Pathways for bicarbonate transfer across the serosal membrane of turtle urinary bladder: Studies with a disulfonic stilbene

Abstract

Bicarbonate is transferred across the serosal (S) membrane of the epithelial cells of the turtle bladder in two directions. Cellular HCO ₃ ⁻ generated behind the H⁺ pump moves across this membrane into the serosal solution. This efflux of HCO ₃ ⁻ is inhibited by SITS (4-isothiocyano-4′-acetamido-2,2′-disulfonic stilbene). When HCO ₃ ⁻ is added to the serosal solution it is transported across the epithelium in exchange for absorbed Cl⁻. This secretory HCO ₃ ⁻ flow traverses the serosal cell membrane in the opposite direction. In this study the effects of serosal addition of 5×10⁻⁴ m SITS on HCO ₃ ⁻ secretion and Cl⁻ absorption were examined. The rate of H⁺ secretion was brought to zero by an opposing pH gradient, and 20mm HCO ₃ ⁻ was added toS. HCO ₃ ⁻ secretion, measured by pH stat titration, was equivalent to the increase inM→S Cl⁻ flux after HCO ₃ ⁻ addition. Neither theS→M flux of HCO ₃ ⁻ nor theM→S flux of Cl⁻ were affected by SITS. In the absence of electrochemical gradients, net Cl⁻ absorption was observed only in the presence of HCO ₃ ⁻ 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 HCO ₃ ⁻ in series with the H⁺ pump and the failure of SITS to affect HCO ₃ ⁻ secretion and Cl⁻ absorption suggest that the epithelium contains at least two types of transport systems for bicarbonate in the serosal membrane.