The positive inotropic action of sulfonylureas. A mechanism independent of cyclic adenosine 3',5'-monophosphate

Haemodynamic and other effects of sulphonylurea drugs on the heart

Antidiabetic sulphonylureas have attracted a great deal of interest in experimental cardiology to evaluate the role of ATP-sensitive potassium channels in the cardiovascular system. It is well established that KATP channels are present in cardiac cells and also in vascular smooth muscle cells and are implicated in the regulation of myocardial and vascular function. It follows that drugs which open, or inhibit the opening of these channels, might profoundly modify cardiovascular function both under physiological and pathophysiological conditions. This paper reviews the evidence for the role of KATP channels in the cardiovascular system and discusses how the different generations of sulphonylurea drugs interfere with cardiac function. We will particularly concentrate on the haemodynamic effects of different sulphonylureas and shortly discuss how these drugs modify ischaemia-reperfusion arrhythmias.

The effects of glyburide and insulin on the decreased beta-adrenergic responsiveness of the gastrointestinal tract in rats with non-insulin-dependent diabetes

1. Decreased beta-adrenergic responses have been reported in gastro-intestinal tract of rats with diabetes mellitus. Effects of glyburide and insulin on the decreased beta-adrenergic responsiveness of the gastro-intestinal tract due to non-insulin-dependent diabetes were investigated using duodenum, jejunum and ileum from rats which were injected with alloxan in their neonatal periods. 2. Insulin treatment of non-insulin-dependent diabetic rats for 10 days corrected the decreased beta-adrenergic responses of the isolated duodenum, jejunum and ileum confirming the previous results obtained from insulin-dependent diabetic rats. 3. Glyburide treatment alone for 3 weeks also reversed the changes in the gastro-intestinal beta-adrenergic responses of non-insulin-dependent diabetic rats. Combination of glyburide with insulin, however, did not cause an additive or supra-additive interaction in terms of beta-adrenergic sensitivities of the diabetic tissues. 4. The results obtained in the present study strongly suggested that non-insulin-dependent diabetes may cause a decrease in the number of gastro-intestinal beta-adrenoceptors, while glyburide and insulin treatments correct the changes related to beta-adrenoceptors. The effect of insulin on the beta-adrenergic sensitivity of diabetic rat duodenum, jejunum and ileum may occur via a direct mechanism, whereas glyburide seems to be effective on the beta-adrenergic responses through the increases in the insulin secretion and/or in the number of gastro-intestinal insulin receptors.

The effects of glyburide and insulin on the cardiac performance in rats with non-insulin-dependent diabetes mellitus

1. The effects of glyburide were studied on the myocardial contractile force and heart rate in the atria isolated from non-diabetic and non-insulin-dependent diabetic rats (diabetic). 2. In order to examine the myocardial changes in the alloxan model of non-insulin-dependent diabetes, atrial functions of 11-week old diabetic rats were evaluated by comparing with the atria from their age-matched controls. 3. Diabetic atria were found to possess an increased contractile force and reduced inotropic responses to isoprenaline as a consequence of non-insulin-dependent diabetes induced by neonatal alloxan injection. 4. However, no significant change was observed in the heart rate of diabetic atria in response to isoprenaline when compared with controls. 5. Since apparent affinity constant (pD2 value) calculated for the inotropic response of diabetic atria to isoprenaline was also reduced, it might be suggested that non-insulin-dependent-diabetes causes a decrease in the beta-adrenoceptor affinity of the rat atria. 6. Glyburide treatment (5 mg/kg/day per os) for 3 weeks was able to improve the reduced responsiveness of rat atria due to non-insulin-dependent diabetes as well. The results obtained in this study indicated that glyburide possesses an improving effect on the decreased beta-adrenergic responses of rat atria with non-insulin-dependent diabetes mellitus.

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.

The effect of the acute administration of tolbutamide on left ventricular function in insulin-dependent diabetics

Despite better control of diabetic patients with morbidity and mortality from heart disease is significantly higher than in non-diabetics. While this may be a result of premature atherosclerosis or primary diabetic cardiopathy, the possibility of oral hypoglycemic agents have adverse cardiac effects must be considered. Using nuclear angiography the effect of the acute administration of 1 g tolbutamide on left ventricular ejection fraction (EF) in a group of 10 Type-1 diabetics was investigated. There was no difference in the response of the EF when compared to the response in 10 diabetic controls given i.v. saline. While i.v. tolbutamide may have adverse cardiac effects when administered to animals or in vitro this does not necessarily apply to human diabetics.

Insulinotropic effects of hypoglycaemic and hyperglycaemic sulphonamides: the ionophoretic hypothesis

Hypoglycaemic sulphonamides stimulate net uptake of 45Ca++ and insulin release in isolated pancreatic islets. These effects are antagonized by organic calcium-antagonists (e.g. suloctidil). In an artificial system, hypoglycaemic sulphonamides, such as gliclazide, stimulate the translocation of calcium into or across a hydrophobic immiscible domain, a process enhanced by the antibiotic ionophore A 23187 and antagonized by suloctidil. In this artificial system, the A 23187-mediated process of calcium countertransport is stimulated by gliclazide and inhibited by diazoxide. It is postulated that the insulinotropic action of hypoglycaemic and hyperglycaemic sulphonamides is primarily attributable to the ionophoretic action of these drugs.

Synergistic effects of hypoglycaemic sulphonylureas and antibiotic ionophores upon calcium translocation

1 Hypoglycaemic sulphonylureas, such as tolbutamide and gliclazide, provoke the translocation of calcium from an aqueous medium into or across a hydrophobic region. The combined effect of sulphonylureas and antibiotic ionophores upon such a process was investigated. 2 The magnitude of the sulphonylurea-induced translocation of calcium was more marked in the presence than in the absence of A23187. Gliclazide and tolbutamide also enhanced, although less markedly, X537A-mediated calcium translocation. The effect of the sulphonylureas was even less marked in the presence of both ionophores, which acted synergistically in causing calcium translocation. 3 A non-hypoglycaemic sulphonylurea and diazoxide failed to affect ionophore-mediated calcium translocation. Gliclazide failed to enhance X537A-mediated sodium translocation. 4 It is proposed that the primary site of action of hypoglycaemic sulphonylureas upon calcium-dependent physiological processes may correspond to a drug-induced facilitation of calcium transport across the plasma membrane, as mediated by native ionophores.