Modulation of cardiac ERG1 K⁺ channels by cGMP signaling

Abstract 

Different K⁺ currents have been implicated in the myocardial action potential repolarization including the I_Kr. ERG1 α subunits, identified as the molecular correlate of I_Kr, have been shown to form heteromultimeric channels in the heart and their activity is modulated by a complex interplay of signal transduction events. Using electrophysiological techniques, we examined the effects of the cGMP-analogue 8-Br-cGMP on rat and guinea-pig papillary action potential duration (APD), on the biophysical properties of heterologously expressed homo- and heteromeric ERG1 channels, and on cardiac I_Kr. 8-Br-cGMP prolonged APD by about 25% after pharmacological inhibition of L-type Ca²⁺ currents and I_Ks. The prolongation was completely abolished by prior application of the hERG channel blocker E-4031 or the protein kinase G (PKG) inhibitor Rp-8-Br-cGMPS. Expression analysis revealed the presence of both ERG1a and -1b subunits in rat papillary muscle. Both 8-Br-cGMP and ANP inhibited heterologously expressed ERG1b and even stronger ERG1a/1b channels, whereas ERG1a channels remained unaffected. The inhibitory 8-Br-cGMP effects were PKG-dependent and involved a profound ERG current reduction, which was also observed with cardiac AP clamp recordings. Measurements of I_Kr from isolated mouse cardiomyocytes using Cs⁺ as charge carrier exhibited faster deactivation kinetics in atrial than in ventricular myocytes consistent with a higher relative expression of ERG1b transcripts in atria than in ventricles. 8-Br-cGMP significantly reduced I_Kr in atrial, but not in ventricular myocytes. These findings provide first evidence that through heteromeric assembly ERG1 channels become a critical target of cGMP-PKG signaling linking cGMP accumulation to cardiac I_Kr modulation.

Keywords: hERG, cGMP, APD, I_Kr, ANP