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 HCO3- generated behind the H+ pump moves 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 x 10(-4) M 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 leads to S Cl- flux after HCO3- addition. Neither the S leads to M flux of HCO3- nor the M leads to 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.

Chloride distribution in the proximal convoluted tubule ofNecturus kidney

“…The inhibitory effect of SITS on transmembrane anion exchange in PCT appears to be small, as estimated from the magnitude of the observed eCl changes. This is not surprising in view of the disparity in responsiveness to disulfonic stilbenes among various tissues: these compounds completely inhibit sulfate fluxes in red blood cells (Cabantchik & Rothstein, 1974), but fail to affect HCO~ secretion and C1 absorption in turtle bladder (Husted, Cohen & Steinmetz, 1979) The occurrence of an uphill entry of C1-into the cell across the basolateral membrane, i.e., against the direction of net NaC1 absorption, may appear at first sight as a useless and energetically wasteful process. To understand its possible role, one must consider in addition the unique pattern of transepithelial C1-distribution in PCT: luminal C1-activity is higher than the theoretical c~ c~ predicted for transepithelial equilibrium in both amphibian and mammalian PCT.…”

Cyclic adenosine monophosphate-stimulated anion transport in rabbit cortical collecting duct. Kinetics, stoichiometry, and conductive pathways.

“…In the absence of cyclic adenosine monophosphate (cAMP), it appears that this HCO3 secretion occurs in both epithelia via 1:1 electroneutral Cl-HCO3 exchange (1)(2)(3)(4). Addition ofbasolateral HCO3 thus induces HCO3 secretion and Cl reabsorption of equal and opposite magnitude (1)(2)(3).…”

“…In the absence of cyclic adenosine monophosphate (cAMP), it appears that this HCO3 secretion occurs in both epithelia via 1:1 electroneutral Cl-HCO3 exchange (1)(2)(3)(4). Addition ofbasolateral HCO3 thus induces HCO3 secretion and Cl reabsorption of equal and opposite magnitude (1)(2)(3). Evidence obtained in the cortical collecting tubule suggests that: (a) the responsible anion exchanger is located at the apical membrane (lowering basolateral Cl concentration acidifies the majority of intercalated cells, presumably by favoring apical Cl entry into and exchange-mediated HCO3 exit out of the cell) (4), and (b) Cl brought into the cell by this exchanger probably exits via a basolateral Cl conductance (5).…”

Electrogenic proton transport in epithelial membranes

“…The current required to nullify this reversed PD is equal to the rate of H + secretion measured simultaneously by pH stat titration. Studies by Schwartz (1976) and Husted, Cohen and Steinmetz (1979) indicate that this equality is preserved in a wide variety of experimental conditions. These studies suggest but do not prove that the acidification pump is electrogenic.…”