NITRIC-OXIDE REGULATES CARDIAC CA2- INVOLVEMENT OF CGMP-INHIBITED ANDCGMP-STIMULATED PHOSPHODIESTERASES THROUGH GUANYLYL CYCLASE ACTIVATION( CURRENT )

The effects of the nitric oxide (NO) donor 3-morpholino-sydnonimine (S IN-1) on the L-type Ca2+ current (I(Ca)) were examined in frog ventric ular myocytes under basal and phosphorylated conditions. SIN-1 was fou nd to exert insignificant effects on basal I(Ca) but to induce a bipha sic action on stimulated I(Ca). Indeed, in the nanomolar range of conc entrations (0.1-10 nM), SIN-1 induced a pronounced (almost-equal-to 40 %) stimulation of I(Ca) elevated by a non-maximal concentration of for skolin (0.3 muM). However, the stimulatory effects of SIN-1 on I(Ca) w ere not additive with those of maximal concentrations (10 muM) of fors kolin or intracellular cAMP. In contrast, at higher concentrations (10 0 nM to 1 mM), SIN-1 strongly reduced I(Ca) (by up to 85%) which had b een previously stimulated by cAMP, forskolin, or isoprenaline. All the effects of SIN-1 appeared to be mediated by the liberation of NO sinc e they were suppressed by methylene blue and LY83583 and were not mimi cked by SIN-1C, a metabolite of SIN-1. The stimulatory or inhibitory e ffects of SIN-1 were absent, respectively, in the presence of milrinon e (10 muM) or when the hydrolysis-resistant cAMP analog 8-bromo-cAMP w as used instead of cAMP to stimulate I(Ca). In addition to its effects on I(Ca), SIN-1 induced a dose-dependent stimulation of guanylyl cycl ase activity in the cytosolic and membrane fractions of frog ventricle . The membrane form of guanylyl cyclase displayed a higher sensitivity to SIN-1 than the cytosolic form, which correlated with SIN-1 sensiti vity of I(Ca). Our data suggest that the activatory and inhibitory eff ects of NO donors on I(Ca). result from an inhibition of the cGMP-inhi bited cAMP-phosphodiesterase and an activation of the cGMP-stimulated cAMP-phosphodiesterase, respectively, both linked to the activation of guanylyl cyclase, possibly a membrane form of the enzyme.