CHOLECYSTOKININ-INDUCED INHIBITION OF ENDOCYTOSIS OF RECEPTOR-BOUND SUBSTANCE-P IN PANCREATIC ACINAR-CELLS

Association of I-125-Bolton-Hunter labelled substance P (I-125-BH-SP) to suspended pancreatic acinar cells of the guinea pig was studied. Cellular association at 37-degrees-C and 22-degrees-C was inhibited by cholecystokinin octapeptide (CCK-8) in concentrations from 10(-9) to 10(-6)M, whereas another pancreatic secretagogue, carbachol, was uneffective. The CCK induced inhibition disappeared at low temperatures. CCK-8 mainly interfered with internalization of I-125-BH-SP into acinar cells. Increased extracellular Ca2+ and the Ca2+ ionophores A23187 and ionomycin reduced association of I-125-BH-SP to cells whereas extracellular Ca2+ chelation with EGTA had the opposite effect. However, extra- and intracellular Ca2+ chelation did not affect the degree of CCK-induced reduction of I-125-B-SP association to acinar cells but eliminated the effect of the calcium ionophore ionomycin. Three agents known to interfere with receptor recycling, namely monensin, methylamine and ammonium chloride reduced cell-associated I-125-BH-SP. In a series of experiments, the cytoplasmic calcium concentrations ([Ca2+]i) during exposure to these three agents, to the CCK-8-analogue caerulein and to ionomycin were determined. In all cases, (Ca2+]i was raised. The results indicate that endocytosis of receptor-bound I-125-BH-SP is regulated by CCK and that the endocytotic process is influenced by calcium.