Adjustments in competitive substrate utilization in stunned myocardium during early reperfusion

Abstract

The purpose of this precis is to state and support a personal viewpoint concerning intermediary metabolism and substrate utilization in stunned and reperfused myocardium following an interval of nonnecrotizing myocardial ischemia. The data in support of this viewpoint were derived from an intact, working pig heart preparation which includes extracorporeal control of regional coronary perfusion, and which has been used in our laboratory for almost two decades. Metabolism was characterized using steady-state labeling of myocardium with tracer isotopes including [U-¹⁴C]palmitate, [9,10-³H]palmitate, [6-¹⁴C] glucose, [5-³H]glucose, [2-¹⁴C]pyruvate, [¹⁴C]lactate, and [1-¹⁴C]acetate either singly or in combination. Myocardium was rendered mild to moderately ischemic for 30–40 min in separate protocols, and then aerobically reperfused for 40–60 min. Rates of substrate utilization and/or oxidation were routinely measured during the preischemic period, the ischemic interval, and as emphasized in this overview the subsequent period of metabolic adjustments during aerobic reperfusion. The composite data compiled from these experiments indicate that in stunned myocardium metabolism is in rapid transition with greater restoration of aerobic oxidation than mechanical function, a hierarchical shift in preferred substrate relationships back toward aerobic utilization with beneficial washout of noxious amphiphiles, and an overshoot in fatty acid oxidation, presumably due to persistent alterations in regulatory mechanisms affected by the preceding ischemic stress.

In select reperfusion studies and depending somewhat on the levels of exogenous fatty acid substrate in perfusate, fatty acid oxidation approximately doubled over preischemic values (6). This augmentation occurred in part from fatty acids made available from release of intra cellular stores, principally triacyclglycerols, as we showed (8) and in part from increased uptake of exogenous suostrate as demonstrated by Saddik and Lopaschuk (12). This rapid ascendancy in fatty acid preference, however, does not imply a substrate immunity from the normal regulatory inhibitions imposed by competing substrates. In separate studies we provided to recovering myocardium excess propionate to influence the kinetcs of anaplerotic entrance to the citric acid cycle (4). This competing substrate effected a 38% decrease in the oxidation of fatty acids which was not explained by the scavenging of coenzyme A units otherwise destined for fatty acid activation It has been argued from data collected in isolated preparations that the resurgency in fatty acid oxidation is of no benefit but rather a negative inotropic intluence on mechanical recovery (1, 7). However, in our intact model using whole blood perfusate this was not the case (6).

A second objective in our experiments was to place in perspective the role of carbohydrate substrate utilizations using glucose, pyruvate, and lactate as representative analogues and to define their relationships to those of fatty acids as regulated substrates. Glucose flux and glucose oxidation were highly suppressed during preischemia, presumably due to allosteric and product inhibitions imposed by the preferred use of fatty acids (2, 3, 9). In moderately ischemic pig hearts with a 60% reduction in regional coronary flow, glycolysis rose nearly tenfold, and even glucose oxidation was increased slightly (2, 9). When multiple exposures of brief ischemic pretreatments were applied in protocols designed to simulate preconditioning (2), flux rates for glycolysis were significantly reduced. During reperfusion, glucose utilization and oxidation fell toward normal preischemic values and lactate release into perfusate, which had been observed during ischemia, reverted to lactate extraction, consistent with aerobic myocardium. Increased levels of glucose in perfusate tended to maintain somewhat higher rates of glycolysis in reperfusion (3). The shift in glucose utilization toward normal preischemic levels were compatible with the allosteric and product inhibitions imposed by the rebound in fatty acid metabolism. To further test this hypothesis, oxfenicine (S-4-hydroxyphenylglycine), an inhibitor of fatty acid utilization and specifically of carnitine palmitoyltransferase I, was administered in separate studies (9). With relief of the regulatory influence of fatty acids, glucose oxidation appropriately increased twofold in stunned myocardium.

It was suggested also that pyruvate and lactate oxidation responded predictably to the regulatory influences of fatty acids (10, 11). Utilization rates of both substrates were decreased from preischemic levels during withdrawal of oxygen supply in moderate ischemia but remained depressed during aerobic reperfusion. Treatment with oxfenicine in separate studies caused an appropriate restoration of pyruvate oxidation during reflow to near preischemic values (10).

It is noteworthy that if one compares the metabolic adjustments in stunned, acutely reperfused myocardium with those of an acutely reperfused, chronic pig model simulating hibemating myocardium (5), important distinctions are evident. In the latter group heart rate was increased, but oxidative capacity was maintained and myocardial oxygen consumption was higher than in acutely developed sham hearts paced at similar rates. Fatty acid oxidation was in the aerobic range compatible with values obtained in acute experiments during early reperfusion following moderate ischemia (6). In chronic hearts, however, in contrast to stunned hearts, glycolytic flux during reflow remained, elevated and was sixfold greater than rates in acute sham hearts. This latter finding suggests a disruption in the normal regulatory controls of fatty acid utilization upon constituent steps within the glycolytic pathway.

In summary, in stunned and reperfused myocardium after a reversible period of moderate ischemia rapid adjustments in substrate utilization occur dominated by the return of the preferred use of long-chain fatty acids for energy metabolism. This recovery of fatty acid oxidation may include an overshoot above preischemic aerobic values. Mechanisms of regulation are maintained among substrates such that glucose, pyruvate, and lactate utilization are normally suppressed during reperfusion but may be up-regulated by the inhibition of fatty acid oxidation. Patterns of glucose utilization appear different between stunned and hibernating myocardium.