Evidence for the induction of repetitive action potentials in synaptosomes by K+-channel inhibitors: an analysis of plasma membrane ion fluxes.

Abstract

The effects of four K+-channel inhibitors on synaptosomal free Ca2+ concentrations and 86Rb+ fluxes are analysed. 4-Aminopyridine, alpha-dendrotoxin, charybdotoxin, and tetraethylammonium all increase the free Ca2+ concentration, although their potencies differ widely. In each case, the elevation in free Ca2+ concentration is reversed by the subsequent addition of tetrodotoxin. The transient 86Rb+ efflux from preequilibrated synaptosomes induced with high concentrations of veratridine is partially inhibited by 4-aminopyridine and alpha-dendrotoxin. In contrast, when 4-aminopyridine or alpha-dendrotoxin is added to polarized synaptosomes, and enhanced 86Rb+ flux is seen, both for uptake and for efflux with no change in the total 86Rb+/K+ content of the synaptosomes and with only a slight time-averaged plasma membrane depolarization (6.4 and 3.3 mV, respectively). The enhancements of flux by 4-aminopyridine or alpha-dendrotoxin are sensitive to ouabain and/or to tetrodotoxin. Furthermore, these flux changes show the same concentration dependencies as the blocked component of veratridine-stimulated 86Rb+ efflux, the elevation of free Ca2+ concentration, and the facilitation of glutamate exocytosis that are elicited by 4-aminopyridine or alpha-dendrotoxin. It is concluded that these findings support the proposal of spontaneous, repetitive firing of synaptosomes evoked by K+-channel inhibitors and that the enhanced 86Rb+ flux is a consequence of the activity of 4-aminopyridine- and alpha-dendrotoxin-insensitive K+ channels during these action potentials.