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 highe r concentrations. Since inhibition of creatine kinase (CK) by NO-induced ni trosylation has been proposed as a possible mechanism of action, we measure d the flux through CK in the intact heart. Methods: In saline perfused, pac ed 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 o f NO. Results: NO (50 muM) doubled coronary flow and induced a rapid drop i n left ventricular developed pressure (39 +/- 10 vs. 81 +/- 10 mmHg) and MV O2 (1.3 +/-0.8 vs. 3.7 +/-0.5 mu mol/min/g) (n=7). This effect was associat ed with an immediate decrease in phosphocreatine (PCr) (-69%) and DeltaG(AT P). During the subsequent 35 min of NO infusion cardiac function and MVO2 r emained depressed, while PCr partially recovered. NO had no effect on the u nidirectional 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 as sociated with an increase in mitochondrial NADH (n=7) as assessed by surfac e fluorescence. The observed change in fluorescence was similar to that obs erved with short term ischemia. Conclusion: The NO-mediated depression of m yocardial function, MVO2 and energy status is not mediated by changes in CK flux. Most likely a partial blockade of mitochondrial oxidative phosphoryl ation at the level of cytochrome c oxidase is responsible for this effect. (C) 2001 Elsevier Science BY All rights reserved.