Nitric oxide inhibits isoproterenol-stimulated adipocyte lipolysis throughoxidative inactivation of the beta-agonist

Nitric oxide has been implicated in the inhibition of catecholamine-stimlul ated lipolysis in adipose tissue by as yet unknown mechanisms. In the prese nt study, it is shown that the nitric oxide donor, 2,2-diethyl-1-nitroso-ox yhydrazine, antagonized isoproterenol (isoprenaline)-induced lipolysis in r at adipocytes, freshly isolated from white adipose tissue, by decreasing th e potency of the beta -agonist without affecting its efficacy. These data s uggest that nitric oxide did not act downstream of the beta -adrenoceptor b ut reduced the effective concentration of isoproterenol. In support of the latter hypothesis, we found that pre-treatment of isoproterenol with nitric oxide abolished the lipolytic activity of the catecholamine. Spectroscopic data and HPLC analysis confirmed that the nitric oxide-mediated inactivati on of isoproterenol was in fact because of the modification of the catechol amine through a sequence of oxidation reactions, which apparently involved the generation of an aminochrome. Similarly, aminochrome was found to be th e primary product of isoproterenol oxidation by 3-morpholinosydnonimine and peroxynitrite. Finally, it was shown that nitric oxide released peroxynitr ite. Finally, it was shown that nitric oxide released from cytokine-stimula ted adipocytes attenuated the lipolytic effect of isoproterenol by inactiva ting the catecholamine. In contrast with very recent findings, which sugges t that nitric oxide impairs the beta -adrenergic action of isoproterenol th rough intracellular mechanisms and not through a chemical reaction between NO and the catecholamine, we showed that nitric oxide was able to attenuate the pharmacological activity of isoproterenol in vitro as well as in a nit ric oxide-generating cellular system through oxidation of the beta -agonist . These findings should be taken into account in both the design and interp retation of studies used to investigate the role of nitric oxide as a modul ator of isoproterenol-stimulated signal transduction pathways.