Focus on "Multiple functional P2X and P2Y receptors in the luminal and basolateral membranes of pancreatic duct cells".

WHEN RELEASED TO EXTRACELLULAR SPACES, ATP and other nucleotides can participate in multiple types of intercellular communication (4). This can involve classical mechanisms of intercellular signaling (similar to those used in neurotransmission and many types of endocrine regulation) that involve exocytotic release of ATP that is copackaged with biogenic amines (or other neurotransmitters) within specialized secretory vesicles or granules. Well-characterized examples include synaptic signaling by vesicular ATP released at nerve junctions or paracrine regulation of endothelial cells and other platelets by the ATP/ADP released from the dense granules of activated platelets. Most physiologists can readily appreciate how such specialized secretory cells can be sources of the extracellular nucleotides used in intercellular communication. It may stretch the credulity of some to consider that all cell types have the potential to release ATP from a store usually considered as sacrosanct, i.e., the cytoplasmic pool of ATP that is used to fuel or regulate virtually all critical intracellular functions. However, diverse types of environmental stress, such as mechanical shear forces (11), stretch (7), and changes in extracellular osmolarity (15), have now been shown to induce the release of nucleotides from cells via mechanisms that do not involve cellular lysis or obvious exocytosis of specialized secretory granules. In turn, the locally released nucleotides can stimulate P2 nucleotide receptors in nearby cells, or the releasing cell itself, with the consequent activation of adaptive or protective responses to the particular environmental stress. This type of autocrine or paracrine regulation based on locally released nucleotides has been observed in several cell types including endothelial cells (13), glial cells (11), and vascular smooth muscle (7, 13). Recent studies suggest that localized increases in extracellular nucleotides also play particularly critical roles in the autocrine or paracrine regulation of cellular fluid and ion homeostasis in epithelial cells derived from the airway (2, 9), intestine (18), liver (15), cervix (6), and other tissues. A noteworthy example is the regulatory volume decrease response of hepatocytes that can be analyzed at the single cell level (15). These cells respond to hypotonic stimulation with a signaling cascade that serially involves a cell-swelling-induced release of ATP, autocrine stimulation of a G proteincoupled nucleotide receptor, and activation of a Cl2 current that ultimately facilitates loss of intracellular KCl and subsequent shrinkage. Although the precise mechanisms that underlie facilitated release of ATP from epithelial cells remain undefined, pharmacological studies suggest that ATP-binding cassette family membrane proteins are somehow involved in this process (16). These known or hypothesized effects of extracellular nucleotides on epithelial function have also prompted interest in defining the nature and location of the receptors that actually bind the released nucleotides and thereby trigger adaptive responses of the particular epithelial tissue. In the current article in focus (Ref. 11a, see page C205 in this issue), Luo et al. present an elegant and informative analysis of P2 nucleotide receptor expression and function in pancreatic duct cells. Their studies show that these cells express a remarkable diversity of nucleotide receptors in both their apical and basolateral membranes. Characterizing P2 receptors is not a trivial issue for several reasons. First, the genes encoding more than a dozen distinct P2 nucleotide receptors (discussed below) have been cloned during the past six years. To make matters more complex, the receptors comprise two major families: the G protein-coupled P2Y receptors (8) and the ATP-gated ion channels of the P2X group (5). Second, all of the identified P2Y and P2X receptors can trigger rapid changes in cytosolic Ca21 due either to mobilization of inositol trisphosphatesensitive Ca21 stores (P2Y subtypes) or to Ca21 influx across the plasma membrane (P2X and P2Y subtypes). Thus most P2 receptor subtypes can trigger similar integrated cellular responses in epithelial cells, such as the activation of Ca21-regulated ion channels or Ca21dependent kinase cascades. Finally, as for any membrane proteins in a polarized epithelium, particular P2 receptors might be selectively localized to the apical or basolateral surfaces or be expressed on both sides of the cell. To date, most investigations of epithelial P2 receptors have utilized tissue-cultured cells (primary isolates or immortalized lines) plated on permeable filters that permit the establishment of polarized monolayers with appropriate transcellular resistances. Such studies of tissue-cultured epithelia have demonstrated that some epithelial cells can express multiple P2 receptor subtypes and that different P2 receptors can be localized on apical, basolateral, or both cellular surfaces (2, 9). However, there is always concern that the pheno-