AJP - Cell Physiology, Vol 259, Issue 1 C35-C40, Copyright © 1990 by American Physiological Society
H+ and HCO3- flux across apical surface of rat distal colon
G. M. Feldman and R. L. Stephenson
Department of Medicine, Veterans Affairs Medical Center, Philadelphia, Pennsylvania.
Colonic ion transport is postulated to occur via simultaneous operation of
Na(+)-H+ exchange and Cl(-)-HCO3- exchange. Accordingly H+ and HCO3- should
be transported simultaneously by the colon. To assess simultaneous H+ and
HCO3- transport, net acid-base flux was measured in isolated segments of
rat distal colon. When both tissue surfaces were bathed in symmetrical
solutions containing Cl-, net base was secreted (-1.0 +/- 0.1 mu
eq.cm-2.h-1). Cl- substitution with gluconate in the mucosal medium caused
net base flux to switch from secretion to absorption (2.0 +/- 0.2 mu
eq.cm-2.h-1). To evaluate whether base absorption was dependent on H+
secretion via Na(+)-H+ exchange, mucosal Na+ was substituted with
N-methylglucamine, and amiloride, an inhibitor of Na(+)-H+ exchange, was
applied. Na+ substitution and 1 mM amiloride inhibited base absorption by
37 and 38%, respectively, suggesting operation of Na(+)-H+ exchange.
Because base absorption persisted, an additional mechanism was considered,
HCO3- absorption via Cl(-)-HCO3- exchange. This was evaluated with an
inhibitor of Cl(-)-HCO3- exchange
4-acetamido-4'-isothiostilbene-2,2'-disulfonic acid (SITS). SITS (1 mM)
inhibited HCO3- absorption by 40%. The effects of amiloride and SITS were
additive, suggesting that the Na(+)-H+ and Cl(-)-HCO3- exchangers operate
simultaneously. Amiloride also inhibited H+ secretion when net HCO3- was
secreted, suggesting that the direction of HCO3- movement does not
influence Na(+)-H+ exchange activity. These data suggest that the colon
transports both H+ and HCO3- across the apical surface via Na(+)-H+
exchange and Cl(-)-HCO3- exchange; H+ is secreted via Na(+)-H+ exchange,
whereas HCO3- can be secreted or absorbed via Cl(-)-HCO3- exchange.
