Sarcoplasmic ATP-sensitive potassium channel blocker HMR1098 protects the ischemic heart: implication of calcium, complex I, reactive oxygen species and mitochondrial ATP-sensitive potassium channel

The aim of this study was to investigate the effects of HMR1098, a selective blocker of sarcolemmal ATP-sensitive potassium channel (sarcK(ATP)), in Langendorff-perfused rat hearts submitted to ischemia and reperfusion. The recovery of heart hemodynamic and mitochondrial function, studied on skinned fibers, was analyzed after 30-min global ischemia followed by 20-min reperfusion. Infarct size was quantified on a regional ischemia model after 2-h reperfusion. We report that the perfusion of 10 microM HMR1098 before ischemia, delays the onset of ischemic contracture, improves recovery of cardiac function upon reperfusion, preserves the mitochondrial architecture, and finally decreases infarct size. This HMR1098-induced cardioprotection is prevented by 1 mM 2-mercaptopropionylglycine, an antioxidant, and by 100 nM nifedipine, an L-type calcium channel blocker. Concomitantly, it is shown that HMR1098 perfusion induces (i) a transient and specific inhibition of the respiratory chain complex I and, (ii) an increase in the averaged intracellular calcium concentration probed by the in situ measurement of indo-1 fluorescence. Finally, all the beneficial effects of HMR1098 were strongly inhibited by 5-hydroxydecanoate and abolished by glibenclamide, two mitoK(ATP) blockers. This study demonstrates that the HMR1098-induced cardioprotection occurs indirectly through extracellular calcium influx, respiratory chain complex inhibition, reactive oxygen species production and mitoK(ATP) opening. Taken together, these data suggest that a functional interaction between sarcK(ATP) and mitoK(ATP) exists in isolated rat heart ischemia model, which is mediated by extracellular calcium influx.     

Incomplete dissociation of glibenclamide from wild-type and mutant pancreatic KATP channels limits their recovery from inhibition

Background and purpose:

The antidiabetic sulphonylurea, glibenclamide, acts by inhibiting the pancreatic ATP-sensitive K⁺ (K_ATP) channel, a tetradimeric complex of K_IR6.2 and sulphonylurea receptor 1 (K_IR6.2/SUR1)₄. At room temperature, recovery of channel activity following washout of glibenclamide is very slow and cannot be measured. This study investigates the relation between the recovery of channel activity from glibenclamide inhibition and the dissociation rate of [³H]-glibenclamide from the channel at 37°C.

Experimental approach:

K_IR6.2, K_IR6.2ΔN5 or K_IR6.2ΔN10 (the latter lacking amino-terminal residues 2–5 or 2–10 respectively) were coexpressed with SUR1 in HEK cells. Dissociation of [³H]-glibenclamide from the channel and recovery of channel activity from glibenclamide inhibition were determined at 37°C.

Key results:

The dissociation kinetics of [³H]-glibenclamide from the wild-type channel followed an exponential decay with a dissociation half-time, t_1/2(D) = 14 min; however, only limited and slow recovery of channel activity was observed. t_1/2(D) for K_IR6.2ΔN5/SUR1 channels was 5.3 min and recovery of channel activity exhibited a sluggish sigmoidal time course with a half-time, t_1/2(R) = 12 min. t_1/2(D) for the ΔN10 channel was 2.3 min; recovery kinetics were again sigmoidal with t_1/2(R) ∼4 min.

Conclusions and implications:

The dissociation of glibenclamide from the truncated channels is the rate-limiting step of channel recovery. The sigmoidal recovery kinetics are in quantitative agreement with a model where glibenclamide must dissociate from all four (or at least three) sites before the channel reopens. It is argued that these conclusions hold also for the wild-type (pancreatic) K_ATP channel.     

Role of K ATP channels in cephalic vasodilatation induced by calcitonin gene-related peptide, nitric oxide, and transcranial electrical stimulation in the rat

Objective.— The objective of this study was to explore the role of K_ATP channels in vasodilatation induced by calcitonin gene‐related peptide (CGRP), nitric oxide (NO), and transcranial electrical stimulation (TES) in intracranial arteries of rat. Background.— Dilatation of cerebral and dural arteries causes a throbbing, migraine‐like pain. Both CGRP and NO are potent vasodilators that can induce migraine. Their antagonists are effective in the treatment of migraine attacks. K_ATP channel openers cause headache in the majority of healthy subjects suggesting a role for K_ATP channels in migraine pathogenesis. We hypothesized that vasodilatation induced by CGRP and the NO donor glyceryltrinitrate (GTN) is mediated via K_ATP channels. Methods.— We examined the effects of the K_ATP channel inhibitor glibenclamide on dural and pial vasodilatation induced by CGRP, NO, and endogenously released CGRP by TES. A rat genuine closed cranial window model was used for in vivo studies and myograph baths for studying the effect in vitro. In the closed cranial window model the diameter of dural vessels was measured directly in anesthetized animals to investigate the vascular effects of infused CGRP, NO, and endogenous CGRP after electrical stimulation. Also diameter changes of pial arteries, mean arterial blood pressure and local cerebral blood flow by Laser Doppler flowmetry (LCBF_Flux) were measured. Results.— CGRP, NO, and TES caused dilatation of the 2 arteries in vivo and in vitro. In anesthetized rats glibenclamide significantly attenuated CGRP induced dural and TES induced dural/pial artery dilatation (P = .001; P = .001; P = .005), but had no effect on dural/pial vasodilatation induced by GTN. In vitro glibenclamide failed to significantly inhibit CGRP‐ and GTN‐induced vasodilatation. Conclusions.— These results show that a K_ATP channel blocker in vivo but not in vitro inhibits CGRP, but not GTN‐induced dilatation of dural and pial arteries, a mechanism thought to be important in migraine.     

Glibenclamide improves postprandial hypertriglyceridaemia in type 2 diabetic patients by reducing chylomicrons but not the very low-density lipoprotein subfraction levels.

AIM: There are scarce data dealing with the degree of postprandial lipaemia after sulphonylurea administration. The aim of this study was to examine the effect of acute glibenclamide administration on postprandial lipaemia in Type 2 diabetic patients. METHODS: Eight randomly selected Type 2 diabetic individuals, aged 43-65 years (mean, 54 years), who had never received any anti-diabetic drug, were included in the study. Each patient was given a 485 kcal mixed meal (45% fat, 40% carbohydrate and 15% protein) twice on separate days after an overnight fast: once with placebo and once with 5 mg glibenclamide, per os, in a random order. The two tests were performed with an interval of 7 days. Venous blood samples were drawn just before and 2 h, 4 h and 6 h after meal consumption. Total triglyceride levels in plasma, in chylomicrons (CM), in CM-deficient plasma, in very low-density lipoprotein (VLDL) subfractions (VLDL-1, VLDL-2) and in intermediate-density lipoprotein (IDL) were determined. Free fatty acid (FFA) and total cholesterol levels in plasma, as well as high-density lipoprotein (HDL) cholesterol and low-density lipoprotein (LDL) cholesterol levels in CM-deficient plasma, were also measured. Finally, serum glucose, insulin and C-peptide concentrations were measured in each sample. RESULTS: As expected there was a significant decrease in postprandial glycaemia after glibenclamide administration compared to placebo (mean area under the curve values: AUC = 53.3 +/- 18.2 and 69.1 +/- 21.6 mm/h, P = 0.00009). In addition, the mean AUC values of insulin and C-peptide were significantly greater after drug administration. The AUC values of total plasma triglyceride and of CM triglyceride following glibenclamide administration were significantly lower compared to placebo, while the AUC values of postprandial triglyceride in CM-deficient plasma and of postprandial triglyceride in VLDL-1, VLDL-2 and IDL were not different after drug administration compared to placebo. Finally, no significant differences were noted in the AUC values of total cholesterol, LDL cholesterol, HDL cholesterol and plasma FFA levels after glibenclamide administration. CONCLUSIONS: These results demonstrate that glibenclamide administration improves postprandial hypertriglyceridaemia acutely by reducing postprandial triglycerides of intestinal origin.     

Monoamines in the pancreatic islets of the mouse

Summary The functional significance of 5-hydroxytryptamine (5-HT) storage in the pancreatic B cells for insulin secreting mechanisms was studied in normal micein vivo. Pretreatment of the animals withL-5-hydroxytryptophan (L-5-HTP) markedly decreased the insulin releasing capacity after sulphonylurea stimulation. This inhibition of insulin release could be abolished by previous administration of an inhibitor of aromatic amino acid decarboxylation. On the other hand, pretreatment with the monoamine oxidase inhibitor nialamide alone, decreased sulphonylurea-induced insulin release. The combined treatment with nialamide andL-5-HTP did not further decrease the insulin response. Insulin release induced byL-isopropylnoradrenaline (L-IPNA) was also found to diminish after previous administration ofL-5-HTP or nialamide; but, unlike the insulin response to sulphonylurea, insulin release induced by IPNA could be totally suppressed by the combined treatment of nialamide or pargyline andL-5-HTP. Insulin release induced by glucose was not significantly influenced with any of the above treatments. Basal levels of plasma insulin were not affected byL-5-HTP injection, and were not consistently diminished by the combined treatment with monoamine oxidase inhibitor andL-5-HTP. The combined treatment with monoamine oxidase inhibitors andL-5-HTP was found to elicit a profound hypoglycaemia in both normal and alloxan-diabetic mice. The hypoglycaemic condition was accompanied by exhaustion of liver and muscle glycogen. The hypoglycaemia could be abolished by previous treatment with an inhibitor of aromatic amino acid decarboxylation. Combined treatment with pargyline and 5-HT brought about a marked hyperglycaemia. It is concluded that: 1. intracellular levels of 5-HT in the pancreatic B cells possess the ability to modify insulin secreting mechanisms; and 2. the hypoglycaemic action of monoamine oxidase inhibitors is brought about by raised intracellular levels of 5-HT, which is accompanied by a markedly increased glucose utilization by the tissues.This work was supported by the Medical Faculty, University of Lund, Sweden. The skilful technical assistance of Mrs. Lena Kvist, Miss Anita Åkesson and Miss Ann-Christin Helander is gratefully acknowledged.     

Mechanism of the vasodilatory action of nicorandil on coronary circulation in dogs

Summary Nicorandil possesses hybrid properties as a nitrate and a potassium (K) channel opener. We have previously reported that large coronary arteries responded to nicorandil at low plasma concentrations, while dilatation of small coronary arteries only occurred at higher plasma concentrations (above 200 ng/ml) in chronically instrumented dogs. In this study we examined the effects of intravenous nicroandil on epicardial coronary artery diameter (CoD) and coronary blood flow (CBF) in the absence and presence of glibenclamide, a K⁺ channel blocker, as well as the effects of nitroglycerin and cromakalim as reference drugs. The increase in CBF induced by nicorandil and cromakalim was significantly suppressed by glibenclamide. However, the increase in CoD induced by nicorandil and nitroglycerin was not suppressed by glibenclamide. These findings suggest that nicorandil-induced dilatation of the large coronary arteries was related to its nitrate action, while nicorandil-induced dilatation of the small coronary arteries was closely related to its effect as a K⁺ channel opener. In addition, the former response to nicorandil occurred at low concentrations, while the latter occurred at higher concentrations.     

ATP敏感性钾通道对狗的冠状动脉的调节

目的　探讨冠状动脉震颤发生的条件及和ATP敏感性钾通道的关系。方法　用优降糖诱发冠状动脉震颤后分别用尼可的尔、硝酸甘油观察两药对震颤的影响。结果　优降糖 3mg/kg可诱发冠状动脉震颤 ,且此现象在细小的动脉表达得更为强烈 ;震颤不能被硝酸甘油抑制 ,但可被尼可的尔终止。结论　ATP敏感性钾通道的阻滞可发生冠状动脉的震颤 ,且在细小动脉中尤为突出 ,当内皮细胞产生NO减少时震颤也将增强。     

Action of some hypoglycaemic sulphonylureas on the oxygen consumption of isolated pancreatic islets of mice

Summary Cartesian divers were used to evaluate the effects in vitro of some hypoglycaemic Sulphonylureas on the oxygen consumption of isolated pancreatic islets. The islet specimens were obtained from obese-hyper-glycaemic mice, and consisted of over 90% B-cells. When incubated with Krebs-Ringer phosphate medium, the islet cells displayed an increased rate of respiration upon addition to the incubation medium of either tolbutamide (D860; 0.1 mg/ml) or glibenclamide (HB 419; 0.1 .g/ ml). The respiratory rate increased with the concentration of HB 419 in the range 0.001–0.1 g/ml, but did not exceed 120% of the respiration in pure phosphate medium. Whereas the physiological excretion product of D 860 did not affect the respiratory rate, the corresponding derivative of HB 419 was still effective in stimulating the oxygen uptake of the islets. When islets were incubated with glucose at a high concentration (3 mg/ml), the oxygen uptake was inhibited by addition of D 860, or its metabolite, or HB 419. The last drug slightly increased the respiration of islets incubated with glucose at a concentration of 1 mg/ml, and a marked stimulation was noted at a still lower glucose concentration, 0.5 mg/ml. Attempts to evaluate the effect of mannoheptulose on the respiratory response of the islets to hypoglycaemic Sulphonylureas produced inconclusive results. It is suggested that Sulphonylureas effect an increased rate of endogenous substrate oxidation in the B-cells of the pancreatic islets.This work was supported by the Swedish Medical Research Council (B68-12X-109-04) and the U.S.Public Health Service (Grant AM-05759-06). The skilful technical assistance of Miss Gunilla Lekselius and Miss Ing-Britt Brolen is greatfully acknowledged.