Determinants of a protective effect of glucose and insulin on the ischemic myocardium. Effects on contractile function, diastolic compliance, metabolism, and ultrastructure during ischemia and reperfusion.

The efficacy of hyperglycemia and insulin therapy for reducing ischemic myocardial injury is controversial and unproven. Accordingly, factors that might influence the effects of hyperglycemia and insulin were studied in isolated perfused rabbit hearts at two degrees of global ischemia, either "severe" or "moderate." During the ischemic period, different groups (n = 15-28/group) received either 100 mg/100 ml glucose-no insulin (control group), 500 mg/100 ml glucose + 100 mU/ml insulin (G + I), or 100 mg/100 ml glucose + 400 mg/100 ml mannitol (osmotic control). During moderate ischemia, effective washout of myocardial lactate was maintained, and hyperglycemia and insulin doubled the glycolytic flux, completely prevented contracture during ischemia, decreased contracture during reperfusion, increased recovery of postischemic contractile function, decreased ultrastructural damage, and increased high energy phosphate levels. Hyperglycemia and insulin increased glycolytic flux only after 30 minutes of ischemia had elapsed, suggesting that endogenous glycogen provided adequate glycolytic substrate prior to this time. The mannitol-glucose substrate had no beneficial effects, indicating that the hyperglycemia and insulin substrate had a metabolic rather than an osmotic mechanism of action. In contrast, during severe ischemia, tissue lactate washout was ineffective; the hyperglycemia and insulin substrate increased glycolytic flux by only 15% and produced no persistent beneficial effects. These results suggest that hyperglycemia and insulin therapy is beneficial to the ischemic myocardium when two conditions are met. First, the degree of myocardial perfusion, although in the ischemic range, must be adequate to prevent the accumulation of high tissue levels of lactate which inhibit glycolysis and prevent any glycolytic stimulation by hyperglycemia and insulin. Second, the ischemic myocardium must be "glucose dependent" for glycolytic substrate; in our studies this occurred after 30-45 minutes of sustained ischemia, probably because myocardial glycogen stores became depleted.