Intestinal HCO3- secretion in Amphiuma: stimulation by mucosal Cl- and serosal Na+

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.