The leak conductance of dialysed oocytes of the ascidian Styela plicata : mode of action of intracellular fluoride

Abstract

Mature oocytes of the ascidian Styela plicata were internally dialysed by the method of Takahashi & Yoshii (1978, J . Physiol ., Lond . 279, 519‒549). Membrane conductance and capacitance of the dialysed eggs were monitored continuously under voltage-clamp, by using a lock-in ampli­fier. After a few minutes of dialysis with a 250 mm fluoride solution, there was a brief, transient conductance increase followed by an apparent recovery. The conductance increase was accompanied by an irreversible loss of membrane capacitance, and a transient low-frequency capacitance dispersion. This was followed by an apparent recovery. Subsequent dialysis with fluoride-free (500 mm Cl¯ ) solutions caused a reversible increase in membrane leak conductance. The effect persisted after fixation of the egg with 1% (by volume) glutaraldehyde. During chloride-induced leak conductance increases, flickering single channels with an unusually high unit conductance of the order of 5000 pS appeared in the membrane. Chloride-induced conductance increases could be abolished by dialysing with 150 mm hexacyanoferrate [III] provided that the extracellular medium contained the precipitating cation cobalt. If the oocyte mem­brane had never been exposed to fluoride, liquefaction of the cytoplasm occurred during chloride dialysis, but membrane conductance was not affected. Under inverted dialysis, in which the cytoplasm was replaced with extracellular medium and vice versa, membrane leak conductance exhibited the same properties as in normal dialysis but responded much faster to solution changes. We conclude that fluoride causes the formation of a leak conductance comprising, in part or in whole, giant membrane channels, and that fluoride and calcium, or hexacyanoferrate [III] and cobalt can precipitate in the membrane and plug these channels, regard­ less of which face of the membrane they are on. The relevance of these results to the preferred composition of intracellular dialysis media is discussed.