Initiation Site of Ca2+ Entry Evoked by Endoplasmic Reticulum Ca2+ Depletion in Mouse Parotid and Pancreatic Acinar Cells

Abstract

PurposeIn 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 MethodsIn 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.ResultsTreatment 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.ConclusionThese 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.