MERCURY BLOCKAGE OF APICAL WATER CHANNELS IN TOAD SKIN (BUFO-MARINUS)

1. Net water flow (J(w)) was continuously monitored across the abdomin al skin of the toad Bufo marinus by means of a volumetric, automatic t echnique. J(w) was either averaged over periods of 2 min or taken cumu latively (10 or 30 min periods). 2. The state of high water permeabili ty induced by vasopressin or isoprenaline was reversed (88-89 % inhibi tion of DELTAJ(w) after 1 h) by the addition of 10(-3) M HgCl2 (or CH3 ClHg) to the external bathing medium. Similarly, pre-exposure of the s kins to Hg2+, totally blocked the induction of the hydrosmotic respons e to the same agents. By itself, Hg2+ exerted only a minor (26 %) stim ulation of basal J(w). 3. There was a sigmoidal dose-response relation ship between the reduction of the hydrosmotic effect of vasopressin (V P) and the concentration of Hg2+ in the extemal medium, with a half-ma ximal 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 ef fect that was reversed, or prevented, by exposure to Hg2+ in a manner indistinguishable from that previously seen with vasopressin or isopre naline. 5. The data point to the presence of a Hg2+-sensitive apical w ater pathway in stimulated epithelia, very probably constituted by wat er channels similar to those reported in red blood cells, amphibian bl adder and mammalian kidney tubules.