BETA-ADRENERGIC REGULATION OF CONSTITUTIVE NITRIC-OXIDE SYNTHASE IN CARDIAC MYOCYTES

Nitric oxide (NO) has been implicated in endogenous control of myocard ial contractility. However, NO release has not yet been demonstrated i n cardiac myocytes. Accordingly, endogenous NO production was measured with a porphyrinic microsensor positioned on the surface of individua l neonatal or adult rat ventricular myocytes (n > 6 neonatal and adult cells per experiment). In beating neonatal myocytes, there was no det ectable spontaneous NO release with each contraction. However, norepin ephrine (NE; 0.25-1 mu M) elicited transient NO release from beating n eonatal (149 +/- 11 to 767 +/- 83 nM NO) and noncontracting adult(157 +/- 13 to 791 +/- 89 nM NO) cells. NO was released by adrenergic agoni sts with the following rank order of potency: isoproterenol (beta(1) b eta(2)) > NE (alpha/beta(1)) > dobutamine (beta(1)) approximate to epi nephrine (alpha/beta(1) beta(2)) > tertbutylene (beta(2)); NO was not released by phenylephrine (alpha). NE-evoked NO release was reversibly blocked by N-G-monomethyl-L-arginine, trifluoperazine, guanosine 5'-O -(2-thiodiphosphate), and nifedipine but was enhanced by 3-isobutyl-1- methylxanthine (0.5 mM = 14.5 +/- 1.6%) and BAY K 8644 (10 mu M = 11.9 +/- 1%). NO was also released by A-23187 (10 mu M = 884 +/- 88 nM NO) , guanosine 5'-O-(3-thiotriphosphate) (1 mu M = 334 +/- 56 nM NO), and dibutyryl adenosine 3',5'-cyclic monophosphate (10-100 mu M = 35 +/- 9 to 284 +/- 49 nM NO) but not by ATP, bradykinin, carbachol, 8-bromog uanosine 3',5'-cyclic monophosphate, or shear stress. This first funct ional demonstration of a constitutive NO synthase in cardiac myocytes suggests its regulation by a beta-adrenergic signaling pathway and may provide a novel mechanism for the coronary artery vasodilatation and enhanced diastolic relaxation observed with adrenergic stimulation.