EXPRESSION AND FUNCTIONAL-CHARACTERIZATION OF THE CARDIAC L-TYPE CALCIUM-CHANNEL CARRYING A SKELETAL-MUSCLE DHP-RECEPTOR MUTATION CAUSING HYPOKALEMIC PERIODIC PARALYSIS

A histidine substitution for the outermost arginine in II/S4 of the al pha(1) subunit of the human skeletal muscle dihydropyridine (DHP) rece ptor has been reported to cause hypokalaemic periodic paralysis (HypoP P). This mutation shifts the voltage dependence of L-type Ca current i nactivation in myotubes from HypoPP patients by -40 mV without affecti ng activation. Based on the strong homology of II/S4 in cardiac and sk eletal muscle alpha(1), we introduced the corresponding mutation into the rabbit cardiac alpha(1) subunit (R65OH). Wild type (WT) and mutant constructs were transiently transfected in HEK cells together with be ta and alpha(2) delta subunits and Ca and Ba currents were studied usi ng the whole-cell patch-clamp technique. In contrast to the results ob tained from human myotubes, R650H produced a small (-5 mV) but signifi cant 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 de protonization of H650, the only difference between WT and mutant chann els was a slightly reduced steepness of the inactivation curve. Additi onal cotransfection of the gamma subunit which is only found in skelet al 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 cor responding residue in the human skeletal muscle L-type channel, since a distinct and selective effect on the midpoint voltage of steady-stat e inactivation could not be found for R650H.