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

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 infinity-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 degrees C, the h infinity-curve had the usual shape, but below 30 degrees 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 infinity-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 degrees C, the separation between the two h infinity-curves was small. At lower temperatures, the separation became larger. This change was almost entirely due to the h infinity-curve of the TTX-insensitive channels shifting towards more negative membrane potentials.