Sodium Conductance Activation without Inactivation in Pronase-perfused Axons

Abstract

A controversy of long standing in membrane electrophysio-logy is whether the sodium ion current (I_Na) and potassium ion current (I_K) pass through the membrane in separate channels, or through a single set of channels which conduct first I_Na and then I_K. In support of the latter hypothesis it has been noted that the sodium conductance (g_Na) decline, called inactivation, proceeds with about the same time course as the potassium conductance (g_K) increase. This could mean that Na⁺ selective channels are being converted into K⁺ selective channels. The hypothesis is especially interesting because of the possibility that the carrier postulated in active transport is convertible from Na⁺ to K⁺ selectivity¹. An explicit statement of the single channel hypothesis and the means for disproving it were given by Mullins². Because a single channel could not simultaneously conduct I_Na and I_K, disproof requires that membrane conductance (g_m) be made somehow to exceed the maximum value of g_Na or g_K. We report here that inactivation of g_Na can be destroyed fairly selectively by the action from inside the axon of the unspecific proteolytic enzymes of pronase. In many cases g_m after pronase treatment is greater than maximum g_K before treatment, making untenable the single channel hypothesis.