Forearm endothelium-dependent vascular responses and the potassium-ATP channel.
ABSTRACT Vasodilation to acetylcholine is mediated at least in part by endothelium-derived hyperpolarising factor (EDHF) which causes membrane hyperpolarisation by activating potassium channels. It is however uncertain which potassium channel mediates this effect. The aim of this study was to determine the role of the potassium-ATP (K(+)-ATP) channel in mediating endothelium-dependent vascular responses to acetylcholine.
In 10 healthy volunteers acetylcholine, an endothelium-dependent vasodilator, and sodium nitroprusside as a control assessing endothelium-independent vasodilatation were infused into the non-dominant brachial artery. Forearm blood flow (FBF) in response to each dose was measured by strain-gauge venous occlusion plethysmography. The K(+)-ATP channel blocker glipizide (2.5 mg) was then administered orally. After 45 min the infusions with FBF measurements were repeated.
Acetylcholine (P < 0.01) and sodium nitroprusside (P < 0.01) both caused an increase in FBF. There was no significant difference in vascular responses to acetylcholine (P > 0.05) or sodium nitroprusside (P > 0.05) following K(+)-ATP channel blockade.
The K(+)-ATP channel does not modulate forearm arteriolar endothelium-dependent responses in healthy volunteers and therefore does not play a role in membrane hyperpolarisation.
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ABSTRACT: The increased perinatal morbidity in diabetes may be partly related to vascular dysfunction. Because potassium channels play an important role in the regulation of vascular tone, this study explores the impact of diabetes on potassium channel function in the fetoplacental vascular bed. Vascular potassium channel function was investigated by ex vivo dual perfusion of isolated placental cotyledons (n = 47). Appropriate control experiments were carried out to exclude nonspecific effects. Glibenclamide (KATP channel blocker) increased perfusion pressure to a maximum fetoplacental arterial pressure of 37 +/- 6 mm Hg in controls versus 15 +/- 6 mm Hg in diabetes (P < .05). 4-Aminopyridine (KV channel blocker) equally increased fetoplacental arterial pressure in controls, and in diabetes (21 +/- 4 mm Hg vs 22 +/- 2 mm Hg). Apamin and charybdotoxin (KCa channel blockers) caused a negligible rise in fetoplacental arterial pressure. In the fetoplacental circulation, KATP channels and KV channels significantly contribute to baseline vascular tone. In diabetes, vascular KATP channel function is impaired.American Journal of Obstetrics and Gynecology 04/2005; 192(3):973-9. · 3.88 Impact Factor
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ABSTRACT: 1. The purine nucleotide adenosine-5'-triphosphate (ATP) exerts pronounced effects on the cardiovascular system. The mechanism of action of the vasodilator response to ATP in humans has not been elucidated yet. The proposed endothelium-derived relaxing factors (EDRFs) were studied in a series of experiments, using the perfused forearm technique. 2. Adenosine 5'-triphosphate (0.2, 0.6, 6 and 20 nmol dl(-1) forearm volume min(-1)) evoked a dose-dependent forearm vasodilator response, which could not be inhibited by separate infusion of the nonselective COX inhibitor indomethacin (5 microg dl(-1) min(-1), n=10), the blocker of Na(+)/K(+)-ATPase ouabain (0.2 microg dl(-1) min(-1), n=8), the blocker of K(Ca) channels tetraethylammonium chloride (TEA, 0.1 microg dl(-1) min(-1), n=10), nor by the K(ATP)-channel blocker glibenclamide (2 microg dl(-1) min(-1), n=10). All blockers, except glibenclamide, caused a significant increase in baseline vascular tone. The obtained results might be due to compensatory actions of unblocked EDRFs. Combined infusion of TEA, indomethacin and l-NMMA (n=6) significantly increased the baseline forearm vascular resistance. The ATP-induced relative decreases in forearm vascular resistance were 48+/-5, 67+/-3, 88+/-2, and 92+/-2% in the absence and 23+/-7, 62+/-4, 89+/-2, and 93+/-1% in the presence of the combination of TEA, indomethacin and l-NMMA (P<0.05, repeated-measures ANOVA, n=6). A similar inhibition was obtained for sodium nitroprusside (SNP, P<0.05 repeated-measures ANOVA, n=6), indicating a nonspecific interaction due to the blocker-induced vasoconstriction. 3. ATP-induced vasodilation in the human forearm cannot be inhibited by separate infusion of indomethacin, ouabain, glibenclamide or TEA, or by a combined infusion of TEA, indomethacin, and l-NMMA. Endothelium-independent mechanisms and involvement of unblocked EDRFs, such as CO, might play a role, and call for further studies.British Journal of Pharmacology 04/2004; 141(5):842-50. · 5.07 Impact Factor
Br J Clin Pharmacol 1997; 44: 292–294
Forearm endothelium-dependent vascular responses and the potassium-
D. McAuley, C. McGurk, A. G. Nugent, C. Hanratty, S. Maguire & G. D. Johnston
The Department of Therapeutics and Pharmacology, The Queen’s University of Belfast, Belfast BT9 7BL, Northern Ireland
Aims Vasodilation to acetylcholine is mediated at least in part by endothelium-
derived hyperpolarising factor (EDHF) which causes membrane hyperpolarisation
by activating potassium channels. It is however uncertain which potassium channel
mediates this effect. The aim of this study was to determine the role of the
potassium-ATP (K+-ATP) channel in mediating endothelium-dependent vascular
responses to acetylcholine.
Methods In 10 healthy volunteers acetylcholine, an endothelium-dependent vaso-
dilator, and sodium nitroprusside as a control assessing endothelium-independent
vasodilatation were infused into the non-dominant brachial artery. Forearm blood
flow (FBF) in response to each dose was measured by strain-gauge venous occlusion
plethysmography. The K+-ATP channel blocker glipizide (2.5 mg) was then
administered orally. After 45 min the infusions with FBF measurements were
Results Acetylcholine (P<0.01) and sodium nitroprusside (P<0.01) both caused
an increase in FBF. There was no significant difference in vascular responses to
acetylcholine (P>0.05) or sodium nitroprusside (P>0.05) following K+-ATP
Conclusions The K+-ATP channel does not modulate forearm arteriolar endo-
thelium-dependent responses in healthy volunteers and therefore does not play a
role in membrane hyperpolarisation.
Keywords: endothelium-dependent vasodilation, forearm blood flow, potassium-ATP
for 6 h and were given a standardised meal 1 h prior to the
study. All gave written informed consent and approval was
obtained from the local ethics committee. The studies took
place in a temperature controlled room (24–26° C). Under
local anaesthesia (1% lignocaine, Antigen Pharmaceuticals,
Ireland) a 27-gauge needle was inserted into the non-
dominant brachial artery. Blood flow at baseline and in
response to each dose of drug infused was measured in both
arms by strain-gauge venous occlusion plethysmography. An
indium-gallium strain gauge coupled to an electronically
calibrated plethysmograph (Medasonics
California, USA) was connected to a two channel chart
recorder (Medasonics model R12B) to record FBF. The
mean of five consecutive measurements was taken for
Using a constant rate infuser (Braun Perfusor pump,
Germany), acetylcholine (Iolab, UK) was infused into the
experimental forearm in incremental doses of 60–240 nmol
min−1for 3min each, to assess endothelium-dependent
vasodilatation. After a washout period basal FBF was again
measured. Sodium nitroprusside (David Bull Laboratories,
UK) was then infused in incremental doses of 3–12 nmol
min−1to assess endothelium-independent vasodilatation. All
infusions were at a rate of 1 ml min−1. Glipizide (2.5 mg,
Farmitalia Carlo Erba Ltd, UK) was then administered
orally. After 45 min to allow absorption FBF was measured
The vascular endothelium plays a central role in the
regulation of vascular tone through the production of
vasoactive substances, including the vasodilators nitric oxide
and prostacyclin. Endothelium-derived hyperpolarising factor
(EDHF) was identified following the recognition that
endothelium-dependent vasodilation is not abolished by
inhibition of both nitric oxide and prostacyclin .
EDHF activates potassium channels causing membrane
hyperpolarisation. This inhibits voltage dependent calcium
channels causing a reduction in intracellular calcium and
vascular smooth muscle relaxation . It is unclear which
of several K+channels mediates this hyperpolarisation. The
purpose of this study was to determine whether the K+-
ATP channel modulates forearm endothelium-dependent
responses to acetylcholine.
Ten healthy non-smoking volunteers (eight male and two
female) of mean age 25.6 years (range 19–30) were studied.
All abstained from alcohol and caffeine containing products
Correspondence: Dr D. McAuley, The Department of Therapeutics and Pharmacology,
The Queen’s University of Belfast, The Whitla Medical Building, 97 Lisburn Road,
Belfast BT9 7BL, Northern Ireland.
©1997 Blackwell Science Ltd
to re-establish baseline. The infusions of acetylcholine and
sodium nitroprusside withFBF responses were thenrepeated.
Results are expressed as mean change in FBF from
baseline (100 ml−1min−1) with 95% confidence intervals
(CI) and were analysed using repeated measures analysis of
variance. Basal blood flows were compared using a paired
The power to detect a change of 50% from mean values
of FBF pre-glipizide at the 5% level of significance was 85%.
FBF did not change significantly in the control arm during
the period of the experiment confirming that drug effects
were confined to the experimental forearm (P>0.05).
Basal FBF did not differ significantly before and after
glipizide for acetylcholine (P>0.05) or sodium nitroprusside
(P>0.05). Acetylcholine (P<0.01) and sodium nitro-
prusside (P<0.01) both caused a dose dependent increase
in FBF. There was no significant difference in forearm
vascular response to either acetylcholine (P>0.05) or
sodium nitroprusside (P>0.05) following glipizide.
Change in FBF from baseline
Figure 1 Change in FBF from baseline in response to
acetylcholine before (&) and after (+) K+channel blockade
(mean and 95% CI).
it has been shown to potentiate vasodilation to acetylcholine
in vivo . In view of this contradictory evidence it is
difficult to conclude confidently that insulin did not
modulate forearm vascular responses to acetylcholine.
Glipizide 2.5 mg reduces fasting blood glucose by 30–34%
and increases immunoreactive insulin by about 100% in
normal volunteers . As insulin release is mediated through
K+-ATP channel blockade in the beta islet cells this
provides a biological assay confirming K+-ATP channel
blockade. To prevent hypoglycaemia this study was carried
out after a standard meal. As blood glucose and immuno-
reactive insulin will be necessarily higher after a meal and
therefore not reflective of insulin release due to glipizide,
measurement of these parameters was unlikely to have been
helpful. Higher doses of sulphonylureas have been associated
with blockade of K+-channels other than the ATP channel
and were therefore avoided . It should be pointed out
that although peak absorption of glipizide occurs 1–3 h after
oral administration, peak plasma concentration of insulin
occurs after 30–60 min. As insulin release is the marker of
K+-ATP channel blockade it is apparent this will be
maximal at 30–60 min after oral administration and therefore
the infusion of acetylcholine was repeated at this time [7, 9].
The finding that the K+-ATP channel is not involved in
modulating endothelium-dependent vasodilation in response
to acetylcholine is not entirely unexpected. The large
conductance calcium-dependent potassium channels, rather
than the K+-ATP channels, account for the large proportion
of potassium ion transport across the cell membrane and are
therefore more likely to be responsible for hyperpolarisation
. Further studies are required to determine the role of
these calcium-dependent potassium channels.
Although our study involved healthy volunteers these
EDHF causing hyperpolarisation through K+
activation, has been shown to account for 20–30% of the
endothelium-dependent vasodilation seen in response to
acetylcholine in the rat aorta . We have demonstrated
that K+-ATP channel blockade with the sulphonylurea,
glipizide does not affect endothelium-dependent vascular
responses suggesting that the K+-ATP channel is not
involved in modulating this response.
As we have previously shown responses to these infusions
are unaltered if are repeated on the same study day in the
same subject without any intervention, it is unlikely that an
order effect or tachyphylaxis occurred to confound these
findings (unpublished data).
It is possible that glipizide or the small standardised meal
(sandwich) could have had a significant haemodynamic
action, causing increased FBF. As basal blood flow was
unchanged before the sodium nitroprusside and acetylcholine
infusions prior to the administration of glipizide (8.7; 95%
CI 3.0–14.4 and 10.0; 95% CI 4.7–16.0 respectively), and
before and after glipizide (8.7; 95% CI 3.0–14.4 and 6.0;
95% CI 3.3–8.7 respectively), this suggests that neither had
a significant haemodynamic effect during the study. It is
possible that insulin release may modulate endothelium-
dependent responses. Although it has been shown that
insulin does not effect endothelium-dependent vasodilation
to acetylcholine in vitro in animal  and human  arteries,
Table 1 Change in FBF from baseline
in response to acetylcholine before and
after K+channel blockade (mean and
© 1997 Blackwell Science Ltd Br J Clin Pharmacol, 44, 292–294
D. McAuley et al.
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non-insulin dependent diabetes (NIDDM). As NIDDM is
associated with impaired nitric oxide mediated endothelium-
dependent vasodilation, hyperpolarisation may be of greater
importance in the regulation of vascular tone. It is reassuring
that K+-ATP channel blockade with glipizide, a sulphonylu-
rea used widely to treat NIDDM, does not effect endo-
In conclusion we have demonstrated in vivo that the
K+-ATP channel does not modulate forearm arteriolar
endothelium-dependent vascular responses to acetylcholine
in healthy volunteers and therefore does not play a role in
haveimplicationsfor themanagement of
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(Received 15 August 1996,
accepted 16 May 1997)
© 1997 Blackwell Science Ltd Br J Clin Pharmacol, 44, 292–294