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Abstract
Nilsson J, Sjödin L, Gylfe E. Supramaximal inhibition of cholecystokinin-induced pancreatic amylase release involves desensitization to cytoplasmic Ca2+. Scand J Gastroenterol 1994;29:561-568.
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 10−9M. At 10−9-10−8M CCK-8 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 10−8M, 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.