Clinical Science (1986)7 0 (Suppl. 14),7s-13s
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The role of smooth muscle and its possible involvement in
diseases of the lower urinary tract
A. F. BRADJNG, J. L. MOSTWIN, G.N. A. SIBLEY AND M. J. SPEAKMAN
University Department of Pharmacology, South Parks Road, Oxford, U.K.
Introduction
Arrangement of smooth muscle cells
A full understanding of the behaviour and control
of the urinary tract requires a knowledge of the
basic physiology of the smooth muscle cells themselves, how their activity can be modulated both
physiologically and pharmacologically, and any
alterations that occur in the diseased state. With this
knowledge a more rational approach to the treatment of urinary tract disorders may be possible.
It is interesting that relatively little effort has been
put into studies of the basic properties of these
smooth muscles until comparatively recently. Much
more attention has been paid to smooth muscles of
the blood vessels, gut and uterus. With a few exceptions, most work on the urinary tract has been
pharmacological, confined to looking at the ability
of agonist drugs to initiate tension changes and
antagonists to antagonize nerve-mediated or
agonist-induced tension changes. We have recently
begun to look more intensively at urinary tract
smooth muscles, building on the tradition set up in
this department by Professor Edith Bulbring.
We are studying the basic properties of the
normal muscle for two reasons: first to define
mechanisms which may be susceptible to modulation by drugs, and second to provide a basis on
which to compare any differences in smooth
muscles from systems which show defects. In this
paper we will discuss what we know of the
properties of the cells, how they generate tone, the
effects of intrinsic nerves on their activity, and the
mechanisms involved in this control.
Urinary tract muscles share the properties common
to most smooth muscles. The cells are very long and
thin, and electrically interconnected in a threedimensional network, so that electrical activity can
spread from cell to cell. The cells are arranged
roughly in parallel in interconnecting bundles,
which may be organized in many different ways
depending on the particular organ. It is, however,
dif€icult to detect distinct layers in detrusor or
ureter analogous to the circular and longitudinal
layers of other tissues.
Key words: bladder, intracellular potentials, smooth
muscle, urethra.
Correspondence: Dr A. F. Brading, Lecturer, University Department of Pharmacology, South Parks Road,
Oxford OX1 3QT, U.K.
Factors affecting smooth muscle tone
In the absence of any pharmacological intervention,
the tone of the smooth muscle will be determined
by the myogenic properties of the cells, the activity
of the nerves innervating the cells, and the presence
of any naturally produced modulatory substances,
such as circulating catecholamines, prostaglandins
etc. Much of the research directed towards drug
treatment of behavioural disorders of the lower
urinary tract has been aimed at affecting the influence of the nerves, for instance by identifying and
selectively blocking the receptors to the various
transmitters involved. However, the intrinsic contractile mechanisms of the smooth muscle itself are
obviously of fundamental importance, and it is possible that drugs directed at the muscle cells could be
equally useful in controlling the tone.
Myogenic activity
We have seen spontaneous mechanical activity in
strips of the detrusor muscle from all the mammals
we have examined (namely guinea-pig, rabbit, pig
and human [l,21). It is true that this activity is seen
less often in the larger mammals, and is not clearly
A . F. Brading et al.
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]
]
10 s
10 mv
0.5
10 s
FIG.1. Mechanical and electrical recording of the activity of a strip of smooth muscle
from the guinea-pig bladder, using the double sucrose-gap technique. The cells in the
gap were alternately depolarized and hyperpolarized by extracellular current application. Note, on the extended time scale tracing, that each action potential initiates a small
increment of tension.
present in every strip in human detrusor, but this
could simply be due to the greater difficulty in dissecting parallel bundles of smooth muscle from the
larger bladders.
Simultaneous recording of electrical and
mechanical activity from detrusor muscle strips of
the guinea-pig in the double sucrose gap, show that,
as expected, the contractions are triggered by action
potentials, and the level of tone is determined by
the frequency of the action potentials (see Fig. 1).
Similar results have been recorded in the rabbit [3].
The spontaneous activity is myogenic. It is little
affected by atropine at doses enough to block
cholinergic activity, and by tetrodotoxin (TTX) at a
concentration that will block conducted activity in
the nerves. The frequency of the action potentials is
very sensitive to the membrane potential, and any
stimulus tending to depolarize the membrane will
increase the frequency leading to contraction, and
hyperpolarization will decrease it, and lead to relaxation (Fig. 2). Unlike much gut muscle, there is no
evidence of a true 'slow wave' mechanism underlying the activity; it seems as if the cells possess
intrinsic pacemaker activity, and that within an
interconnected bundle of cells, the cell that reaches
threshold first will fire the bundle.
Electrical recordings from microelectrodes show
that in some cells the action potential takes off without a prepotential, and in others a clear pacemakertype depolarization is seen. Spontaneous activity
has also been seen in guinea-pig urethra [4], but was
less regular than in the bladder.
The action potential mechanisms seem to be
similar to other smooth muscles [5].Fig. 3 illustrates
some of the properties of the action potential in
A
0
U
5s
FIG. 2. Microelectrode recording from cells of the
guinea-pig bladder. A, Depolarizing current was
applied for the duration of the bar, causing a
marked increase in spontaneous spike frequency. B,
Hyperpolarizing current, applied during the bar,
caused a cessation of spike activity.
guinea-pig bladder smooth muscle recorded with
intracellular microelectrodes. The upstroke is not
due to activation of the classical voltage-sensitive
Na+ channels since the spikes persist in the absence
of Na+ (sucrose replacing NaCl), and in fact show a
faster rate of rise. They are also insensitive to TTX.
The rate of rise and size of the spikes is, however,
diminished as extracellular Ca2+ is lowered, and if
Ca2+ antagonists are applied, suggesting that the
upstroke depends on the activation of the slower
voltage-sensitive Ca2+ channels. There is also a
pronounced negative after-potential in guinea-pig,
Role of smocbth muscle
A
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This spontaneous activity is of great importance
to the function of the organs, and allows both
contractions and relaxations of the muscle to be
activated by nerves or circulating modulators.
a
Efsects of transmural nerve stimulation
20 ms
B
In small strips of smooth muscle, the nerves
running between the muscle fibres can be selectively activated by extracellular electrical stimulation. This is due to the electrical properties of the
muscle cells, which have a very long time constant,
so that applied current takes much longer to change
the membrane potential of the muscle than the
nerves [6,7]. We have found, however, that in bladder very short stimuli are needed to be sure that
there is no direct effect on the smooth muscle: it
seems that some of the cells are poised so close to
threshold, that very small depolarizations can fire
them. We use 0.05 ms for nerve stimulation, and at
this duration, except at very high stimulus frequencies, effects are abolished by TTX, suggesting that
they are entirely nerve mediated [l].
In studies of the smooth muscles of bladder
detrusor and trigone, bladder neck and urethra,
there is evidence of three types of excitatory innervation, and one of the inhibitory innervation,
depending on the species and muscle. All bladders
we have studied show powerful excitatory cholinergic innervation of the detrusor, less powerful of the
trigone and bladder neck. In all species except man,
there is also a non-cholinergic non-adrenergic excitatory innervation of the detrusor; we have no evidence of this in the human material we have studied
[1], although other groups believe that this type of
innervation may be present in humans. In the trigone, bladder neck and urethra of man and other
mammals there is a non-cholinergic, possibly
adrenergic, excitatory innervation that in bladder
neck is often more powerful than the cholinergic
one, and we have also recently seen a non-adrenergic, non-cholinergic inhibitory innervation of the
trigone in humans and pigs.
.
20 ms
a.
nn
- 7 1 I
.
\
50 ms
FIG.3. Microelectrode recording of action potentials from cells of the guinea-pig bladder. A, Effect
of lowering the extracellular Ca2+concentration: (a)
control (2.5 mmolfl); (b) after 10 min in 10% Ca2+
(0.25 mmolfl); (c) shortly after reducing the calcium
to 5% and adding ethyleneglycol-bis(B-aminoethyl
ether)-N, N', N'-tetra-acetic acid (EGTA: 1 mmolfl);
(d)in the same solution as (c),2 min later. B, Effect
of lowering extracellular Na (replacing NaCl with
sucrose): (a) control (136 mmolA Na+);(b) after 10
min in 10% Na+ (13.6 mmolfl). C, Effect of TEA:
(a) control without TEA; (b)after 10 min in TEA (9
mmolfl). Stimulus duration 12 ms (partition field
stimulation method).
+
which may be due to activation of Ca2+-dependent
K + channels, since it is diminished as extracellular
Ca2+is lowered. The repolarization phase probably
involves voltage-sensitive K + channels, as suggested by the prolongation of the spike duration by
the K + channel blocker tetraethylammonium
(TEA).
Postsynaptic potentials
There have been relatively few electrophysiological
studies of neuromuscular transmission in urinary
tract smooth muscles and, as far as we are aware,
none in human muscle. The results that have been
obtained in rabbit and guinea-pig tissues are
described below.
Cholinergic
Evidence is accumulating that acetylcholine
released from the cholinergic nerves does not pro-
A . F. Brading et al.
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duce clearly defined rapid junction potentials in
response to each nerve stimulus, but may produce a
slowly developing small depolarization, which can
lead to an increase in the frequency of spontaneous
action potentials. This depolarization increases with
higher frequencies of stimulation, is blocked by
atropine and enhanced by cholinesterase inhibitors.
This type of behaviour has been recorded from
guinea-pig bladder (this study), rabbit bladder [3],
and recently in rabbit bladder neck and proximal
urethra [S].
Non-cholinergic, non-adrenergic( excitatory)
In contrast, the non-cholinergic, non-adrenergic
excitatory nerves cause discrete excitatory junction
potentials (e.j.p.s.), which, if large enough, trigger
spike production, as seen in guinea-pig bladder
(Fig. 4), and rabbit bladder and bladder neck [3, 81.
This suggests that these nerves must form quite
close junctions with the muscle. The tension behaviour of isolated strips seems to support this suggestion, since a non-cholinergic response can be
triggered by a single stimulus, whereas atropine
more effectively blocks the response to higher frequencies of stimulation [9].
A
Adrenergic (excitatory)
In rabbit proximal urethra, Ito & Kimoto [S] have
found nerve evoked e.j.p.s. that are reduced (but not
abolished) by a-receptor blockers or guanethidine.
Non-cholinergic, non-adrenergic( inhibitoty)
Ito & Kimoto [8] have also recorded inhibitory
junction potentials (i.j.p.s.) in rabbit proximal
urethra that are not affected by a - or j3-receptor
blockers, atropine or guanethidine.
The mechanisms underlying these postsynaptic
membrane potential changes have not so far been
investigated in urinary tract smooth muscle, but in
analogy with other smooth muscles, opening or
closing of various ‘receptor operated’ ionic channels
is likely to be involved. Studies with exogenous
application of transmitters in the double sucrosegap apparatus allows the membrane resistance to be
monitored, and this technique should be able to
throw light on the mechanisms involved.
Postsynaptic potentials can either alter the frequency of action potentials or initiate them de ~ I O V O .
In the case of the cholinergic innervation which
does not evoke discrete e.j.p.s. it is likely that the
predominant effect is to increase action potential
frequency, as a consequence of membrane depolarization. Application of acetylcholine to
detrusor muscle does indeed depolarize the membrane and cause an increase in the firing rate, associated with increased tension.
In those species showing a non-adrenergic, noncholinergic excitatory response, tension records
show that single nerve impulses will produce a small
contractile response which is atropine resistant, and
presumably due to a spike evoked by an e.1.p. The
fact that e.j.p.s. can be recorded suggests that the
nerves must be making close synaptic contacts with
the muscle cells. This innervation has been suggested to be purinergic, with ATP as the transmitter
(e.g. [lo]).
The excitatory adrenergic innervation of the trigone, bladder neck and urethra is of obvious importance to this Symposium, but unfortunately has not
been studied so extensively at a basic level. These
tissues respond to noradrenaline often with a
vigorous contraction, which is blocked by a receptor blockers. It is interesting, however, that
recent reports from the Department of Urology and
Physiology at Nottingham, on work from human
bladder neck, have shown that the nerve-evoked
non-cholinergic contraction is not blocked by doses
of a-receptor blockers that completely abolish the
noradrenaline-evoked contraction of the same preparation. This situation is very reminiscent of the
behaviour of the vas deferens of most species
i
B
I
u
2s
FIG.4. Microelectrode recording of nerve-evoked
responses of guinea-pig bladder muscle. A, Recording at the normal membrane potential. An excitatory junction potential (e.j.p.) is evoked which
triggers a spike. B, Stimulation applied during a
conditioning hyperpolarization using the partition
field stimulation method. Note that this eliminates
the evoked action potential, revealing the time
course of the e.j.p., and initiates an off response.
Nerve stimulation applied through ring electrodes
(0.1 s, 100 V).
Role of smooth muscle
studied, and of several blood vessels, Such results
have led to controversy in their interpretation. On
the other hand, a separate class of catecholamine
receptor (the y-receptor) not blocked by a- or j3adrenoceptor blockers, has been postulated (for
review see [ll]),and on the other hand, purinergic
receptors activated by co-release of ATP from the
nerve terminal, have been suggested to be involved
[12, 131.
With reference to the i.j.p.s. recorded by Ito &
Kimoto [8] in rabbit proximal urethra, we have
recently found an inhibitory response to transmural
nerve stimuli during mechanical recordings from
the human trigone. The response is a rapid relaxation which can be elicited by single stimuli, and thus
is likely to be due to a close inhibitory innervation
which might be causing i.j.p.s. similar to those in the
rabbit. This type of inhibitory innervation appears
similar to that found in gut muscle (e.g. [14]) and
again it has been suggested to be purinergic.
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Calcium may also enter the tissue through calcium channels other than those involved in the
spike mechanism. Two types have been postulated,
channels specifically coupled to receptors, and
slow, voltage-dependent channels that will be activated whenever the cells are depolarized. These
mechanisms are thought to be responsible for the
tonic contractions to agonists. In studies of the
contractile responses of detrusor muscle, however,
it is difficult to get the tissue to maintain any reasonable degree of tone; it is a remarkably phasic tissue,
and although peak responses to agonists may be
large, they fade rapidly and are also often very
spiky, the components of tension associated with
each action potential being visible [2]. This is in
contrast to many other muscles, where high doses
of agonists seem to depolarize the tissue into a
range where the spike mechanism is activated, but
tension is maintained apparently through the slow
voltage-sensitive Ca2+ channels. It is possible that
the detrusor muscle either has less of the Ca2+
channels, or has very efficient Ca2+ buffering and
Mechanisms involved in modulation of muscle
extrusion mechanisms. Presumably the phasic
tone
nature of the muscle, and the fact that spike rate is
the main determinant of the tone, is an important
Although recent studies on the contractile profeature of this muscle, and will allow the precise
cesses of smooth muscle have suggested that
nervous control necessary.
changes in tension could under some circumstances
In the bladder neck and urethra, one would
occur in the absence of changes in free intracellular
expect that maintenance of tone would be of greater
Ca2+,nevertheless, under normal conditions alteraimportance. It has been suggested by It0 & Kimoto
tions in Ca2+are the most likely cause of changes in
[8]that in rabbit urethra endogenous prostaglandins
the degree of activation of the contractile
may play a physiological role in maintaining the
machinery.
muscle tone, through a mechanism not involving
The very tight coupling of action potentials to
changes in the membrane potential.
phasic tension response seen in detrusor muscle
Many urinary tract smooth muscles possess
suggests that most of the control is achieved
adrenergic j3-receptors, which mediate inhibition.
through alteration of the frequency of action potenThe mechanisms of action probably involve cyclic
tials. Exactly how an action potential initiates actiAMP-mediated phosphorylation of several provation of the contractile machinery is not yet clear.
teins, causing such effects as activation of plasma
Ca2+carrying the inward current will elevate intramembrane calcium pumps and active lowering of
cellular Ca2+ levels, but it is unlikely that this is a
free cell calcium concentration. In detrusor muscle
major source of activator Ca2+, because most
smooth muscles have considerable Ca2+ buffering the j3-receptor response to catecholamines is the
predominant one, although in pig and rabbit
power. The smooth muscle cells also contain sarcodetrusor, but not in human muscle, if the j3-receptor
plasmic reticulum, usually close to the plasma membrane, and the spikes may also release these Ca2+ response is blocked, some strips show a very small
contractile response to a-adrenoceptor activation.
stores.
In trigone and bladder neck the excitatory aIt is a common feature of smooth muscles that
receptor response is dominant, but in the presence
some agonists can directly release Ca2+from these
of a-receptor blockers, a /?-receptor response can
stores without necessarily altering the membrane
often be uncovered.
potential, and such direct effects may have to be
considered when accounting for the action of drugs
and transmitters. In particular excitatory aAbnormalities of smooth muscle
receptor activation in several tissues, especially
Finally, we would like briefly to discuss the posblood vessels, is known to involve release of such
sibility that, in diseases of the lower urinary tract,
stores. Stores have been demonstrated in guinea-pig
there may be changes in the smooth muscles thembladder [2], although their physiological role has not
selves, which merit investigation. One example of
yet been elucidated.
12s
A . F. Brading et al.
such a condition is the finding [9] that, in detrusor
instability, particularly that resulting from urinary
outflow obstruction, the smooth muscle becomes
supersensitive to a range of different agonists. This
behaviour is reminiscent of the classical denervation supersensitivity of smooth muscles [ 141, and is
associated with a reduced effectiveness of intramural nerve stimulation to activate the muscle. Under
these conditions, it would seem sensible to investigate the underlying mechanism of this supersensitivity, and to look for drug treatment which reduces
the excitability of the cells.
It would be interesting to see if smooth muscle
abnormalities are associated with other disorders.
The more we understand the behaviour of the
normal cells, the more easily we will be able to pick
up and exploit any abnormalities which are present.
A treatment which leaves relatively intact the
normal control mechanisms, and operates by
modulating the excitability of the smooth muscle
cell would seem most desirable.
Acknowledgments
This work is supported by the Medical Research
Council, the Welcome Foundation, and the American Urological Association.
References
1. Sibley, G.N.A. (1984) A comparison of spontaneous
and nerve-mediated activity in bladder muscle from
man, pig and rabbit. Journal of Physiology (London),
354,431-443.
2. Mostwin, J.L. (1985) Intracellular receptor-operated
calcium stress in the smooth muscle of the guinea-pig
bladder. Journal of Urology, 133 (In press).
3. Creed, K.E., Ishikawa, S. & Ito, Y. (1983) Electrical
and mechanical activity recorded from rabbit urinary
bladder in response to nerve stimulation. Journal of
Physiology (London),338,149- 164.
4. Callahan, S.M. & Creed, K.E. (1981) Electrical and
mechanical activity of isolated lower urinary tract of
the guinea-pig. British Journal of Pharmacology, 74,
353-358.
5. Tomita, T. (1981) Electrical activity (spikes and slow
waves) in gastrointestinal smooth muscle. In: Smooth
Muscle: an Assessment of Current Knowledge. Ed.
Bulbring, E., Brading, A.F., Jones, A.W. & Tomita, T.
Edward Arnold, London.
6 . Tomita, T. (1966) Membrane capacity and resistance
of mammalian smooth muscle. Journal of Theoretical
Biology, 12,216-227.
7. Tomita, T. (1975) Electrophysiology of mammalian
smooth muscle. Progress in Biophysics and Molecular Biology, 30, 185-203.
8 Ito, Y. & Kimoto, Y. (1985) The neural and nonneural mechanisms involved in urethral activity in
rabbits. Journal of Physiology( London) (In press).
9. Sibley, G.N.A. (1985) An experimental model of
detrusor instability in the obstructed pig. Brirish
Journal of Urology(1npress).
10. Burnstock, G., Cocks, T., Crowe, R. & Kasakov, L.
(1978) Purinergic innervation of the guinea-pig
urinary bladder. British Journal of Pharmacology,
63,121-138.
11. Neild, T.D. & Zelcer, E. (1982) Noradrenergic
muscular transmission with special reference to
arterial. smooth muscle progress in neuropathology.
Progress in Neurology, 19,141-158.
12. Sneddon, P. & Burnstock, G. (1984) ATP as a
cotransmitter in rat tail artery. European Journal of
Pharmacology, 106,149-152.
13. Sneddon, P. & Westfall, D.P. (1984) Pharmacological
evidence that adenosine triphosphate and noradrenaline are co-transmitters in the guinea-pig vas
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561-580.
14. Tomita, T. & Watanabe, H. (1973) A comparison of
the effects of adenosine triphosphate with noradrenaline and with the inhibitory potential of the guineapig taenia coli. Journal of Physiology (London),231,
167-177.
15. Westfall, D.P. (1981) Supersensitivity of smooth
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DISCUSSION
Mundy: Thank you very much, Dr Brading. Do
we have any questions?
Gosling: I was very interested in your observations concerning the possibility of denervation
supersensitivity, because we have been doing some
work on bladders that have been unstable and
obstructed and have found that there is reduction in
the distribution of nerves within the muscle, when
measured objectively compared with age-matched
controls, so there does appear in those bladders to
be a real reduction in nerves. The thing that was
noticeable, however, was that we have not been able
to demonstrate a similar reduction in patients who
have got instability in the absence of obstruction. I
wondered whether you, under your title of ‘instability’, distinguished between those whose instability might be related to obstruction and the
idiopathic type, where we have not been able to
demonstrate any change in the autonomic innervation?
Brading: This is Dr Sibley’s work. He has not
looked at very many of the unstable non-obstructed
patients because they have not come our way all
that often. But he certainly had a group of, I think,
some half a dozen bladders that he looked at, and
we found the same pattern. We were expecting to
find differences, but at the moment I do not think
we have any solid statistical evidence with the
Role of smooth muscle
rather few samples where they really are supersensitive. But there was no clear difference between
that group of the unstables and the obstructed ones.
We hope to get more material.
Mundy: Richard Kinder in our laboratory did
some similar studies to those that have been
described, in patients with both idiopathic instability and post-obstructive instability. In our experience the proportion of patients is more the other
way round. We see far more of the idiopathic instability. Following Gary Sibley and Alison Brading’s
report of this we could also find a reduced response
to electrical stimulation and an increased sensitivity
to acetylcholine in obstructed instability. But in
non-obstructed detrusor instability there was no
change in sensitivity to acetylcholine, and the
response to electrical stimulation was quite
markedly different with an increased sensitivity to
lower frequency stimulation. So there does indeed
appear to be quite a definite distinction on this and,
as you are aware from other things that we have
done, in other aspects, between obstructed instability and idiopathic instability, and it stresses the
clinical observation frequently made that these two
should be distinguished the one from the other.
13s
Kirby: Thank you for a very interesting talk, Dr
Brading. In your obstructed patients in whom you
have demonstrated these findings suggestive of denervation, do you think that the changes you find
indicate some change in receptor density on the
smooth muscle cells, or some intrinsic change of
threshold electrical activity of the muscle cells
themselves?
Brading: I can only really answer that in terms of
what other people have found. We have looked at
receptor binding studies, but when you get the
hypertrophy that you see and the large increase in
the cell sizes, to interpret the receptor binding data
is exceedingly difficult. We see no obvious immediate changes. But the careful studies that have been
done by Dave Westfall and his group on supersensitive smooth muscle, produced mechanically by
cutting the nerves, have not shown any evidence of
either a change in the sensitivity or in the number of
receptors. The increased excitability of the smooth
muscle seems to reside in the membrane properties,
and possibly in the conduction between the cells
rather than at the level of the receptors.