Acute insulin treatment normalizes the resistance to the cardiotoxic effect of isoproterenol in streptozotocin diabetic rats. A morphometric study of isoproterenol induced myocardial fibrosis.

The acute effect of insulin treatment on the earlier reported protective effect of streptozotocin diabetes against the cardiotoxic effect of high doses of isoproterenol (ISO) was investigated in rats. Thirty to 135 min after the injection of crystalline insulin, ISO was given subcutaneously and when ISO induced fibrosis in the myocardium was morphometrically analyzed 7 days later, a highly significant correlation (r = 0.83, 2 p = 0.006) to the slope of the fall in blood glucose after insulin treatment appeared. The myocardial content of catecholamines was estimated in these 8 day diabetic rats. The norepinephrine content was significantly increased while epinephrine remained unchanged. An enhanced sympathetic nervous system activity with a consequent down regulation of the myocardial beta-adrenergic receptors could, therefore, explain this catecholamine resistance. The rapid reversion after insulin treatment excludes the possibility that streptozotocin in itself causes the ISO resistance and points towards a direct insulin effect on myocardial catecholamine sensitivity in diabetic rats. The phenomenon described might elucidate pathogenetic mechanisms behind toxic myocardial cell degeneration and may possibly have relevance for acute cardiovascular complications in diabetic patients.