The nitric oxide-induced reduction in cardiac energy supply is not due to inhibition of creatine kinase.
OBJECTIVE
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 31P 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 microM) doubled coronary flow and induced a rapid drop in left ventricular developed pressure (39+/-10 vs. 81+/-10 mmHg) and MVO(2) (1.3+/-0.8 vs. 3.7+/-0.5 micromol/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 MVO(2) remained depressed, while PCr partially recovered. NO had no effect on the unidirectional forward flux through CK (98 +/- 21 vs. 99 +/- 20 micromol/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.


CONCLUSIONS
The NO-mediated depression of myocardial function, MVO(2) 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.
Decking
U K
UK
Department of Physiology Heinrich-Heine-University Düsseldorf, Düsseldorf, Germany. decking@uni-duesseldorf.de
Williams
J P
JP
Dahmann
R
R
Stumpe
T
T
Kelm
M
M
Schrader
J
J
D000818:Animals; D003402:Creatine Kinase / Q000378:metabolism*; D004734:Energy Metabolism; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D009682:Magnetic Resonance Spectroscopy; D008856:Microscopy, Fluorescence; D008929:Mitochondria, Heart / Q000201:enzymology; D009245:NADH Dehydrogenase / Q000032:analysis; D009569:Nitric Oxide / Q000494:pharmacology*; D010085:Oxidative Phosphorylation; D010477:Perfusion
D009569:Nitric Oxide; D009245:NADH Dehydrogenase; D003402:Creatine Kinase