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 (NADH(c)/NAD(c)(+)), mitochondrial NADH (NADH(m)), and [ATP]/([ADP] x [P-i]) 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 NADH, 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 P-31 metabolites were measured using nuclear magnetic resonance spectroscopy. The addition of pyruvate to the perfusion medium caused increases in myocardial oxygen consumption (MVO(2)), NADH(m) fluorescence, phosphocreatine (PCr), and [ATP]/([ADP] x [P-i]) and a decrease in NADH(c)/NAD(c)(+) (decreased G-3-P/DHAP). Although the addition of lactate to the perfusion medium caused an increase in NADdH(m) similar to pyruvate, MVO(2) and PCr did not change significantly, [ATP]/([ADP] x [P-i]) increased less than with pyruvate, and there was an increase in NADH(c)/NAD(c)(+). 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]I([ADP] x [P-i]). This implies that under the conditions of this study an equilibrium is not maintained between [ATP]/([ADP] x [P-i]) and NADH(c)/NAD(c)(+). Furthermore, similar levels of NADH, can be associated with different values for [ATP]/([ADP] x [P-i]) and MVO(2), depending on the substrates available to the heart.