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 microM) 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 2s. 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 pathways, 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 Ca2+ ATPase might play an important role in the polarization of the Ca2+ response.
Toescu
E C
EC
Physiological Laboratory, Liverpool University, United Kingdom.
Petersen
O H
OH
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000819:agonists / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002462:Cell Membrane / Q000201:enzymology; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D008564:Membrane Potentials; D051379:Mice; D010179:Pancreas / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D015398:Signal Transduction
D015544:Inositol 1,4,5-Trisphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium