Cyclic AMP-induced changes in membrane conductance ofNecturus gallbladder epithelial cells

Summary Enhanced cellular cAMP levels have been shown to increase apical membrane Cl^– and HCO₃^– conductances in epithelia. We found that the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) increases cAMP levels inNecturus gallbladder. We used conventional open-tip and double-barreled Cl^–-selective microelectrodes to study the effects of IBMX on membrane conductances and intracellular Cl^– activities in gallbladders mounted in a divided chamber and bathed with Ringer's solutions at 23°C and pH 7.4. In HCO₃^–-free media, 0.1mM IBMX added to the mucosal medium depolarized the apical membrane potentialV_a, decreased the fractional resistanceF_R, and significantly reduced intracellular Cl^– activity (a_Clⁱ). Under control conditions,a_Clⁱ was above the value corresponding to passive distribution across the apical cell membrane. In media containing 25mM HCO₃^–, IBMX caused a small transient hyperpolarization ofV_a followed by a depolarization not significantly different from that observed in HCO₃^–-free Ringer's. Removal of mucosal Cl^–, Na⁺ or Ca²⁺ did not affect the IBMX-induced depolarization inV_a. The basolateral membrane ofNecturus gallbladder is highly K⁺ permeable. Increasing serosal K⁺ from 2.5 to 80mM, depolarizedV_a. Mucosal IBMX significantly reduced this depolarization. Addition of 10mM Ba²⁺, a K⁺ channel blocker, to the serosal medium depolarizedV_a and, essentially, blocked the depolarization induced by IBMX. These results indicate that mucosal IBMX increases apical HCO₃^– conductance and decreases basolateral K⁺ conductance in gallbladder epithelial cells via a cAMP-dependent mechanism. The latter effect, not previously reported in epithelial tissues, appears to be the major determinant of the IBMX-induced depolarization ofV_a.