The Effects of Extracellular Protons on the hERG Potassium Channel

Abstract

The α-subunit of channels mediating the cardiac rapid delayed rectifier current (IKr) is encoded by the human Ether-à-go-go-Related Gene (hERG). Macroscopic hERG current (IhERG) amplitude is reduced and deactivation kinetics are accelerated with extracellular acidosis. We have investigated the single channel basis for the effects of acidic external pH (pHe) on the isoforms of IhERG expressed in myocytes (hERG1a and 1b). Patch clamp recordings were made at room temperature with the extracellular superfusate (whole-cell) or pipette solution (cell attached) acidified to pH 6.3 compared with control (pH 7.4). A decrease in pHe to 6.3 caused acceleration in deactivation and a reduction in maximal whole-cell conductance of ∼34% for IhERG1a (n=8 cells) and of ∼36% for IhERG1b (n=5 cells). Single channel recordings were made with isotonic potassium (140 mM) bathing the cells and in the electrode. Channel amplitude and open state kinetics were measured at a series of repolarisation voltages following a depolarising command to +40mV. Slope conductance values derived from amplitude current-voltage relationships between −120 and −40mV were 12.3±0.2pS for pH 7.4 (n=10 cells) and 9.3±0.1pS for pH 6.3 (n=9 cells) (P<0.01, two-tailed t-test) for hERG1a. The corresponding values for hERG1b were 11.4±0.2pS for pH 7.4 (n=6 cells) and 7.6±0.4pS for pH 6.3 (n=5 cells) (P<0.0001; two-tailed t-test). Open-time kinetics at −120mV for hERG1a were reduced from 8.49±1.0ms in control (n=8 cells) to 4.2±0.4ms in pHe 6.3 (n=7 cells) (P<0.05; two-tailed t-test). The hERG1b open state kinetics were 5.7±0.6ms in pHe 7.4 (n=6 cells) and reduced to 3.1±0.7ms in pHe 6.3 (n=5 cells) (P<0.05; two-tailed t-test). Thus, it can be concluded that a reduction in the single channel conductance and acceleration of open-times contribute to the attenuation of macroscopic IhERG when exposed to acidic pHe.