Effects of Cholecystokinin on Cytosolic Calcium in Pancreatic Duct Segments and Ductal Cells

Although the gastrointestinal peptide cholecystokinin (CCK) has been shown to increase bicarbonate and water secretion and potentiate the effects of secretin on pancreatic ducts, CCK receptors have not been identified on pancreatic ductal epithelium. The effects of CCK octapeptide (CCK-8) on cytosolic calcium were evaluated on microscopically dissected rat pancreatic duct segments and single rat duct cells from the established ARIP cell line. By use of fluorescence microscopy in fura-2-loaded duct segments or single cells, intracellular calcium concentration was measured in response to CCK-8 (4 x 10(-12)-4 x 10(-8) M). CCK-8 significantly increased cytosolic calcium up to 50-fold over baseline. The greatest increase occurred with the highest concentration of CCK-8 (4 x 10(-8) M). Oscillations were observed in experiments performed in buffer containing 0.68 mM physiological calcium. In another series of experiments performed in the presence of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid to deplete extracellular calcium, CCK-8 treatment still resulted in significant increases in cytosolic calcium; however, oscillations were abolished. Since cytosolic calcium increased in the absence of extracellular calcium, the initial calcium rise most likely was from cytosolic stores. Our findings of CCK-8-stimulated increases in cytosolic calcium in microdissected pancreatic ducts suggest the presence of CCK receptors, a characteristic that was not lost in cultured pancreatic ductal cells. In addition, in ARIP cells, the CCK-8-induced increase in cytosolic calcium was abolished by pretreatment with the selective CCK-B receptor antagonist L-365,260 but not by the CCK-A receptor antagonist L-364,718.