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-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(-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.
Nilsson
J
J
Dept. of Medical Cell Biology, Uppsala University, Sweden.
Sjödin
L
L
Gylfe
E
E
D000681:Amylases / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000032:analysis / Q000502:physiology*; D002766:Cholecystokinin / Q000494:pharmacology*; D003593:Cytoplasm / Q000737:chemistry*; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D066298:In Vitro Techniques; D015759:Ionomycin / Q000494:pharmacology; D010179:Pancreas / Q000187:drug effects / Q000201:enzymology*; D012844:Sincalide / Q000494:pharmacology
D015759:Ionomycin; D002766:Cholecystokinin; D000681:Amylases; D012844:Sincalide; D002118:Calcium