The effects of acidosis on the hERG potassium channel

Abstract

The human ether-a-go-go-related gene (hERG) encodes channels mediating the rapid delayed rectifier K+ current (IKr). IKr participates in cardiac action potential (AP) repolarisation and may also protect the ventricles against premature stimulation. The heart is exposed to acidosis (low pH) in a number of pathological conditions including myocardial ischemia. Extracellular acidosis is known to modulate hERG current (IhERG) function, although a number of aspects of the modulation remain incompletely understood. The aims of this investigation were to establish the effects of acidosis on: (i) IhERG amplitude, kinetics and the response to premature stimulation at mammalian physiological temperature; (ii) the hERG blocking potency of selected anti arrhythmic drugs. Whole-cell patch-clamp recordings of IhERG were made from mammalian cells (CHO or HEK 293) at 37 QC. Lowering external pH from 7.4 to 6.3 reduced the magnitude of IhERG by reducing macroscopic hERG conductance and modulating IhERG kinetics, with positively shifted activation and accelerated deactivation. Results from experiments using an acidic pipette solution showed that the actions of protons occurred from the external surface and not from secondary intracellular acidosis. Experimental and computer simulation work demonstrated that acidosis impairs the protective role of IhERG against premature stimulation. The effects of extracellular acidosis on IhERG kinetics were preserved when the shortened hERG I b isoform was studied, indicating that a full-length N-terminus is not necessary for acidic modulation of hERG channel function. Interestingly, the inhibitory effect of acidosis on IhERG was greater for hERG 1 band hERG lall b than for hERG la. Extracellular acidosis decreased the hERG blocking potency of flecainide, dofetilide and ranolazine, whilst the potency of amiodarone was unaffected. IhERG inactivation was found to be important for ranolazine's inhibitory action and a series of S6 and inner helix residues (Y652, F656, T623, S624 and V625) were identified as contributing to ranolazine binding. I.EThOS - Electronic Theses Online ServiceGBUnited Kingdo