Salivary gland nucleotide receptors: Evidence for functional expression of both P2X and P2Y subtypes

A growing body of information now supports the suggestion that P2 receptors for extracellular nucleotides (primarily ATP) have a role in regulating salivary gland function. There is solid pharmacological and molecular evidence for the presence of P2X ligand-gated ion channel nucleotide receptors (P2X(4) and P2X(7)/P-2Z). More recently, our group and others have obtained evidence that multiple P2Y G protein-coupled nucleotide receptors (P2Y(1) and P2Y(2)) are also expressed. Our studies have focused on defining the conditions under which P2Y receptors are expressed, the functional consequences of their activation, and the importance of co-expression of P2X and P2Y receptors. Functional and molecular approaches have been used to identify the P2 subtypes in salivary glands and in salivary cell lines. Assays include measurement of changes in [Ca2+](i), changes in transcellular short circuit current in monolayers, and RT-PCR to assess changes in receptor mRNA levels. The main observations are: (1) P2Y(1) receptor activity is present in the submandibular gland (SMG) of immature rats but decreases over the first four weeks following birth, although mRNA levels remain relatively constant, (2) P2Y(2) receptors are present in the cell lines and are upregulated during short-term culture of normal parotid, sublingual, and SMG cells and following ligation of the main excretory duct of SMG; and (3) the P2X subtypes, P2X(4) and P2X(7), and the P2Y subtypes, P2Y(1) and P2Y(2), are coexpressed in salivary glands and salivary cell lines, and exhibit distinct basolateral versus apical localization in polarized cell monolayers as well as discrete patterns of intracellular signaling.