol. Rev. 1989; 69,864-989.
11. Stajarne, L. Basic mechanisms and local modulation of
nerve impulse-induced secretion of neurotransitters from
individual synaptic nerve varicosities. Rev. Physiol.
Biochem. Pharmacol. 1989; 112,l-137.
12. Furchgott, R.F. & Zawadzki, J.V. The obligatory role of
endothelial cells in the relaxation of arterial smooth muscle
by acetylcholine. Nature (London) 1980; 288,373-6.
13. Furchgott, R.F. Studies on relaxation of rabbit aorta by
sodium nitrate: the basis for the proposal that the acidactivatable factor from bovine retractor penis is inorganic
nitrate and the endothelium-derived relaxing oxide. In:
Vanhoutte, P.M., ed. Mechanisms of vasodilation. New
York: Raven Press, 1988: 401 - 14.
14. Komori, K. & Suzuki, H. Electrical responses of smooth
muscle cells during cholinergic vasodilation in the rabbit
saphenous artery. Circ. Res. 1987; 61,586-93.
15. Komori, K. & Suzuki, H. Heterogenous distribution of
muscarinic receptors in the rabbit saphenous artery. Br. J.
Pharmacol. 1987; 92,657-64.
16. Moncada, S., Ferriera, S.H., Bunting, S. & Vane, J.R. An
enzyme isolated from arteries transformed prostaglandin
endoperoxides to an unstable substance that inhibits platelet aggregation. Nature (London) 1977; 263,663-5.
17. Moncada, S. & Vane, J.R. Pharmacology and endogenous
roles of prostaglandin endoperoxides, thromboxane A?,
and prostacyclin. Pharmacol. Rev. 1979; 30,293-331.
18. Vanhoutte, P.M., Rubanyi, G.M., Miller, V.M. & Houston,
D.S. Modulation of vascular smooth muscle contraction by
the endothelium. Annu. Rev. Physiol. 1986; 48,307-20.
19. Shirahase, H., Usui, H., Karahashi, K., Fujiwara, M. &
Fukui, K. Possible role of endothelial thromboxane A? in
the resting tone and contractile responses to acetylcholine
and arachidonic acid in canine cerebral arteries. J. Cardiovasc. Pharmacol. 1987; 10,5 17-22.
20. Yanagisawa, M., Kurihara, H., Kimura, S. et al. A novel
potent vasoconstrictor peptide produced by vascular
endothelial cells. Nature (London) 1988; 332,411-15.
2 I. Takeshima, H., Nishimura, S., Matsumoto, T. et al. Primary
structure and expression from complementary DNA of
skeletal muscle rvanodine receDtor. Nature (London)
1989; 339,439-45.
22. Furuichi. T.. Yoshikawa. S.. Mivawaki. A.. Wada. K..
Maeda, 'N. ' & Mikoshiba, 'K. Primary structure ' and
functional expression of the inositol 1,4,5-trisphosphatebinding protein P,,,,. Nature (London) 1989; 342, 32-8.
23. Edwards, G. & Weston, A.H. Structure-activity
relationships of Kf channel openers. Trends Pharmacol.
Sci. 1990; 11,417-22.
24. Uchida, Y., Yoshimoto, N. & Murao, S. Effects of SG-75
(2-nicotinamido ethyl nitrate) on coronary circulation. Jpn.
Heart. J. 1978; 19, 112-24.
25. Uchida, Y. Decreasing potassium conductance - a
possible mechanism of phasic coronary vasospasm. Jpn.
Circ. J. 1989; 49,128-39.
26. Taira, N., Satoh, K., Yanagisawa, T., Imai, Y. & Hiwatari,
M. Pharmacological profile of a new coronary vasodilator
drug, 2-nicotinamido-ethyl nitrate (SG-75). Clin. Exp.
Pharmacol. Phvsiol. 1979: 6.301-16.
L I . Furukawa, K., Itoh, T., Kajiwara, M. et al. Vasodilating
I
??
,
actions of 2-nicotinamidoethyl nitrate on porcine and
guinea-pig coronary arteries. J. Pharmacol. Exp. Ther.
1981; 218,248-59.
28. Itoh, T., Furukawa, K., Kajiwara, M. et al. Effects of 2-nicotinamidoethyl nitrate on smooth muscle cells and on
adrenergic transmission in the guinea-pig and porcine
mesenteric arteries. J. Pharmacol. Exp. Ther. 1981; 218,
260-70.
29. Weir, S.W. & Weston, A.H. Effects of apamin on responses
to BRL 34915, nicorandil and other relaxants in the
guinea-pig taenia caeci. Br. J. Pharmacol. 1986; 88,
113-20.
30. Weir, S.W. & Weston, A.H. The effects of BRL 34915 and
nicorandil on electrical and mechanical activity and on
X6Rbefflux in rat blood vessels. Br. J. Pharmacol. 1986;
88,121-8.
31. Karashima, T., Itoh, T. & Kuriyama, H. Effects of 2nicotinamidoethyl nitrate on smooth muscle cells of the
guinea-pig mesenteric and portal veins. J. Pharmacol. Exp.
Ther. 1982: 221,472-80.
32. Inoue, T., Ito, Y. & Takeda, T. The effects of 2-nicotinamidoethyl nitrate on smooth muscle cells of the dog
mesenteric artery and trachea. Br. J. Pharmacol. 80,
459-70.
33. Inoue, T., Kammuura, Y., Fujiwara, T., Itoh, T. &
Kuriyama, H. Effects of 2-nicotinamidoethylnitrate
(nicorandil; SG 75) and its derivatives on smooth muscle
cells of the canine mesenteric artery. J. Pharmacol. Exp.
Ther. 1984; 229,293-302.
34. Kaiiwara. M.. Droonmans. G. & Casteels, R. Effects of
2-nicotin'midoethyl- nitrate (nicorandil) on excitation-contraction coupling in the smooth muscle cells of
rabbit ear artery. J. Pharmacol. Exp. Ther. 1984; 230,
462-8.
35. Allen, S.L., Boyle, J.P., Cortijo, J., Foster, R.W., Morgan,
G.P. & Small, R.C. Electrical and mechanical effects of
BRL 34915 in guinea-pig isolated trachealis. Br. J.
Pharmacol. 1986; 89,395-405.
36. Allen, S.L., Foster, R.W., Morgan, G.P. & Small, R.C. The
relaxant action of nicorandil in guinea-pig isolated
trachealis. Br. J. Pharmacol. 1986; 87, 117-27.
37. Sumimoto, K., Domae, M., Yamanaka, K. et al. Actions of
nicorandil on vascular smooth muscles. J. Cardiovasc.
Pharmacol. 1987; 1 0 (Suppl. 8), S66-75.
38. Hamilton, T.C. & Weston, A.H. Cromakalim, nicorandil
and pinacidil: novel drugs which open potassium channels
in smooth muscles. Gen. Pharmacol. 1989; 20, 1-9.
39. Clapham, J.C. & Wilson, C. Anti-spasmogenic and
spasmolytic effects of RL 34915: a comparison with
nifedipine and nicorandil. J. Autonom. Pharmacol. 1987;
I, 1-10.
40. Holzmann, S. Cyclic G M P a possible mediator of
coronary arterial relaxation by nicorandil (SG-75). J.
Cardiovasc. Pharmacol. 1983; 5,364-70.
41. Schmidt, K., Reich, R. & Kukovetz, W.R. Stimulation of
coronary guanylate cyclase by nicorandil (SG-75) as a
mechanism of its vasodilating action. J. Cyclic Nucleotide
Res. 1985; 10,43-53.
42. Murad, M., Mittal, C.K., Arnold, W.P., Katsuki, S. &
Kimura, H. Guanylatc cyclase activation by azido, nitro
compounds, nitric oxide, and hydroxyl radical and inhibition by hemoglobin and myoglobin. Adv. Cyclic Nucleotide Protein Phosphorylation Res. 1978; 9, 131-43.
43. Ignarro, L.J. Biological actions and properties of endothelium-derived nitric oxide formed and released from
artery and vein. Circ. Res. 1989; 65, 1-21.
44. Sakanashi, M., Araki, H., Horio, Y. & Takenaka, F. Effects
of 2-nicotinamidoethyl nitrate (SG-75) on dog coronary
artery and cat papillary muscle. Jpn. Heart. J. 1979; 20,
549-56.
45. Sakanashi. M.. Nonuchi. K.. Kato. K., Sunanawa, R.. Ito. H.
& Nakasone,' J. %sodilating effects of nicorandil and
L
.
K+-channel activators in vascular tissues
nitroglycerin in anaesthetized open-chest dogs. NaunynSchmiedeberg’sArch. Pharmacol. 1986; 333,439-44.
46. Mizukami, M., Tomoike, H., Inoue, T., Watanabe, K.,
Kikuchi, Y. & Nakamura, M. Effects of 2-nicotinamidoethyl nitrate (SG-75) and/or nitroglycerin on systemic
hemodynamics and coronary blood flow in conscious
dogs. Arzneim-Forsch. 1981; 31 (Suppl. 11),1244-7.
47. Sakai, K., Shiraki, Y. & Nabata, H. Cardiovascular effects
of a new coronary vasodilator, N-(2-hydroxyethy1)nicotinamide nitrate (SG-75): comparison with nitroglycerine and
diliazem.J. Cardiovasc. Pharmacol. 1981; 3, 139-50.
48. Shiraki, Y.,Akima, M., Nabata, H., Ohba, Y., Hoshimo, E.
& Sakai, K. The hypotensive mechanisms of the antianginal drug N-(2-hydroxyethyl)nicotinamidenitrate (SG75) in Beagle dogs. Jpn. J. Pharmacol. 1981; 31,921-39.
49. Abiko, Y.,Nishimura, T. & Sakai, K. Nicorandil attenuates
myocardial acidosis during coronary occlusion in dogs. Br.
J. Pharmacol. 1984; 81,409-11.
50. Lamping, K.A., Warltier, D.C., Hardman, H.F. & Gross,
G J . Effects of nicorandil, a new antianginal agent, and
nifedipine on collateral blood flow in a chronic coronary
occlusion model. J. Pharmacol. Exp. Ther. 1984; 229,
359-63.
51. Korb, H., Hoeft, A., Hunneman, D.H., Schraeder, R.,
Wolpers, H.G. & Hellige, G. Effectiveness of nicorandil in
the preservation of myocardium stressed by transient
ischemia and its influence on cardiac metabolism during
coronary artery occlusion with subsequent reperfusion: a
comparison with isosorbide dinitrate. Naunyn-Schmiedebergs’ Arch. Pharmacol. 1985; 329,440-6.
52. Kuromaru, 0. & Sakai, K. Comparison of development of
tolerance between nicorandil and nitroglycerin in
anaesthetized, open-chest dogs. Jpn. J. Pharmacol. 1986;
42,199-205.
53. Shirnshak, T.M., Preuss, K.C., Gross, G.J., Brooks, H. &
Warltier, D.C. Recovery of contractile function in postischemic reperfused myocardium of conscious dogs:
influence of nicorandil, a new antianginal agent. Cardiovasc. Res. 1986; 20,621-6.
54. Kashiwabara, T., Odai, H., Kaneta, S., Tanaka, Y.,
Fukushima, H. & Nishikori, K. Vasodilating and hypotensive properties of KRN2391. Eur. J. Pharmacol. 1990;
183,1266.
55. Ashwood, V.A., Cassidy, F., Evants, J.M., Faruk, E.A. &
Hamilton, T.C. Trans-4-cyclic amido-3,4-dihydro-2-Hbenzopyran-3-01s as antihypertensive agents. Proc. VIIIth
Int. Med. Chem. Symp. 1984; 1,316-17.
56. Ashwood, V.A., Buckingham, R.E., Cassidy, F. et al.
Synthesis and antihypertensive activity of 4-(cyclic
amido)-2-H-l-benzopyrans.J. Med. Chem. 1986; 29,
2194-201.
57. Buckingham, R.E., Clapharn, J.C., Hamilton, T.C.,
Longman, S.D., Norton, J. & Poyser, R.H. BRL 34915, a
novel antihypertensive agent: comparison of effects on
blood pressure and other haemodynamic parameters with
those of nifedipine in animal models. J. Cardiovasc.
Pharmacol. 1986; 8,798-804.
58. Hamilton, T.C., Weir, S.W. & Weston, A.H. Comparison of
the effects of BRL 34915 and verapamil on electrical and
mechanical activity in rat portal vein. Br. J. Pharmacol.
1986;88,103-11.
59. Coldwell, M.C. & Howlett, D.R. BRL 34915 induced
potassium channel activation in rabbit isolated mesenteric
artery is not calcium dependent. Br. J. Pharmacol. 1986;
88,443P.
60. Coldwell, M.C. & Howlett, D.R. Specificity of action of the
novel antihypertensive agent, BRL 34915, as a potassium
channel activator; comparison with nicorandil. Biochem.
Pharmacol. 1987; 36,3663-9.
61. Kreye, V.A.W., Gerstheimer, F. & Weston, A.H. Effect of
BRL 34915 on resting membrane potential and “Rb
efflux in rabbit tonic vascular smooth muscle. NaunynSchmiedeberg’sArch. Pharmacol. 1987; 335, R64.
137
62. Kreye, V.A.W., Gerstheimer, F. & Weston, A.H. Effects of
the antihypertensive, BRL 34915, on membrane potential
and “Rb efflux in rabbit tonic vascular smooth muscle.
Pflugers Arch. 1987; 408, R79.
63. Quast, U. Effect of the K+ efflux stimulating vasodilator
BRL 34915 on shRb efflux and spontaneous activity in
guinea pig portal vein. Br. J. Pharmacol. 1987; 91,
569-78.
64. Southerton, J.S. & Weston, A.H. Some effects of CaZ+and
K+-channel-blocking agents on responses to BRL 349 15
and pinacidil in the isolated rat portal vein. J. Physiol.
(London) 1987; 391,77P.
65. Southerton, J.S., Weston, A.H., Bray, K.M., Newgreen,
D.T. & Taylor, S.G. The potassium channel opening action
of pinacidil; studies using biochemical, ion flux and
microelectrode
techniques. Naunyn-Schmiedeberg’s
Arch. Pharmacol. 1988; 338,310-18.
66. Cook, N.S., Quast, U., Hof, R.P., Baumlin, Y. & Pally, C.
Similarities in the mechanism of action of the two new
vasodilator drugs, pinacidil and BRL 34915. J. Cardiovasc. Pharmacol. 1988; 11,90-9.
67. Vandenburg, M.J., Kakad, J., Wiseman, W.T. et al. BRL
34915, a novel potassium channel activator: duration of
antihypertensive action. Cardiovasc. Drugs Ther. 1978; 1,
299.
68. Petersen, H.J., Kaergaard-Nielsen, C. & Arrigoni-Martelli,
E. Synthesis and hypotensive activity of N-alkyl-N”cyano-N1-pyridylguanidines. J. Med. Chem. 1978; 21,
773-81.
69. Cohen, M.L. Pinacidil monohydrate - a novel vasodilator.
Drug Dev. Res. 1986; 6,1-10.
70. Bray, K.M., Newgreen, D.T., Small, R.C., Southerton, J.S.,
Taylor, S.G., Weir, S.W. & Weston, A.H. Evidence that the
mechanism of the inhibitory action of pinacidil in rat and
guinea-pig smooth muscle differs from that of glyceryl
trinitrate. Br. J. Pharmacol. 1987; 91,421-9.
71. Goldberg, M.R. Clinical pharmacology of pinacidil, a
prototype for drugs affecting potassium channels. J.
Cardiovasc. Pharmacol. 1988; 12 (Suppl. 2), 541-9.
72. Meisheri, K.D. & Cipkus, L.A. Minoxidil sulphate acts as a
K+ channel agonist to produce vasodilatation [Abstract].
Fed. Proc. Fed. Am. SOC.Exp. Biol. 1987; 46,1383.
73. Meisheri, K.D., Cipkus, L.A. & Taylor, C.J. Mechanism of
action of minoxidil sulphate-induced vasodilation: a role
for increased K + permeability. J. Pharmacol. Exp. Ther.
1988; 245,751-60.
74. Newgreen, D.T., Longmore, J. & Weston, A.H. The effect
of glibenclamide on the action of cromakalim, diazoxide
and minoxidil on rat aorta. Br. J. Pharmacol. 1989; 95,
116P.
75. Mondot, S., Mestre, M., Caillard, C.G. & Cavero, I.
RP49356: a vasorelaxant agent with potassium channel
activating properties. Br. J. Pharmacol. Proc. 1988: Suppl.
9 5 , s 139.
76. Thuringer, D. & Escando, D. The potassium channel
opener RP49356 modifies the ATP-sensitivity of K+-ATP
channels in cardiac myocytes. Pflugers Arch. 1989; 414
(Suppl. 1),S175.
77. De Peyer, L.E., Lues, I., Gericke, R. & Hausler, G. Characterization of a novel K+ channel activator, EMD52962, in
electrophysiological and pharmacological experiments.
Pflugers Arch. 1989; 414 (Suppl. l), S191.
78. Ashcroft, M.J. Adenosine 5’-triphosphate-~ensitivepotassium channels. Annu. Rev. Neurosci. 1988; 11,97-118.
79. Quast, U. & Cook, N.S. Moving together: K+ channel
openers and ATP-sensitive K+ channels. Trends Pharmacol. Sci. 1989; 10,431-5.
80. Escande, D. The pharmacology of ATP-sensitive K+
channels in the heart. Pflugers Arch. 1989; 414 (Suppl. l ) ,
93-8.
81. Benham, C.D., Bolton, T.B., Lang, R.J. & Takewaki, T.
The mechanism of action of Ba2+and TEA on single Ca2+
138
S. Kajioka e t al.
activated K + channels in arterial and intestinal smooth
muscle cell membrane. Pfliigers Arch. 1985; 403, 120-7.
82. Ohya, Y., Terada, K., Kitamura, K. & Kuriyama, H.
Membrane currents recorded from a fragment of rabbit
intestinal smooth muscle cells. Am. J. Physiol. 1986; 251,
C335-46.
83. Ohya, Y., Terada, K., Yamaguchi, K. et al. Effects of
inositol phosphates on the membrane activity of smooth
muscle cells of the rabbit portal vein. Pflugers Arch. 1988;
412,382-9.
84. Sakai, T., Terada, K., Kitamura, K. & Kuriyama, H.
Ryanodine inhibits the Ca-dependent KL current after
depletion of Ca stored in smooth muscle cells of the rabbit
ileal longitudinal muscle. Br. J. Pharmacol. 1988; 95,
1089-100.
85. Bolton, T.B. & Lim, S.P. Properties of calcium stores and
transient outward currents in single smooth muscle cells of
rabbit intestine. J. Physiol. (London) 1989; 409, 385-401.
86 Hume, J.R. & Leblanc, N. Macroscopic K+ currents in
single smooth muscle cells of the rabbit portal vein. J.
Physiol. (London) 1989; 413,49-73.
87 Okabe, K., Kitamura, K. & Kuriyama, H. Features of
4-aminopyridine sensitive outward current observed in
single smooth muscle cells from the rabbit artery. Pfliigers
Arch. Eur. J. Physiol. 1987; 409, 56 1-8.
88. Edwards, F.R. & Hirst, G.D.S. Inward rectification in submucosal arterioles of the guinea-pig ileum. J. Physiol.
(London) 1988; 404,437-55.
89. Benham, C.D. & Bolton, T.B. Spontaneous transient
outward currents in single visceral and vascular smooth
muscle cells of the rabbit. J. Physiol. (London) 1986; 381,
38 5-406.
90. Lang, R.J. Identification of the major membrane currents
in freshly dispersed single smooth muscle cells of the
guinea-pig ureter. J. Physiol. (London) 1989; 41 2,
375-95.
91. Bregestovski, P.D., Printseva, O.Y., Serebryakov, V.,
Stinnakre, J., Turmin, A. & Zamoski, V. Comparison of
Ca2+-dependent K+ channels in the membrane of smooth
muscle cells isolated from adult and foetal human aorta.
Pfliigers Arch. 1988; 413,8-13.
92. Supattapone, S., Worley, P.F., Baraban, J.M. & Snyder, S.H.
Solubilization, purification and characterization of an
inositol trisphosphate receptor. J. Biol. Chem. 1988; 263,
1530-4.
93. Supattapone, S., Strittmatter, S.M., Fricker, L.D. & Snyder,
S.H. Characterization of a neural divalent cation-sensitive
endopeptidase: a possible role in neuropeptide processing.
Brain Res. 1989; 427, 173-81.
94. Inui, M., Saito, A. & Fleischer, S. Purification of the
ryanodine receptor and identity with feet structures of
junctional terminal cisternae of sarcoplasmic reticulum
from fast skeletal muscle. J . Biol. Chem. 1987; 262,
1740-7.
95. Imagawa, T., Smith, J.S., Coronado, R. & Campbell, K.P.
Purified ryanodine receptor from skeletal muscle sarcoplasmic reticulum is the Ca*+-permeable pore of the
calcium release channel. J. Biol. Chem. 1987; 262,
16636-43.
96. Lai, F.A., Erickson, H.P., Rousseau, E., Liu, Q.-Y. &
Meissner, G. Purification and reconstitution of the calcium
release channel from skeletal muscle. Nature (London)
1988;331,315-19.
97. Itoh, T., Ucno, H. & Kuriyama, H. Calcium-induced
calcium release mechanism in vascular smooth muscles assessments based on contractions evoked in intact and
saponin-treated skinned muscles. Experientia 1985; 41,
989-96.
98. Inoue, R., Kitamura, K. & Kuriyama, H. Two Ca-dependent K-channels classified by the application of tetraethylammonium distribute to smooth muscle membranes of the
rabbit portal vein. Pfliigers Arch. 1985; 405, 173-9.
99. Inoue, R., Okabe, K., Kitamura, K. & Kuriyama, H. A
newly-identified Caz+-dependent K+-channel in the
smooth muscle membrane of single cells dispersed from
the rabbit portal vein. Pflugers Arch. 1986; 406, 138-43.
100. Standen, N.B., Quayle, J.M., Davis, N.W., Brayden, J.E.,
Huang, Y. & Nelson, M.T. Hyperpolarizing vasodilators
activate ATP-sensitive K+-channels in arterial smooth
muscle. Science 1989; 245, 177-80.
101. Kajioka, S., Oike, M. & Kitamura, K. Nicorandil opens a
calcium-dependent potassium-channel in smooth muscle
cells of the rat portal vein. J. Pharmacol. Exp. Ther. 1990;
254,905-13.
102. Kajioka, S., Oike, M., Kitamura, K. & Kuriyama, H.
Nicorandil opens a Ca-dependent and ATP-sensitive
potassium channel in the smooth muscle cells of the rat
portal vein. Jpn. J. Pharmacol. 1990; 52 (Suppl. I), SIP.
103. Okabe, K., Kajioka, S., Nakao, K., Kitamura, K., Kuriyama,
H. & Weston, A.H. Actions of cromakalim on ionic
currents recorded from single smooth muscle cells of the
rat portal vein. J. Pharmacol. Exp. Ther. 1990; 250,
832-9.
104. Sims, S.M., Singer, J.J. & Walsh, J.V., Jr. Cholinergic
agonists suppress a potassium current in freshly dissociated smooth muscle cells of the toad. J. Physiol.
(London) 1985; 367,503-29.
105. Noma, A. ATP-regulated K+-channels in cardiac muscle.
Nature (London) 1983; 305, 147-8.
106. DeWeille, J.R., Fosset, M., Mourre, C., Schmid-Antomarchi, H., Bernardi, H. & Lazdunski, M. Pharmacology
and regulation of ATP-sensitive K+ channels. Pfliigers
Arch. 1989; 414 (Suppl. I), 80-7.
107. Schmid-Antomarchi, H., deWeille, J.R., Fosset, M. &
Lazdunski, M. The receptor for antidiabetic sulfonylureas
controls the activity of the ATP-modulated K + channel in
insulin-secreting cells. J. Biol. Chem. 1987; 262, 15840-4.
108. Schmid-Antomarchi, H., deWeille, J.R., Fosset, M. &
Lazdunski, M. The antidiabetic sulfonylurea glibenclamide
is a potent blocker of the ATP-modulated K+ channel in
insulin-secreting cells. Biochem. Biophys. Res. Commun.
1987; 146,21-5.
109. Beech, D.J. & Bolton, T.B. Properties of the cromakaliminduced potassium conductance in smooth muscle cells
isolated from the rabbit portal vein. Br. J. Pharmacol.
1989; 98,851-64.
110. Williams, D.L., Jr., Katz, G.M., Roy-Contancin, L. &
Reuben, J.P. Guanosine 5'-monophosphate modulates
gating of high-conductance Caz+-activated K+-channels in
vascular smooth muscle cells. Proc. Natl. Acad. Sci. U.S.A.
1988; 85,9360-4.
111. Kusano, K., Barros, F., Katz, G., Garcia, M., Kaczorowski,
G. & Reuben, J.P. Modulation of K channel activity in
aortic smooth muscle by BRL 34915 and a scorpion toxin.
Biophys. J. 1987; 51,55a.
112. Hermsmeyer, R.K. Pinacidil actions on ion channels in
vascular smooth muscle. J. Cardiovasc. Pharmacol. 1988;
12 ( S ~ p p l2),
. 517-22.
113. Gelband, C.H., Lodge, N.J., Talvenheime, J.A. & vanBreemens, C. BRL 34915 increases Pope,, of the large conductance Ca2+ activated K+ channel isolated from rabbit
aorta in planar lipid bilayers. Biophys. J. 1988; 53, 149a.
114. Inoue, I., Nakaya, Y., Nakaya, S. & Mori, H. Extracellular
Ca'+-activated K channel in coronary artery smooth
muscle cells and its role in vasodilation. FEBS Lett. 1989;
255,281-4.
115. Nakao, K., Okabe, K., Kitamura, K., Kuriyama, H. &
Weston, A.H. Characteristics of cromakalim-induced
relaxations in smooth muscle cells of guinea-pig mesenteric artery and vein. Br. J. Pharmacol. 1988; 95,795-804.
116. Yamanaka, K., Furukawa, K. & Kitamura, K. The different
mechanisms of action of nicorandil and adenosine triphosphate on potassium channels of circular smooth
muscle of the guinea-pig small intestine. NaunynSchmiedeberg's Arch. Pharmacol.; 1985; 331,96-103.
K+-channel activators in vascular tissues
117. Cook, D.L. & Hales, C.N. Intracellular ATP directly
blocks K+ channels in pancreatic /?-cells.Nature (London)
1984;311,271-3.
118. Ashcroft, EM., Ashcroft, S.J.H. & Harrison, D.E. The
glucose-sensitivepotassium channel in rat Dancreatic betacells is inhibited by intracellular ATP. J. P6ysiol. (London)
1985;369.101P.
119. Ashcroft, F.M., Kakei, M., Kelly, R.P. & Sutton, R. ATPsensitive K-channels in isolated-human pancreatic /?-cells.
FEBS Lett. 1987; 215.9-12.
120. Rorsman, P. & Trube, G. Glucose dependent K+ channels
in pancreatic /?-cells are regulated by intracellular ATP.
Pflugers Arch. 1985; 405, 305-9.
121. Misler, D.S., Falke, L.C., Gillis, K. & McDaniel, M.L. A
metabolite regulated potassium channel in rat pancreatic /?
cells. Proc. Natl. Acad. Sci. U.S.A. 1986; 83,7119-23.
122. Ashcroft, EM. & Kakei, M. ATP-sensitive K+ channels in
rat pancreatic /?-cells: modulation by ATP and Mg2+ ions.
J. Physiol. (London) 1989; 416,349-67.
123. Trube, G. & Hescheler, J. Inward-rectifying channels in
isolated patches of the heart cell membrane: ATP-dependence and comparison with cell-attached patches. Pflugers
Arch. 1984;401,178-84.
124. Trube, G., Rorsman, P. & Ohno-Shosaku. T. Opposite
effect of tolbutamide and diazoxide on the ATP-dependent
K+ channel in mouse pancreatic /?-cells. Pflugers Arch.
1986; 407,493-9.
125. Kakei, M. & Noma, A. Adenosine 5'-triphosphatesensitive single potassium channel in the atrioventricular
node cell of the rabbit heart. J. Physiol. (London) 1984;
352,265-84.
126. Noma, A. & Shibasaki, T. Membrane current through
adenosine-triphosphate regulated potassium channels in
guinea-pig ventricular cells. J. Physiol. (London) 1985;
363,463-80.
127. Kakei, M., Noma, A. & Shibasaki, T. Properties of
adenosine-triphosphate regulated potassium channels in
guinea pig ventricular cells. J. Physiol. (London) 1985;
363,441-62.
128. Arena, J.P. & Kass, R.S. Activation of ATP-sensitive K
channels in heart cells bv Dinacidik deDendence of ATP.
Am. J. Physiol. 1989; 26,H2092-6.
129. Kakei. M.. Yoshinam. M.. Saito. K. & Tanaka. H. The
potassiumcurrent activated by 2-nicotinamidoethyl nitrate
(nicorandil) in single ventricular cells of guinea pigs. Proc.
R. SOC.London Ser. B 1986; 229,331-43.
130. Hiraoka, M. & Fan, Z. Activation of ATP-sensitive outward K + current by nicorandil (2-nicotinamidoethyl
nitrate) in isolated ventricular myocytes. J. Pharmacol.
Exp. Ther. 1989; 250,278-85.
131. Fan, Z., Nakayama, K. & Hiraoka, M. Pinacidil activates
the ATP-sensitive K+ channel in inside-out and cell
attached patch membranes of guinea-pig ventricular
myocytes. Pflugers Arch. 1990; 415,387-94.
132. Sturgess, N.C., Ashford, M.L.J., Cook, D.L. & Hales, C.N.
The sulphonylurea receptor may be an ATP-sensitive
potassium channel. Lancet 1985; ii, 474-75.
133. Carmeliet, E. K+ channels in cardiac cells: mechanisms of
activation, inactivation, rectification and K f , sensitivity.
Pflugers Arch. 1989; 414 (Suppl. l), 88-92.
134. Ohno-Shosaku, T., Zunckler, B. & Trube, G. Dual effects
of ATP on K f currents of mouse pancreatic @-cells.
Pflugers Arch. 1987; 408,133-8.
139
135. Spuler, A., Lehmann-Horn, F. & Grafe, P. Cromakalim
(BRL 34915) restores in vitro the membrane potential of
depolarized human skeletal muscle fibres. NaunynSchmiedeberg's Arch. Pharmacol. 1989; 339,327-31.
136. Ashcroft, EM., Ashcroft, S.J.H., Berggren, P.O. et al.
Expression of K channels in Xenopus laevis oocytes
injected with poly(A+) mRNA from the insulin-secreting
/?-cell lines, HITTl5. FEBS Lett. 1988; 239, 185-9.
137. Dunne, M.T., Yule, D.I., Gallacher, D.V. & Petersen, O.H.
Cromakalim (BRL 34915) and diazoxide activate ATPregulated potassium channels in insulin-secreting cells.
Pflugers Arch. 1989; 414 (Suppl. l), 154-5.
138. Arrigoni-Martelli, E., Nielsen, C.K., Bang Olse, U.B. &
Petersen, H.J. N"-Cyano-N-4-pyridyI-N'-1,2,2-trimethylpropyl-guanidine monohydrate (P1134): a new, potent
vasodilator. Experientia 1980; 36,445-7.
139. Itoh, T., Seki, N., Kajikuri, T. & Kuriyama, H. Effects of
pinacidil on electrical and mechanical activities in the
rabbit mesenteric artery. Eur. J. Pharmacol. 1990; 183,
1739.
140. Cowlrick, I.S., Paciorek, P.M. & Waterfall, J.F. Cromakalim
and calcium movements in vascular smooth muscle. Br. J.
Pharmacol. 1988; 95,640P.
141. Cook, N.S., Quast, U. & Eier, S.W. In vitro and in vivo
comparison of two K+ channel openers diazoxide and
BRL 34915. Pflugers Arch. 1988; 411, R46.
142. Wilson, C. & Hicks, F. Comparison of K+ channel
activators with other vasorelaxants as inhibitors of
caffeine-induced arterial contraction. Br. J. Pharmacol.
1989; 96,222P.
143. Bray, K.M. & Weston, A.H. Effects of K+ channel openers
on the spasmogenic component of caffeine-induced
responses in rabbit aorta. Br. J. Pharmacol. 1989; 96,
220P.
144. Chiu, P.J.S., Tezloff, G. & Sybertz, E.J. Effects of BRL
34915 [( + )6-cyano-3,4-dihydro-2,2-dimethyl-tr~ns-4-(20x0- l-pyrolidyl)-2H-benzo( b)pyran-3-ol]
on
transmembrane 45Ca movements in rabbit aortic smooth
muscle. Hypertension 1987; 10,360.
145. Cox, R.H. & Ferrone, R.A. Effects of BRL 34915 on X6Rb
efflux and active stress in rat vascular smooth muscle.
FASEB J. 1988; 2, A756.
146. Wilson, C. Comparative effects of cromakalim on contractions to noradrenline and caffeine in rabbit isolated
renal artery. Br. J. Pharmacol. 1988; 95,570P.
147. Nakashima, M., Li, Y., Seki, N. & Kuriyama, H. Pinacidil
indirectly inhibits neuromuscular transmission in the
guinea-pig and rabbit mesenteric arteries. Br. J. Pharmacol.
1990; 101,581-6.
148. Nedergaard, O.A. Effects of pinacidil on sympathetic
neuroeffector transmission in rabbit blood vessels.
Pharmacol. Toxicol. 1989; 65,287-94.
149. Cook, N.S. & Quast, U. Potassium channel pharmacology.
In: Cook, N.S., ed. Potassium channels - structure, classification, function and therapeutic potential. Chichester: Ellis
Harwood, 1990: 181-55.
150. Sims, S.M., Singer, J.J. & Welsh, J.V., Jr. Antagonistic
adrenergic-muscarinic regulation of M current in smooth
muscle cells. Science 1988; 239, 190-3.
151. Benham, C.D., Bolton, T.B., Denbigh, J.S. & Lang, R.J.
Inward rectification in freshly isolated single smooth
muscle cells of the rabbit jejunum. J. Physiol. (London)
1987; 383,461-76.