Cyclic AMP and intracellular ionic activities innecturus gallbladder
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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 Na i ,a K i ,a Cl i ) inNecturus gallbladder under open-circuit conditions. Transepithelial potential difference (E Tr ) was also measured. In the presence of cAMP,a Cl i 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 Na i decreased anda K i increased,E m was unchanged andE Tr increased from virtually zero to a small but significant serosal positive value. The cAMP-induced increase ina K i 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 Cl i . 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.
Key wordsNecturus gallbladder cyclic AMP calcium ionophore intracellular ionic activities membrane potentials liquid ion-exchanger microelectrodes
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