Cyclic adenosine monophosphate-stimulated bicarbonate secretion in rabbit cortical collecting tubules

We studied the effects of cyclic AMP (cAMP) on HCO₃^- transport by rabbit cortical collecting tubules perfused in vitro. Net HCO₃^- secretion was observed in tubules from NaHCO₃^- loaded rabbits. 8-Bromo-cAMP-stimulated net HCO₃^- secretion, whereas secretion fell with time in control tubules. Both isoproterenol and vasopressin (ADH) are known to stimulate adenylate cyclase in this epithelium; however, only isoproterenol stimulated net HCO₃^- secretion. The mechanism of cAMP-stimulated HCO₃^- secretion was examined. If both HCO_3- and H⁺ secretion were to occur simultaneously in tubules exhibiting net HCO₃^- secretion, cAMP might increase the net HCO₃^- secretory rate by inhibiting H⁺ secretion, by stimulating HCO₃^- secretion, or both. These possibilities were examined using basolateral addition of the disulfonic stilbene (4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS). In acidifying tubules from NH₄Cl-loaded rabbits, DIDS eliminated HCO₃^- reabsorption, a result consistent with known effects of DIDS as an inhibitor of H⁺ secretion. In contrast, cAMP left acidification (H⁺ secretion) intact. DIDS applied to HCO₃^- secretory tubules failed to increase the HCO₃^- secretory rate, indicating minimal H⁺ secretion in HCO₃^- secreting tubules. Thus, inhibition of H⁺ secretion by cAMP could not account for the cAMP-induced stimulation of net HCO₃^- secretion. cAMP-stimulated HCO₃^- secretion was reversibly eliminated by 0 Cl perfusate, whereas luminal DIDS had no effect. Bath amiloride (1 mM) failed to eliminate cAMP-stimulated HCO₃^- secretion when bath [Na⁺] was 145 mM or 5 mM. cAMP depolarized the transepithelial voltage. The collected fluid [HCO₃^-] after cAMP could be accounted for by electrical driving forces, suggesting that cAMP stimulates passive HCO₃^- secretion. However, cAMP did not alter HCO₃^- permeability measured under conditions expected to inhibit transcellular HCO₃^- movement (0 Cl^- solutions and bath DIDS). This measured HCO₃^- permeability was not high enough to account, by passive diffusion, for the HCO₃^- fluxes observed in Cl^- containing solutions. We conclude the following: 1) cAMP increased net HCO₃^- secretion by stimulating HCO₃^- secretion and not by inhibiting H⁺ secretion; 2) this HCO₃^- secretion may have occurred by Cl-HCO₃^- exchange; 3) Na⁺-H⁺ exchange appeared not to play a role in basolateral H⁺ extrusion under these conditions; and 4) the stimulation of HCO₃^- secretion by isoproterenol, but not ADH, suggests the existence of separate cell cAMP pools or cellular heterogeneity in this cAMP response.