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

Full-length deoxyribonucleic acid, complementary (cDNA) constructs enc oding the alpha-subunit of the adult human skeletal muscle Na+ channel , hSkM1, were prepared. Functional expression was studied by electroph ysiological recordings from cRNA-injected Xenopus oocytes and from tra nsiently transfected tsA201 cells. The Na+ currents of hSkM1 had abnor mally slow inactivation kinetics in oocytes, but relatively normal kin etics when expressed in the mammalian cell line. The inactivation kine tics of Na+ currents in oocytes, during a depolarization, were fitted by a weighted sum of two decaying exponentials. The time constant of t he fast component was comparable to that of the single component obser ved in mammalian cells. The block of hSkM1 Na+ currents by the extrace llular toxins tetrodotoxin (TTX) and alpha-conotoxin (mu CTX) was meas ured. 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 r SkM1, but the potency of mu CTX is 22-fold lower in hSkM1, primarily d ue to a higher rate of toxin dissociation in hSkM1. Single-channel rec ordings were obtained from outside-out patches of oocytes expressing h SkM1. The single-channel conductance, 24.9 pS, is similar to that obse rved for rSkM1 expressed in oocytes.