The Complexities of Coupling Nitric Oxide Synthase Pathways in the Heart

The roles of nitric oxide (NO) in the heart have been studied intensely ever since the first report nearly 2 decades ago showing that endogenous NO modulates cardiac myocyte function.1 All 3 NO synthase (NOS) isoforms have been found in mammalian heart tissues (see reviews in Massion et al2 and Belge et al3). A complex array of myocyte and nonmyocyte cells in the heart express NOS isoforms, and the local generation of NO2,3 and reactive oxygen species (ROS)4 may exert both autocrine and paracrine effects on cellular function. Not only is the endothelial isoform of NOS (eNOS, or NOS3) robustly expressed in the endothelial cells of the cardiac vasculature, but eNOS is also present within cardiac myocytes, where the enzyme associates with the scaffolding/regulatory protein caveolin-3 in T tubules in plasmalemmal caveolae.5 The neuronal NOS (nNOS, or NOS1) is also expressed in cardiac myocytes, where the enzyme appears to be localized in the sarcoplasmic reticulum and modulates phospholamban phosphorylation.6 Although both eNOS and nNOS appear to be physiologically expressed in cardiac tissues, the inflammation-related NOS isoform (iNOS, or NOS2) only appears in the heart after immunoactivation; iNOS may modulate the decline in myocardial function seen in sepsis.7 There have been innumerable studies of the cardiac effects of various NOS inhibitors, NO-donating drugs, and NO itself.2 The cardiac phenotypes of NOS “knockout” mouse models have been characterized extensively, and mice lacking 1, 2, or all 3 NOS isoforms have been generated and characterized.8 In addition, the roles of NOS substrates, cofactors, and allosteric modulators have been explored exhaustively in diverse cardiac model systems. Emerging from this broad range of experimental evidence is an increasingly clear view that NO and NOS are key determinants of cardiac function. Yet the molecular mechanisms and pathways whereby this ubiquitous, reactive, and pluripotent signaling system modulates cardiac contraction and relaxation remain incompletely defined.