The nitric oxide-induced reduction in cardiac energy supply is not due to inhibition of creatine kinase

Objectives: While nitric oxide (NO) is a potent vasodilator already in the nM range, a cGMP-independent negative inotropic effect is observed at higher concentrations. Since inhibition of creatine kinase (CK) by NO-induced nitrosylation has been proposed as a possible mechanism of action, we measured the flux through CK in the intact heart. Methods: In saline perfused, paced guinea pig hearts P-31 NMR spectroscopy was employed to directly assess the cardiac energy status, i.e. free energy of ATP hydrolysis (DeltaG(ATP)) and flux through CK using magnetization transfer in absence and presence of NO. Results: NO (50 muM) doubled coronary flow and induced a rapid drop in left ventricular developed pressure (39 +/- 10 vs. 81 +/- 10 mmHg) and MVO2 (1.3 +/-0.8 vs. 3.7 +/-0.5 mu mol/min/g) (n=7). This effect was associated with an immediate decrease in phosphocreatine (PCr) (-69%) and DeltaG(ATP). During the subsequent 35 min of NO infusion cardiac function and MVO2 remained depressed, while PCr partially recovered. NO had no effect on the unidirectional forward flux through CK (98 +/- 21 vs. 99 +/- 20 mu mol/min/g, n=7) which was 5- to 10-fold greater than the rate of ATP turnover. Upon cessation of NO infusion both cardiac function and PCr rapidly returned to baseline values. The NO-induced fall in the myocardial energy status was associated with an increase in mitochondrial NADH (n=7) as assessed by surface fluorescence. The observed change in fluorescence was similar to that observed with short term ischemia. Conclusion: The NO-mediated depression of myocardial function, MVO2 and energy status is not mediated by changes in CK flux. Most likely a partial blockade of mitochondrial oxidative phosphorylation at the level of cytochrome c oxidase is responsible for this effect. (C) 2001 Elsevier Science BY All rights reserved.