PATHWAYS FOR BICARBONATE TRANSFER ACROSS THE SEROSAL MEMBRANE OF TURTLE URINARY-BLADDER - STUDIES WITH A DISULFONIC STILBENE

Bicarbonate is transferred across the serosal (S) membrane of the epithelial cells of the turtle bladder in two directions. Cellular HCO3- generated behind the H+ pump moves across this membrane into the serosal solution. This efflux of HCO3- is inhibited by SITS (4-isothiocyano-4′-acetamido-2,2′-disulfonic stilbene). When HCO3- is added to the serosal solution it is transported across the epithelium in exchange for absorbed Cl-. This secretory HCO3- flow traverses the serosal cell membrane in the opposite direction. In this study the effects of serosal addition of 5×10-4m SITS on HCO3- secretion and Cl- absorption were examined. The rate of H+ secretion was brought to zero by an opposing pH gradient, and 20 mm HCO3- was added to S. HCO3- secretion, measured by pH stat titration, was equivalent to the increase in M→S Cl- flux after HCO3- addition. Neither the S→M flux of HCO3- nor the M→S flux of Cl- were affected by SITS. In the absence of electrochemical gradients, net Cl- absorption was observed only in the presence of HCO3- in the media; under such conditions, unidirectional and net fluxes of Cl- were not altered by serosal or mucosal SITS. H+ secretion, however, measured simultaneously as the short-circuit current in ouabain-treated bladders decreased markedly after serosal SITS. The inhibition of the efflux of HCO3- in series with the H+ pump and the failure of SITS to affect HCO3- secretion and Cl- absorption suggest that the epithelium contains at least two types of transport systems for bicarbonate in the serosal membrane. © 1979 Springer-Verlag New York Inc.