Forearm vascular reactivity is differentially influenced by gliclazide and glibenclamide in chronically treated type 2 diabetic patients

Sulphonylureas (SUs) act by inhibition of beta-cell K(ATP) channels after binding to the sulphonylurea receptor SUR1. K(ATP) channels are also expressed in cardiac and vascular myocytes coupled to SUR2A and SUR2B involved into adaptations of vascular tone and myocardial contractility. Different influence of SUs on vascular function is based on different binding to the SUR family. Few data on the effect of different SUs, used in patients in therapeutic doses, on vascular function are currently available. We investigated possible effects of acute and chronic treatment with glibenclamide and gliclazide on forearm postischaemic reactive hyperaemia (RH) in type 2 diabetic patients. To that purpose a double-blind, randomized, cross-over study with gliclazide (80 mg, b.i.d.) and glibenclamide (5 mg, b.i.d.) was performed in 15 type 2 diabetic patients. Forearm vascular reactivity was measured after 5 min of ischaemia by plethysmography before and after 4 weeks treatment. After acute administration of gliclazide (80 mg) or glibenclamide (5 mg) RH was not influenced. After 4 weeks of treatment, no influence of either drug was seen in the steady state before dosing. After dosing glibenclamide induced a significant (P = 0.004) reduction of RH from 26.4 +/- 6.9 to 21.9 +/- 7.6 ml min(-1)/100 ml after 4 h. Gliclazide, conversely, did not induce a reduction of RH (23.9 +/- 6.0 to 23.3 +/- 6.6 ml min(-1)/100 ml). No influence of HbA1c or actual glycaemia on RH was observed. Our results indicate that in chronically treated patients with type 2 diabetes ingestion of glibenclamide but not gliclazide results in sustained reduction of postischaemic RH. This difference is most probably based on different SUR binding.     

Endogenous adenosine mediates coronary vasodilation during exercise after K(ATP)+ channel blockade.

The mechanism of coronary vasodilation produced by exercise is not understood completely. Recently, we reported that blockade of vascular smooth muscle K(ATP)+ channels decreased coronary blood flow at rest, but did not attenuate the increments in coronary flow produced by exercise. Adenosine is not mandatory for maintaining basal coronary flow, or the increase in flow produced by exercise during normal arterial inflow, but does contribute to coronary vasodilation in hypoperfused myocardium. Therefore, we investigated whether adenosine opposed the hypoperfusion produced by K(ATP)+ channel blockade, thereby contributing to coronary vasodilation during exercise. 11 dogs were studied at rest and during exercise under control conditions, during intracoronary infusion of the K(ATP)+ channel blocker glibenclamide (50 micrograms/kg per min), and during intracoronary glibenclamide in the presence of adenosine receptor blockade. Glibenclamide decreased resting coronary blood flow from 45 +/- 5 to 35 +/- 4 ml/min (P < 0.05), but did not prevent exercise-induced increases of coronary flow. Glibenclamide caused an increase in myocardial oxygen extraction at the highest level of exercise with a decrease in coronary venous oxygen tension from 15.5 +/- 0.7 to 13.6 +/- 0.8 mmHg (P < 0.05). The addition of the adenosine receptor antagonist 8-phenyltheophylline (5 mg/kg intravenous) to K(ATP)+ channel blockade did not further decrease resting coronary blood flow but did attenuate the increase in coronary flow produced by exercise. This was accompanied by a further decrease of coronary venous oxygen tension to 10.1 +/- 0.7 mmHg (P < 0.05), indicating aggravation of the mismatch between oxygen demand and supply. These findings are compatible with the hypothesis that K+ATP channels modulate coronary vasomotor tone both under resting conditions and during exercise. However, when K(ATP)+ channels are blocked, adenosine released from the hypoperfused myocardium provides an alternate mechanism to mediate coronary vasodilation in response to increases in oxygen demand produced by exercise.     

Vascular K(ATP) channel blockade by glibenclamide, but not by acarbose, in patients with Type II diabetes

Glibenclamide inhibits the opening of vascular ATP-sensitive potassium (K(ATP)) channels, which represents a protective mechanism during ischaemia. This effect may imply harmful cardiovascular effects of glibenclamide when used under conditions of ischaemia in patients with Type II diabetes. Acarbose is not associated with effects on the cardiovascular system, because the drug is not absorbed from the bowel. Therefore we hypothesized that treatment of Type II diabetes patients with glibenclamide will impair the vasodilator function of K(ATP) opening, unlike treatment with acarbose. A double-blind randomized cross-over study in 12 patients with Type II diabetes was performed to compare the effects of glibenclamide with those of acarbose on the vasodilator responses to K(ATP) channel opening in the forearm vascular bed. The study consisted of two periods: 8 weeks of treatment with orally administered glibenclamide (10 mg x day(-1)) followed by 8 weeks of treatment with acarbose (300 mg x day(-1)), or vice versa. At the end of each treatment period, forearm blood flow (venous occlusion plethysmography) in response to intra-arterially administered diazoxide, acetylcholine and dipyridamole and to forearm ischaemia was measured. The diazoxide-mediated increase in the forearm blood flow ratio (infused/control arm) was significantly less pronounced after glibenclamide than after acarbose (290 +/- 58% and 561 +/- 101% respectively; P<0.0005). Forearm blood flow responses to acetylcholine, dipyridamole and forearm ischaemia were similar during glibenclamide and acarbose treatment. Thus, in patients with Type II diabetes mellitus, treatment with glibenclamide is associated with an attenuated response to K(ATP) opening as compared with treatment with acarbose. This implies that glibenclamide may affect defensive mechanisms under conditions of K(ATP) channel activation.     

Basal and exercise-induced skeletal muscle blood flow is augmented in type I diabetes mellitus

Hyperaemia occurs early in the renal and retinal microcirculation of patients with type I (insulin-dependent) diabetes mellitus, and may be critical in the development of nephropathy and retinopathy. We therefore sought to determine whether resting and exercise-induced hyperaemia was also apparent in the skeletal muscle circulation of young subjects with type I diabetes. Blood flow was assessed by venous occlusion plethysmography in 18 diabetic (DM) subjects and 20 matched controls. Exercise entailed 2 min of isotonic exercise against no load. Endothelium-dependent and -independent vasodilator function was assessed following intra-arterial infusion of acetylcholine and sodium nitroprusside respectively. Forearm blood flow (FBF) was higher in DM subjects than in controls (3.3+/-0.3 and 2.2+/-0.2 ml x min(-1) x 100 ml(-1) forearm respectively; P<0.005). This was not due to differences in forearm or body size, blood pressure, heart rate, lipid status or glycaemic control. Peripheral insulin levels were higher in DM subjects than in controls (48.5+/-8 and 15.5+/-1.5 micro-units/ml respectively; P<0.005). Resting FBF was closely correlated with insulin levels (r(2)=0.4; P<0.005). Parameters of exercise-induced hyperaemia [including peak flow (16.4+/-1.4 and 12.0+/-0.7 ml x min(-1) x 100 ml(-1) forearm in DM and control subjects respectively; P<0.01) and the volume repaid to the forearm at 5 min post-exercise (32.1+/-3.1 and 23.1+/-1.4 ml x 100 ml(-1) forearm respectively; P<0.05)] were also significantly greater in DM subjects, even when differences in resting FBF were taken into account. Peak hyperaemic blood flow and the volume repaid at 5 min were also related to insulin levels (r(2)=0.16; P<0.05 and r(2)=0.27; P<0.005 respectively). The vasodilator response to acetylcholine was reduced in DM subjects (P<0.05; analysis of variance), and the slope of this dose-flow relationship was inversely related to insulin levels (r(2)=0.2; P<0.05). These data show that both resting and exercise-induced skeletal muscle blood flow are augmented in young patients with type I diabetes, possibly due to the vasodilatory effect of increased insulin levels. Diminished vasodilator responses to acetylcholine may also, in part, be a consequence of insulin-augmented resting muscle blood flow.     

Vasodilator prostanoids, but not nitric oxide, may account for skeletal muscle hyperaemia in Type I diabetes mellitus

We and others have previously documented increased resting and exercise-induced skeletal muscle blood flow in young subjects with Type I (insulin-dependent) diabetes mellitus compared with healthy controls. Both NO and prostanoids are important regulators of vascular tone and may therefore contribute to this hyperaemia. The aim of the present study was to determine the contribution of NO and vasodilator prostanoids to this skeletal muscle hyperaemia in diabetes. We assessed the effects of infusion into the intrabrachial artery of the cyclo-oxygenase inhibitor acetylsalicylic acid (ASA; aspirin) and of the L-arginine analogue N(G)-monomethyl-L-arginine (L-NMMA) on skeLetal muscle blood flow in subjects with Type I diabetes mellitus (DM subjects) and control subjects. Blood flow was measured by venous occlusion plethysmography. Isotonic forearm exercise involved 2 min of wrist flexion and extension. Resting flow (forearm blood flow; FBF) was augmented in DM subjects, as was peak exercise-related blood flow (PFBF) and the volume repaid to the forearm 5 min after exercise (AUC 5, where AUC is area under the flow-time curve) (P<0.05), even when accounting for differences in basal flow. Infusion of L-NMMA reduced resting flow by 48% in controls (P<0.005) and by 12% in DM subjects (not significant). L-NMMA reduced PFBF and AUC 5 by 29% (P<0.05) and 39% (P<0.0005) respectively in controls, but had no significant effect on these parameters in DM subjects. Infusion of ASA reduced FBF, PFBF and AUC 5 in both DM (P<0.05) and control (P<0.05) subjects, but the magnitude of this reduction was greater in DM than in control subjects (ANOVA, P<0.05), even when differences in resting FBF were accounted for. Indeed, ASA eliminated the differences in FBF, PFBF and AUC 5 between DM and control subjects. Thus increased release of vasodilator prostanoids, rather than of NO, appears to account for skeletal muscle hyperaemia in Type I diabetes.     

Sulfonylurea treatment of type 2 diabetic patients does not reduce the vasodilator response to ischemia

OBJECTIVE: Sulfonylureas block the activation of vascular potassium-dependent ATP channels and impair the vasodilating response to ischcmia in nondiabetic individuals, but it is not know whether this occurs in type 2 diabetic patients under chronic treatment with these drugs. Glimepiride, a new sulfonylurea, apparently has no cardiovascular interactions. The aim of our study was to compare the effect of the widely used compound glibenclamide, the pancreas-specific glimepiride, and diet treatment alone on brachial artery response to acute forearm ischemia. RESEARCH DESIGN AND METHODS: Brachial artery examination was performed by a high-resolution ultrasound technique on 20 type 2 diabetic patients aged mean +/- SD) 67 +/- 2 years and on 18 nondiabetic patients matched for age, hypertension, and dislipidemia. Diabetic subjects underwent three separate evaluations at the end of each 8-week treatment period, during which they received glibenclamide, glimepiride, or diet alone according to crossover design. Scans were obtained before and after 4.5 min of forearm ischemia. Postischemic vasodilation and hyperemia were expressed as percent variations in vessel diameter and blood flow. RESULTS: Postischemic vasodilation and hyperemia were, respectively, 5.42 +/- 0.90 and 331 +/- 38% during glibenclamide, 5.46 +/- 0.69 and 326 +/- 28% during glimepiride, and 5.17 +/- 0.64 and 357 +/- 35% during diet treatment (NS). These results were similar to those found in the nondiabetic patients (6.44 +/- 0.68 and 406 +/- 42%, NS). CONCLUSIONS: In type 2 diabetic patients, the vasodilating response to forearm ischemia was the same whether patients were treated with diet treatment alone or with glibenclamide or glimepiride at blood glucose-lowering equipotent closes.     

Occlusion cuff position is an important determinant of the time course and magnitude of human brachial artery flow-mediated dilation

Non-invasive ultrasound techniques to assess flow-mediated vasodilation (FMD) are frequently used to assess arterial endothelial vasodilator function. However, the range of normal values varies considerably, possibly due to differences in methodological factors. We sought to determine the effect of occlusion cuff position on the time course and magnitude of brachial artery blood flow and flow-mediated dilation. Twelve healthy subjects underwent measurements of forearm blood flow using venous occlusion plethysmography (VOP) before and after 5 min of susprasystolic cuff inflation, using two randomly assigned occlusion cuff positions (upper arm and forearm). An additional 16 subjects underwent two brachial ultrasound studies, using the two cuff positions, to assess the extent and time course of changes in brachial artery diameter and blood flow. Maximum increase in blood flow (peak reactive hyperaemia), measured by VOP, occurred immediately upon each cuff deflation, but was greater after upper arm compared with forearm arterial occlusion (33.1+/-3.1 versus 22.8+/-2.2 ml/min per forearm tissue, P=0.001). Maximal brachial artery FMD was significantly greater following upper arm occlusion (9.0+/-1.2%, mean +/- S.E.M.) compared with forearm occlusion (5.9+/-0.7%, P=0.01). The time course of the change in brachial artery diameter was affected differently in response to each protocol. The time to peak dilation following upper arm occlusion was delayed by 22 s compared with forearm occlusion. Occlusion cuff position is thus a powerful determinant of peak reactive hyperaemia, volume repaid and the extent and time course of brachial artery FMD. Positioning the cuff on the upper arm produces a greater FMD. These results highlight the need for comparisons between FMD studies to be made with care.     

Adenosine triphosphate-sensitive potassium channels prevent extension of myocardial ischemia to subepicardium during hemorrhagic shock

Cardiac dysfunction during hemorrhagic shock (HS) is associated with myocardial ischemia, during which adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels can be activated. We investigated the role of K(ATP) channels in HS-induced myocardial ischemia. Canine HS was induced using an aortic reservoir to maintain the aortic pressure at a constant 40 mmHg. To visualize the myocardial ischemia as a nicotinamide adenine dinucleotide (NADH) - fluorescent area, the beating hearts were rapidly cross-sectioned (120 ms) and freeze-clamped (-190 degrees C) using a sampling device after 10 min of HS. The effect of a K(ATP) channel blocker, glibenclamide (1 mg/kg, i.v.), on myocardial ischemia was also quantified. Regional myocardial blood flow was measured using heavy element-loaded nonradioactive microspheres. Myocardial ischemia developed in the subendocardium in the HS alone group, whereas it extended through all the cardiac layers in the glibenclamide-treatment group. The coadministration of a K(ATP) channel opener, cromakalim (50 microg/kg, i.v.), with glibenclamide prevented the extension of myocardial ischemia to the subepicardium. Glibenclamide decreased the myocardial ATP concentration selectively in the subepicardium during HS. The HS decreased myocardial blood flow transmurally, and following the administration of glibenclamide, further decreased the blood flow selectively in the subepicardium. These results suggest that K(ATP) channels are activated during HS, enabling selective subepicardial coronary dilatation and protecting the myocardium from the extension of myocardial ischemia to the subepicardium.     

Differential effects of sulphonylureas on the vasodilatory response evoked by K(ATP) channel openers

The potency of three sulphonylureas, glibenclamide, glimepiride and gliclazide in antagonizing the vasorelaxant action of openers of adenosine triphosphate (ATP)-regulated K+ channel (KATP) was studied in vivo and in vitro in micro- and macrovessels, respectively. In the hamster cheek pouch, the vasodilatation and the increase in vascular diameter and blood flow induced by diazoxide were markedly reduced by the addition of either glibenclamide or glimepiride (0.8 microm) while they were not affected by gliclazide up to 12 microm. Similarly, in rat and guinea-pig isolated aortic rings, glibenclamide, glimepiride and gliclazide reduced the vasodilator activity of cromakalim. However, the inhibitory effect of gliclazide was considerably less when compared with either glimepiride or glibenclamide. These results suggest that, in contrast to glibenclamide and glimepiride, therapeutically relevant concentrations of gliclazide do not block the vascular effects produced by KATP channel openers in various in vitro and in vivo animal models.     

Randomized, double-blind, placebo-controlled crossover pilot study of a potassium channel blocker in patients with septic shock

BACKGROUND: Marked potassium efflux prevents calcium entry into vascular smooth muscle cells and may be responsible for the "vasoplegia" of septic shock. Blockade of adenosine triphosphate (ATP)-sensitive potassium channels restores vascular tone in animal studies of septic shock. The effect of such potassium channel blockade has not been previously studied in humans. OBJECTIVE: To test whether the administration of an ATP-sensitive potassium (K(ATP)) channel blocker restores norepinephrine responsiveness in patients with septic shock. DESIGN: Randomized, double-blind, placebo-controlled crossover pilot study. SETTING: Intensive care unit of a university hospital. PATIENTS: Ten patients with septic shock requiring invasive hemodynamic monitoring and infusion of norepinephrine to maintain adequate mean arterial pressure. INTERVENTION: In addition to standard therapy, patients were randomized to initially receive either the K(ATP) channel blocker glibenclamide (20 mg) or placebo. Then, after 24 hrs, each patient crossed over to receive the alternative therapy. MEASUREMENTS AND MAIN RESULTS: After the administration of the K(ATP) channel blocker glibenclamide, median norepinephrine requirements decreased from 13 to 4 microg/min compared with a change from 19 to 7 microg/min after placebo. The two changes represented a decrease of 78.9% and 71.1% in dose, respectively (p = .57, not significant). There were also no significant changes in heart rate, mean arterial blood pressure, and lactate concentration when comparing the study drug with placebo. Glibenclamide, however, induced a significant decrease in median blood glucose concentration (5.4 [inter-quartile range, 4.5-7.0] vs. 7.0 mmol/L [5.2-9.3], p < .0001) compared with placebo and increased the need for parenteral glucose administration. CONCLUSIONS: The K(ATP) channel blocker glibenclamide failed to achieve a greater reduction in norepinephrine dose than placebo in septic shock patients, although it caused a reduced glucose concentration. Our observations suggest that, in such patients, blockade of K(ATP) channels does not have a potent effect on vasomotor tone.     

Role of K+ ATP channels and adenosine in the regulation of coronary blood flow during exercise with normal and restricted coronary blood flow.

Regulation of coronary vasomotor tone during exercise is incompletely understood. We investigated the contributions of K+ ATP channels and adenosine to the coronary vasodilation that occurs during exercise in the normal heart and in the presence of a coronary artery stenosis. Dogs that were chronically instrumented with a Doppler flow probe, hydraulic occluder, and indwelling catheter on the left anterior descending coronary artery were exercised on a treadmill to produce heart rates of approximately 200 beats/min. By graded inflation of the occluder to produce a wide range of coronary stenosis severities, we determined the coronary pressure-flow relation. K+ atp channel blockade with intracoronary glibenclamide (10-50 microgram/kg per min) decreased coronary blood flow during exercise at coronary pressures within and below the autoregulatory range, indicating that coronary K+ ATP channel activation is critical for producing coronary vasodilation with either normal arterial inflow or when flow is restricted by a coronary artery stenosis. Adenosine receptor blockade with intravenous 8-phenyltheophylline (5 mg/kg) had no effect on coronary flow at pressures within the autoregulatory range but decreased flow at pressures < 55 mmHg. In contrast, in the presence of K+ ATP channel blockade, the addition of adenosine receptor blockade further decreased coronary flow even at coronary pressures in the autoregulatory range, indicating increased importance of the vasodilator influence of endogenous adenosine during exercise when K+ atp channels are blocked. Intracoronary adenosine (50 microgram/kg per min) increased coronary flow at perfusion pressures both within and below the autoregulatory range. In contrast, selective K+ ATP channel activation with intracoronary pinacidil (0.2-5.0 microgram/kg per min) increased flow at normal but not at lower coronary pressures (< 55 mmHg). This finding demonstrates that not all K+ ATP channels are activated during exercise at pressures in the autoregulatory range, but that most K+ ATP channels are recruited as pressures approach the lower end of the autoregulatory plateau. Thus, K+ ATP channels and endogenous adenosine play a synergistic role in maintaining vasodilation during exercise in normal hearts and distal to a coronary artery stenosis that results in myocardial hypoperfusion during exercise.     

Calcitonin gene-related peptide: exploring its vasodilating mechanism of action in humans

OBJECTIVE: In vitro studies suggest that the vasodilator mechanism of action of calcitonin gene-related peptide (CGRP) involves various endothelium-dependent and endothelium-independent mechanisms. An in vivo analysis of the contribution of nitric oxide, prostaglandins, calcium-sensitive potassium channels (K(+)(Ca) channels), and adenosine triphosphate (ATP)-sensitive potassium channels (K(+)(ATP) channels) to CGRP-induced vasodilation in humans was performed. METHODS: CGRP (3, 10, and 30 ng x min(-1) x dL(-1) forearm) was infused into the brachial artery of 40 healthy subjects. Forearm vascular responses were measured by venous occlusion plethysmography. First, dose-response curves were constructed during coinfusion of CGRP with placebo (sodium chloride, 0.9%). After washout, in 5 subgroups (n = 8 each), the infusions of CGRP were repeated with placebo (time-control experiments), N(G)-monomethyl-L-arginine (L-NMMA, a nitric oxide-synthase inhibitor), indomethacin (a cyclooxygenase inhibitor), tetraethylammonium chloride (TEAC) (a K(+)(Ca)-channel blocker), and glyburide (INN, glibenclamide) (a K(+)(ATP)-channel blocker), respectively. RESULTS: CGRP induced a dose-dependent and reproducible decrease in forearm vascular resistance (P < .001). Compared with placebo, L-NMMA reduced the decrease in forearm vascular resistance induced by CGRP (P < .001) (3 and 10 ng x min(-1) x dL(-1) forearm). The absence of an inhibitory effect of L-NMMA on CGRP-induced vasodilation at the highest dose of CGRP suggests that still other mechanisms are involved. The vasodilator response to CGRP was not affected by coinfusion of indomethacin, tetraethylammonium chloride, or glyburide. CONCLUSIONS: The intrabrachial infusion of CGRP results in a dose-dependent and reproducible forearm vasodilator response. CGRP-induced vasodilation is dependent at least in part on the release of nitric oxide and does not involve the release of prostaglandins or the activation of K(+)(Ca) channels or K(+)(ATP) channels in humans.     

Insulin-mediated vasodilatation, but not glucose uptake or endothelium-mediated vasodilatation, is enhanced in young females compared with males

In order to evaluate possible differences between men and women with regard to the ability of insulin to induce vasodilatation, promote glucose uptake and enhance endothelium-dependent vasodilatation, 12 young (22-28 years), non-obese women and 15 corresponding males were subjected to 2 h of euglycaemic hyperinsulinaemia (insulin infusion rate of 56 m-units x min(-1) x m(-2)). Forearm blood flow was measured by venous occlusion plethysmography. Endothelium-dependent vasodilatation was evaluated by the local intra-arterial infusion of methacholine into the brachial artery (2-4 microg/min). The cardiac index was measured by thoracic bioimpedance. A 2 h period of hyperinsulinaemia increased the plasma insulin concentration to a similar degree in both sexes (females, 84 +/- 8.8 m-units/l; males, 87 +/- 7.5 m-units/l), but induced a more marked increase in forearm blood flow in females than in males (+104 +/- 67% and +52 +/- 30% respectively; P<0.01; 95% confidence interval for difference 11-94%). Furthermore, a significant decrease in total peripheral resistance (-20 +/- 6.9%; P<0.01) and an increase in cardiac index (+23 +/- 13%; P<0.01) were seen in women only (P<0.05 compared with men). Blood pressure and heart rate were not altered in either sex. Whole-body insulin-mediated glucose uptake and forearm glucose uptake did not differ between the sexes, and the ability of insulin to enhance endothelium-dependent vasodilatation (+19%; P<0.01) was similar in men and women. In conclusion, the present study shows that the ability of insulin to cause vasodilatation was greater in non-obese young women compared with men. However, no differences between the sexes were seen with regard to insulin-mediated glucose uptake and the ability of insulin to enhance endothelium-dependent vasodilatation.     

Intracellular ATP can regulate afferent arteriolar tone via ATP-sensitive K+ channels in the rabbit.

Studies were performed to assess whether ATP-sensitive K+ (KATP) channels on rabbit preglomerular vessels can influence afferent arteriolar (AA) tone. K+ channels with a slope conductance of 258 +/- 13 (n = 7) pS and pronounced voltage dependence were demonstrated in excised patches from vascular smooth muscle cells of microdissected preglomerular segments. Channel activity was markedly reduced by 1 mM ATP and in a dose-dependent fashion by glibenclamide (10(-9) M to 10(-6) M), a specific antagonist of KATP channels. 10(-5) M diazoxide, a K+ channel opener, activated these channels in the presence of ATP, and this effect was also blocked by glibenclamide. To determine the role of these KATP channels in the control of vascular tone, diazoxide was tested on isolated perfused AA. After preconstriction from a control diameter of 13.1 +/- 1.1 to 3.5 +/- 2.1 microns with phenylephrine (PE), addition of 10(-5) M diazoxide dilated vessels to 11.2 +/- 0.7 microns, which was not different from control. Further addition of 10(-5) M glibenclamide reconstricted the vessels to 5.8 +/- 1.5 microns (n = 5; P less than 0.03). In support of its specificity for KATP channels, glibenclamide did not reverse verapamil induced dilation in a separate series of experiments. To determine whether intracellular ATP levels can effect AA tone, studies were conducted to test the effect of the glycolytic inhibitor 2-deoxy-D-glucose. After preconstriction from 13.4 +/- 3.2 to 7.7 +/- 1.3 microns with PE, bath glucose was replaced with 6 mM 2-deoxy-D-glucose. Within 10 min, the arteriole dilated to a mean value of 11.8 +/- 1.4 microns (n = 6; NS compared to control). Subsequent addition of 10(-5) M glibenclamide significantly reconstricted the vessels to a diameter of 8.6 +/- 0.5 micron (P less than 0.04). These data demonstrate that KATP channels are present on the preglomerular vasculature and that changes in intracellular ATP can directly influence afferent arteriolar tone via these channels.     

Glyburide inhibits dipyridamole-induced forearm vasodilation but not adenosine-induced forearm vasodilation

BACKGROUND: The mechanism of the vasodilator response to adenosine has not been elucidated in humans. Stimulation of adenosine receptors on endothelial and vascular smooth muscle cells with subsequent endothelial release of nitric oxide and opening of adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels has been suggested. AIM: The aim of this study was to investigate the involvement of K(ATP) channels in the vasodilator response to adenosine and the nucleoside transport inhibitor dipyridamole.Methods and results In healthy male volunteers, adenosine (0.6, 1.9, 5.6, 19, 57, and 190 nmol. min(-1). dL(-1)) was infused into the brachial artery, and forearm blood flow (FBF) was measured by use of strain-gauge plethysmography. Adenosine increased the FBF ratio (FBF in experimental arm/FBF in control arm) from 1.3 +/- 0.2 to 1.2 +/- 0.2, 1.5 +/- 0.2, 2.8 +/- 0.4, 7.3 +/- 2.3, 11.1 +/- 4.1, and 12.9 +/- 3.7 for the six increasing adenosine doses, respectively. Simultaneous infusion of glyburide (INN, glibenclamide), a blocker of K(ATP) channels, did not affect this response (from 1.7 +/- 0.4 to 1.5 +/- 0.2, 2.2 +/- 0.3, 4.0 +/- 1.0, 9.3 +/- 4.0, 13.5 +/- 6.4, and 15.9 +/- 5.3 for the 6 increasing doses of adenosine, respectively; P =.439, n = 6). The increase in FBF ratio during infusion of the nucleoside transport inhibitor dipyridamole (20, 60, and 200 nmol. min(-1). dL(-1)) was significantly reduced by glyburide, as follows: from 1.2 +/- 0.1 to 1.7 +/- 0.2, 2.4 +/- 0.5, and 2.9 +/- 0.4, respectively, during saline solution and from 1.6 +/- 0.2 to 1.8 +/- 0.2, 2.1 +/- 0.3, and 2.2 +/- 0.4, respectively, during glyburide (P =.010 for effect of glyburide on response from baseline, ANOVA for repeated measures; n = 8). The vasodilator response to dipyridamole was significantly inhibited by the adenosine receptor antagonist theophylline. CONCLUSION: Opening of vascular K(ATP) channels is involved in the forearm vasodilator response to dipyridamole but not to adenosine. Differences in stimulated cell type (endothelium for adenosine versus smooth muscle cells for dipyridamole) may underlie this divergent pharmacologic profile.     

Acute and chronic effects of flavanol-rich cocoa on vascular function in subjects with coronary artery disease: a randomized double-blind placebo-controlled study

Evidence suggests that flavonoid-containing diets reduce cardiovascular risk, but the mechanisms responsible are unclear. In the present study, we sought to determine the effect of flavanol-rich cocoa on vascular function in individuals with CAD (coronary artery disease). Forty subjects (61+/-8 years; 30 male) with CAD were recruited to a 6-week randomized double-blind placebo-controlled study. Subjects consumed either a flavanol-rich chocolate bar and cocoa beverage daily (total flavanols, 444 mg/day) or matching isocaloric placebos daily (total flavanols, 19.6 mg/day) for 6 weeks. Brachial artery FMD (flow-mediated dilation) and SAC (systemic arterial compliance) were assessed at baseline, 90 min following the first beverage and after 3 and 6 weeks of daily consumption. Soluble cellular adhesion molecules and FBF (forearm blood flow) responses to ACh (acetylcholine chloride; 3-30 microg/min) and SNP (sodium nitroprusside; 0.3-3 microg/min) infusions, forearm ischaemia and isotonic forearm exercise were assessed at baseline and after 6 weeks. FMD, SAC and FBF responses did not differ between groups at baseline. No acute or chronic changes in FMD or SAC were seen in either group. No difference in soluble cellular adhesion molecules, FBF responses to ischaemia, exercise, SNP or ACh was seen in the group receiving flavanol-rich cocoa between baseline and 6 weeks. These data suggest that over a 6-week period, flavanol-rich cocoa does not modify vascular function in patients with established CAD.     

Role of neuronal KATP channels and extraneuronal monoamine transporter on norepinephrine overflow in a model of myocardial low flow ischemia

Global myocardial low flow ischemia results in an uniform suppression of norepinephrine (NE) overflow from the heart. We hypothesized that opening of neuronal ATP-sensitive potassium (K(ATP)) channels as well as activation of the extraneuronal monoamine transporter (EMT) mediates attenuation of NE overflow during low flow ischemia. Isolated rat hearts were subjected to low coronary flow of 0.4 ml min(-1). Release of endogenous NE was induced by electrical field stimulation. EMT activity was measured as the transport rate of the substrate N-[methyl-3H]4-phenylpyridinium ([3H]MPP+). NE overflow decreased by 57 +/- 2% within 120 min of low flow. Five minutes of reperfusion at normal flow (8 ml min(-1)) restored NE overflow to baseline. K(ATP) channel blockade with glibenclamide as well as EMT blockade with corticosterone increased NE overflow 1.5- and 2-fold at 120 min of low flow, whereas neither drug affected NE overflow in the absence of flow reduction. At normal flow, K(ATP) channel opening with cromakalim suppressed NE overflow, both in the presence and absence of EMT blockade (14 +/- 4 and 9 +/- 1%). However, cromakalim had no effect on EMT activity as indicated by an unaffected [3H]MPP+ overflow. In conclusion, activation of both K(ATP) channels and EMT mediate suppression of NE overflow during low flow ischemia. K(ATP) channels impair NE release directly at presynaptic nerve endings, whereas EMT increases NE elimination in a manner independent of K(ATP) channels.     

Age-independent forearm vasodilatation by acetylcholine and adenosine 5'-triphosphate in humans

1. Forearm vasodilator responses to acetylcholine, ATP and sodium nitroprusside were examined in healthy young (20 +/- 1 years, n = 9), middle-aged (46 +/- 2 years, n = 6) and old (57 +/- 1 years, n = 6) subjects. 2. A brachial artery was cannulated with a 20-gauge cannula through which drugs at graded doses were locally infused for 2 min at each dose. During drug infusions, forearm blood flow was continuously measured at 15 s intervals using a plethysmograph. Forearm vascular resistance was calculated from forearm blood flow and mean blood pressure obtained in the opposite arm. Basal forearm blood flow and forearm vascular resistance did not differ between the three groups. 3. Acetylcholine and ATP were used to examine endothelium-dependent vasodilatation, and sodium nitroprusside was used to examine endothelium-independent vasodilatation. All three drugs caused dose-dependent increases in forearm blood flow (P less than 0.01) and decreases in forearm vascular resistance (P less than 0.01). The increases in forearm blood flow or decreases in forearm vascular resistance in response to infusions of the three drugs did not differ between the three groups. 4. These results suggest that endothelium-dependent and endothelium-independent vasodilatation in forearm resistance arteries do not alter with ageing in humans.     

The new K+ channel opener Aprikalim (RP 52891) reduces experimental infarct size in dogs in the absence of hemodynamic changes.

The objective of the present study was to determine the effect of a novel K+ channel opener, Aprikalim (RP 52891; [trans-(-)-N-methyl-2-(3-pyridyl)-2-tetrahydrothio-pyran carbothiamide-1-oxide]), on myocardial infarct size in barbital-anesthetized dogs subjected to 90 min of left circumflex coronary artery occlusion followed by 5 hr of reperfusion. To determine if RP 52891 is mediating its effects by opening adenosine triphosphate regulated potassium channels (KATP), glibenclamide, a KATP channel antagonist was used. Dogs were pretreated with vehicle, a nonhypotensive dose of RP 52891 (10 micrograms/kg + 0.1 microgram/kg/min i.v.), glibenclamide (1 mg/kg; i.v. bolus) or RP 52891 (10 micrograms/kg and 0.1 microgram/kg/min i.v.) after pretreatment with glibenclamide (1 mg/kg i.v. bolus). At the end of reperfusion, myocardial infarct size was determined by triphenyltetrazolium staining. There were no significant differences in systemic hemodynamics, myocardial oxygen demand, collateral blood flow or ischemic bed size among groups with the exception of an increase in coronary blood flow to the ischemic area at 3 and 5 hr of reperfusion in both RP-treated groups. However, myocardial infarct size, expressed as a percentage of the area at risk, was significantly (P less than .05) reduced (38%) by RP 52891 and significantly increased (38%) by glibenclamide (vehicle, 39 +/- 4%; RP 52891, 24 +/- 2%; and glibenclamide, 54 +/- 5%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of graded leg cycling on postischaemic forearm blood flow in healthy subjects

This study assessed in healthy subjects, the effect of leg cycling on the forearm vascular responses to ischaemia to confirm previous results showing that exercise-induced sympathetic activation during leg cycling reduced postischaemic forearm hyperaemia. Seven young healthy subjects performed two bouts of cycling exercises at 50% and 80% of their maximal aerobic capacity (Ex(50), Ex(80) respectively) during which forearm arterial blood flow was successively occluded for 40, 90 and 180 s. Control forearm blood flow (FBF) and postischaemic forearm blood flow (pi-FBF) measured at the release of arterial occlusions were assessed using plethysmography. Digital arterial pressure was continuously monitored allowing calculation of control and postischaemic forearm conductance (FC and pi-FC respectively). At rest, pi-FBF increased with the duration of ischaemia (5 +/- 1, 19 +/- 3, 29 +/- 3, 31 +/- 4 ml min(-1) 100 ml(-1) after 0, 40, 90 and 180 s of ischaemia respectively). During Ex(50), FBF and pi-FBF did not change significantly although pi-FC was significantly reduced (Deltapi-FC = -39%, -33%, -27% for 40, 90, 180 s of ischaemia respectively). During Ex(80), there was a further dramatic decrease in pi-FC (-53%, -66%, -62% from rest) and pi-FBF were largely blunted (13 +/- 4 versus 19 +/- 3, 14 +/- 4 versus 29 +/- 3, 17 +/- 5 versus 31 +/- 4 ml min(-1) 100 ml(-1)). These results demonstrated that forearm responses to ischaemia depended on leg activities. It was suggested that exercise-induced sympathetic activation may have interfered on local vasodilatation because of ischaemia.