The nitric oxide donors, SNAP and DEA/NO, exert a negative inotropic effect in rat cardiomyocytes which is independent of cyclic GMP elevation.

The role of guanosine 3',5'-cyclic monophosphate (cGMP) in the regulation of cardiac contractility remains controversial. The present study has examined the effects of high concentrations of the nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine (SNAP) and 1,1-diethyl-2-hydroxy-2-nitroso-hydrazine (DEA/NO), on cGMP levels and isoproterenol-induced increases in contractility in rat cardiomyocytes before and after selective inhibition of soluble guanylyl cyclase with 1 H -[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). In control myocytes, 100 microm SNAP or 100 microm DEA/NO increased cGMP levels by more than 15-fold at 2 and 6 min and produced marked attenuations of isoproterenol-mediated increases in maximal cell shortening over the same time period. The NO donors had no significant effect on basal cell shortening (in the absence of isoproterenol). Pretreatment of myocytes with 25 microm ODQ for 30 min resulted in a complete blockade of the SNAP- or DEA/NO-induced increases in cGMP with no reversal of negative inotropy. ODQ did not affect basal contractility, basal cGMP levels or isoproterenol-induced increases in cell shortening. Furthermore, myocytes exposed to the cGMP analog, 8-bromo-cGMP (100 microm), did not exhibit significant differences in basal contractility or isoproterenol-induced increases in cell shortening. These results suggest that attenuation of cardiac contractility by NO donors in rat cardiomyocytes occurs by a mechanism independent of increases in cGMP levels.