Acute Negative Inotropic Effect in Unstimulated Cardiac Muscle

We examined the effect of several nitric oxide (NO) donors, authentic NO gas, and L-arginine in isolated cat and rat papillary muscles. We did not observe significant inotropic effects in response to any NO donor (ie, SPM-5185, C87-3754, and S-nitroso-N-acetylpenicillamine [SNAP]) from 1 nmol/L to 100 ,umol/L. Similarly, authentic NO, at concentrations far in excess of those that maximally dilate the coronary vasculature (ie, 500 nmol/L), also failed to exert a detectable inotropic effect in these preparations. However, in the presence of 5 ,umol/L norepinephrine, 500 nmol/L NO exerted a 12±3% decrease in isolated rat papillary muscle contractility (P<.05). Addition of L-arginine up to 25 mmol/L exerted no inotropic effects in isolated rat papillary muscles. However, at 50 mmol/L, L-arginine decreased contractile force by 21±4% (P<.01). On further examination, the negative inotropic effect of 50 mmol/L L-arginine appeared to be nonspecific, since the inactive stereoisomer, D-arginine, at 50 mmol/L exerted the same effect. Further studies in isolated itric oxide synthase (NOS) exists in several N isoforms, all of which use L-arginine as the substrate to produce nitric oxide (NO).12 Endothelium-derived NO is synthesized by a constitutive NOS and is critical for the preservation of vascular homeostasis.1,3 The constitutive NO synthase is associated with low levels of NO formation, which are continuously generated (ie, basal production).4 Despite these modest levels of NO production, NO released via the constitutive pathway is sufficient to maintain vascular tone and inhibit neutrophil and platelet adhesion to the vascular endothelium under control conditions.56 These physiological concentrations apparently are between 0.1 and 1 nmol/L in the coronary circulation.7 Similarly, therapeutic concentrations (ie, -500 nmol/L) of agents that release physiological concentrations of NO (ie, NO donors) also inhibit neutrophil and platelet adhesion to endothelial cells and preserve the ischemic/reperfused myocardium.8-10 Although low concentrations ofNO clearly have cardioprotective effects Received March 10, 1994; accepted June 30, 1994. From the Departments of Physiology and Pathology (A.S.W., X.M., M.B., T.M., V.E.A., A.M.L., J.M.N., A.P.T.), Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pa, and the Department of Cardiothoracic Surgery (D.J.L., J.V.-J.), Bowman Gray School of Medicine, Winston-Salem, NC. Correspondence to Dr Allan M. Lefer, Department of Physiology, Jefferson Medical College, 1020 Locust St, Philadelphia, PA 19107. adult rat cardiac myocytes elicited similar results, in that 50 mmol/L of Land D-arginine equally decreased contraction amplitude and the underlying cytosolic calcium transient. Moreover, 500 nmol/L of the NO donor SPM-5185 only modestly decreased contraction amplitude or intracellular calcium in isolated rat cardiac myocytes. These results indicate that administration of physiological concentrations of exogenous NO does not acutely depress the inotropic state of the rat or cat heart to a physiologically significant extent. Only in the presence of high concentrations of norepinephrine did NO exert a statistically significant negative inotropic effect, and this effect was a modest one. These data demonstrate that physiological levels of NO do not exert a major regulatory effect on cardiac contractility. (Circ Res. 1994;75:692-700.)