Cyclic AMP inhibits Na+/H+ exchange at the apical membrane of Necturus gallbladder epithelium.

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Abstract
The effects of elevating intracellular cAMP levels on Na+ transport across the apical memebrane of Necturus gallbladder epithelium were studied by intracellular and extracellular microelectrode techniques. Intracellular cAMP was raised by serosal addition of the phosphodiesterase inhibitor theophylline (3 mM) or mucosal addition of either 8-Br-cAMP (1 mM) or the adenylate cyclase activator forskolin (10 .mu.M). During elevation of intracellular cAMP, intracellular Na+ activity (aNai) and intracellular pH (phi) decreased significantly. Acidification of the mucosal solution, which contained either 100 or 10 mM Na+, was inhibited by .apprx. 50%. The inhibition was independent of the presence of Cl- in the bathing media. The rates of change of aNai upon rapid alterations of mucosal [Na+] from 100 to 10 mM and from 10 to 100 mM were both decreased, and the rate of pHi recovery upon acid loading was also reduced by elevated cAMP levels. Inhibition was .apprx. 50% for all of these processes. cAMP inhibits apical membrane Na+/H+ exchange. The results of measurements of pHi recovery at 10 and 100 mM mucosal [Na+] and a kinetic analysis of recovery as a function of pHi suggest that the main or sole mechanism of the inhibitory effect of cAMP is a reduction in the maximal rate of acid extrusion. In conjunction with the increase in apical membrane electrodiffusional Cl- permeability, produced by cAMP, which causes a decrease in net Cl- entry (Petersen, K.-U., and L. Reuss, 1983), inhibition of Na+/H+ exchange contributes to the reduction of fluid absorption elicited by this agent. Similar mechanisms may account for the effects of cAMP in other epithelia with similar transport properties. Inhibition of Na+/H+ exchange by cAMP may play a role in the regulation of pHi in other cell types.