Nitric oxide signalling and the regulation of cardiac repolarisation

Nitric oxide (NO) signalling has been recently linked with cardiac repolarisation. Studies from isolated heart preparations have demonstrated a protective effect from ventricular fibrillation following vagus nerve stimulation, which is NO- dependent. Genome-wide association studies have linked NO signalling components to arrhythmias. A role for NO signalling has also been described in the diurnal variation of cardiac muscle responses to sympathetic stimulation. The cellular and molecular mechanisms for NO regulation of ventricular repolarisation by cGMP-dependent signalling were investigated using BAY 60-2770, a novel NO/haem-independent soluble guanylyl cyclase (sGC) activator and compared with the NO donor SNAP. Experiments were performed on guinea pig isolated left ventricular myocytes. Cellular cGMP was selectively quantified by radioimmunoassay. Action potentials and the slow (I[subscript Ks]) and rapid (I[subscript Kr]) delayed rectifier K[superscript +] currents were recorded using the perforated patch-clamp technique. BAY 60-2770 applied under basal conditions, modestly increased cellular cGMP levels and shortened action potential duration (APD), but failed to modulate I[subscript Ks] or I[subscript Kr]. In contrast, SNAP failed to modulate APD despite greater increases in cGMP compared with BAY 60-2770. When phosphodiesterases (PDEs) were inhibited, BAY 60-2770 increased cGMP levels much more, prolonged APD and inhibited I[subscript Ks] (but not I[subscript Kr]). The mechanism for inhibition of I[subscript Ks] did not involve protein kinase G. These results demonstrate that PDE activity suppresses elevations of cGMP in response to sGC activation, and also compartmentalises cGMP-dependent signalling. Thus, PDEs uncouple changes in cGMP levels from ion channels located at the sarcolemma. In addition, APD shortening in response to isoprenaline exhibited diurnal variation, with greater shortening in active compared to resting period myocytes. However, this effect was not blocked by nitric oxide synthase inhibition. In conclusion, NO signalling pathways may be able to regulate cardiac repolarisation through a cGMP-dependent mechanism, but PDEs act to limit these responses in the healthy myocardium.