Heterogeneous expression of NO-activated soluble guanylyl cyclase in mammalian heart: implications for NO- and redox-mediated indirect versus direct regulation of cardiac ion channel function.

Nitrogen oxides exert significant but diverse regulatory effects on cardiac myocytes. Many of these effects are due to modulation of voltage-sensitive ion channel function. The redox-status of NO-related compounds is a critical factor in determining whether indirect (cGMP-dependent) versus direct (cGMP-independent) effects are dominant. However, molecular mechanisms by which different cardiac myocyte types, and associated different ion channel types expressed within them, could achieve selectivity between NO-related indirect versus direct effects are unclear We have previously demonstrated heterogeneous expression gradients of Type III NO synthase (eNOS) and sarcolemmal superoxide dismutase (ECSOD) in ferret and human ventricle, with both enzymes being highly expressed in right ventricle and left ventricular subepicardium but markedly reduced in left ventricular subendocardium. In this study we extend this previous analysis by analyzing NO-activated soluble guanylyl cyclase (sGC) expression in the heart (ferret and human). We demonstrate that, at both tissue and single myocyte levels, sGC protein expression is heterogeneous, being high in sinoatrial node, right atrium, right ventricle and left ventricular subepicardium, but markedly reduced to absent in left atrium and left ventricular subendocardium. Thus, there is a significant overlap in expression gradients of sGC, eNOS, and ECSOD among distinct cardiac tissue and myocyte types. These gradients positively correlate with both: (i) experimentally measured basal NO production levels; and (ii) expression gradients of specific voltage-gated ion channels (particularly Kv1 and Kv4 channels). Our results provide the first demonstration in the heart of an expressed coupled multienzymatic system for selective regulation of indirect (sGC-dependent) versus direct (sGC-independent) NO- and redox-related modulation of voltage-gated ion channel function in different myocyte types. Our results also have functional implications for NO(*)/redox-related modulation of ion channels expressed in other cell types, including neurons, skeletal muscle and smooth muscle.