Phosphatidylinositol 4‐phosphate 5‐kinase (PIP5K), which is composed of three isozymes (α, β and γ), catalyzes the production of phosphatidylinositol bisphosphate (PIP2). This phospholipid functions in membrane trafficking, as an anchor for actin cytoskeletons and as a regulator of intramembranous channels/transporters. It is also a precursor of such second messengers as diacylglycerol, inositol triphosphate and phosphatidylinositol (3,4,5)‐triphosphate. In the present study, the expression and localization of endogenous PIP5Ks were examined in the three major salivary glands of young adult mice in situ. In western blotting of normal control glands, immunoreactive bands for individual PIP5Ks were detectable, with the highest density in the parotid gland and the weakest density in the submandibular gland. In immuno‐light microscopy under non‐stimulated condition, weak immunoreactivity for PIP5Kα was confined to the apical plasmalemma in parotid, but not sublingual or submandibular, acinar cells. Immunoreactivity for PIP5Kβ was weak to moderate and confined to ductal cells but not acinar cells, whereas that for PIP5Kγ was selectively and intensely detected in myoepithelial cells but not acinar cells, and it was weak in ductal cells in the three glands. In western blot of the parotid gland stimulated by isoproterenol, a β‐adrenoceptor agonist, no changes were seen in the intensity of immunoreactive bands for any of the PIP5Ks. In contrast, in immuno‐light microscopy, the apical immunoreactivity for PIP5Kα in parotid acinar cells was transiently and distinctly increased after the stimulation. The increased immunoreactivity was ultrastructurally localized on most apical microvilli and along contiguous plasma membrane, where membranous invaginations of various shapes and small vesicles were frequently found. It was thus suggested that PIP5Kα is involved in post‐exocytotic membrane dynamics via microvillous membranes. The present finding further suggests that each of the three isoforms of PIP5K functions through its product PIP2 discretely in different cells of the glands to regulate saliva secretion.

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To clarify the signal transduction mechanism in the differentiation and secretion of salivary glandular cells, the present study was attempted to examine in the submandibular gland (SMG) of mice, the expression and localization of phospholipase D1 (PLD1), one of the important effector molecules working in response to the activation of intramembranous receptors by first messengers. In immunoblotting analysis, the expression of PLD1 was high at postnatal 4 weeks (P4W) and decreased at P8W, and it was at negligible levels at newborn stage (P0W) and postnatal 2 weeks (P2W). The expression of PLD1 was greater in females, and it was suppressed by administration of testosterone to female mice. In immuno-light microscopy, immunoreactivity for PLD1 at P4W was moderate to intense, in the forms of dots and globules mainly in the apical domains of immature granular convoluted tubule (GCT)-cells localized largely in the proximal portion of the female GCT. By P8W, it decreased in intensity and remained weak to moderate along the apical plasmalemma of cells throughout the course of the female GCT, whereas it was faint throughout the GCT of the male SMG at P4W and negligible at P8W. In immuno-electron microscopy, immature GCT-cells characterized by electron-lucent granules were immunoreactive and the immunoreactive materials were deposited close to, but not within, those granules. Typical GCT cells, characterized by electron-dense granules, were immunonegative. No significant immunoreaction for PLD1 was seen in acini of SMGs of either sex at any time point examined. It is suggested that PLD1 is involved in the signaling for secretion of immature GCT cells and influences differentiation of these cells, probably through their own secretory substances.

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