A double-membrane model for urinary bicarbonate secretion

Abstract

To define the transport pathway for HCO-3 secretion (JHCO3) across the apical and basolateral membranes of turtle bladder, we examined the effects of cAMP, isobutylmethylxanthine (IBMX), the Cl- channel blocker 9-anthroic acid (9-AA), and the disulfonic stilbene DIDS (4,4'-diisothiocyanostilbene-2,2'-sulfonic acid) on the electroneutral and electrogenic components of JHCO3. Total JHCO3 was measured by pH stat titration of the mucosal compartment after Na+ absorption and H+ secretion were abolished by ouabain and a delta pH, respectively. Addition of cAMP or IBMX increased total JHCO3 and induced a short-circuit current (ISC), accounting for a large part of JHCO3; net Cl- absorption was reduced. Mucosal 9-AA inhibited the IBMX-induced electrogenic component of JHCO3, whereas mucosal DIDS inhibited the electroneutral component and acetazolamide reduced both. We suggest that HCO-3 is generated within the cell by a Na-independent primary active acid-base transport at the basolateral membrane (H+ extrusion into the serosal compartment). Cellular HCO-3 accumulation drives JHCO3 via a Cl-HCO3 exchanger at the luminal membrane. IBMX and cAMP activate a 9-AA-sensitive anion conductance parallel to the exchanger. The apparent reversal of the transport elements between the two cell membranes (compared with H+-secreting cells) led to an ultrastructural examination of the carbonic anhydrase-rich cells.

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

“…In this regard, cAMP has been found to increase the serosal-tomucosal Cl flux and therefore decrease net Cl reabsorption in two separate studies in turtle bladder (6,26). If cAMP were to increase an apical Cl conductance as proposed by Stetson et al.…”

“…In the presence of a phosphodiesterase inhibitor (isobutylmethylxanthine, IBMX), Ehrenspeck found that HCO3 addition significantly increased the mean transepithelial conductance of turtle bladders from 0.35 to 0.61 mS/cm2 (25). Similarly, Satake et (6).…”

“…University ofIowa, (6,7). Because apical application of the Cl channel blocker anthracene-9-carboxylic acid reduces the effect of cAMP on electrogenic HCO3 secretion, and because cAMP reduces net Cl reabsorption, it has been suggested that the putative cAMP-induced apical anion conductance in turtle bladder is poorly selective, allowing for either: (a) electrogenic HCO3 secretion, or (b) recycling of Cl (brought into the cell by an apical Cl-HCO3 exchanger) across the apical membrane (6).…”

“…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). The overall process of Cl-HCO3 exchange thus must be considered in terms of such a "double-membrane" model (6).…”

Effect of secretion on intracellular pH regulation in isolated rat bile duct epithelial cells.

“…In different cell types Cl/HCO-exchangers may be functionally coupled with C1-channels (28)(29)(30)(31)(32)(33). In BDE cells both cAMP-and Ca++-regulated C1-channels have been recently described (34).…”