Inactivation of TTX-sensitive and TTX-insensitive sodium channels of rat myoballs

Abstract

Myoballs, spherical muscle cell regenerates, have two types of Na + channels, the adult type with a high sensitivity to the Na + channel blocker tetrodotoxin (TTX), and the juvenile type with 10 times less TTX sensitivity. The potential dependence of the steady-state inactivation of the Na + channels ( h ∞-curve) was studied in 50 rat myoballs with the whole-cell patch-clamp method which provides overall results from all the Na + channels in an investigated myoball. At 37°C, the h ∞-curve had the usual shape, but below 30°C it was indented in a way suggesting contributions from two populations of Na + channels with a different potential dependence of inactivation. A more distinct separation of the two components was possible by differentiating the h ∞-curve with respect to the prepulse potential. The population requiring a more negative potential for the removal of inactivation had a low sensitivity to TTX while the other population had TTX sensitivity similar to that of Na + channels in adult muscle. At 37°C, the separation between the two h ∞-curves was small. At lower temperatures, the separation became larger. This change was almost entirely due to the h ∞-curve of the TTX-insensitive channels shifting towards more negative membrane potentials.