FUNCTIONAL EXPRESSION AND PROPERTIES OF THE HUMAN SKELETAL-MUSCLE SODIUM-CHANNEL

Full-length deoxyribonucleic acid, complementary (cDNA) constructs encoding the alpha-subunit of the adult human skeletal muscle Na+ channel, hSkM1, were prepared. Functional expression was studied by electrophysiological recordings from cRNA-injected Xenopus oocytes and from transiently transfected tsA201 cells. The Na+ currents of hSkM1 had abnormally slow inactivation kinetics in oocytes, but relatively normal kinetics when expressed in the mammalian cell line. The inactivation kinetics of Na+ currents in oocytes, during a depolarization, were fitted by a weighted sum of two decaying exponentials. The time constant of the fast component was comparable to that of the single component observed in mammalian cells. The block of hSkM1 Na+ currents by the extracellular toxins tetrodotoxin (TTX) and alpha-conotoxin (mu CTX) was measured. The IC50 values were 25nM (TTX) and 1.2 mu M (mu CTX) in oocytes. The potency of TTX is similar to that observed for the rat homolog rSkM1, but the potency of mu CTX is 22-fold lower in hSkM1, primarily due to a higher rate of toxin dissociation in hSkM1. Single-channel recordings were obtained from outside-out patches of oocytes expressing hSkM1. The single-channel conductance, 24.9 pS, is similar to that observed for rSkM1 expressed in oocytes.