Mercury blockage of apical water channels in toad skin (Bufo marinus).

1. Net water flow (Jw) was continuously monitored across the abdominal skin of the toad Bufo marinus by means of a volumetric, automatic technique. Jw was either averaged over periods of 2 min or taken cumulatively (10 or 30 min periods). 2. The state of high water permeability induced by vasopressin or isoprenaline was reversed (88-89% inhibition of delta Jw after 1 h) by the addition of 10(-3) M HgCl2 (or CH3ClHg) to the external bathing medium. Similarly, pre-exposure of the skins to Hg2+, totally blocked the induction of the hydrosmotic response to the same agents. By itself, Hg2+ exerted only a minor (26%) stimulation of basal Jw. 3. There was a sigmoidal dose-response relationship between the reduction of the hydrosmotic effect of vasopressin (VP) and the concentration of Hg2+ in the external medium, with a half-maximal effect at 1.2 x 10(-4) M HgCl2. 4. Total replacement of Na+ by K+, Rb+ or Cs+ in the Ringer solution, caused a VP-like, hydrosmotic effect that was reversed, or prevented, by exposure to Hg2+ in a manner indistinguishable from that previously seen with vasopressin or isoprenaline. 5. The data point to the presence of a Hg(2+)-sensitive apical water pathway in stimulated epithelia, very probably constituted by water channels similar to those reported in red blood cells, amphibian bladder and mammalian kidney tubules.