Effect of substrate on mitochondrial NADH, cytosolic redox state, and phosphorylated compounds in isolated hearts

The effect of metabolic substrates on the relation among cytosolic redox state (NADHc/NAD+) mitochondrial NADH (NADHm), and [ATP]/([ADP] x [Pi]) was studied in isolated working rabbit hearts. Substrates were varied from 5.6 mM glucose alone to glucose in combination with 10 mM lactate and/or 10 mM pyruvate while afterload and preload were held constant. Changes in NADHm were determined from epicardial NADH fluorescence. The ratio of glycerol 3-phosphate (G-3-P) to dihydroxyacetone phosphate (DHAP), determined from tissue extracts, was used as an index of cytosolic redox. Myocardial 31P metabolites were measured using nuclear magnetic resonance spectroscopy. The addition of pyruvate to the perfusion medium caused increases in myocardial oxygen consumption (MVo2), NADHm fluorescence, phosphocreatine (PCr), and [ATP]/([ADP] x [Pi]) and a decrease in NADHc/NADc+ (decreased G-3-P/DHAP). Although the addition of lactate to the perfusion medium caused an increase in NADHm similar to pyruvate, MVo2 and PCr did not change significantly, [ATP]/([ADP] x [Pi]) increased less than with pyruvate, and there was an increase in NADHc/NADc+. The findings suggest that variations in the cytosolic redox state do not necessarily result in obligatory changes in either the mitochondrial redox state or in the [ATP]/([ADP] x [Pi]). This implies that under the conditions of this study an equilibrium is not maintained between [ATP]/([ADP] x [Pi]) and NADHc/NADc+. Furthermore, similar levels of NADHm can be associated with different values for [ATP]/([ADP] x [Pi]) and MVo2, depending on the substrates available to the heart.