Myocyte Nitric Oxide Synthase 2 Contributes to Blunted β-Adrenergic Response in Failing Human Hearts by Decreasing Ca ²⁺ Transients

Abstract

Background-Human heart failure (HF) usually exhibits blunted response to ␤-adrenergic receptor (AR) stimulation.Here, we examined whether expression of nitric oxide synthase-2 (NOS2, or inducible NOS) contributes to this loss of inotropic reserve in human HF. Methods and Results-Failing human hearts were obtained at transplantation. Contraction and [Ca 2ϩ ] i measurements were performed in isolated cardiac myocytes and trabeculae. In HF myocytes and muscle, isoproterenol (ISO), a ␤-AR agonist, led to small inotropic and lusitropic responses. Specific inhibition of NOS2 by aminoguanidine (AG) or L-NIL dramatically increased the ISO-induced inotropy and lusitropy, such that the ISOϩAG response in HF approached that seen with ISO alone in nonfailing human myocytes or muscles. Ca 2ϩ transient data directly paralleled these results, indicating that altered cellular Ca 2ϩ handling is responsible. In nonfailing human hearts, NOS2 inhibition had no effects. In addition, NOS2 inhibition also had no effect in 30% of failing hearts, but in these myocytes and muscles, the ISO response alone was similar to that of nonfailing hearts. In line with these functional findings, NOS2 protein expression measured by Western blotting was induced in HF when AG/L-NIL had a functional effect but not when AG/L-NIL had no effect on contractility and Ca 2ϩ transients. Conclusions-NOS2 expression strongly limited ISO-induced increases in contraction, twitch ⌬[Ca 2ϩ ] i , and lusitropy in trabeculae and isolated myocytes from failing human hearts. Thus, the ␤-AR hyporesponsiveness in human HF is mediated in large part by NO (or related congeners) produced within cardiac myocytes via NOS2.

Arginase modulates myocardial contractility by a nitric oxide synthase 1-dependent mechanism

“…However, a coherent hypothesis detailing the role of specific NOS isoforms and the locus of action of NO in heart failure has not yet emerged. Some studies indicate that cytokine-induced NOS2 and NO production cause suppression of myocyte Ca 2ϩ transients (35)(36)(37)(38). However, it has more recently been shown that constitutive NOS isoforms contribute to the heart failure phenotype.…”

Biphasic effect of SIN-1 is reliant upon cardiomyocyte contractile state

“…The biphasic nature of SIN-1 may relate to the physiological regulation of basal and submaximal β-adrenergic-stimulated contractility with low peroxynitrite production (i.e., NOS1) [40], and the pathophysiological regulation of chronic β-adrenergic stimulation with high peroxynitrite production (i.e., NOS2), as occurs in heart failure [3]. Thus, it appears that low SIN-1 may potentially mimic the peroxynitrite production of NOS1, which is hypothesized to produce low concentrations of peroxynitrite due to a co-localization with xanthine oxidase [4].…”

“…NOS1 is considered a physiological regulator of cardiac function and is also thought to increase myocardial contractility [41]. Conversely, high SIN-1 may be mimicking the peroxynitrite production of NOS2, which is considered to be a pathophysiological regulator of cardiac function by reducing β-adrenergic responsiveness [3,36]. During many pathophysiological conditions of the myocardium, nitric oxide production is increased with NOS2 expression [3,42] and superoxide production is increased through NADPH oxidase and/or xanthine oxidoreductase [8,9], leading to supraphysiological peroxynitrite production and cardiac dysfunction.…”

“…Conversely, high SIN-1 may be mimicking the peroxynitrite production of NOS2, which is considered to be a pathophysiological regulator of cardiac function by reducing β-adrenergic responsiveness [3,36]. During many pathophysiological conditions of the myocardium, nitric oxide production is increased with NOS2 expression [3,42] and superoxide production is increased through NADPH oxidase and/or xanthine oxidoreductase [8,9], leading to supraphysiological peroxynitrite production and cardiac dysfunction. Additionally, this physiological and pathophysiological regulation of cardiac contractility by peroxynitrite likely relies on signaling pathways that target the excitationcontraction coupling protein PLB as well as other targets, thus resulting in the biphasic effect of SIN-1.…”

“…is produced by three distinct isoforms of nitric oxide synthase (NOS) [2]. Neuronal NOS (nNOS, NOS1) and endothelial NOS (eNOS, NOS3) are constitutively expressed, while inducible NOS (iNOS, NOS2) is only expressed during immune responses and many pathophysiological conditions of the myocardium, such as heart failure [3]. NOS1 co-localizes with xanthine oxidase [4], a superoxide (O 2 .-) producing enzyme, and may potentially lead to the production of low levels of peroxynitrite (ONOO − ), as nitric oxide and superoxide react with a very high rate constant [5].…”