In exocrine acinar cells a variety of neurotransmitters (e.g. acetylcholine) stimulate phosphatidylinositol 4,5-bisphosphate hydrolysis elevating intracellular calcium to activate calcium-dependent membrane currents (outward K+ and inward Cl−). This study shows that in lacrimal acinar cells extracellular application of ATP is also associated with outward and inward current responses; these, however, are not the result of phosphoinositide metabolism. ATP directly activates receptor-operated cation channels which permit influx of Na+ and Ca+ (the inward current). The elevation in [Ca2+], which results is sufficient to activate the outward K+ current. ATP thus promotes Ca+ influx in the absence of phosphoinositide metabolism.
