Agonist‐induced changes in cell membrane capacitance and conductance in dialysed pancreatic acinar cells of rats.

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Abstract
1. Single acinar cells enzymatically isolated from the rat pancreas were subjected to tight-seal whole-cell recordings. Changes in cell membrane capacitance and conductance were simultaneously recorded using a phase-sensitive detection method. 2. Acethylcholine (ACh, 0.05-0.5 .mu.M) and cholecystokinin octapeptide (CCK-8, 10-50 pM) concomitantly induced transient increases in cell membrane current, capacitance and conductance only when cytosolic Ca2+ was weakly chelated by EGTA (70 .mu.M). These responses were prolonged when the cells were dialysed with a solution containing GTP.gamma.S (a stable analogue of GTP, 50-100 .mu.M), whereas they were inhibited by dialysing with that containing GDP.beta.S (a stable analogue of GDP). These results suggest that a type of guanine-nucleotide-binding protein (G-protein) could be involved in ACh- or CCK-receptor signalling. 3. The ACh- or CCK-induced responses (with or without GTP.gamma.S in the cytosol) were all abolished when a high dose of EGTA (1-2 mM) was injected into the acinar cells. In addition, A23187, a calcium ionophore, induced sustained responses when the cytosolic Ca2+ was weakly buffered by 70 .mu.M-EGTA. These results suggest that the secretagogues regulate the changes in cell membrane capacitance and conductance via an increase and decrease of cytosolic Ca2+ concentration. 4. Oscillatory changes in cell membrane conductance and capacitance were consistently observed even without applying secretagogues when the cells were dialysed with a solution containing GTP.gamma.S (50-100 .mu.M) and cytosolic free Ca2+ ions weakly buffered at about 10-6 M with a low dose of EGTA and CaCl2. 5. The peak amplitude of changes in cell membrane capacitance induced by ACh or CCK-8, with or without GTP.gamma.S in the cytosol, varied between 200 and 1000 fF, thereby suggesting that 20-100 zymogen granules can fuse with the luminal cell membrane in response to these agonists in rat exocrine pancreatic acinar cells.

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