Large conductance channel in plasma membranes of astrocytic cells is functionally related to mitochondrial VDAC-channels

Abstract

Large conductance anion channels with similar electrophysiological characteristics were found in plasma membranes and in outer mitochondrial membranes of various cell types. Although their large conductance and their peculiar voltage dependence point to a close relation, it was questioned whether they belong to the same family. We therefore compared some biochemical features of a plasmalemmal channel with those known from the mitochondrial channel. Current events were recorded from excised patches of plasma membranes of a rat astrocytic cell line (RGCN). The underlying channels exhibited a conductance of 401±50 pS. Open probability was highest between ±10 mV and gradually approached zero beyond ±25 mV. Activity as induced by voltage ramps between ±40 mV appeared after a delay of up to several min. The delay could be reduced by bathing either side of the patch in an acidic Ringer solution (pH 6.2). 1 mM Al 3+ increased the open time at potentials more positive than 20 mV. 10 mM dextran sulfate (MW 8000) caused reversible flickering, increasing the closed probability. 4,4′-diisothiocyanatostilbene-2,2′ disulfonic acid (DIDS) also caused a reversible flickering into the closed state, reducing the apparent single channel amplitude by up to 70% at 0.5 mM DIDS. Application of 5 mM ATP resulted in reversible blockade; ATP was more effective from the outside than from the inside (blocking activity 65% vs. 16% of the patches). We conclude that the large conductance anion channel from astrocytic cells displays electrophysiological and pharmacological characteristics that resemble those of VDAC (Voltage Dependent Anion Channel) from the outer mitochondrial membrane.