AJP - Renal Physiology, Vol 252, Issue 6 1022-F1027, Copyright © 1987 by American Physiological Society
Selectivity of basolateral anion exchange in the acidification pathway of turtle bladder
R. F. Husted and J. L. Fischer
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.
