Electrophysiological effects of hypoglycaemic sulphonylureas on rabbit heart

Influence of hypoglycaemic sulphonylureas on the electrophysiological parameters of the heart

The effects of different ATP-sensitive potassium channel blocker sulphonylurea drugs (0.01-1000 mumol/l, or kg) were investigated in vitro on the electrical threshold, conduction time, effective refractory period and automaticity of left atrium, right ventricle and Purkinje fibers as well as in vivo on strophanthidin-, ischaemia- and reperfusion-induced arrhythmias and on vagal nerve stimulation in rabbits, rats and dogs. They proved to exert different actions not only quantitatively but also qualitatively. In vitro, glibenclamide diminished the electrical activity of the heart muscle preparations, while chlorpropamide stimulated it, whereas glimepiride does not seem to affect it markedly. In vivo, glibenclamide and glimepiride decrease, while gliclazide and tolbutamide increase, the amount of strophanthidin- and ischaemia-induced ventricular ectopic beats and the duration of ventricular fibrillation. In the case of glimepiride the effect was dependent on the metabolic state. The different actions of sulphonylureas on the electrophysiological properties of the heart cannot be explained solely by their ATP-dependent potassium channel blocking potencies.

What kind of cardiovascular alterations could be influenced positively by oral antidiabetic agents?

Since the first publication of the University Group Diabetes Programme in 1970 the possible deleterious cardiovascular effects of certain hypoglycaemic sulphonylurea products has been well known. In contrast, international knowledge of the advantageous cardiovascular effects of certain sulphonylurea compounds became available recently. Glibenclamide decreases the incidence of fatal myocardial infarction and the development of ventricular fibrillation in patients suffering from acute myocardial infarction. It also lowers the incidence of ventricular ectopic beats in digitalized patients compared with patients treated with other investigated sulphonylurea compounds. The survival of patients treated with glibenclamide, insulin or diet alone is longer after the first attack of angina pectoris or after first acute myocardial infarction compared with those on other investigated sulphonylurea therapy. Glibenclamide decreases arrhythmogenesis during acute myocardial infarction in rats and strophanthin cardiotoxicity in rabbits. Arterial blood pressure and myocardial contractile force are not influenced by glibenclamide whereas these parameters are increased by other investigated sulphonylurea compounds. Consequently, deleterious cardiovascular effects of certain hypoglycaemic sulphonylurea drugs may contribute to the high cardiovascular mortality rate in diabetes mellitus, partly due to the effect on membrane channels and partly due to independent cardiac and extracardiac actions. Finally, recent observations suggest that glimepiride has a more advantageous cardiovascular effect than glibenclamide.

Direct effect of hypoglycemic sulphonylureas on the cardiovascular system of dogs

The effects of first generation sulphonylurea compounds carbutamide, gliclazide and tolbutamide as well as second generation compounds glibenclamide and glipizide on the cardiovascular system were investigated in dogs. Six dogs received each compound intravenously at cumulative dose levels of 74, 296, 1184 mumol/kg of carbutamide and tolbutamide, 0.4, 2.0, 10.0 mumol/kg of glibenclamide and glipizide, and 16, 48 and 144 mumol/kg of gliclazide. Mean arterial blood pressure, myocardial contractile force, cardiac output and heart rate were measured. The rate of change of myocardial contractile force development (positive dF/dt), as well as of myocardial relaxation (negative dF/dt) were measured. The first generation sulphonylureas were found, in dogs, to exert a positive inotropic effect in contrast to second generation compounds. The clinical importance of our findings may be in the potential for the malfunction of the cardiovascular system (based on cardiopathy, neuropathy, atherosclerosis, and obesity), developing in diabetes, to be further impaired by the first generation sulphonylureas. Therefore, second generation sulphonylureas should be preferred in the therapy of type 2 diabetics, if satisfactory metabolic control cannot be achieved by dietary management alone and sulphonylurea treatment becomes necessary.

Modification by glibenclamide of the electrophysiological consequences of myocardial ischaemia in dogs and rabbits

Glibenclamide has been shown to block ATP-dependent K+ channels in the heart and prevent the shortening of cardiac action potentials caused by hypoxia in vitro. The present study examines the ability of glibenclamide to modify the effect of acute ischaemia on monophasic action potential duration in pentobarbital-anaesthetized rabbits, and on monophasic action potential duration and ventricular fibrillation threshold in pentobarbital-anaesthetized dogs. Left ventricular endocardial monophasic action potential duration was measured using a contact electrode catheter, and ventricular fibrillation threshold was measured by the single pulse method. Ischaemia was produced in rabbits by occluding the circumflex coronary for 5 min and in dogs by occluding the left anterior descending coronary artery for 40 min. In rabbits, glibenclamide (0.3-3 mg/kg, i.v.) had no effect on baseline monophasic action potential duration, but attenuated action potential shortening during ischaemia in a dose-related manner. In dogs, monophasic action potential duration did not shorten during ischaemia in the vehicle group, but tended to increase in the glibenclamide group (0.5 mg/kg, i.v.) both before and during ischaemia (7 +/- 5% and 14 +/- 8%, respectively, NS). Likewise, ventricular effective refractory period was significantly increased by glibenclamide prior to ischaemia (5 +/- 1%). Ventricular fibrillation threshold tended to increase during 40 min of ischaemia in vehicle-treated dogs (40 +/- 29%, NS), but was unchanged during ischaemia in the glibenclamide-treated dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Divergent cardiac effects of the first and second generation hypoglycemic sulfonylurea compounds

The effects of first and second generation hypoglycemic sulfonylureas on the incidence of ventricular ectopic beats and on the duration of transitional ventricular fibrillation in the ischemic rat heart were investigated. First generation sulfonylurea compounds (tolbutamide, carbutamide and gliclazide) in 105 preparations increased, while second generation sulfonylurea compounds (glibenclamide and glipizide) in 50 preparations decreased in a dose-dependent manner both the number of ventricular ectopic beats and the duration of transitional ventricular fibrillation during the first 30 min after ligation of the left anterior descending coronary artery. Therefore, second generation sulfonylureas should be preferred in the treatment of type 2 diabetics with ischemic heart diseases, if satisfactory metabolic control cannot be achieved by a treatment regimen and diet alone.

Glyburide increases cytosolic-free calcium concentrations in normal rat pancreatic islet cells

We have attempted to delineate the effect of glyburide on the regulation of cytosolic-free calcium concentrations, [Ca2+]i, in normal rat pancreatic islet cells. In the presence of extracellular calcium (1 mmol/L), glyburide increased [Ca2+]i from 70 nmol/L to 260 nmol/L in a dose-dependent manner. The maximal effect was seen at a concentration of 2 mumol/L with half-maximal stimulation observed at .25 mumol/L. The effect of glyburide (.25 mumol/L) was inhibited 90% by the calcium channel blocker, verapamil (30 mumol/L). At a maximally effective concentration of glyburide (2 mumol/L), the inhibitory effect of verapamil was only 17%. In the absence of extracellular calcium, glyburide increased [Ca2+]i from 55 nmol/L to 107 nmol/L, indicating its ability to mobilize intracellular calcium stores. These results correlated well with the ability of glyburide (2 mumol/L) to stimulate insulin secretion both in the presence (from 38 +/- 5 mumol/L/10 islets to 131 +/- 28 microU/10 islets) and in the absence (from 49 +/- 4 microU/10 islets to 93 +/- 7 microU/10 islets) of extracellular Ca2+. The present observations suggest that glyburide promotes calcium influx via voltage-dependent calcium channels and may mobilize intracellular calcium stores.

Adverse cardiovascular effects of sulphonylurea drugs. Clinical significance

Sulphonylureas are widely used in the treatment of diabetes mellitus. Since the publication of the University Groups Diabetes Program (UGDP) results the discussion on their possible cardiovascular side effects has been lively and sometimes even passionate. The initial UGDP findings about the adverse effects of tolbutamide on the cardiovascular system have been criticised, particularly for shortcomings in the study design. The results of other epidemiological studies of the sulphonylurea effects on cardiovascular morbidity and mortality published this far have been contradictory. This is understandable because the factors involved are very complex. Most of these studies have used tolbutamide only, and the findings cannot necessarily be directly extrapolated to other sulphonylureas. Only properly performed prospective studies may provide further information on this issue. High concentrations of several sulphonylureas may have inotropic effects on heart muscle in in vitro animal models, but human studies performed in vivo do not support the view of clinically significant inotropy for sulphonylureas. High concentrations of tolbutamide or glibenclamide (glyburide) may affect the myocardial metabolism in isolated organs, but the possible clinical significance of these findings remains unknown. Some epidemiological and experimental studies have associated oral antidiabetic treatment with the occurrence of cardiac arrhythmias or increased digitalis toxicity. Only a few results are available, and there may be differences between the sulphonylureas in this respect. Antiaggregatory properties have been postulated for some sulphonylureas. Gliclazide, in particular, has been studied, but some other compounds of this class have also been effective in short term studies. If confirmed, these effects on haemostasis would be noteworthy. The sulphonylurea effects on serum lipids, especially on HDL-cholesterol, have been discussed widely during the last few years. Decreases in HDL-cholesterol concentrations were suggested to be associated with sulphonylurea therapy. However, these findings were not confirmed in recent cross-sectional and longitudinal studies performed with different sulphonylureas. Chlorpropamide, and to a lesser extent tolbutamide, may cause dilutional hyponatraemia and aggravate existing heart failure. Glibenclamide may increase the clearance of water in the kidney.

Effects of various hypoglycaemic sulphonylureas on the cardiotoxicity of glycosides

In diabetic patients it has been shown that tolbutamide and carbutamide enhanced and glibenclamide did not influence the incidence of digitalis intoxication, or that of multifocal ectopic beats or coupling due to premature ectopic ventricular beats during digitalis therapy. In rabbits glibenclamide decreased and tolbutamide and carbutamide increased strophanthidin toxicity in a dose dependent manner. It was concluded that glibenclamide should be preferred to tolbutamide or carbutamide in digitalis-treated diabetics, when satisfactorily metabolic control is not achieved with a dietary regime alone.