Initiation site of Ca(2+) entry evoked by endoplasmic reticulum Ca(2+) depletion in mouse parotid and pancreatic acinar cells

Purpose

In non-excitable cells, which include parotid and pancreatic acinar cells, Ca²⁺ entry is triggered via a mechanism known as capacitative Ca²⁺ entry, or store-operated Ca²⁺ entry. This process is initiated by the perception of the filling state of endoplasmic reticulum (ER) and the depletion of internal Ca²⁺ stores, which acts as an important factor triggering Ca²⁺ entry. However, both the mechanism of store-mediated Ca²⁺ entry and the molecular identity of store-operated Ca²⁺ channel (SOCC) remain uncertain.

Materials and Methods

In the present study we investigated the Ca²⁺ entry initiation site evoked by depletion of ER to identify the localization of SOCC in mouse parotid and pancreatic acinar cells with microfluorometeric imaging system.

Results

Treatment with thapsigargin (Tg), an inhibitor of sarco/ endoplasmic reticulum Ca²⁺-ATPase, in an extracellular Ca²⁺ free state, and subsequent exposure to a high external calcium state evoked Ca²⁺ entry, while treatment with lanthanum, a non-specific blocker of plasma Ca²⁺ channel, completely blocked Tg-induced Ca²⁺ entry. Microfluorometric imaging showed that Tg-induced Ca²⁺ entry started at a basal membrane, not a apical membrane.

Conclusion

These results suggest that Ca²⁺ entry by depletion of the ER initiates at the basal pole in polarized exocrine cells and may help to characterize the nature of SOCC.