REGULATION OF CARDIAC MYOCYTE CONTRACTILE FUNCTION BY INDUCIBLE NITRIC-OXIDE SYNTHASE (INOS) - MECHANISMS OF CONTRACTILE DEPRESSION BY NITRIC-OXIDE

Inflammatory cytokines have been implicated in the reversible depressi on of cardiac contractile function accompanying local or systemic immu ne stimulation. Incubation of cardiac myocytes with soluble components in the supernatant from cultured rat lung macrophages activated with endotoxin decreases their contractile response to beta-adrenergic stim ulation through the induction of iNOS and the subsequent production of nitric oxide by these cells. In the present study, we characterize th e mechanisms underlying NO's attenuation of adrenergic responsiveness in cardiac myocytes. iNOS was induced in cultured ventricular myocytes from adult rats by incubation for 20 h with conditioned medium from l ipopolysaccharide (LPS)-activated macrophages. iNOS induction did not induce any alteration in beta-adrenergic receptor density or affinity, G(alpha i) protein abundance, or adenylyl cyclase activity in culture d myocytes. Myocyte exposure to activated macrophage-conditioned mediu m markedly attenuated the elevation of cAMP in response to isoproteren ol (Iso, 2 nM). Induction of iNOS with the macrophage-conditioned medi um also potentiated the Iso-induced increase in myocyte cGMP, This cGM P increase was totally abolished by NOS inhibitors. NOS inhibition als o returned the attenuated cAMP response to 2 nM Iso to levels observed in control cells. Preincubation of the cells in isobutylmethylxanthin e (IBMX), a phosphodiesterase inhibitor, also partly reversed the atte nuation of cAMP increase with 2 nM Iso in cells expressing iNOS. Brief (15 min) exposure of myocytes to the NO donor, S-nitrosoacetylcystein e (SNAG, 100 mu M) which produced a three-fold increase in intracellul ar cGMP, also decreased by half the contractile response of cardiac my ocytes to Iso (2 nM). We conclude that NO endogenously produced by iNO S decreases the intracellular levels of cAMP in response to beta-adren ergic stimulation in isolated cardiac myocytes, in part through a cGMP -mediated mechanism. This effect may participate in the NO-dependent d epression of cardiac function following cytokine exposure. (C) 1998 Ac ademic Press Limited.