Expression and functional characterization of the cardiac L-type calcium channel carrying a skeletal muscle DHP-receptor mutation causing hypokalaemic periodic paralysis

Abstract

A histidine substitution for the outermost arginine in II/S4 of the α₁ subunit of the human skeletal muscle dihydropyridine (DHP) receptor has been reported to cause hypokalaemic periodic paralysis (HypoPP). This mutation shifts the voltage dependence of L-type Ca current inactivation in myotubes from HypoPP patients by −40 mV without affecting activation. Based on the strong homology of II/S4 in cardiac and skeletal muscle α₁, we introduced the corresponding mutation into the rabbit cardiac α₁ subunit (R650H). Wild type (WT) and mutant constructs were transiently transfected in HEK cells together with β and α₂δ subunits and Ca and Ba currents were studied using the whole-cell patch-clamp technique. In contrast to the results obtained from human myotubes, R650H produced a small (−5 mV) but significant shift of both the steady-state activation and inactivation curves. When external pH was increased from 7.4 to 8.4 in order to favour deprotonization of H650, the only difference between WT and mutant channels was a slightly reduced steepness of the inactivation curve. Additional cotransfection of the γ subunit which is only found in skeletal but not in heart muscle, shifted the inactivation curves of both WT and R650H by −20 mV. We conclude that R650 plays a different role in voltage-dependent gating of the cardiac L-type Ca channel than the corresponding residue in the human skeletal muscle L-type channel, since a distinct and selective effect on the midpoint voltage of steady-state inactivation could not be found for R650H.