Effects of anions and/or cell volume on the permeance of an apical water pathway induced by Hg in toad skin epithelium.

Hg compounds block membrane transport units behaving as water channels. Here we show that Hg induces an apical water pathway in toad skins pretreated with 10(-3) M CH3ClHg or HgCl2, added to the outer bathing medium. Washing with SO4-Ringer caused a several-fold increase in net water flow (Jw) and osmotic permeability coefficient (Pf) that was reversed by re-exposure to Cl- or NO3-Ringer and mimicked by gluconate-Ringer. These Pf changes could be elicited repeatedly and were present if, and only if, anion replacements took place in the inner bathing solution. Such inner polarity was related to the anion permeability of the epidermal basolateral membrane: impermeant anions (SO4, gluconate) increased Pf; permeant anions (Cl, NO3) did not change basal Pf but reversed the high Pf induced by impermeant anions. Hg induced the appearance of aggregates that persisted despite repeated washings of the skins during 4-5 h, and whether Pf was high (SO4-Ringer) or low (Cl-Ringer) before skin fixation. The Hg-induced apical water pathway in toad skin appears to be a unique model for studying the interplay between cell volume, cell ionic composition and water permeability.