AJP - Gastrointestinal and Liver Physiology, Vol 241, Issue 5 389-G396, Copyright © 1981 by American Physiological Society
Bicarbonate absorption by in vitro amphibian small intestine
J. F. White and M. A. Imon
Isolated segments of jejunum from Amphiuma bathed in Cl--free (SO42(-))
media usually generated serosa-negative electrical potentials when HCO3(-)
was present in the media. Bidirectional isotope fluxes under short circuit
revealed a negligible absorption of Na+ and a residual flux consistent with
anion absorption. Acetazolamide (10(-4) M) eliminated the short-circuit
current and the residual flux. Segments of jejunum exposed on the mucosal
surface to HCO3(-) (pH 7.4) alkalinized the unbuffered serosal fluid at a
rate of about 1.1 mueq . h-1 . cm-2, as measured by the pH-stat while
clamped at zero transepithelial potential. Acetazolamide, anoxia, and
2,4-dinitrophenol lowered the rate of alkalinization and simultaneously
reduced the short-circuit current by an equal amount. Absorption was
constant above a [HCO3(-)] of 35 meq/l and uninfluenced by applied
transepithelial voltage gradients. HCO3(-) absorption was not reduced after
replacement of media Na+ or Cl- but was reduced on addition of ouabain or
removal of K+. It is concluded that the jejunum actively absorbs HCO3(-) by
an electrogenic process that does not involve Na+-H+ exchange.
