Cholecystokinin (CCK) is one of several active peptides that have been isolated from both the gut1 and the brain2. CCK evokes membrane depolarization and conductance increase in both pancreatic acinar cells3,4 and pyramidal neurones of hippocampus5. In the acinar cells, activation of cholinergic muscarinic and two different peptidergic receptor sites (for CCK and bombesin) result in the same type of conductance increase6. The application of intracellular Ca mimics the action of the agonists7 and Ca has been proposed as an intracellular mediator for the membrane conductance increase8. A cation channel activated by internal Ca has been directly demonstrated in single-channel current recording experiments on isolated patches of plasma membrane from the basolateral surface of pancreatic acini9. We now demonstrate that CCK and acetylcholine (ACh) can activate these inward current channels indirectly. Single-channel currents were observed in electrically isolated membrane patches in situ when micropipette application of CCK or ACh was made outside the patch area. Single-channel currents from the same membrane patches were subsequently recorded after they had been excised (inside-out) and identical values for the single-channel conductance were obtained (∼35 pS). No effect of CCK was obtained in the presence of the specific receptor antagonist dibutyryl cyclic GMP10-12 although in all such cases single-channel currents were subsequently recorded from the excised patch (Ca-activated). These results demonstrate directly that CCK and ACh control ion channel opening via an intracellular messenger (Ca).
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