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EDITORIAL FOCUS
Abteilung für Kardiologie und Pneumologie, Zentrum Innere Medizin, Universitätsklinikum Göttingen, Göttingen, Germany
WHEN NITRIC OXIDE (NO) donors were first administered to isolated, normal cardiomyocytes, a negative, cGMP-dependent inotropic effect was observed, whereas inhibitors of NO synthases (NOS) had no effect. Further studies then demonstrated a biphasic contractile response to exogenous NO and cGMP, explained by dose-dependent inhibition or activation of phosphodiesterases modulating cAMP. Also, exogenous NO can, via cGMP, activation of protein kinase G, and phosphorylation of troponin I, decrease myofilamental calcium responsiveness and exert cGMP-independent positive inotropic effects by nitrosylation. These in part contradictory findings are not surprising given the multiple mechanisms (via cGMP, nitrosylation, antioxidation), sites of production (cardiomyocytes, endothelial cells, nerve endings), and targets (mitochondrial respiration, glucose transport, cAMP turnover, L-type calcium channels, sarco(endo)plasmic reticulum Ca2+-ATPase, ryanodine receptor, myofilaments) of myocardial NO signaling known to date (11).
Large animal studies demonstrated that there is a net positive inotropic effect of constitutive myocardial NO synthesis during normoperfusion and ischemia (6) and that basal NO release supports myocardial efficiency, i.e., the amount of myocardial work produced at a given level of oxygen consumption, during normoperfusion, ischemia (6), exercise (2), and pacing-induced heart failure (15). This was ascribed to a counterbalance of xanthine oxidase-derived oxygen radical production by NO (15). Importantly, heart failure represents a NO-deficient state by decreased myocardial production (14) and/or increased oxidative inactivation (1) of NO, whereas in the exercised heart, as the conceptual opposite of the failing myocardium, NO production and antioxidative capacity are increased (4, 19). In line with that, upregulation of NO synthesis by statins attenuated heart failure after myocardial infarction in rats (13) or during tachycardic pacing in dogs (20).
Knockout models of the constitutive NOS isoforms in mice showed that both endothelial NOS (eNOS) (17) and neuronal NOS (nNOS) (3, 16) support cardiac function after myocardial infarction and that the presence of eNOS is a prerequisite for the salutary effect of statins (9) and angiotensin-converting enzyme antagonism (10). In detail (11), eNOS is subcellularly localized to caveolae in the sarcolemma controlling ß-adrenergic signaling transduction at the level of cAMP and L-type calcium channels. nNOS colocalizes with the cardiac sarcoplasmic reticulum, limiting calcium cycling in the basal state but facilitating it during adrenergic and frequency (8) stimulation, although the precise mechanisms are still under debate. Taken together, constitutive myocardial NO synthesis is protective, sensitive to oxidative stress, and modulates myocardial function at multiple sites.
It has been known for some time that in the vasculature, vasodilation by NO does not only depend on the presence of eNOS and is limited by oxidative inactivation of NO but also can be increased by administration of the sole substrate of NOS, L-arginine. Surprisingly, endothelial L-arginine levels (near the millimolar range) by far exceed the Km of eNOS (<10 µM). This "L-arginine paradox" has been explained by subcellular compartmentation of L-arginine stores, rendering L-arginine supply to eNOS dependent on transport proteins such as cationic amino acid transporter-1 or by the presence of endogenous NOS inhibitors such as asymmetric dimethylarginine that compete with L-arginine for binding to eNOS. However, L-arginine is also a substrate of arginase that competes with NOS for L-arginine. Arginase exists in two isoforms, the liver type I and the nonliver type II, and catalyzes the conversion of L-arginine to L-ornithine and urea in the urea cycle to excrete nitrogen. Ornithine is converted to proline, feeding collagen synthesis, and to polyamines involved in cellular growth and differentiation. Of note, an intermediate product of NO synthesis, N-hydroxy-L-arginine, inhibits arginase activity (5). Arginase I activity has recently been shown to be upregulated and thus to limit NOS-induced vasodilation in the vasculature of aged (21) and hypertensive (7) animals. Whether arginase may affect cardiomyocyte NO signaling has not been tested so far.
In the current issue of the American Journal of Physiology-Heart and Circulatory Physiology, Jung et al. (7a) demonstrate in a clear and straightforward study that arginase I is constitutively expressed in normal feline cardiomyocytes and impacts on cardiomyocyte NO signaling. Inhibition of arginase with boronoethyl chloride increased normal cardiomyocyte cGMP threefold, decreased the calcium transient, and exerted a pronounced negative inotropic effect mediated by cGMP. Although direct NOS inhibition was not applied, these data strongly imply that in isolated normal cardiomyocytes, NO production is regulated by L-arginine availability toward NOS, such that the term L-arginine paradox could from now also comprise cardiomyocytes.
Given the numerous studies that did not find an effect of endogenous NO synthesis on cardiac function in vitro (as in contrast to in vivo), the question arises whether there might just have been not enough fuel for NOS in vitro. In the perfused, beating heart, basal NO release is increased by shear stress and stretch, which might inhibit arginase activity by N-hydroxy-L-arginine, thus shifting more L-arginine toward NOS. Such a mechanism would not be active in nonperfused cardiac preparations. Clearly, experiments in integrated models are needed to assess whether constitutive arginase activity indeed limits cardiomyocyte NO production in vivo and in which direction it may shift cardiac function in normal and diseased states.
Jung et al. (7a) also report that in compensated hypertrophy, arginase expression is decreased, thus facilitating NO signaling. It is tempting to hypothesize that in decompensated failure, cardiomyocyte arginase activity might be upregulated and, by attenuating NOS signaling, could worsen cardiac disease. Such a mechanism has been proposed to promote asthmatic disease (12); however, the first clinical data about dietary L-arginine supplementation in patients after myocardial infarction showed no benefit (18). Thus cardiomyocyte arginase has now been introduced as a new modulator of myocardial function via limiting substrate supply to NOS, but many more studies are needed before its beneficial or harmful role can be assessed.
FOOTNOTES
Address for reprint requests and other correspondence: H. Post, Abteilung für Kardiologie und Pneumologie, Zentrum Innere Medizin, Universitätsklinikum Göttingen, Robert-Koch Strasse 40, 37077 Göttingen, Germany (e-mail: heiner.post{at}med.uni-goettingen.de)
REFERENCES
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