Calcium Current Properties and Leak Conductance in Mouse Muscle Fibers Overexpressing a Type 1 Hypokalemic Periodic Paralysis Mutant L-Type Calcium Channel

Abstract

Missense mutations in the gene encoding the alpha1 subunit of the L-type calcium channel CaV1.1 induce type 1 Hypokalemic Periodic Paralysis (HypoPP1). These mutations mainly occur at arginine residues in the fourth transmembrane segment of voltage-sensor domains. Very few studies have investigated the acute effects of these mutations on channel function and muscle membrane electrical properties because of the difficulty to express Cav1.1 in heterologous systems. In the present study we successfully transferred by electroporation the genes encoding the turboGFP-tagged human wildtype (WT) and R1239H HypoPP1 mutant Cav1.1 into hind limb mouse muscles. The expression profile of the two channels showed a regular striated pattern indicative of the localization of the channels in the t-tubule membrane. Measurement of the L-type current using the silicone-clamp technique showed that the maximal conductance and the voltage-dependence of the Cav1.1 channel were significantly reduced and shifted towards negative potentials respectively in fibers expressing R1239H Cav1.1 as compared to fibers expressing WT Cav1.1. Applying voltage ramps from a holding potential of 0, −20, −40 or −60 mV to −120 mV in the presence of an external low-chloride, sodium-free and potassium-free solution revealed a significant higher leak conductance measured between −80 and −120 mV in fibers expressing R1239H Cav1.1. Acidification of the external solution significantly increased the leak inward current, leak conductance and the fluorescence of an internally loaded pH indicator in fibers expressing R1239H. These data suggest that an elevated leak inward current, likely carrying protons, flows at resting membrane potentials in fibers expressing R1239H Cav1.1 and that acidification of the external medium following muscle exercise could contribute to favor the onset of muscle paralysis in HypoPP1.