Polarized Expression of Ca2+ Channels in Pancreatic and Salivary Gland Cells

Abstract

In polarized epithelial cells [Ca 2؉ ] i waves are initiated in discrete regions and propagate through the cytosol. The structural basis for these compartmentalized and coordinated events are not well understood. In the present study we used a combination of [Ca 2؉ ] i imaging at high temporal resolution, recording of Ca 2؉ -activated Cl ؊ current, and immunolocalization by confocal microscopy to study the correlation between initiation and propagation of [Ca 2؉ ] i waves and localization of Ca 2؉ release channels in pancreatic acini and submandibular acinar and duct cells. In all cells Ca 2؉ waves are initiated in the luminal pole and propagate through the cell periphery to the basal pole. All three cell types express the three known inositol 1,4,5-trisphosphate receptors (IP 3 Rs). Expression of IP 3 Rs was confined to the area just underneath the luminal and lateral membranes, with no detectable receptors in the basal pole or other regions of the cells. In pancreatic acini and SMG ducts IP 3 R3 was also found in the nuclear envelope. Expression of ryanodine receptor was detected in submandibular salivary gland cells but not pancreatic acini. Accordingly, cyclic ADP ribose was very effective in mobilizing Ca 2؉ from internal stores of submandibular salivary gland but not pancreatic acinar cells. Measurement of [Ca 2؉ ] i and localization of IP 3 Rs in the same cells suggests that only a small part of IP 3 Rs participate in the initiation of the Ca 2؉ wave, whereas most receptors in the cell periphery probably facilitate the propagation of the Ca 2؉ wave. The combined results together with our previous studies on this subject lead us to conclude that the internal Ca 2؉ pool is highly compartmentalized and that compartmentalization is achieved in part by polarized expression of Ca 2؉ channels.

Polarized Expression of Ca2+ Pumps in Pancreatic and Salivary Gland Cells

“…The results with the mAb were not conclusive, because of low signal/noise with this mAb in SMG cells (not shown). The pattern of expression of SERCA2b in SMG cells resembled the labeling obtained with all Ca 2ϩ release channels (7).…”

“…The most problematic aspect of the mechanism described above is that we could not find Ca 2ϩ release channels in any part of the cells, except underneath the luminal and lateral membranes and in the nuclear envelope (7). This raises the question of how [Ca 2ϩ ] i increases uniformly in all parts of the cells without redistribution of Ca 2ϩ release channels.…”

“…Thus, the cells express all three types of IP 3 R and the SMG cells also express the ryanodine receptor. Expression of the receptors appears to be confined to the area just underneath the luminal and lateral membranes (7).…”

“…In the preceding manuscript (7) we showed similar phenomena in SMG acinar and duct cells. Characterization and localization of Ca 2ϩ release channels in the three cell types (7) suggests that the channels determine the initiation site and pattern of the [Ca 2ϩ ] i waves. Thus, the cells express all three types of IP 3 R and the SMG cells also express the ryanodine receptor.…”

Biosynthesis of inositol trisphosphate receptors: selective association with the molecular chaperone calnexin

“…The primary sequences also show differences in consensus phosphorylation and Nglycosylation sites [9]. In addition, it has been suggested that individual isoforms may be localized to discrete regions of the cell [10][11][12] or that specific isoforms may regulate distinct cellular processes such as Ca# + entry across the plasma membrane [10] or apoptosis [11,13]. Overall, the presence of differentially regulated IP $ R isoforms with the ability to form hetero-oligomers provides the cell with considerable flexibility in the regulation of Ca# + signals.…”