SUPRAMAXIMAL INHIBITION OF CHOLECYSTOKININ-INDUCED PANCREATIC AMYLASE RELEASE INVOLVES DESENSITIZATION TO CYTOPLASMIC CA2+

Background: Cholecystokinin (CCK) is a major stimulant of pancreatic enzyme secretion. The dose-response relationship for CCK-induced secretion is bell-shaped, with a characteristic supramaximal inhibition. The mechanism for this inhibition has now been studied. Methods: The kinetics of amylase release and the changes of the cytoplasmic Ca2+ concentration ([Ca2+](i)) were recorded during stimulation of guinea-pig pancreatic acinar cells with different concentrations of cholecystokinin octapeptide (CCK-8) and the Ca2+ ionophore ionomycin. Results: Individual cells reacted with [Ca2+](i) oscillations at 10(-11)-10(-10) M CCK-8 and with an initial peak followed by a sustained suprabasal level at 10(-9)-10(-8) M of the agonist. The latter response was also seen in suspensions of acinar cells at all tested concentrations of CCK-8 and at 10(-6)-10(-5) M Of ionomycin. With increases of extracellular Ca2+ from 0.5 to 5.0 mM there was a rise of [Ca2+](i) during exposure to 10(-9)-10(-8) M CCK-8 or 10(-5) M ionomycin but a paradoxical decrease at lower concentrations of CCK-8 or ionomycin. A dose-dependent increase of amylase release was seen at CCK-8 concentrations from 10(-11) to 10(-9) M. At 10(-9)-10(-8) M CCK-EI secretion was characterized by an initial peak followed by a sustained phase. Whereas the initial peak of secretion remained unaffected by increasing CCK-8 from 10(-9) to 10(-8) M, the sustained phase was inhibited (supramaximal inhibition). Increasing extracellular Ca2+ from 0.5 to 5.0 mM transiently enhanced secretion in response to 10(-9) M but lacked effect during supramaximal inhibition of secretion by 10(-8) M CCK-8. Conclusions: Both initial and sustained CCK-8-stimulated amylase release increase with [Ca2+](i). However, supramaximal inhibition of secretion was not due to a decrease of [Ca2+](i) but was characterized by desensitization to the stimulatory effect of [Ca2+](i).