AJP - Gastrointestinal and Liver Physiology, Vol 249, Issue 5 607-G613, Copyright © 1985 by American Physiological Society
pH gradient-stimulated sulfate transport by rabbit ileal brush-border membrane vesicles: evidence for SO4-OH exchange
C. M. Schron, R. G. Knickelbein, P. S. Aronson, J. Della Puca and J. W. Dobbins
In the presence of a pH gradient (7.7 inside, 5.5 outside), the initial
velocity of SO4 uptake by rabbit ileal brush-border membrane (BBM) vesicles
was markedly stimulated compared with uptake in the absence of a pH
gradient. Under pH gradient conditions, SO4 was transiently accumulated at
a concentration 13-fold higher than at equilibrium ("overshoot").
Superimposition of a HCO3 gradient did not further stimulate the initial
velocity of SO4 uptake compared with a pH gradient alone. Evidence that
this pH gradient-stimulated SO4 uptake represented SO4-OH exchange included
lack of sensitivity of SO4 transport to alterations of the membrane
potential; 85-95% inhibition of SO4 uptake by the anion exchange inhibitors
DIDS and SITS; and saturation kinetics (Km for SO4 = 0.475 +/- 0.054 mM;
Vmax = 4.1 +/- 0.1 nmol SO4 X mg prot-1 X min-1). Sulfate did not inhibit
pH gradient-stimulated 36Cl uptake, indicating that SO4-OH and Cl-HCO3(OH)
are different exchangers. When BBM vesicles were compared with basolateral
membrane (BLM) vesicles, pH gradient-stimulated SO4 uptake was found
predominantly in the BBM preparation. Brush-border SO4-OH exchange was
further localized by demonstrating Na-stimulated SO4 efflux from vesicles
loaded under pH gradient conditions, suggesting that Na-SO4 cotransport and
SO4-OH exchange are on the same BBM vesicles. In conclusion, a SO4-OH
exchanger (or H-SO4 cotransporter) exists on the brush border of rabbit
ileum which is distinct from the brush-border Cl-HCO3(OH)
exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)
