P2X receptors are ATP-gated ion channels that play a role in intracellular calcium signaling. Not required for the purinergic response to extracellular nucleotides. Inward currents evoked by intracellular ATP and ATP analogs. Exclusively selective for ATP over other nucleotides. Insensitive to P2 receptor antagonists PPADS, suramin and 2',3'-O-(2,4,6-trinitrophenyl)-ATP but inhibited by nanomolar concentrations of copper and sodium ion. More permeable to ammonium than either sodium or potassium ions and less permeable to choline. It has been reported that p2xA is not essential for osmoregulation (PubMed:19833731), however this information is in contradiction with another source (PubMed:17625565) which indicates that p2xA is required for osmoregulation. Found to be permeable to chloride ions. Inhibited by copper and sodium ions. Ligand binding domain within the lumen of the vacuole. Response to extracellular ATP remains the same in p2xA null strains. p2xA null cells have been reported to have compromised contractile vacuoles and defective osmoregulation, and exhibit cell swelling (PubMed:17625565). This is contradictory to other reports (PubMed:19833731). Quintuple p2xA/p2xB/p2XC/p2xd/p2xE null cells exhibit a slight delay in their osmoregulatory response, but are ultimately still capable of regulating their cell volume in water. Extracellular purinergic response to ATP persists in the quintuple null cells with no alteration in the kinetics of the response, but the magnitude of the response is lower. Responses to the calmodulin antagonist calmidazolium are reduced and intracellular calcium signaling is disrupted in quintuple null cells. The presence of copper prevents both wild type and quintuple null cells from undergoing an osmoregulatory decrease in cell volume. The quintuple null strains grow slightly slower than wild type in shaking axenic cultures. Belongs to the P2X receptor family.