Selectivity of basolateral anion exchange in the acidification pathway of turtle bladder

Abstract

The turtle urinary bladder in vitro acidifies the solution bathing its luminal surface. Protons are actively extruded across the apical membrane by an H+-ATPase. Bicarbonate ion exits the cell across the basolateral membrane via a stilbene-sensitive, anion exchange for chloride. Chloride then exits the cell via a conductive pathway. The present studies were undertaken to define the specificity of the basolateral anion exchange mechanism for chloride. Turtle bladders were mounted on chambers in vitro, short-circuited, and treated with ouabain. The current remaining after inhibition of sodium transport was used to measure the acidification rate. Ion replacement studies with bromide, isethionate, sulfate, and nitrate indicated that only bromide supported acidification at rates comparable to chloride. In separate experiments, kinetic analysis of anion interaction with the exchanger indicates that maximal acidification rates decrease in the order: Cl greater than Br greater than SO4 greater than methyl sulfate = gluconate. The affinity of the exchanger decreases in the order: Cl greater than SO4 greater than Br greater than HCO3 greater than methylsulfate greater than gluconate. These selectivity sequences indicate "strong" interaction of the anions with the selectivity site. The differences in position of the polyatomic anions in the two sequences indicates that the "binding" site is accessible but that transport is limited by steric factors.