SUMMARY1. Isolated Amphiuma small intestine exposed on both surfaces to buffered or unbuffered media generated gradients of pH under short-circuited conditions consistent with secretion of HCO3-.2. When unbuffered mucosal medium was maintained at pH 7-4 by addition of acid, alkalinization of the mucosal medium occurred at a rate of 1-2 #tequiv/hr cm2 under short-circuit conditions (Is,) and was reduced by anoxia, acetazolamide or removal of CO2. 3. The rate of HCO3-secretion (JHCO3-) was reduced at a mucosal pH above or below 7-4 and was proportional to serosal HCO3-.4. JHCO3 was reduced in Na+-free (choline) and Cl--free (SO42-) media and after exposure to the stilbene SITS.5. The difference JHCO3 -lS was consistent with net Cl-absorption.6. The tissue resistance (Rt) was elevated upon exposure to serosal HCO3-and lowered by mucosal HCO3-.7. The intestinal mucosa exhibited carbonic anhydrase activity that was sensitive to ethoxazolamide.8. It is concluded that HCO3-secretion is active, influenced by intracellular carbonic anhydrase activity and coupled to Cl-and possibly Na+ absorption.

The requirement for Na+ and Cl- in the bathing media to obtain a maximal HCO3- secretory flux (JHCO3-) across isolated short-circuited Amphiuma duodenum was investigated using titration techniques and ion substitution. Upon substitution of media Na+ with choline, HCO3- secretion was markedly reduced. Replacement of media Cl- produced a smaller reduction of JHCO3-. The presence of Cl- enhanced HCO3- secretion only if Na+ was also in the media. Elevation of media Na+ or Cl- in the presence of the other ion produced a saturable increase of JHCO3-. In the presence of Na+, Cl- stimulated JHCO3- when added to the mucosal but not the serosal medium. In the presence of Cl-, Na+ elevated JHCO3- when added to the serosal but not the mucosal medium. The ability of mucosal Cl- to stimulate JHCO3- was not apparently dependent on mucosal Na+. Simultaneous addition of 10 mM Cl- to the Na+ -free mucosal medium and 10 mM Na+ to the Cl- -free serosal medium stimulated JHCO3- above levels produced by serosal Na+ alone. In conclusion, intestinal HCO3- secretion required mucosal Cl- and serosal Na+ and did not involve mucosal NaCl cotransport. The results are consistent with a mucosal Cl- absorptive mechanism in series with parallel basolateral Na+ -H+ and Cl- -HCO3- exchange mechanisms.

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.

Isolated short-circuited segments of jejunum from Amphiuma absorbed HCO-3 at a rate determined by the anion composition of the bathing media. The rate of HCO-3 absorption (JHCO-3) measured by pH-stat was high when the major anion was Cl-, Br-, I-, or SO2-(4) but was lower when gluconate or benzene sulfonate (SO-3) was the anion. The disulfonic stilbenes SITS, DIDS, and DNDS at 1 mM reduced JHCO-3 and short-circuit current when added to the serosal bathing medium. Inhibition by SITS was comparable whether Cl-, Br-, or SO2-(4) was the major anion, and SITS produced a small inhibition of JHCO-3 in gluconate-based media and no effect in benzene SO3-based media. Under conditions in which tissue conductance was low SITS lowered tissue conductance further. In Cl- media SITS reduced short-circuit current consistent with inhibition of Cl- absorption as well as HCO-3 absorption. Inhibition of the Cl- current by DIDS was not reversed by washout, although the inhibition of JHCO-3 was reversed. The rate of HCO-3 absorption in gluconate media could be increased by serosal addition of Cl-, Br-, or I-. It is concluded that the process that results in net jejunal HCO-3 absorption entails anion-HCO-3 exchange at the basolateral membrane and is distinct from the basolateral Cl(-)-HCO-3 exchange process involved in Cl- absorption.

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