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.