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

Abstract

In polarized epithelial cells [Ca²⁺]_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 [Ca²⁺]_i imaging at high temporal resolution, recording of Ca²⁺-activated Cl⁻ current, and immunolocalization by confocal microscopy to study the correlation between initiation and propagation of [Ca²⁺]_iwaves and localization of Ca²⁺ release channels in pancreatic acini and submandibular acinar and duct cells. In all cells Ca²⁺ 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 (IP₃Rs). Expression of IP₃Rs 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 IP₃R3 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 Ca²⁺ from internal stores of submandibular salivary gland but not pancreatic acinar cells. Measurement of [Ca²⁺]_i and localization of IP₃Rs in the same cells suggests that only a small part of IP₃Rs participate in the initiation of the Ca²⁺ wave, whereas most receptors in the cell periphery probably facilitate the propagation of the Ca²⁺ wave. The combined results together with our previous studies on this subject lead us to conclude that the internal Ca²⁺ pool is highly compartmentalized and that compartmentalization is achieved in part by polarized expression of Ca²⁺ channels.