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 (CCK 8)-induced changes in free cytosolic Ca 2+ concentration, [Ca 2+] i, in large numbers of individual acinar cells. CCK 8 typically evoked a switchlike increase in [Ca 2+] i which was preceded by a concentration-dependent latency. The threshold concentration for the CCK 8-induced rise in [Ca 2+] i differed greatly among individual acinar cells, resulting in the dose-dependent recruitment of acinar cells in terms of CCK 8-induced Ca 2+ mobilization. The EC 50 value for CCK 8-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 CCK 8 concentration of 100 pM, virtually all cells responded to the hormone with an increase in [Ca 2+] i and the number of responding cells remained unchanged upon further increase of the CCK 8 concentration. The dose-response curve for cell-recruitment coincides with that of the apparent [Ca 2+] 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 [Ca 2+] i but an increase in the number of responding cells. The initial rise in [Ca 2+] i, which was transient by nature, was followed by repetitive [Ca 2+] i transients of long duration. The dose-response curve for the effect of CCK 8 on the percentage of acinar cells displaying these distinct [Ca 2+] i oscillations was biphasic. A maximum of 99% of the cells showing oscillatory behaviour was reached at 100 pM CCK 8, 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 CCK 8 an increasing number of acinar cells became desensitized towards hormonal induction of oscillatory changes in [Ca 2+] i. CCK 8 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 CCK 8 concentration with a rapid change in amplitude and frequency of the oscillations. Thus, with increasing CCK 8 concentration the frequency gradually increased, whereas the amplitude only slightly increased at first, reached a maximum, and decreased thereafter.