Myocardial infarction in young vs old male rats: pathophysiologic changes.

Young (90 days) and old (15 months) male, Sprague-Dawley rats were subjected to an acute and massive myocardial infarct by giving them two injections of a large dose of isoproterenol. The animals were autopsied at sequential time intervals to ascertain the similarities or dissimilarities in the pathophysiologic events which attend acute myocardial infarction and repair in young vs old rats. Although the signs and severity of hypotensive shock appeared to be equal, mortality was higher in the old rats, especially during the acute necrosis phase. The older rats also manifested more severe and persistent congestive heart failure, i.e., hydrothorax. Serum enzymes (CPK, SGOT, SGPT, and LDH), lipids (triglycerides, free fatty acids, and cholesterol), glucose, and BUN levels manifested a dynamic rise and fall concomitant with the induced myocardial necrosis and repair phases with distinct differences in these metabolic changes between young and old rats. Despite initially higher circulating levels of corticosterone in the old vs young rats, the older animals manifested little or no increase in circulating corticosterone levels during the acute stress of myocardial infarction. This apparent lack of adrenocortical responsiveness was accentuated by the concomitant finding of greatly hypertrophied, hemorrhagic, and lipid-depleted adrenal glands in the old rats vs a dynamic increase in circulating corticosterone levels and alterations in the weight of adrenal and thymus glands of the young rats. During the myocardial repair phase, the young rats manifested extensive endocardial fibrosis whereas the old rats displayed little or no endocardial fibrosis but copous and persistent myocardial edema and ground substance in keeping with their higher concentration of cardiac hexosamine. The pathophysiologic course of events which attends myocardial necrosis and repair is quite different in young vs old rats and may be related to the degree of responsiveness of the pituitary-adrenal axis which changes with age.