Effects of Temperature on Heteromeric Kv11.1a/1b and Kv11.3 Channels.

Kv11.1 channels are crucial in cardiac physiology, and there is increasing evidence of physiological roles of different Kv11 channels outside the heart. The HERG (human Kv11.1a) channel has previously been shown to carry substantially more current at elevated temperatures, and we have now comparably investigated the temperature dependence of neuronal Kv11.3 channels and the more ubiquitous heteromeric Kv11.1a/1b channels. Transiently expressed rat Kv11 channels were studied at 21°C, 30°C, and 35°C. At near-physiological temperature, the maximal sustained outward current density was almost three times the mean value obtained at room temperature for Kv11.1a/1b, and increased by ∼150% for Kv11.3. For both channels, reduced inactivation contributed to the current increase at higher temperature. Elevated temperature moved Kv11.1a/1b isochronal activation curves to more negative potentials, but shifted the potential of half-maximal Kv11.3 channel activation to more depolarized values and reduced its voltage sensitivity. Thus, increased temperature stabilized the open state over the closed state of Kv11.1a/1b channels and exerted the opposite effect on Kv11.3 channel activation. Both Kv11 channels exhibited an overall high temperature sensitivity of most gating parameters, with remarkably high Q10 factors of ∼5 for the rate of Kv11.1a/1b activation. The Q10 factors for Kv11.3 gating were more uniform, but still higher for activation than for inactivation kinetics. The results demonstrate that characteristic differences between Kv11.1a/1b and Kv11.3 determined at room temperature do not necessarily apply to physiological conditions. The data provided here can aid in the design of models that will enhance our understanding of the role of Kv11 currents in excitable cells.

Modulation of cardiac ERG1 K(+) channels by cGMP signaling.

Different K(+) currents have been implicated in the myocardial action potential repolarization including the I(Kr). ERG1 alpha 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 Ca(2+) 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.