Sensitization by Extracellular Ca2+of Rat P2X5Receptor and Its Pharmacological Properties Compared with Rat P2X1.

The recombinant rat P2X5(rP2X5) receptor, a poorly understood ATP-gated ion channel, was studied under voltage-clamp conditions and compared with the better understood homomeric rP2X1receptor with which it may coexist in vivo. Expressed in defolliculated Xenopus laevisoocytes, rP2X5responded to ATP with slowly desensitizing inward currents that, for successive responses, ran down in the presence of extracellular Ca2+(1.8 mM). Replacement of Ca2+with either Ba2+or Mg2+prevented rundown, although agonist responses were very small, whereas reintroduction of Ca2+for short periods of time (<300 s) before and during agonist application yielded consistently larger responses. Using this Ca2+-pulse conditioning, rP2X5responded to ATP and other nucleotides (ATP, 2-methylthio-ATP, adenosine-5′-O-(thiotriphosphate), 2′-&-3′-O-(4-benzoylbenzoyl)-ATP, α,β-methylene-ATP, P1-P(4)-diadenosine-5′-phosphate, and more) with pEC50values within 1 log unit of respective determinations for rP2X1. Only GTP was selective for rP2X5, although 60-fold less potent than ATP. At rP2X5, lowering extracellular pH reduced the potency and efficacy of ATP, whereas extracellular Zn2+ions (0.1–1000 μM) potentiated then inhibited ATP responses in a concentration-dependent manner. However, these modulators affected rP2X1receptors in subtly different ways–with increasing H+and Zn2+ion concentrations reducing agonist potency. For P2 receptor antagonists, the potency order at rP2X5was pyridoxal-5-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) > 2′,3′-O-(2,4,6-trinitrophenyl)ATP (TNP-ATP) > suramin > reactive blue 2 (RB-2) > diinosine pentaphosphate (Ip5I). In contrast, the potency order at rP2X1was TNP-ATP = Ip5I > PPADS > suramin = RB-2. Thus, the Ca2+-sensitized homomeric rP2X5receptor is similar in agonist profile to homomeric rP2X1—although it can be distinguished from the latter by GTP agonism, antagonist profile, and the modulatory effects of H+and Zn2+ions.