Effects of extracellular ATP on cytosolic Ca2+ concentration and secretory responses in rat parotid acinar cells

At concentrations > 50 μM, extracellular ATP dose-dependently increased cytosolic Ca2+ concentration ([Ca2+]i) via activation of P2Z purinoceptors in rat parotid acinar cells. This increase in [Ca2+]i is primarily due to entry of Ca2+ from the extracellular medium. Inositol trisphosphate formation and Ca2+ release from intracellular Ca2+ stores were slightly stimulated by ATP. These responses seem to result from the activation of a P2 purinoceptor subtype different from P2Z because the maximum responses were induced by much lower concentrations (< 0.25 mM) of ATP than those (> 1 mM) required to produce the maximum activation of Ca2+ entry. ATP did not stimulate amylase release, supporting the view that an increase in [Ca2+]i is not a sufficient stimulus. In addition, ATP evoked a marked K+ release in the presence and absence of extracellular Ca2+, while in carbachol-stimulated cells, only a transient release of K+ was observed in the absence of extracellular Ca2+. The ATP-induced K+ release may be mediated by a mechanism independent of the opening of the Ca2+-sensitive K+ channels.