Effects of Exogenous and Endogenous Nitric Oxide on the Contractile Function of Cultured Chick Embryo Ventricular Myocytes

Nitric oxide (NO) has been shown to be a ubiquitous intercellular autacoid in the heart and, in cultured rat ventricular myocytes, to decrease the contractile responsiveness to isoproterenol (ISO). The aim of the present study was to investigate whether exogenous (sodium nitroprusside, SNP) or endogenous nitric oxide generated from -arginine modulated the response to ISO in cultured chick embryo ventricular myocytes. SNP 1μ or -arginine 1 m had no effect on baseline contractile function. Superfusion with ISO 100 n significantly increased myocyte amplitude of shortening to 1.31±0.06 (ratio to baseline amplitude). Initial superfusion with SNP 1μ or -arginine 1 m attenuated the response to ISO to 0.89±0.05 and 1.09±0.07 respectively (P<0.05). Potassium ferrocyanide which is not a NO donor and -arginine the inactive substrate of NO synthase did not attenuate the response to ISO. Myocyte cGMP content was significantly increased by incubation with SNP 1μ (31.65±3 fmol/well) but not by -arginine 1 m (11.1±0.3 fmol/well) as compared to myocytes incubated in control medium (11±0.9 fmol/well). Preincubation with SNP 1μ or -arginine 1 m significantly attenuated the ISO mediated-increase in cAMP content from 4.33±0.2 pmol/well (ISO 100 n alone) to 1.48±0.36 fmol/well and 1.72±0.21 pmol/well respectively. Potassium ferrocyanide and -arginine had no effect on myocyte cGMP or cAMP content. Chick embryo myocytes have measurable and NMMA-inhibited NO synthase activity as measured by the conversion of [3H] -arginine to [3H] -citrulline. In conclusion, these results demonstrate that in cultured chick embryo ventricular myocytes both exogenous and endogenous NO elevate cGMP. This may account for the inhibition ofβ-adrenergic agonist-stimulated increases in cAMP and amplitude of shortening via an unidentified intracellular negative coupling.