Cyclic AMP and intracellular ionic activities innecturus gallbladder

Summary Open-tip and liquid ion-exchanger microelectrodes were used to study the effects of cAMP (6mm, added to the serosal medium) on apical membrane potential (E_m) and intracellular sodium, potassium, and chloride activities (a_Naⁱ,a_Kⁱ,a_Clⁱ) inNecturus gallbladder under open-circuit conditions. Transepithelial potential difference (E_Tr) was also measured. In the presence of cAMP,a_Clⁱ fell from about 1.5 times its equilibrium value to a level that corresponded to electrochemical equilibrium across the apical and basolateral cell membranes. Under these conditionsa_Naⁱ decreased anda_Kⁱ increased,E_m was unchanged andE_Tr increased from virtually zero to a small but significant serosal positive value. The cAMP-induced increase ina_Kⁱ was abolished when Cl^–-free incubation media were used. Addition of the Ca⁺⁺-ionophore A23187 (0.5 g/ml) to the serosal medium had no effect onE_m,E_Tr, ora_Clⁱ. When A23187 was added to the mucosal medium,E_m and the basolateral membrane potential hyperpolarized by about 20 mV and an increase in the outwardly directed electrochemical driving force for Cl^– was observed. These results indicate that cAMP inhibits coupled transapical Na–Cl entry into epithelial cells ofNecturus gallbladder and suggest that this inhibition may not be mediated by an increase in intracellular Ca⁺⁺ concentration.