Abstract

We searched for sites on the α-subunit of the fast Na⁺channel responsible for the difference in GTX (grayanotoxin) sensitivity of the skeletal- and cardiac-muscle Na⁺current. cDNA clones, encoding the skeletal or cardiac isoforms of the α-subunit, were inserted into a mammalian expression vector and transiently transfected into human embryonic kidney cells. The expressed channels were measured using whole-cell patch-clamp techniques and examined for GTX sensitivity. As a measure of GTX sensitivity, we used relative chord conductance (ratio of maximum chord conductance of noninactivating GTX-modified Na⁺ currents to that of unmodified peak currents). Wild-type channels from skeletal muscle (μ1) were more sensitive to GTX modification than wild-type cardiac channels (rH1) by a factor of 1.6. To facilitate exploration of α-subunit sites determining GTX sensitivity, we used SHHH, a chimera of skeletal muscle (S) domain D1 and heart muscle (H) domains D2D3D4 with supernormal sensitivity to GTX I (1.5-fold of wild-type μ1). Successive replacement of Ser-251 (D1S4–S5 intracellular loop) and Ile-433 (D1S6 transmembrane segment), with corresponding rH1 residues Ala and Val, reduced, in a stepwise manner, the GTX sensitivity of the chimera and related mutants to that of wild-type rHl. We concluded that, in addition to Ile-433, known as the GTX-binding site, Ser-251 represents a novel site for GTX modification.