Extracellular ATP raises cytosolic calcium and activates basolateral chloride conductance in Necturus proximal tubule

Extracellular ATP concentration is known to be low compared with its intracellular level, but it may reach higher values locally since this nucleotide can be extruded from the cytosol by various mechanisms (Dubyak & El-Moatassim, 1993) and may thus act as a paracrine factor. There is growing evidence that extracellular ATP modulates ion transport systems in various cells, including epithelial cells. Most of these effects have been related to the binding of ATP to P2-type receptors or of its catabolite, adenosine, to P1 (also called A)-type receptors. In the case of ATP binding to P2 receptors, the modulation of ionic transport systems is either subsequent to the activation of a signal transduction system (ATP binds to a P2 receptor coupled to a G protein) or direct (ATP binds to a channel-receptor), as detailed in the review by Dubyak & El-Moatassim (1993). In both cases, an increase in intracellular calcium concentration ([Ca2+]i) is usually observed. Recently, it has been pointed out that extracellular ATP could activate Cl− conductances in various secretory epithelia (Chan, Zhou & Wang, 1995; Hwang, Schwiebert & Guggino, 1996). In most cases, the transductional system underlying this response was identified, but in a few cases (Stutts, Chinet, Mason, Fullton, Clarke & Boucher, 1992; Stutts, Fitz & Paradiso, 1994; Guo, Merlin, Harvey, Laboisse & Hopfer, 1995), the effect of extracellular ATP on Cl− conductance could not be related to any investigated signal transduction pathway, raising the possibility of a direct effect of the nucleotide on Cl− channels, or of the involvement of some unknown transduction system. In reabsorbing epithelia, it has also been observed that extracellular ATP modulates Cl− transport in the distal part of the nephron (Middleton, Mangel, Basavappa & Fitz, 1993). In contrast, little is known concerning the effects of extracellular ATP on [Ca2+]i or on ionic transport in the proximal tubule of the nephron, the tubular segment of which reabsorbs about 60 % of the filtered NaCl load. This relative lack of information might be due in part to the difficulty of applying Ca2+ measurement techniques in the intact proximal tubule (McCarthy & O'Neil, 1990). Thus, most studies focusing on the effects of extracellular ATP in proximal tubular cells have been performed on cell cultures: ATP increases [Ca2+]i in a primary culture of proximal cells (Cejka, Bidet, Tauc & Poujeol, 1993; Cejka, Le Maout, Bidet, Tauc & Poujeol, 1994) and in LLC-PK1, a cell line which shares common features with proximal tubular cells (Weinberg, Davis, Shayman & Knight, 1989). A recent study demonstrated that in isolated rabbit proximal tubule, extracellular ATP also increases [Ca2+]i via basolateral P2y receptors (Yamada, Seki, Taniguchi, Uwatoko, Suzuki & Kurokawa, 1996), without significant effect on the electrophysiological properties of the cell membrane. We investigated the effects of extracellular ATP on [Ca2+]i and basolateral membrane potential (Vm) in the proximal convoluted tubule (PCT) of the amphibian nephron. Our results are consistent with the presence of a P2y-type receptor and with the activation by extracellular ATP of a basolateral Cl− conductance, GCl. However, GCl activation does not seem to be related to the [Ca2+]i increase or to the other investigated signal transduction systems.