Endogenous nitric oxide enhances coupling between O2 consumption and ATP synthesis in guinea pig hearts

Endogenous nitric oxide (eNO) modulates tissue respiration. To test whether eNO modulates myocardial O2 consumption (MVO2), ATP synthesis, and metabolic efficiency, we used isolated isovolumic guinea pig hearts perfused at a constant flow. N(omega)-nitro-L-arginine (L-NNA; 5 x 10(-5) mol/l) was used to inhibit eNO production. MVO2 was measured at different levels of cardiac work, estimated as the rate-pressure product (RPP). ATP content and synthesis rate were determined using (31)P NMR and magnetization transfer during high cardiac work. L-NNA increased coronary vascular resistance (19 +/- 3%, P < 0.05) and MVO2 (12 +/- 3%, P < 0.05) without an increase in the RPP. In contrast, vehicle infusion resulted in insignificant changes in coronary vascular resistance (3 +/- 2%, P > 0.05) and MVO2 (-2 +/- 1%, P > 0.05). Compared with vehicle, L-NNA caused a higher MVO2 both during KCl arrest (L-NNA 5.6 +/- 0.5 vs. vehicle 3.0 +/- 0.4 micromol x min(-1) x mg x dry wt(-1), P < 0.05) and during increased cardiac work elicited by elevating perfusate Ca2+, indicating an upward shift in the relationship between contractile performance (measured as RPP) and MVO2. However, neither ATP contents nor ATP synthesis rates were different in the two groups during high cardiac work. Thus, because inhibition of eNO production by L-NNA increased MVO2 without a change in the ATP synthesis rate, these data suggest that eNO increases myocardial metabolic efficiency by reducing MVO2 in the heart.