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Summary Enhanced cellular cAMP levels have been shown to increase apical membrane Cl− and HCO 3 − 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 HCO 3 − -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 Cl i ). Under control conditions,a Cl i was above the value corresponding to passive distribution across the apical cell membrane. In media containing 25mM HCO 3 − , IBMX caused a small transient hyperpolarization ofV a followed by a depolarization not significantly different from that observed in HCO 3 − -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 HCO 3 − 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 .
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