Protein kinase C activation inhibits receptor-evoked inositol trisphosphate formation and induction of cytosolic calcium oscillations by decreasing the affinity-state of the cholecystokinin receptor in pancreatic acinar cells.
Digital-imaging microscopy of Fura-2-loaded pancreatic acinar cells revealed that the C-terminal octapeptide of cholecystokinin (CCK8) dose-dependently recruited 94% of freshly isolated acinar cells in terms of receptor-evoked Ca2+ mobilization. Maximal and half-maximal cell-recruitment were reached with 0.1 nM and 16.8 pM CCK8, respectively. The upstroke of the dose-recruitment curve consisted of cells displaying oscillatory changes in free cytosolic Ca2+ concentration ([Ca2+]i). After having reached its maximum, the percentage oscillating cells dose-dependently decreased upon further increasing of the CCK8 concentration. Pretreatment of the acinar cells with 0.1 microM TPA caused a rightward shift of the dose-recruitment curve but did not change the maximal effect of CCK8 on the recruitment of oscillating cells. Half-maximal recruitment was obtained with 287 pM CCK8. This observation demonstrates that high levels of protein kinase C activation do not inhibit Ca2+ oscillations at a level downstream to receptor activation. Moreover, this observation demonstrates that protein kinase C-mediated inhibition of Ca2+ oscillations evoked by submaximal CCK8 concentrations occurs at the receptor level, converting it from a high-affinity state into a low-affinity state. This conclusion is supported by the observation that TPA completely inhibited the recruitment of acinar cells in response to the high-affinity receptor agonist JMV-180. The inhibitory action of TPA on CCK8-evoked cell-recruitment was paralleled by an inhibitory effect of the phorbol ester on the CCK8-evoked peak increase in average inositol trisphosphate concentration in a population of acinar cells. This observation indicates that low concentrations of CCK8 interact with the high-affinity CCK receptor to increase [Ca2+]i through the intermediation of inositol trisphosphate.
Willems
P H
PH
Department of Biochemistry, University of Nijmegen, The Netherlands.
Smeets
R L
RL
Bosch
R R
RR
Garner
K M
KM
Van Mackelenbergh
M G
MG
De Pont
J J
JJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000201:enzymology; D004305:Dose-Response Relationship, Drug; D015544:Inositol 1,4,5-Trisphosphate / Q000819:agonists / Q000037:antagonists & inhibitors* / Q000096:biosynthesis; D010179:Pancreas / Q000737:chemistry / Q000166:cytology*; D010507:Periodicity; D011493:Protein Kinase C / Q000502:physiology*; D011817:Rabbits; D011949:Receptors, Cholecystokinin / Q000378:metabolism*; D012815:Signal Processing, Computer-Assisted; D015398:Signal Transduction / Q000502:physiology; D012844:Sincalide / Q000031:analogs & derivatives / Q000494:pharmacology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D011949:Receptors, Cholecystokinin; C058451:JMV 180; D015544:Inositol 1,4,5-Trisphosphate; D011493:Protein Kinase C; D012844:Sincalide; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium