β-Adrenergic regulation of constitutive nitric oxide synthase in cardiac myocytes

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Abstract
Nitric oxide (NO) has been implicated in endogenous control of myocardial contractility. However, NO release has not yet been demonstrated in cardiac myocytes. Accordingly, endogenous NO production was measured with a porphyrinic microsensor positioned on the surface of individual neonatal or adult rat ventricular myocytes ( n > 6 neonatal and adult cells per experiment). In beating neonatal myocytes, there was no detectable spontaneous NO release with each contraction. However, norepinephrine (NE; 0.25–1 μM) elicited transient NO release from beating neonatal (149 ± 11 to 767 ± 83 nM NO) and noncontracting adult (157 ± 13 to 791 ± 89 nM NO) cells. NO was released by adrenergic agonists with the following rank order of potency: isoproterenol (β1β2) > NE (α/β1) > dobutamine (β1) ≈ epinephrine (α/β1β2) > tertbutylene (β2); NO was not released by phenylephrine (α). 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 μM = 11.9 ± 1%). NO was also released by A-23187 (10 μM = 884 ± 88 nM NO), guanosine 5′- O-(3-thiotriphosphate) (1 μM = 334 ± 56 nM NO), and dibutyryl adenosine 3′,5′-cyclic monophosphate (10–100 μM = 35 ± 9 to 284 ± 49 nM NO) but not by ATP, bradykinin, carbachol, 8-bromoguanosine 3′,5′-cyclic monophosphate, or shear stress. This first functional demonstration of a constitutive NO synthase in cardiac myocytes suggests its regulation by a β-adrenergic signaling pathway and may provide a novel mechanism for the coronary artery vasodilatation and enhanced diastolic relaxation observed with adrenergic stimulation.