Upregulation of P2Y2 nucleotide receptors in rat salivary gland cells during short-term culture

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Abstract
In contrast to the widespread expression of G protein-coupled P2Y2 receptors for extracellular nucleotides in permanent cell lines of salivary gland origin, there is less evidence for robust P2Y2 receptor activity in normal rat salivary gland cells assayed immediately after isolation. We examined the effect of short-term culture (3 h to 6 days) of normal rat submandibular gland (SMG) cells on P2Y2 receptor activity and mRNA expression. Results indicate that increases in the intracellular free Ca2+ concentration in SMG cells in response to the P2Y2 receptor agonist UTP (100 microM) were detectable after 3 h in culture and that after 3 days in culture the magnitude of the response to UTP was similar to that obtained with maximal muscarinic cholinoceptor activation. The Ca2+ mobilization response exhibited the pharmacological profile (UTP = ATP > 2-methylthioadenosine 5'-triphosphate) typical of the P2Y2 receptor subtype and was accompanied by enhanced production of inositol phosphates, reflecting the activation of phospholipase C ubiquitously associated with P2Y2 receptors. The time-dependent increase in P2Y2 receptor activity was accompanied by an increase in the steady-state level of P2Y2 receptor mRNA, as assessed by reverse transcription-polymerase chain reaction. Other studies revealed that the increased P2Y2 receptor activity was independent of cell proliferation, was similar in serum-containing and defined culture media, and was blocked by inhibitors of transcription and translation. Upregulation of the P2Y2 receptor was observed in both acinar cell- and ductal cell-enriched cultures of the SMG and in cells isolated from rat parotid and sublingual glands but not in cells isolated from the pancreas. These in vitro results were complemented by in vivo studies in which P2Y2 receptor activity and mRNA levels were increased in SMG after ligation of the main excretory duct but were not increased in the contralateral, nonligated gland. These findings suggest that changes in the expression and activity of the P2Y2 receptor in salivary gland cells may be related to pathological challenges to the gland in vivo.