AJP - Gastrointestinal and Liver Physiology, Vol 249, Issue 5 614-G621, Copyright © 1985 by American Physiological Society
Effects of cations on pH gradient-stimulated sulfate transport in rabbit ileal brush-border membrane vesicles
C. M. Schron, R. G. Knickelbein, P. S. Aronson, J. Della Puca and J. W. Dobbins
In brush-border membrane vesicles from rabbit ileum, we previously reported
pH gradient-stimulated SO4 uptake and presented evidence that this
represents carrier-mediated SO4-OH exchange. In the present study
inhibitors of SO4-OH exchange (H-SO4 cotransport) were shown not to inhibit
Na-SO4 cotransport, suggesting that these are two separate carrier-mediated
transport mechanisms. While pH gradient-stimulated SO4 uptake was inhibited
87% by 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, disodium
salt (DIDS) and 79% by 1.0 mM furosemide, Na+-stimulated SO4 uptake was
only inhibited 11 and 0%, respectively. K+ (20 mM), Cl (5 mM), and oxalate
(0.25 mM) inhibited pH gradient-stimulated SO4 uptake (38-65%) but had no
effect on Na+-stimulated SO4 uptake. Finally, at Na+ concentrations (10 mM)
significantly less than that required for Na+-stimulated SO4 uptake (60-100
mM), external Na+ inhibited pH gradient-stimulated SO4 uptake, suggesting
two independent effects of this cation. SO4 uptake was also inhibited by
external K+ both in the presence and absence of a pH gradient. A Dixon plot
of the DIDS-sensitive SO4 uptake under pH gradient conditions yielded a
straight line, indicating a single site of interaction between external K+
and the SO4-OH carrier (apparent Ki = 7.2 mM). In contrast to the
inhibition by external K+, internal K+ stimulated SO4 uptake. This effect
was DIDS sensitive and not enhanced by valinomycin, suggesting an
interaction of internal K+ with the SO4-OH exchanger independent of a
K+-induced electrical potential. SO4 uptake and the effects of K+ were pH
modulated with less SO4 uptake and less K+ effect at higher pH.(ABSTRACT
TRUNCATED AT 250 WORDS)
