Dose-dependent recruitment of pancreatic acinar cells during receptor-mediated calcium mobilization.
Digital-imaging microscopy of Fura-2-loaded rabbit pancreatic acinar cells was used to simultaneously monitor the cholecystokinin-octapeptide (CCK8)-induced changes in free cytosolic Ca2+ concentration, [Ca2+]i, in large numbers of individual acinar cells. CCK8 typically evoked a switchlike increase in [Ca2+]i which was preceded by a concentration-dependent latency. The threshold concentration for the CCK8-induced rise in [Ca2+]i differed greatly among individual acinar cells, resulting in the dose-dependent recruitment of acinar cells in terms of CCK8-induced Ca2+ mobilization. The EC50 value for CCK8-induced cell-recruitment was estimated to be 15 pM. The hormone was equally potent in stimulating amylase secretion from acinar cells in suspension. At a CCK8 concentration of 100 pM, virtually all cells responded to the hormone with an increase in [Ca2+]i and the number of responding cells remained unchanged upon further increase of the CCK8 concentration. The dose-response curve for cell-recruitment coincides with that of the apparent [Ca2+]i increase in a suspension of acinar cells. This suggests that the most likely interpretation of the latter dose-response curve is not a generalized increase in [Ca2+]i but an increase in the number of responding cells. The initial rise in [Ca2+]i, which was transient by nature, was followed by repetitive [Ca2+]i transients of long duration. The dose-response curve for the effect of CCK8 on the percentage of acinar cells displaying these distinct [Ca2+]i oscillations was biphasic. A maximum of 99% of the cells showing oscillatory behaviour was reached at 100 pM CCK8, beyond which concentration the number of oscillating cells dose-dependently decreased again. The latter decrease was paralleled by a dose-dependent increase of the percentage responding but non-oscillating cells, indicating that beyond 100 pM CCK8 an increasing number of acinar cells became desensitized towards hormonal induction of oscillatory changes in [Ca2+]i. CCK8 was approximately 100-fold more potent in reducing the percentage of oscillating cells than in inhibiting amylase secretion. Oscillating acinar cells responded to a stepwise increase of the medium CCK8 concentration with a rapid change in amplitude and frequency of the oscillations. Thus, with increasing CCK8 concentration the frequency gradually increased, whereas the amplitude only slightly increased at first, reached a maximum, and decreased thereafter. In some cells full extinction was reached. Again, large differences in dose-dependency were observed among individual acinar cells. The observations presented demonstrate the existence of a marked functional heterogeneity among pancreatic acinar cells in terms of CCK8-induced Ca2+ mobilization.
Willems
P H
PH
Department of Biochemistry, University of Nijmegen, The Netherlands.
Van Emst-De Vries
S E
SE
Van Os
C H
CH
De Pont
J J
JJ
D000681:Amylases / Q000378:metabolism; D000703:Analog-Digital Conversion; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D016257:Fura-2 / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D008856:Microscopy, Fluorescence; D010179:Pancreas / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D010784:Photomicrography / Q000379:methods; D011817:Rabbits; D011949:Receptors, Cholecystokinin / Q000187:drug effects* / Q000378:metabolism; D012844:Sincalide / Q000494:pharmacology*; D014741:Video Recording / Q000379:methods
D011949:Receptors, Cholecystokinin; D000681:Amylases; D012844:Sincalide; D002118:Calcium; D016257:Fura-2