Activation of epithelial Na(+) channel activity in the rabbit urinary bladder by cAMP

The rabbit urinary bladder actively absorbs Na(+) from the urine. The rate-limiting step in this process is the diffusion of Na(+) across the apical membrane of bladder epithelial cells, mediated by amiloride-sensitive epithelial Na(+) channels. We have investigated the effects of cAMP on epithelial Na(+) channel activity in the rabbit bladder by measuring the amiloride-sensitive short-circuit current across bladders mounted in Ussing chambers. Three agents that raise intracellular cAMP levels (forskolin, dibutyryl-cAMP and 3-isobutyl-1-methylxanthine (IBMX)) increased the amiloride-sensitive short-circuit current relative to control preparations. The forskolin-induced increase in amiloride-sensitive short-circuit current was significantly inhibited by the vesicle fusion inhibitor brefeldin A and the protein synthesis inhibitor cycloheximide. These findings, together with the magnitude and protracted time course of the cAMP effects, suggests that cAMP stimulates the insertion of new Na(+) channels into the apical membrane of the rabbit bladder epithelium.