Am J Physiol Heart Circ Physiol 284: H711–H718, 2003;
10.1152/ajpheart.00315.2002.
Inhibition of vascular ATP-sensitive K⫹ channels
does not affect reactive hyperemia in human forearm
H. M. OMAR FAROUQUE AND IAN T. MEREDITH
Cardiovascular Research Centre, Monash Medical Centre and
Monash University, Melbourne, Victoria, 3168, Australia
Submitted 8 April 2002; accepted in final form 23 October 2002
regional blood flow; ion channels; ischemia
channels are activated by an increase in intracellular
ADP and potassium channel-opening drugs, such as
the antihypertensive agent diazoxide resulting in
membrane hyperpolarization and vasodilation (40). In
view of the biophysical characteristics of ATP-sensitive
K⫹ channels, these channels may be active under conditions of ischemia and hypoxia. Indeed, studies in
animals have demonstrated a role for ATP-sensitive
K⫹ channels in coronary and skeletal muscle reactive
hyperemia (2, 14, 17, 52).
Recent experiments have provided evidence for the
existence of these channels in human smooth muscle
cells (23, 33). We have demonstrated that ATP-sensitive K⫹ channels in the human coronary circulation
may contribute to the regulation of resting coronary
blood flow, adenosine-induced hyperemia, and metabolic coronary vasodilation (21, 22). In contrast, these
channels may not contribute to exercise-induced forearm hyperemia (20). A few clinical studies have examined the role played by vascular ATP-sensitive K⫹
channels in skeletal muscle reactive hyperemia; however, these investigations have yielded conflicting results. These studies suggest that ATP-sensitive K⫹
channels may be involved in the initial phase (8, 31), or
sustained phase of hyperemia (3, 4) but not both. Moreover, the magnitude of the contribution of ATP-sensitive K⫹ channels has varied among these studies. Thus
the extent to which ATP-sensitive K⫹ channels contribute to peripheral blood flow regulation in human vasculature is unresolved. Therefore, the principal aim of
this study was to determine the contribution of vascular ATP-sensitive K⫹ channels in reactive hyperemia
by examining the effect of acute ATP-sensitive K⫹
channel inhibition with glibenclamide on postischemic
forearm vasodilation in healthy humans.
to the phenomenon of increased blood flow that follows relief of ischemia and is
a result of conduit and resistance vessel dilatation.
Typically, maximal hyperemia is observed immediately after restoration of blood flow (initial phase),
which then declines exponentially toward baseline flow
over several minutes (sustained phase). Clarification of
the mechanisms involved in reactive hyperemia is of
importance in understanding the pathophysiology and
treatment of myocardial and limb ischemia (34). Several factors have been implicated in the genesis of
reactive hyperemia, including mechanical (7, 11) and
neurogenic mechanisms (32), endothelium-derived vasoactive factors (13, 18, 39), adenosine (11), and membrane-bound ion channels (14, 41). Among the ion
channels, there has been interest in the ATP-sensitive
K⫹ channel and its role in blood flow regulation. These
The study population was 31 healthy subjects (age 23 ⫾ 6
yr; 20 males and 11 females). All subjects were screened at an
initial visit to determine suitability for the study. Exclusion
criteria included the presence of conventional cardiovascular
Address for reprint requests and other correspondence: I. T.
Meredith, Cardiovascular Research Centre, Monash Medical Centre,
246 Clayton Road, Clayton, Melbourne, Victoria, 3168, Australia
(E-mail: [email protected]).
The costs of publication of this article were defrayed in part by the
payment of page charges. The article must therefore be hereby
marked ‘‘advertisement’’ in accordance with 18 U.S.C. Section 1734
solely to indicate this fact.
REACTIVE HYPEREMIA REFERS
http://www.ajpheart.org
MATERIALS AND METHODS
Subjects
0363-6135/03 $5.00 Copyright © 2003 the American Physiological Society
H711
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Farouque, H. M. Omar and Ian T. Meredith. Inhibition
of vascular ATP-sensitive K⫹ channels does not affect reactive
hyperemia in human forearm. Am J Physiol Heart Circ Physiol
284: H711–H718, 2003; 10.1152/ajpheart.00315.2002.—The extent to which ATP-sensitive K⫹ channels contribute to reactive hyperemia in humans is unresolved. We examined the
role of ATP-sensitive K⫹ channels in regulating reactive
hyperemia induced by 5 min of forearm ischemia. Thirty-one
healthy subjects had forearm blood flow measured with venous occlusion plethysmography. Reactive hyperemia could
be reproducibly induced (n ⫽ 9). The contribution of vascular
ATP-sensitive K⫹ channels to reactive hyperemia was determined by measuring forearm blood flow before and during
brachial artery infusion of glibenclamide, an ATP-sensitive
K⫹ channel inhibitor (n ⫽ 12). To document ATP-sensitive
K⫹ channel inhibition with glibenclamide, coinfusion with
diazoxide, an ATP-sensitive K⫹ channel opener, was undertaken (n ⫽ 10). Glibenclamide did not significantly alter
resting forearm blood flow or the initial and sustained phases
of reactive hyperemia. However, glibenclamide attenuated
the hyperemic response induced by diazoxide. These data
suggest that ATP-sensitive K⫹ channels do not play an important role in controlling forearm reactive hyperemia and
that other mechanisms are active in this adaptive response.
H712
ATP-SENSITIVE K⫹ CHANNELS AND REACTIVE HYPEREMIA
Table 1. Subject characteristics
Age, yr
Gender, M/F
Body mass index, kg/m2
Total cholesterol, mmol/l
LDL cholesterol, mmol/l
HDL cholesterol, mmol/l
Triglycerides, mmol/l
Heart rate, beats/min
Mean arterial pressure, mmHg
Protocol 1
Protocol 2
22 ⫾ 4
7/5
22 ⫾ 2
3.9 ⫾ 0.6
2.4 ⫾ 0.5
1.1 ⫾ 0.1
0.9 ⫾ 0.5
61 ⫾ 9
80 ⫾ 3
22 ⫾ 6
5/5
23 ⫾ 3
3.7 ⫾ 0.5
2.2 ⫾ 0.5
1.2 ⫾ 0.2
0.9 ⫾ 0.5
61 ⫾ 2
91 ⫾ 2
Values are means ⫾ SE; n ⫽ 22 subjects. M, male; F, female.
Venous Occlusion Plethysmography
All studies were performed at the same time each morning
in a quiet, temperature-controlled (22–23°C) vascular research laboratory as previously described (16, 39). In brief,
subjects were fasted overnight and had abstained from caffeine- and alcohol-containing products for at least 12 h before
the study. Brachial arterial cannulation was performed in
the nondominant arm under aseptic conditions after adequate local anesthesia was achieved. Physiological saline was
infused into the brachial artery at 0.4 ml/min to keep the
catheter patent. Subjects were routinely rested in the supine
position for a minimum of 30 min before the first forearm
blood flow measurements were taken. Bilateral forearm
blood flow was measured using the technique of venous
occlusion plethysmography with a calibrated mercury-in-Silastic strain gauge (D. E. Hokanson, Bellevue, WA). A collecting cuff was wrapped around the upper arm and connected to a Hokanson E-20 rapid cuff inflator. The upper limb
was supported above the level of the heart to ensure free
venous drainage. The collecting cuff was rapidly inflated with
air to a pressure of 40 mmHg in a cyclical fashion to occlude
Blood Flow Measurements
Resting forearm blood flow was measured for a minimum
of 2 min and an average of at least five stable measurements
was used for analysis. Forearm reactive hyperemia was measured after deflation of an upper arm cuff that had been
inflated to a pressure of 190 mmHg for 5 min. Flow measurements were recorded every 7 s for the first 2 min and then
every 12 s for the next 3 min. Plethysmographic data and
intra-arterial blood pressure were digitized on-line using an
eight-channel analog-to-digital converter (MacLab/8s System, ADInstruments, Castle Hill, NSW, Australia) and analyzed off-line (Chart version 4.0.1, ADInstruments). Forearm
blood flow-was measured for 5 min after the ischemic stimulus and a flow versus-time curve constructed (Fig. 1).
Reproducibility of Reactive Hyperemia
To determine the reproducibility of the forearm ischemia
protocol, blood flow was assessed during three periods of reactive hyperemia each separated by 15 min in nine healthy
subjects. Five minutes of brachial artery occlusion induced
reproducible reactive hyperemic blood flow responses (Table 2).
Drugs
Glibenclamide lyophilisate (kindly supplied by Aventis
Pharma Deutschland, Frankfurt, Germany) was used as an
inhibitor of vascular ATP-sensitive K⫹ channels in this
study. Its specificity for ATP-sensitive K⫹ channels in vascular tissue has been demonstrated in humans (9) and animals
(17). Glibenclamide lyophilisate is a stable preparation suitable for parenteral human use and does not require the
addition of an alkaline vehicle to ensure solubility. Glibenclamide was dissolved in 0.9% saline (vehicle) and infused at
15 ␮g/min into the brachial artery by computerized syringe
pump (Terumo, Tokyo, Japan) at 0.4 ml/min. Assuming resting forearm blood flow is 3 ml 䡠 100 ml⫺1 䡠 min⫺1, this infusion
regimen would result in a regional plasma concentration of
⬃500 ng/ml. This level is at the upper end of the concentration range observed in venous blood at a mean of 3 h after the
administration of a single 20-mg oral dose of glibenclamide to
patients with Type 2 diabetes mellitus (12). Diazoxide (David
Fig. 1. Forearm blood flow-time curve after 5
min of forearm ischemia. The graph demonstrates a typical individual example of the hyperemic response after ischemia. The occlusive upper arm cuff is deflated at time 0, and blood flow
measurements begin shortly thereafter. Shaded
areas indicate the total volume of blood repaid
between 0–60 and 0–300 s.
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risk factors, a history of significant cardiac or noncardiac
medical illnesses and the use of vasoactive medication. Twenty-two subjects underwent drug infusions, and baseline characteristics for these participants are displayed in Table 1.
Nine subjects underwent reproducibility experiments (mean
age 26 ⫾ 8 yr; 8 males). The study was approved by the
Southern Health Human Research Ethics Committee and
written informed consent was obtained from all participants.
venous outflow. Hand blood flow was prevented by inflating a
wrist cuff to a pressure of 200 mmHg during forearm blood
flow recordings.
ATP-SENSITIVE K⫹ CHANNELS AND REACTIVE HYPEREMIA
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Table 2. Reproducibility of reactive hyperemia
Peak RHBF, ml 䡠 100 ml 䡠 min
Minimum FVR, units
Volume repaid at 1 min, ml/100 ml
Volume repaid at 5 min, ml/100 ml
⫺1
⫺1
RHBF 1
RHBF 2
RHBF 3
P
ICC
30.4 ⫾ 3.0
3.2 ⫾ 0.4
13.5 ⫾ 1.4
27.0 ⫾ 1.8
29.3 ⫾ 3.2
3.3 ⫾ 0.4
13.3 ⫾ 1.2
27.6 ⫾ 2.0
30.0 ⫾ 2.7
3.3 ⫾ 0.4
12.8 ⫾ 1.2
26.3 ⫾ 1.7
0.52*
0.36*
0.69*
0.56*
0.97
0.98
0.83
0.88
Values are means ⫾ SE; n ⫽ 9 subjects. RHBF, reactive hyperemic blood flow; ICC, intraclass correlation coefficient. * Repeated-measures
ANOVA.
Bull Laboratories, Melbourne, Australia), a vasodilator that
acts by opening vascular ATP-sensitive K⫹ channels (43),
was diluted in 5% dextrose and infused in graded doses of 1.9,
3.8, 7.5, and 15 mg/min into the brachial artery in protocol 2
(see below).
Experimental Design
Calculations and Statistical Analysis
All values are expressed as means ⫾ SE and demographic
data, as means ⫾ SD. Forearm vascular resistance (expressed
RESULTS
Effect of Glibenclamide on Resting Forearm
Blood Flow
Glibenclamide infused for 30 min at 15 ␮g/min did
not alter resting forearm blood flow or forearm vascular resistance (Table 3). Compared with vehicle, glib-
Fig. 2. Study timelines for each protocol. Protocol 1 was designed to assess the contribution of
vascular ATP-sensitive potassium channels to
reactive hyperemia. Protocol 2 was designed to
demonstrate the efficacy of glibenclamide in attenuating the hyperemia induced by diazoxide,
an ATP-sensitive K⫹ channel opener. A minimum time of 45 min elapsed before the first and
second stages of this protocol. Diazoxide infusions are indicated by D1, D2, D3, and D4 corresponding to infusion rates of 1.9, 3.8, 7.5, and 15
mg/min, respectively. FBF, resting forearm blood
flow; RHBF, reactive hyperemic blood flow.
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Blood flow studies are schematically represented in Fig. 2.
Protocol 1. The contribution of vascular ATP-sensitive K⫹
channels to postischemic vasodilation was determined by
measuring reactive hyperemia before and during infusion of
glibenclamide lyophilisate, an inhibitor of the ATP-sensitive
K⫹ channel (44, 50). Glibenclamide was infused at 15 ␮g/min
into the brachial artery of 12 subjects by syringe pump for 30
min before blood flow measurements and continued during
the period of ischemia and reactive hyperemia. Blood samples were taken before and after glibenclamide infusion for
measurement of glucose, insulin, and C-peptide concentrations. C-peptide, an enzymatic cleavage product of proinsulin
secreted from pancreatic ␤-cells with insulin in equimolar
concentrations, was used as a confirmatory index of pancreatic insulin release.
Protocol 2. To assess the efficacy of ATP-sensitive K⫹
channel inhibition by glibenclamide lyophilisate, blood flow
responses to graded infusions of diazoxide, an ATP-sensitive
K⫹ channel opener, were determined in 10 subjects before
and during coinfusion with glibenclamide.
as units indicating mmHg䡠ml⫺1 䡠100 ml tissue⫺1 䡠min⫺1 forearm tissue) was calculated from the quotient of mean arterial
blood pressure and forearm blood flow. The blood volume repaid
(or total hyperemic volume) after 1 and 5 min after release of
arterial occlusion (Fig. 1) was determined by estimating the
area under the flow-versus-time curve (35). To account for
changes in resting blood flow, the absolute increase in peak
reactive hyperemic blood flow and absolute volume repaid at 1
and 5 min was calculated by subtracting baseline resting forearm blood flow from the blood volume repaid at these time
intervals. The sample size in protocol 1 was on the basis of a
calculation showing that 12 subjects would be required to demonstrate a 30% difference in the 5-min hyperemic volume at a P
value of 0.05 with 90% power. Reproducibility of reactive hyperemia was assessed using repeated-measures ANOVA and
the intraclass correlation coefficient. The paired Student’s t-test
was used to compare biochemical data and hemodynamic variables at rest and after ischemia before and after glibenclamide
lyophilisate infusion. Two-way repeated-measures ANOVA was
used to compare the effect of glibenclamide lyophilisate on the
dose-flow response relationship to graded infusions of diazoxide. Statistical significance was accepted when P was ⬍ 0.05.
ATP-SENSITIVE K⫹ CHANNELS AND REACTIVE HYPEREMIA
H714
enclamide did not change mean arterial pressure (78 ⫾
1 vs. 79 ⫾ 1 mmHg; P ⫽ 0.40) after 30 min of infusion.
Effect of Glibenclamide on Forearm
Reactive Hyperemia
Effect of Glibenclamide on DiazoxideInduced Vasodilation
Diazoxide elicited a dose-related increase in blood
flow in the infused forearm (P ⫽ 0.004; Fig. 3). Forearm
blood flow increased nearly fivefold, from 3.5 ⫾ 0.4
ml 䡠 100 ml⫺1 䡠 min⫺1 during vehicle infusion to 15.3 ⫾
2.9 ml 䡠 100 ml⫺1 䡠 min⫺1 at the highest diazoxide dose.
Coinfusion of glibenclamide attenuated the vasodilator
response induced by diazoxide (P ⫽ 0.02, ANOVA; Fig.
3). A downward shift of the dose-response curve was
noted with the reduction in diazoxide-induced hyperemia being apparent at the upper end of the doseresponse relationship. Coinfusion of glibenclamide resulted in a 20% decrease in stimulated blood flow at the
highest diazoxide dose (15 mg/min; P ⫽ 0.002) and a
23% decrease at the preceding diazoxide dose (7.5 mg/
min; P ⫽ 0.02). There was no significant alteration to
mean arterial pressure or contralateral forearm blood
flow during the dose-response study.
DISCUSSION
In this study, we demonstrate that glibenclamide, a
specific inhibitor of the ATP-sensitive K⫹ channel, does
Table 3. Effect of glibenclamide on resting forearm
blood flow and reactive hyperemia
Resting FBF, ml 䡠 100 ml 䡠 min
Resting FVR, units
Peak RHBF, ml 䡠 100 ml⫺1 䡠 min⫺1
⌬ Peak RHBF, ml 䡠 100 ml⫺1 䡠 min⫺1
Minimum FVR, units
Volume repaid at 1 min, ml/100 ml
⌬ Volume repaid at 1 min,
ml/100 ml
Volume repaid at 5 min, ml/100 ml
⌬ Volume repaid at 5 min,
ml/100 ml
Ischemia MAP, mmHg
⫺1
⫺1
Isotonic
Saline
Glibenclamide
P
1.8 ⫾ 0.3
57.0 ⫾ 9.5
26.8 ⫾ 1.4
25.1 ⫾ 1.3
3.0 ⫾ 0.2
14.2 ⫾ 1.5
1.7 ⫾ 0.3
60.4 ⫾ 9.5
25.9 ⫾ 1.3
24.3 ⫾ 1.2
3.9 ⫾ 0.6
13.3 ⫾ 1.7
0.55
0.67
0.59
0.62
0.10
0.14
12.6 ⫾ 1.3
25.9 ⫾ 2.7
11.7 ⫾ 1.6
23.6 ⫾ 2.9
0.17
0.12
17.5 ⫾ 2.1
78.6 ⫾ 1.3
15.7 ⫾ 2.1
81.2 ⫾ 1.3
0.30
0.03
Values are means ⫾ SE. FBF, forearm blood flow; FVR, forearm
vascular resistance; ⌬, hyperemic blood flow minus baseline flow.
AJP-Heart Circ Physiol • VOL
Fig. 3. Forearm blood flow responses to brachial artery infusion of
diazoxide with coinfusion of vehicle (E) and glibenclamide (■). There
was a significant difference between the dose response curves (P ⫽
0.02, ANOVA). *P ⬍ 0.05 compared with vehicle infusion.
not significantly alter resting blood flow or the hyperemic response to 5 min of ischemia in the forearm
circulation of healthy humans. Our findings suggest
that vascular ATP-sensitive K⫹ channels do not significantly contribute to the regulation of forearm blood
flow at rest or during reactive hyperemia in healthy
subjects.
Resting Forearm Blood Flow
Many animal studies using different experimental
approaches have demonstrated that basal coronary
arterial tone is, in part, dependent on the activity of
ATP-sensitive K⫹ channels (17, 27, 49). However, the
contribution of ATP-sensitive K⫹ channels to regulation of resting tone in animal skeletal muscle vasculature is less certain with some studies (29, 52) but not
all (48), implicating ATP-sensitive K⫹ channels. In
other vascular beds, such as the cerebral and renal
circulations, ATP-sensitive K⫹ channels do not appear
to contribute to the maintenance of basal tone but may
be an important mechanism of vasodilation in certain
pathophysiological states (5, 19, 46).
The contribution of ATP-sensitive K⫹ channels to
blood flow regulation in humans has been examined
using the technique of venous occlusion plethysmography. In one of the early studies, a single oral dose of
glibenclamide was found to induce a significant reduction of resting calf blood flow in healthy subjects at 1
and 2 h after drug ingestion (31). This finding is in
contrast to the data presented in our study and from
other investigators in which no alteration to resting
blood flow with ATP-sensitive K⫹ channel inhibition
was noted (3, 4, 8). It may be of relevance that these
studies have examined forearm blood flow rather than
calf blood flow. Although lower and upper limb blood
flow measurements with plethysmography primarily
reflect skeletal muscle perfusion, there may be differences in blood flow regulatory mechanisms between
these vascular beds (47). Moreover, in these negative
studies, sulfonylurea ATP-sensitive K⫹ channel inhib-
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Glibenclamide did not affect the peak or sustained
phase of reactive hyperemia (Table 3). There was a
trend to increase in minimum forearm vascular resistance with glibenclamide (3.0 ⫾ 0.2 vs. 3.9 ⫾ 0.6 units;
P ⫽ 0.10) due to a small rise in mean arterial pressure.
Glibenclamide induced a decrease in serum glucose
from 4.7 ⫾ 0.1 to 3.8 ⫾ 0.1 mmol/l (P ⬍ 0.001) and an
increase in plasma insulin from 8.3 ⫾ 0.9 to 11.2 ⫾ 0.7
mU/l (P ⬍ 0.01). There was a concomitant rise in
plasma C-peptide concentration from 0.7 ⫾ 0.1 to 0.8 ⫾
0.1 nmol/l (P ⫽ 0.001). Contralateral forearm blood
flow was unchanged.
ATP-SENSITIVE K⫹ CHANNELS AND REACTIVE HYPEREMIA
itors have been infused directly into the forearm circulation of the experimental arm, thereby minimizing
any potential systemic effects that may occur with oral
administration. The finding that ATP-sensitive K⫹
channels do not appear to contribute to basal vascular
tone in the human forearm may reflect inactivity of
these channels under resting conditions due to the
presence of physiological concentrations of intracellular ATP. However, the absence of a role for ATPsensitive K⫹ channels in mediating basal forearm vascular tone should not necessarily be extrapolated to
other vascular beds in humans, or to other situations,
such as chronically ischemic muscle, in which blood
flow may be more dependent on ATP-sensitive K⫹
channels.
The contribution of vascular ATP-sensitive K⫹ channels to early and late phases of reactive hyperemia in
animals is well established. Hyperemia after brief or
prolonged coronary occlusions (ⱕ45 min) can be attributed to ATP-sensitive K⫹ channel activation (2, 14, 15,
17). These channels have also been implicated in skeletal muscle reactive hyperemia (52). In light of the
experimental data, a small number of studies have
examined the role played by ATP-sensitive K⫹ channels in human reactive hyperemia. Kosmas et al. (31)
examined calf reactive hyperemia after 10 min of femoral arterial occlusion before and after oral glibenclamide in healthy subjects. Glibenclamide was associated with a significant reduction in peak reactive
hyperemia of 28% at 2 h after drug ingestion, but there
was no change in reactive hyperemic volume after
accounting for differences in baseline flow. These findings are in contrast to another study, which examined
the contribution of ATP-sensitive K⫹ channels to forearm reactive hyperemia, induced by 5 min of ischemia
(3). Blood flow responses were assessed before and
during brachial artery infusion of the sulfonylurea
tolbutamide in healthy subjects. This group reported
no change in peak blood flow but a significant 27%
reduction in reactive hyperemic volume at 5 min after
cuff deflation.
Bijlstra et al. (8) studied forearm reactive hyperemia
before and during brachial artery infusion of glibenclamide (infusion rate of ⬃3 ␮g/min) in 12 healthy
subjects. After 2 min of forearm ischemia, there was a
significant 9% reduction in peak hyperemia and a 19%
reduction in total flow debt repayment at 1 min after
cuff deflation. Surprisingly, longer periods of forearm
ischemia (5 and 13 min) did not result in significant
changes to peak hyperemia or total flow debt repayment, although a small increase in minimum forearm
vascular resistance was noted after the longer occlusion period. More recently, Bank et al. (4) reported that
brachial arterial glibenclamide infusion at 100 ␮g/min
did not reduce peak hyperemic blood flow after 5 min of
forearm ischemia but did attenuate reactive hyperemic
volume by 19% at 2 min after cuff deflation in nine
healthy subjects. The findings of these studies, while
AJP-Heart Circ Physiol • VOL
conflicting as to the magnitude and timing of contribution by ATP-sensitive K⫹ channels, have, in general,
supported a small-to-modest role for these channels in
reactive hyperemia. Our data do not support these
observations.
Evidence for Vascular ATP-Sensitive K⫹
Channel Inhibition
Glibenclamide is a potent and specific inhibitor of
ATP-sensitive K⫹ channels (17, 44, 50) and is widely
used to examine the contribution of ATP-sensitive K⫹
channels in blood flow regulation. The ability of glibenclamide to reduce the dilator response to potassium
channel opening drugs, such as pinacidil and diazoxide, is often used in experimental settings to confirm
ATP-sensitive K⫹ channel inhibition (17, 29, 48). Given
that glibenclamide infused intra-arterially at 15 ␮g/
min did not alter forearm blood flow at rest or after
ischemia in this study, it was important to demonstrate that vascular ATP-sensitive K⫹ channel inhibition was achieved. We observed a modest reduction in
diazoxide-induced vasodilation at the upper end of the
dose-response curve in keeping with ATP-sensitive K⫹
channel inhibition. These findings concur with a previous study (9) in humans using brachial arterial infusions of glibenclamide. Although the vasodilator action
of diazoxide has been attributed in large part to an
opening of vascular ATP-sensitive K⫹ channels (37, 43,
44), the persistence of vasodilator activity in the presence of glibenclamide in this and other studies suggests that ATP-sensitive K⫹ channel-independent effects may also be operative, as has been demonstrated
for other drugs in the class (38).
Methodological Considerations
We observed a small but significant rise in plasma
insulin and C-peptide levels and a reduction in plasma
glucose consistent with the effect of glibenclamide on the
pancreas. Insulin is known to augment skeletal muscle
blood flow and its vasodilator properties are primarily
mediated by endothelial nitric oxide release (6), although
other mechanisms including the activation of ATP-sensitive K⫹ channels may be involved (36). It is possible that
the increase in insulin opposed the vasoconstrictor effects
of vascular ATP-sensitive K⫹ channel inhibition. However, the vasoactive effects of insulin are delayed in onset
and tend to occur at higher concentrations than the low
physiological levels observed during this study (6). Moreover, in previous experiments, we have found no correlation between plasma insulin levels in the low physiological range and forearm blood flow (H. M. O. Farouque,
unpublished observations). Insulin-induced hypoglycemia is also known to increase forearm blood flow and this
effect appears to be mediated by ␤-adrenoreceptors (24).
However, it is unlikely that a greater hyperemia would
have resulted at the glucose levels observed in the
present study. Accordingly, the small changes observed
in these humoral parameters are unlikely to have contributed to the lack of effect by glibenclamide on reactive
hyperemia.
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Reactive Hyperemia
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ATP-SENSITIVE K⫹ CHANNELS AND REACTIVE HYPEREMIA
Physiological and Clinical Implications
Results of this study do not provide support for an
important role of vascular ATP-sensitive K⫹ channels
in the control of forearm reactive hyperemia in healthy
young human subjects. We (20) have recently shown
that ATP-sensitive K⫹ channels also do not contribute
to forearm metabolic vasodilation after isotonic forearm exercise. However, in the human coronary circulation of patients with atherosclerosis, these channels
appear to be active and to participate in the regulation
of coronary blood flow (21, 22). Thus there appear to be
differences in the mechanisms of vasoregulation between peripheral and coronary circulations in vivo,
which might be a reflection of the unique characteristics of each tissue and its associated vascular bed. The
physiological relevance of these differences with reAJP-Heart Circ Physiol • VOL
spect to ATP-sensitive K⫹ channels is unclear. It might
be that vascular ATP-sensitive K⫹ channels are more
active in tissues that are vulnerable to ischemia, such
as the myocardium, although this is speculative. The
impact of variables, such as age, gender, and risk
factors for atherosclerosis and other diseases on the
activity of vascular ATP-sensitive K⫹ channels in the
human circulation is unknown. It is possible that some
of these factors may contribute to the differences in
response to ATP-sensitive K⫹ channel inhibition between coronary and peripheral circulations.
Our data raise the possibility of a pharmacodynamic
interaction between sulfonylureas and the ATP-sensitive K⫹ channel openers. ATP-sensitive K⫹ channel
openers, such as nicorandil, are a relatively new class
of drug introduced for the treatment of angina (51a).
The efficacy of nicorandil is related to its coronary and
peripheral vasodilator actions (25, 51). Given that diabetes mellitus is one of the major risk factors for
ischemic heart disease, the potential for combined administration of sulfonylureas and nicorandil exists. A
recent study has shed light on the molecular basis for
an interaction between nicorandil and sulfonylureas
(45). With the use of electrophysiological techniques
in cells made to express recombinant ATP-sensitive
K⫹ channels, nicorandil was found to activate smooth
muscle and cardiac muscle variants of the ATP-sensitive
K⫹ channel. Glibenclamide and glimepiride inhibited
both smooth muscle and cardiac ATP-sensitive K⫹
channels. On the basis of these data, it is conceivable
that clinically significant drug interactions may occur
with coadministration of certain sulfonylureas and
ATP-sensitive K⫹ channel openers.
We have demonstrated that acute vascular ATPsensitive K⫹ channel inhibition does not significantly
alter resting blood flow or the peak and sustained
phases of reactive hyperemia in the forearm circulation
of healthy young individuals.
We thank Kais Hamza, Department of Mathematics and Statistics, Monash University, for assistance with statistical analyses. We
are grateful to Michael Zhang and Mauro Baldi for technical assistance.
H. M. O. Farouque is supported by a National Heart Foundation
of Australia Medical Postgraduate Research Scholarship PM98M0006.
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