The salivary glands secrete saliva, which plays a role in the maintenance of a healthy oral environment. Under physiological conditions, saliva secretion within the acinar cells of the gland is regulated by stimulation of specific calcium (Ca2+) signaling mechanisms such as increases in the intracellular Ca2+ concentration ([Ca2+]i) via storeoperated Ca2+ entry, which involves components such as Orai1, transient receptor potential (TRP) canonical 1, stromal interaction molecules, and inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs). Homer proteins are scaffold proteins that bind to G protein-coupled receptors, IP3Rs, ryanodine receptors, and TRP channels. However, their exact role in Ca2+ signaling in the salivary glands remains unknown. In the present study, we investigated the role of Homer2 in Ca2+ signaling and saliva secretion in parotid gland acinar cells under physiological conditions. Deletion of Homer2 (Homer2−/−) markedly decreased the amplitude of [Ca2+]i oscillations via the stimulation of carbachol, which is physiologically concentrated in parotid acinar cells, whereas the frequency of [Ca2+]i oscillations showed no difference between wild-type and Homer2−/− mice. Homer2−/− mice also showed a significant decrease in amylase release by carbachol in the parotid gland in a dose-dependent manner. These results suggest that Homer2 plays a critical role in maintaining [Ca2+]i concentration and secretion of saliva in mouse parotid gland acinar cells.

The effects of adenosine triphosphate(ATP) on salivary glands have been recognized since 1982. The presence of purinergic recepetors(P2Rs) that mediate the effects of ATP in various tissues, including parotid and submandibular salivary gland, has been supported by the cloning of receptor cDNAs and the expression of the receptor proteins. P2Rs have many subtypes, and the activation of these receptor subtypes increase intracellular Cα²+, a key ion in the regulation of the secretion in the salivary gland. The apical pores of taste buds in circumvallate and foliate papillae are surrounded by the saliva from von Ebner salivary gland(vEG). Thus, it is important how the secretion of vEG is controlled. This study was designed to elucidate the roles of P2Rs on salivary secretion of vEG. Male Sprague-Dawley rats (about 200 g) were used for this experiment. vEG-rich tissues were obtained from dissecting 500-1,000μm thick posterior tongue slices under stereomicroscope view. P2Rs mRNA in vEG acinar cells were identified with RT-PCR. To observe the change in intracellular Cα²+ activity, we employed Cα²+-ion specific fluorescence analysis with fura-2. Single acinar cells and cell clusters were isolated by a sequential trypsin/collagenase treatment and were loaded with 10 μM fura -2 AM for 60 minutes at room temperature. Several agonists and antagonists were used to test a receptor specificity. RT-PCR revealed that the mRNAs of P2X₄, P2Y₁, P2Y₂ and P2Y₃ are expressed in vEG acinar cells. The intracellular calcium activity was increased in response to 10 μM ATP, a P2Rs agonist, and 2-MeSATP, a P2Y₄ and P2Y₂R agonist. However, 300 μM αβ-MeATP, a P2X₁ and P2X₃R agonist, did not elicit the response. The responses elicited by 10 μM ATP and UTP, a P2Y₂R agonists, were maintained when extracellular calcium was removed. 10 μM suramin, a P2XR antagonist, and reactive blue 2, a P2YR antagonist, partially blocked ATP-induced response. However, when extracellular calciums were removed, suramin did not abolish the responses elicited by ATP. These results suggest that P2Rs play an important role in salivary secretion of vEG acinar cells and the effects of ATP on vEG salivary secretion may be mediated by P2X₄, P2Y₁, P2Y₂, and/or P2Y₃.