1.A.7.2.1
The osmoregulatory intracellular P2X receptor, P2XA gated by ATP (present in the osmoreulatory organelle, the contractile vacuole) (Fountain et al., 2007). One of five P2X receptors in D. discoideum is localized to the contractile vacuole with the ligand binding domain facing the lumen. Plays a role in Ca²⁺ signaling, but also is Cl^- permeable. May function in osmoregulation (Ludlow et al., 2009).  Four of the five receptors operate as ATP-gated channels (P2XA, P2XB, P2XD, and P2XE). For the P2XA receptor, ATP was the only effective agonist, but extracellular sodium, compared with potassium, strongly inhibited ATP responses in P2XB, P2XD, and P2XE receptors. Increasing the proton concentration (pH 6.2) accelerated desensitization at P2XA receptors and decreased currents at P2XD receptors, but increased the currents at P2XB and P2XE receptors. Dictyostelium lacking P2XA receptors showed an impaired regulatory volume decrease in hypotonic solution. This phenotype was rescued by overexpression of P2XA and P2XD receptors, partially rescued by P2XB and P2XE receptors, and not rescued by P2XC receptor which appeared to be inactive (Baines et al. 2013).