Rubidium efflux from neural cell lines through voltage-dependent potassium channels

Abstract

Cloned cell lines derived from the rat and mouse nervous systems were screened for the presence of voltage-dependent K+ channels by assaying for an increase in 86Rb+ efflux under depolarizing conditions. Two types of depolarizing stimuli, (a) 100 mM KCl and (b) a combination of veratridine and scorpion venom, produced parallel results in all of 11 excitable cell lines tested; i.e., a cell line either responded to both sets of stimuli (8 examples) or to neither (3 examples). In the case of three cell lines where it was possible to compare the Rb+ efflux results with electrophysiological measurements of K+ currents the two methods gave qualitatively parallel results; i.e., cell lines that exhibited delayed rectification responded positively to depolarization in the Rb+ efflux assay, while cell lines that failed to exhibit K+ currents did not respond in the efflux assay. KCl-induced 86Rb+ efflux from the excitable cell lines did not appear to involve a Ca2+-dependent K+ channel, since efflux was not blocked by lowering extracellular Ca2+ from 2 to 0.1 mM. Efflux also was not blocked by tetraethylammonium or 4-aminopyridine, but could be blocked by scorpion venom. Of 14 cell lines previously classified as glial, 7 failed to show evidence of depolarization-induced 86Rb+ efflux while 7 others responded positively to 100 mM K+. Efflux from the latter 7 lines could be distinguished from efflux from the excitable lines in three ways. (1) Veratridine plus scorpion venom did not stimulate efflux. (2) Scorpion venom did not block KCl-induced efflux. (3) Tetraethylammonium and 4-aminopyridine were effective in blocking KCl-induced efflux from 4 of the 7 lines. These results extend the usefulness of ion flux measurements as a means of assessing the electrical properties of cells and raise some interesting questions concerning the nature of cells that appear to have K+ channels but not Na+ channels.