NITRIC-OXIDE INHIBITS CREATINE-KINASE AND REGULATES RAT-HEART CONTRACTILE RESERVE

Cardiac myocytes express both constitutive and cytokine-inducible nitr ic oxide synthases (NOS). NO and its congeners have been implicated in the regulation of cardiac contractile function. To determine whether NO could affect myocardial energetics, P-31 NMR spectroscopy was used to evaluate high-energy phosphate metabolism in isolated rat hearts pe rfused with the NO donor S-nitrosoacetylcysteine (SNAC). All hearts we re exposed to an initial high Ca2+ (3.5 mM) challenge followed by a re covery period, and then, either in the presence or absence of SNAC, to a second high Ca2+ challenge. This protocol allowed us to monitor sim ultaneously the effect of SNAC infusion on both contractile reserve (i .e., baseline versus high workload contractile function) and high-ener gy phosphate metabolism. The initial high Ca2+ challenge caused the ra te-pressure product to increase by 74 +/- 5% in all hearts. As expecte d, ATP was maintained as phosphocreatine (PCr) content briefly dropped and then returned to baseline during the subsequent recovery period. Control hearts responded similarly to the second high Ca2+ challenge, but SNAC-treated hearts did not demonstrate the expected increase in r ate-pressure product. In these hearts, ATP declined significantly duri ng the second high Ca2+ challenge, whereas phosphocreatine did not dif fer from controls, suggesting that phosphoryl transfer by creatine kin ase (CK) was inhibited. CK activity, measured biochemically, was decre ased by 61 +/- 13% in SNAC-treated hearts compared to controls. Purifi ed CK in solution was also inhibited by SNAC, and reversal could be ac complished with DTT, a sulfhydryl reducing agent. Thus, NO can regulat e contractile reserve, possibly by reversible nitrosothiol modificatio n of CK.