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 HCO3- 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.
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