Apical Vesicles Bearing Inositol 1,4,5-trisphosphate Receptors in the Ca(2+)Initiation Site of Ductal Epithelium of Submandibular Gland

In polarized epithelial cells, agonists trigger Ca(2+) waves and oscillations. These patterns may be caused by the compartmentalization of inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) pools into specific regions. We have investigated the relationship between the distribution of IP(3) receptors (IP(3)Rs) and the spatiotemporal pattern of Ca(2+) signaling in the duct cells of the rat submandibular gland (SMG). Using immunofluorescence, although labeling was somewhat heterogeneous, the IP(3)Rs were colocalized to the apical pole of the duct cells. Immunoelectron microscopy identified small apical vesicles bearing IP(3)R2 in some types of duct cells. Real-time confocal imaging of intact ducts demonstrated that, after carbachol stimulation, an initial Ca(2+) spike occurred in the apical region. Subsequently, repetitive Ca(2+) spikes spread from the apical to the middle cytoplasm. These apical Ca(2+) initiation sites were found only in some “pioneer cells,” rather than in all duct cells. We performed both Ca(2+) imaging and immunofluorescence on the same ducts and detected the strongest immunosignals of IP(3)R2 in the Ca(2+) initiation sites of the pioneer cells. The subcellular localization and expression level of IP(3)Rs correlated strongly with the spatiotemporal nature of the intracellular Ca(2+) signal and distinct Ca(2+) responses among the rat SMG duct cells.