Hyperinsulinemia Induces Myocardial Infarctions and Arteriolar Medial Hypertrophy in Spontaneously Hypertensive Rats

To investigate the effects of hyperinsulinemia on the myocardial vessels, long acting insulin (mixtard, a combination of 30% regular human insulin and 70% NPH human insulin) was injected daily for 8 weeks, intraperitoneally, in two strains of rats, normotensive WKY and hypertensive SHR. There were four groups in all, a control group, and an insulin-injected group in each strain. The drinking water contained 10% glucose to prevent hypoglycemia in the insulin-injected rats. At the end of the 8 weeks experimental period, after measuring blood pressure and taking blood for the determination of glucose, urea, creatinine, and insulin, the rats were killed. The organs were fixed in formaldehyde. The blood glucose levels were higher at the end of the experiment, in both the placebo- (saline)-injected and the insulin-injected rats. Blood pressure rose significantly only in the insulin-injected SHR. The intramyocardial arterioles in the insulin-injected SHR had a significantly thicker vascular wall than the placebo-injected SHR, as represented by the vessel wall to lumen ratio, because of hypertrophy of the media. When compared with the placebo injected WKY rats, there was a higher wall/lumen ratio of the intramyocardial arterioles in the insulin-injected WKY, but the difference did not reach significance. Heart weights factored by body weights was significantly higher in insulin-injected as compared with placebo-injected SHR. Myocardial infarctions were observed in four of eight rats in the insulin-injected SHR group despite the fact that there were no signs of atherosclerosis or intimal thickening. It is possible that the increase in heart weight and the probable increase in metabolic activity resulting from hyperinsulinemia, together with the increased oxygen demand of the myocardium and the arteriolar narrowing, may have contributed to the occurence of myocardial infarctions in the absence of atherosclerotic coronary occlusion.