APICAL VESICLES BEARING INOSITOL 1,4,5-TRISPHOSPHATE RECEPTORS IN THECA2-GLAND( INITIATION SITE OF DUCTAL EPITHELIUM OF SUBMANDIBULAR)

In polarized epithelial cells, agonists trigger Ca2+ waves and oscilla tions. These patterns may be caused by the compartmentalization of ino sitol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pools into specific reg ions. We have investigated the relationship between the distribution o f IP3 receptors (IP(3)Rs) and the spatiotemporal pattern of Ca2+ signa ling in the duct cells of the rat submandibular gland (SMG), Using imm unofluorescence, although labeling was somewhat heterogeneous, the IP( 3)Rs were colocalized to the apical pole of the duct cells. Immunoelec tron microscopy identified small apical vesicles bearing IP(3)R2 in so me types of duct cells. Real-time confocal imaging of intact ducts dem onstrated that, after carbachol stimulation, an initial Ca2+ spike occ urred in the apical region. Subsequently, repetitive Ca2+ spikes sprea d from the apical to the middle cytoplasm, These apical Ca2+ initiatio n sites were found only in some ''pioneer cells,'' rather than in all duct cells. We performed both Ca2+ imaging and immunofluorescence on t he same ducts and detected the strongest immunosignals of IP(3)R2 in t he Ca2+ initiation sites of the pioneer cells. The subcellular localiz ation and expression level of IP(3)Rs correlated strongly with the spa tiotemporal nature of the intracellular Ca2+ signal and distinct Ca2responses among the rat SMG duct cells.