Differential Regulation of Slow and Rapid Delayed Rectifier Potassium Currents by cGMP Dependent Nitric Oxide Signalling Pathways in Isolated Adult Guinea Pig Ventricular Myocytes

Abstract

Nitric oxide (NO) signalling pathways are reported to regulate cardiac repolarisation. This was investigated using BAY 60-2770, which activates soluble guanylyl cyclase (sGC) in a NO and haem independent manner to generate cGMP. Action potentials (APs) and currents were recorded from isolated guinea pig ventricular myocytes at 37oC using the perforated patch clamp technique. APs were stimulated at 2 Hz. The slow and rapid delayed rectifier potassium currents, IKs and IKr were measured as tail currents elicited by a voltage step from +40 mV to −50 mV. The following experiments were carried out in the presence of 100 µM IBMX (3-Isobutyl-1-methylxanthine) a non-selective phosphodiesterase (PDE) inhibitor. 1 µM BAY 60-2770 lengthened APD90 by 15.7 ms (p<0.05, n=7), suggesting that sGC activation and an increase in cGMP inhibits repolarising currents. Further experiments showed that IKs was inhibited by BAY 60-2770. IBMX alone enhanced IKs by 77 ± 9 % (p<0.001, n=8) and subsequent addition of BAY 60-2770 reduced the IBMX dependent enhancement of IKs by 68 ± 2 % (p<0.001, n=8). This suggests that PDE inhibition permits IKs to be inhibited by cGMP dependent NO signalling pathways. The sGC mediated inhibition of IKs by BAY 60-2770 is unlikely to be due to phosphorylation by protein kinase G (PKG) because the inhibition remained in the presence of 100 nM KT-5823, a PKG inhibitor and because PKG activation with 100 µM 8-bromo-cGMP had no effect on IKs. IKr was not regulated by PDE inhibition, sGC activation or PKG activation. Overall, these findings suggest that cGMP dependent NO signalling pathways regulate IKs, but not IKr, via a PKG independent mechanism.