REGION-SPECIFIC ACTIVITY OF THE PLASMA-MEMBRANE CA2+ PUMP AND DELAYED ACTIVATION OF CA2+ ENTRY CHARACTERIZE THE POLARIZED, AGONIST-EVOKED CA2+ SIGNALS IN EXOCRINE CELLS

The initial release of Ca2+ from the intracellular Ca2+ stores is followed by a second phase during which the agonist-dependent Ca2+ response becomes sensitive to the extracellular Ca2+, indicating the involvement of the plasma membrane (PM) Ca2+ transport systems. The time course of activation of these transport systems, which consist of both Ca2+ extrusion and Ca2+ entry pathways, is not well established. To investigate the participation of these processes during the agonist-evoked Ca2+ response, isolated pancreatic acinar cells were exposed to maximal concentrations of an inositol 1,4,5-trisphosphate-mobilizing agonist (acetylcholine, 10 mu M) in different experimental conditions. Following the increase of [Ca2+](i), there was an almost immediate activation of the PM Ca2+ extrusion system, and maximal activity was reached within less than 2 s. The rate of Ca2+ extrusion was dependent on the level of [Ca2+](i), with a steep activation at values just above the resting [Ca2+](i) and reached a plateau value at 700 nM Ca2+. In contrast, the PM Ca2+ entry pathway was activated with a much slower time course. There was also a delay of 3-4 s between the maximal effective depletion of the intracellular Ca2+ stores and the activation of this entry pathway. By use of digital imaging data, the PM Ca2+ transport systems were also analyzed independently in two regions of the cells, the lumenal and the basal poles. With respect to the activation of the Ca2+ entry path ways, no significant difference existed between these two regions. In contrast, the PM Ca2+ pump displayed a different pattern of activity in these regions. In the basal pole, the pump activity was more sensitive to changes of [Ca2+](i) and had a higher maximal activity. Also, in the lumenal pole, the pump became saturated at values of [Ca2+](i) around 700 nM, whereas at the basal pole [Ca2+](i) had a biphasic effect on the pump activity, and higher [Ca2+](i) inhibited the pump. It is argued that these differences in sensitivity to the levels of [Ca2+](i) and the different relationship between [Ca2+](i) and the rate of extrusion at the two functional poles of the pancreatic acinar cells indicate that the plasma membrane Ca(2+)ATPase might play an important role in the polarization of the Ca2+ response.