KATP Channel Blockade Research Articles

Vascular KATP 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.

Ischemic shortening of action potential duration as a result of KATP channel opening attenuates myocardial stunning by reducing calcium influx.

Action potential duration (APD) shortening due to opening of sarcolemmal ATP-dependent potassium (KATP) channels has been postulated to protect the myocardium against postischemic damage by reducing Ca2+ influx. This hypothesis was assessed, assuming that increased postischemic stunning due to KATP channel inhibition with glibenclamide could be reverted by the addition of the Ca2+ channel blocker diltiazem. Percent wall thickening fraction (%WTh, conscious sheep) and APD (open-chest sheep) were obtained from the following groups: control: 12 min ischemia by anterior descending coronary artery occlusion followed by 2 h reperfusion; glibenclamide: same as control, with glibenclamide (0.4 mg/kg) infused 30 min before ischemia; diltiazem: same as control, with diltiazem (100 microg/kg) administered prior to ischemia; glibenclamide+diltiazem: both drugs infused as in glibenclamide and diltiazem groups. APD was reduced in control ischemia. Conversely, KATP-channel blockade by glibenclamide lengthened APD and increased postischemic stunning (p < 0.01 vs. control); glibenclamide+diltiazem did not shorten APD but enhanced functional recovery (p < 0.01 vs. glibenclamide). Ca2+ channel blockade improvement of increased stunning provoked by KATP channel inhibition supports the hypothesis that APD shortening due to opening of KATP channels protects against postischemic stunning by limiting Ca2+ influx.

Selective blockade of mitochondrial K(ATP) channels does not impair myocardial oxygen consumption.

Opening of mitochondrial ATP-sensitive potassium (K(ATP)) channels has been postulated to prevent inhibition of respiration resulting from matrix contraction during high rates of ATP synthesis. Glibenclamide, which blocks K(ATP) channels on the sarcolemma of vascular smooth muscle cells and myocardial myocytes as well as on the inner mitochondrial membrane, results in a decrease of myocardial oxygen consumption (MVO2) both at rest and during exercise. This study examined whether this represents a primary effect of blockade of mitochondrial K(ATP) channels or occurs secondary to coronary resistance vessel constriction with a decrease of coronary blood flow (CBF) and myocardial oxygen availability. MVO2 was measured at rest and during treadmill exercise in 10 dogs during control conditions, after selective mitochondrial K(ATP) channel blockade with 5-hydroxydecanoate (5-HD), and after nonselective K(ATP) channel blockade with glibenclamide. During control conditions, exercise resulted in progressive increases of CBF and MVO2. Glibenclamide (50 microg x kg(-1) x min(-1) ic) resulted in a 17 +/- 6% decrease of resting CBF with a downward shift of CBF during exercise and a decrease of coronary venous PO2, indicating increased myocardial oxygen extraction. In contrast with the effects of glibenclamide, 5-HD (0.7 mg x kg(-1) x min(-1) ic) had no effect on CBF, MVO2, or myocardial oxygen extraction. These findings suggest that glibenclamide decreased MVO2 by causing resistance vessel constriction with a decrease of CBF and oxygen available to the myocardium rather than to a primary reduction of mitochondrial respiration.

Invited commentary

Potassium-based cardioplegic solutions have long been used by surgeons to reduce the damaging effects of ischemia. The basic premise has been that depolarized arrest reduces cardiac work and myocardial energy demands, thereby increasing ischemic tolerance. This manuscript by Toyoda and colleagues provides new data that suggests that other mechanisms may be involved. In a carefully controlled series of experiments they suggest a role for ATP-sensitive potassium (KATP) channels in the cardioprotection afforded by cardioplegia consisting of K+ and Mg++. This work builds on studies from other investigators that have suggested that KATP channels serve an important role in modulating the effects of myocardial ischemia. Two KATP subtypes have been identified—sarcolemmal and mitochondrial—but it is the latter that appears to be most important to myocardial protection. Some recent experiments have demonstrated that KATP activation may be an important mechanism behind the endogenous cardioprotective phenomenon known as preconditioning. These results have led to attempts to manipulate KATP channels as a therapy for acute myocardial ischemia. In general, administration of potassium channel openers in pharmacologic doses prior to the onset of ischemia has led to a reduction in ischemic damage. In present study, manipulations of KATP channels had dramatic effects on the degree of infarct reduction induced by K/Mg cardioplegia. In particular, the addition of diazoxide (a potent mitochondrial KATP channel opener) effectively reduced infarct size beyond that observed with cardioplegia alone, while 5-hydroxydecanoate (a channel blocker) reduced the effectiveness of cardioplegia. The effects of cardioplegia were not altered by blockade of sarcolemmal KATP channels. Thus potassium-based cardioplegia may do much more than reduce cardiac energy demands. It may exert an effect on ion channels within the mitochondria, an effect which may influence mitochondrial depolarization, calcium accumulation, matrix volume, or ATP production. These effects may be very important in the overall picture of ischemic tolerance. More work needs to be done to determine the molecular mechanisms of cardioprotection provided by KATP activation and by potassium cardioplegia. However, this line of investigation appears promising and may suggest strategies to extend our ability to protect the heart during cardiac surgery.

Characterization of a KATP channel-independent pathway involved in potentiation of insulin secretion by efaroxan.

Efaroxan, like several other imidazoline reagents, elicits a glucose-dependent increase in insulin secretion from pancreatic beta-cells. This response has been attributed to efaroxan-mediated blockade of KATP channels, with the subsequent gating of voltage-sensitive calcium channels. However, increasing evidence suggests that, at best, this mechanism can account for only part of the secretory response to the imidazoline. In support of this, we now show that efaroxan can induce functional changes in the secretory pathway of pancreatic beta-cells that are independent of KATP channel blockade. In particular, efaroxan was found to promote a sustained sensitization of glucose-induced insulin release that persisted after removal of the drug and to potentiate Ca2+-induced insulin secretion from electropermeabilized islets. To investigate the mechanisms involved, we studied the effects of the efaroxan antagonist KU14R. This agent is known to selectively inhibit insulin secretion induced by efaroxan, without altering the secretory response to glucose or KCl. Surprisingly, however, KU14R markedly impaired the potentiation of insulin secretion mediated by agents that raise cAMP, including the adenylate cyclase activator, forskolin, and the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX). These effects were not accompanied by any reduction in cAMP levels, suggesting an antagonistic action of KU14R at a more distal point in the pathway of potentiation. In accord with our previous work, islets that were exposed to efaroxan for 24 h became selectively desensitized to this agent, but they still responded normally to glucose. Unexpectedly, however, the ability of either forskolin or IBMX to potentiate glucose-induced insulin secretion was severely impaired in these islets. By contrast, the elevation of cAMP was unaffected by culture of islets with efaroxan. Taken together, the data suggest that, in addition to effects on the KATP channel, imidazolines also interact with a more distal component that is crucial to the potentiation of insulin secretion. This component is not required for Ca2+-dependent secretion per se but is essential to the mechanism by which cAMP potentiates insulin release. Overall, the results indicate that the actions of efaroxan at this distal site may be more important for control of insulin secretion than its effects on the KATP channel.

Role of KATP Channel During Sustained Ventricular Fibrillation

Objectives:To investigate the role of ATP-sensitive potassium channel (KATP) during sustained ventricular fibrillation (VF), the effects of gliburide, a specific blocker of KATP channel and PCO400, an KATP opener, were studied in isolated and perfused swine right ventricular free walls (n=8). Methods:Recording of single cell transmembrane potentials was performed and constructed action potential duration restitution (APDR) curve by plotting APD 90% (APD90) versus preceding diastolic interval (DI). Results:All isolated tissues fibrillated spontaneously. In this preparation, stable VF could persist over a 4-hour period if it was allowed to continue undisturbed (n=1). Gliburide (1-5 μM) increased DI without significant changes in APD 90 during VF, resulting in more regularization of VF. Higher concentration (10-20 μM) increased both APD 90 and DI, and converted to monomorphic ventricular tachycardia (MVT) through the transitional period characterized by APD alternans. PCO400 (1-2.5 μM) caused a significant shortening of APD during MVT and a period of APD alternans became more evident before conversion from MVT to VF. Gliburide eliminated profibrillatory effect of PCO400. This antifibrillatory action of gliburide was accompanied by gradual decrease in the maximum slope of APDR curve during VF. Conclusion:KATP channel blockade causes a transition from VF to MVT via lengthening of DI and APD alternans, concomitantly with a reduction of the slope of APD restitution curve. (Korean Circulation J 2001;31(3):359-369)

Effect of combined K(ATP) channel activation and Na(+)/H(+) exchange inhibition on infarct size in rabbits.

We tested if combining treatment with cariporide, an Na(+)/H(+) exchange inhibitor, and diazoxide, a mitochondrial ATP-sensitive K(+) (K(ATP)) channel opener, would reduce myocardial infarct size (IS) to a greater extent than either intervention alone. Four groups of rabbits were studied (n = 10 each): cariporide (0.3 mg/kg), diazoxide (10 mg/kg), both drugs, and saline control, given 15 min before a 30-min coronary artery occlusion and 3 h reperfusion. IS in controls comprised 47 +/- 6% of the risk region. Cariporide reduced IS by 55% compared with control (21 +/- 3%), but diazoxide did not significantly reduce IS compared with controls (37 +/- 6%). Combined treatment resulted in an IS of 18 +/- 5%. Also we determined that diazoxide did not potentiate a subthreshold dose of cariporide nor did a mitochondrial K(ATP) channel blocker, 5-hydroxydecanoate (5-HD), prevent cariporide from reducing IS. Thus cariporide reduced necrosis by >50% in this model, both in the presence and absence of K(ATP) channel blockade. There was no significant difference in IS reduction between the group receiving cariporide alone and the group receiving combined treatment. Because the effect of cariporide was not blocked by 5-HD, it is unlikely that K(ATP) channels play a role as an end effector in cariporide's mechanism.

Role of potassium channels in the central neurogenic neuroprotection elicited by cerebellar stimulation in rat

Electrical stimulation of the cerebellar fastigial nucleus (FN) in spontaneously hypertensive (SHR), Wistar–Kyoto (WKY) and Fisher rats reduced, by ∼50%, the infarctions produced by occlusion of the middle cerebral artery. Blockade of ATP-dependent potassium (K-ATP) channels with glibenclimide (i.c.v.) abolished salvage only in the SHR rat. While blockade of K-ATP channels failed to abolish salvage in WKY and Fisher rats, participation of potassium channels in neurogenic neuroprotection cannot be excluded.

Inhibition of Na+-K+Pump Alleviates the Shortening of Action Potential Duration caused by Metabolic Inhibition via Blockade of KATPChannels in Coronary Perfused Ventricular Muscles of Guinea-pigs

The Na+-K+pump is a consumer of intracellular ATP. We therefore examined whether blockade of the Na+-K+pump by cardiac glycosides could inhibit ATP-sensitive K+(KATP) channels and prolong the action potential duration (APD) of the guinea-pig ventricular muscles perfused with Tyrode's solution via the coronary artery and stimulated at 3 Hz. The metabolic inhibition (MI) achieved by application of 0.1μm carbonyl cyanidep-(trifluoromethoxy) phenylhydrazone (a mitochondrial uncoupler) shortened the APD in a time-dependent manner. When dihydroouabain (DHO, 5μm) was introduced 5 min but not 10 min after introduction of MI, the APD shortening was significantly attenuated. Application of glibenclamide (1μm), a blocker of KATPchannels, introduced both 5 and 10 min after MI also alleviated the APD shortening; DHO did not alleviate the APD shortening effect produced by cromakalim (5μm), a KATP-channel opener. In separate experiments using whole-cell patch-clamp techniques, we found that this concentration of DHO (5μm) depressed the Na+-K+pump current of the guinea-pig ventricular myocytes from 210 to 100 pA (at 0 mV) or by 49.5%. We conclude that, during early phase (5 min) of MI, the APD shortening mostly results from the activation of KATPchannels, and that even a 50% inhibition of the Na+-K+pump by DHO leads to the blockade of KATPchannels and eventual lengthening of the APD

Normothermic Transfer Times up to 3 Min Will Not Precondition the Isolated Rat Heart

Isolated, perfused heart preparations suffer an inevitable peri-operative delay (POD) before retrograde perfusion restores coronary flow. By varying this ischaemic period we investigated the threshold of POD-induced inadvertent preconditioning (PC) in the rat heart. Hearts subjected to POD at 37°C increasing from 1, 2, 3, 5, 10 up to 15 min prior to 20 min retrograde perfusion were further subjected to 30 min global, normothermic ischaemia and 30 min reperfusion (index I/R). The functional recovery was 32±4.1% in hearts subject to 1 min POD. After 3 min POD functional recovery started to improve and peaked at 10 min POD (78±7.1%,P<0.001). At 4°C functional recovery started to improve after 5 min POD and peaked at 10 min POD. To demonstrate that the POD-mediated protection was true PC, two conventional models of PC were established. In both models, hearts were retrogradely perfused within 1 min POD prior to a standard PC protocol (one episode of 10 min ischaemia, or four episodes of 5 min ischaemia). In the conventional PC models protection against the index I/R was abolished using 100μm5-hydroxydecanoate (5-HD), the mitrochondrial KATPchannel inhibitor. Likewise, 10 min POD-mediated recovery at 37°C (70±3.2%) was reversed by 100μm5-HD perfusion (36±5.9%; NS vs 2 min POD). We conclude: (1) the threshold for PC is greater than 3 min at 37°C and greater than 5 min at 4°C; (2) blockade of the mitochondrial KATPchannel abolishes protection in three models of PC in the rat heart, including prolonged POD.

Mitochondrial ATP-sensitive K+ channels modulate cardiac mitochondrial function.

Discovered in the cardiac sarcolemma, ATP-sensitive K+ (KATP) channels have more recently also been identified within the inner mitochondrial membrane. Yet the consequences of mitochondrial KATP channel activation on mitochondrial function remain partially documented. Therefore, we isolated mitochondria from rat hearts and used K+ channel openers to examine the effect of mitochondrial KATP channel opening on mitochondrial membrane potential, respiration, ATP generation, Ca2+ transport, and matrix volume. From a mitochondrial membrane potential of -180 +/- 15 mV, K+ channel openers, pinacidil (100 microM), cromakalim (25 microM), and levcromakalim (20 microM), induced membrane depolarization by 10 +/- 7, 25 +/- 9, and 24 +/- 10 mV, respectively. This effect was abolished by removal of extramitochondrial K+ or application of a KATP channel blocker. K+ channel opener-induced membrane depolarization was associated with an increase in the rate of mitochondrial respiration and a decrease in the rate of mitochondrial ATP synthesis. Furthermore, treatment with a K+ channel opener released Ca2+ from mitochondria preloaded with Ca2+, an effect also dependent on extramitochondrial K+ concentration and sensitive to KATP channel blockade. In addition, K+ channel openers, cromakalim and pinacidil, increased matrix volume and released mitochondrial proteins, cytochrome c and adenylate kinase. Thus, in isolated cardiac mitochondria, KATP channel openers depolarized the membrane, accelerated respiration, slowed ATP production, released accumulated Ca2+, produced swelling, and stimulated efflux of intermembrane proteins. These observations provide direct evidence for a role of mitochondrial KATP channels in regulating functions vital for the cardiac mitochondria.

Coronary Vascular Responsiveness to Adenosine Is Impaired Additively by Blockade of Nitric Oxide Synthesis and a Sulfonylurea

Objectives. We sought to define effects of glibenclamide, a sulfonylurea known to block ATP-dependent potassium (KATP) channels, and Nω-nitro-l-arginine methyl ester (l-NAME), an l-arginine analog known to block nitric oxide (NO) synthesis, on coronary vascular responsiveness to adenosine.Background. The role of adenosine in coronary flow regulation becomes increasingly important when KATPchannel function or NO synthesis is impaired. Both variables are potentially altered in patients with coronary artery disease taking a sulfonylurea.Methods. Dose-response curves relating coronary conductance to plasma adenosine concentration were obtained by using intracoronary infusions of adenosine (10 to 1,000 μg/min) in chronically instrumented dogs.Results. ED50, the plasma concentration of adenosine needed to produce 50% of the maximal increase in conductance under baseline conditions, increased threefold after either 1 or 10 mg/kg of l-NAME. ED50also increased in response to glibenclamide in a dose-related fashion (5.7-fold increase per 1 mg/kg body weight of glibenclamide). Effects of combined blockade of KATPchannels and NO synthesis were additive, with increases in ED50as high as 15-fold. Both l-NAME and glibenclamide increased systemic pressure and reduced coronary conductance, confirming the roles of NO and KATPchannels in regulating coronary and systemic vascular tone under rest conditions as well as during stress.Conclusions. Coronary vascular responsiveness to adenosine is blunted in vivo by both l-NAME and glibenclamide. Effects of the sulfonylurea and blockade of NO synthesis are additive and can limit coronary vasodilation as well as other responses involving KATPchannels and NO.

Sulfonylureas and Mortality in Diabetic Patients After Myocardial Infarction

To the Editor: A long-standing controversy that relates to the potentially harmful effects of sulfonylurea drugs within the ischemic myocardium was presented recently in Circulation .1 The controversy arises from the fact that sulfonylurea drugs inhibit ATP-sensitive potassium (KATP) channels.2 Although blockade of KATP channels in the pancreas promotes the release of insulin, conductance through KATP channels in the heart is thought to be critical to the protection afforded by ischemic preconditioning, which limits infarct size.2 3 4 Yet, despite experimental data that demonstrate that sulfonylurea drugs prevent ischemic preconditioning,3 5 …

Acceleration of stiffness in underperfused diabetic rat hearts by glyburide, a KATP channel blocker, and its prevention by levcromakalim and insulin.

To clarify the role of the KATP channels in myocardial dysfunction during underperfusion with norepinephrine (NE) in the diabetic heart, particularly the heart treated with sulphonylurea derivatives. Isolated 6-week streptozotocin-diabetic rat hearts with a balloon in the left ventricle (LV) were paced and perfused with normoxic Krebs-Henseleit solution. Agents were infused for 15-25 min before as well as during 60-min underperfusion (2 ml/min/g heart weight) with 10(-6) M NE. Regional myocardial flow distribution was measured using dye microspheres. The effects of ex vivo glyburide (10(-6) M, a sulphonylurea anti-diabetic drug and a specific KATP channel inhibitor) on contractile dysfunction and abnormal regional myocardial energy metabolism were examined during underperfusion with NE in the absence of presence of levcromakalim (10(-4) M, a selective K+ channel opener) and insulin (2 mU/min/g heart weight). The flow rate was greater in the LV subendocardium than the subepicardium during normal perfusion, and smaller at 60-min underperfusion with NE. The LV diastolic tension and pressure during underperfusion with NE increased more rapidly in the presence of glyburide. At 60-min underperfusion with NE, the diastolic pressure elevation was still higher in the glyburide-treated heart, and decreases in tissue ATP, creatine phosphate (CP), energy charge, phosphorylation potential and CP/inorganic phosphate (P(i)) ratio, and increases in AMP, P(i) and lactate were more marked in the glyburide-treated heart, particularly in the LV subendocardium. Thus, ex vivo glyburide enhanced the increase in LV stiffness and abnormal myocardial energy metabolism during underperfusion with NE in diabetic hearts. These changes were reduced by levcromakalim to the level during underperfusion with NE without glyburide. Insulin did not prevent the glyburide-induced earlier exacerbation of the increase in LV stiffness during underperfusion with NE, but reduced the detrimental effects 20 min after the onset of underperfusion. KATP channels in the diabetic myocardium probably open during underperfusion with NE, and it helps delay the initiation of the increase in cardiac stiffness. Glyburide may have harmful effects in the ischemic diabetic heart; the myocardial KATP channel blockade during underperfusion with NE enhanced the increase in LV stiffness and abnormal myocardial energy metabolism. The glyburide-induced detrimental effects in the ischemic diabetic heart are prevented by levcromakalim and partly by insulin.

Blockade of ATP‐sensitive K+ channel abolishes the anti‐ischemic effects of isoflurane in dog hearts

Although isoflurane is reported to have a protective effect against ischemic damage on the myocardium, the mechanisms of this effect are not clear. Activation of adenosine triphosphate sensitive potassium (KATP) channels is indicated to protect myocardium during ischemia. Thus, it was hypothesized that if isoflurane could activate KATP channels, blockade of KATP channels would decrease its cardioprotective effect. Mongrel dogs, anesthetized with morphine, urethane, and chloralose, were subjected to 15 min of left anterior descending coronary artery occlusion followed by 60 min reperfusion. The dogs were divided into three groups: the control group (n = 8), ISO group (n = 8) and ISOGC group (n = 8). In the ISO and ISOGC groups, 1 MAC of isoflurane was administrated during ischemia and reperfusion. In the ISOGC group, 0.3 mg/kg of glibenclamide, the KATP channel blocker, was given 45 min before ischemia. Full-thickness samples of myocardium were obtained and the concentrations of adenosine monophosphate, adenosine diphosphate, adenosine triphosphate (ATP), creatine phosphate and lactate in the endocardial portion of the myocardium were measured. The ischemia-reperfusion caused a 25.4% and 27.6% reduction of myocardial ATP in the control and ISOGC groups, respectively. In contrast, the ISO group showed only 11.0% reduction of ATP, which was significantly lower compared to the other groups (P < 0.01). Our results shows that blockade of the KATP channel abolishes cardioprotective effects of isoflurane in myocardial ischemia-reperfusion. The KATP channel may play a role in the ATP-sparing effect of isoflurane.

Beta 2-adrenergic dilation of resistance coronary vessels involves KATP channels and nitric oxide in conscious dogs.

We considered that beta 2-adrenergic stimulation may dilate resistance coronary vessels by opening ATP-sensitive potassium (KATP) channels, thereby triggering NO formation. In conscious instrumented dogs after beta 1-adrenergic blockade, intracoronary (IC) injections of acetylcholine (ACh), nitroglycerin (NTG), and pirbuterol (PIR), a selective beta 2-adrenergic agonist, were performed before and after blockade of NO formation with IC N omega-nitro-L-arginine methyl ester (L-NAME, 50 micrograms.kg-1.min-1 x 12 minutes) or blockade of KATP channels with IC glibenclamide (25 micrograms.kg-1.min-1 x 12 minutes followed by 2 micrograms.kg-1.min-1). PIR (50.0 ng/kg) increased coronary blood flow (CBF) by 32 +/- 6 from 43 +/- 7 mL/min and by only 11 +/- 2 (P < .01) from 40 +/- 7 mL/min after L-NAME. Increases in CBF to ACh were also reduced by L-NAME, but NTG responses were not. Before glibenclamide, PIR increased CBF by 33 +/- 5 from 45 +/- 7 mL/min and by only 14 +/- 3 (P < .01) from 36 +/- 5 mL/min thereafter. CBF responses to ACh and NTG were maintained after glibenclamide. Lemakalim, a selective opener of KATP channels, caused dose-dependent increases in CBF that were partially inhibited by L-NAME. In experiments in which CBF was controlled, the fall in distal coronary pressure caused by PIR was less after L-NAME or glibenclamide than before. beta 2-Adrenergic dilation of resistance coronary vessels involves both the opening of KATP channels and NO formation. L-NAME antagonized lemakalim responses consistent with a link between the opening of KATP channels and NO formation in canine resistance coronary vessels.

The ischemia-selective KATP channel antagonist, 5-hydroxydecanoate, blocks ischemic preconditioning in the rat heart.

Although the KATP channel has been demonstrated to be involved in ischemic preconditioning (IPC) in most species, controversy still exists as to the role of this channel as a mediator of PC in the rat heart. Previously, the authors' laboratories have shown that glibenclamide blocks IPC in the intact rat heart, in a time-dependent manner; however, since glibenclamide has been shown to have non-selective effects unrelated to KATP channel blockade, a structurally dissimilar and ischemia-selective KATP channel blocker, 5-hydroxydecanoate (5-HD), was used to further elucidate the role of KATP channels in mediating IPC in the rat heart. Anesthetized, open-chested Sprague-Dawley rats were subjected to one of four protocols. In Group I, control (C), rats were subjected to 30 min of left coronary artery occlusion and 90 min of reperfusion. In Group II, IPC was elicited by 1 x 5 min occlusion followed by 10 min of reperfusion, prior to the 30 min occlusion and 90 min reperfusion periods, 5-HD (5 mg/kg, i.v.) was given 15 min prior to the 30 min occlusion period in non-preconditioned animals, or given 15 min prior to IPC (5-HD+IPC) (Groups III and IV, respectively). Infarct size (IS), as a percentage of the area at risk (AAR), was determined by triphenyltetrazolium staining. Ischemic preconditioning produced a marked reduction in infarct size (47.5 +/- 3.8% to 7.9 +/- 1.9%, *P < 0.01), which was completely abolished by 5-HD (50.5 +/- 2.6%). These data further suggest that the opening of KATP channels is an important component of IPC in the intact rat heart, similar to that observed in other species.

Blockade of KATP Channels With Glibenclamide Does Not Abolish Preconditioning During Demand Ischemia

The role of adenosine triphosphate-sensitive potassium channels in the adaptive response to demand ischemia was tested in 22 patients treated with placebo or glibenclamide before sequential exercise testing or atrial pacing. Glibenclamide did not affect the improvement in signs of ischemia in both protocols, indicating that opening of these channels is not a mechanism of this adaptive response in humans.

Nonspecific inhibition of adenosine-activated K+ current by glibenclamide in guinea pig atrial myocytes.

The ATP-sensitive K+ (KATP) channel blocker glibenclamide was reported to inhibit the K+ current activated by adenosine. This study investigated whether the inhibition by glibenclamide of the adenosine-induced current is due to a specific blockade of KATP channels in guinea pig atrial myocytes. In the absence of adenosine, the basal background current was an inward rectifier K+ current (IK1). Glibenclamide at concentrations of 10, 30, and 100 microM reduced the basal background K+ current by 15 +/- 6, 43 +/- 10, and 63 +/- 11%, respectively. In the presence of adenosine (10 microM), glibenclamide (30 microM) decreased the adenosine-induced K+ current by 39 +/- 3%. A similar inhibitory effect of glibenclamide on the outward K+ currents activated by either the muscarinic agonist carbachol or the nonhydrolyzable GTP analogue guanosine 5'-[gamma-thio]triphosphate was observed. A low concentration (1 microM) of glibenclamide did not significantly attenuate the current elicited by adenosine, although it completely abolished the outward K+ current activated by pinacidil, a KATP channel opener. Thus we conclude that the inhibition of adenosine-induced K+ current by glibenclamide at high concentrations (> 1 microM) is not due to a specific blockade of KATP channels, but rather, resulted from a blockade of IK1.

Effects of tolbutamide on vascular ATP-sensitive potassium channels in humans. Comparison with literature data on glibenclamide and glimepiride.

Sulfonylurea (SU) derivatives exert their hypoglycemic effect by blockade of adenosine-5'-triphosphate-sensitive potassium (KATP) channels in the beta-cell of the pancreas. Interestingly, KATP channels also occur in the cardiovascular system, where they are thought to play an important role in cardioprotective mechanisms against ischemia. We have recently shown that the classical second generation SU-derivative glibenclamide is able to block vascular KATP channels in man, whereas the newly developed second generation derivative glimepiride was devoid of this property. The aim of this study was to determine whether the first generation SU derivative tolbutamide has KATP channel blocking properties in humans. In a group of 12 healthy male non-smoking volunteers, we investigated whether therapeutic concentrations of tolbutamide were able to inhibit the forearm vasodilation in response to the infusion of the KATP channel opening drug diazoxide into the brachial artery. Changes in forearm blood flow were recorded by venous occlusion mercury-in-silastic strain-gauge plethysmography. Diazoxide alone increased the forearm blood flow ratio dose-dependently by ultimately 691 +/- 198%. A second diazoxide infusion in the presence of tolbutamide revealed a comparable vasodilator response with a percentage increase in forearm blood flow ratio of ultimately 542 +/- 111%. This response did not differ from the vasodilator response to diazoxide alone. The present study shows that therapeutic concentrations of tolbutamide are not able to attenuate the vasodilation caused by the KATP channel opener diazoxide in man. When compared with published data on second generation SU derivatives, tolbutamide shows an intermediate position between glibenclamide (with significant blockade of vascular KATP channels) versus glimepiride (with no blockade at all). It remains to be determined whether these acute effects of SU derivatives on pharmacological opening of forearm vascular KATP channels can be extrapolated to the chronic effects of these drugs on ischemia-mediated opening of myocardial KATP channels during treatment of NIDDM patients.