Search

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.

The recombinant rat P2X(5) (rP2X(5)) receptor, a poorly understood ATP-gated ion channel, was studied under voltage-clamp conditions and compared with the better understood homomeric rP2X(1) receptor with which it may coexist in vivo. Expressed in defolliculated Xenopus laevis oocytes, rP2X(5) responded to ATP with slowly desensitizing inward currents that, for successive responses, ran down in the presence of extracellular Ca(2+) (1.8 mM). Replacement of Ca(2+) with either Ba(2+) or Mg(2+) prevented rundown, although agonist responses were very small, whereas reintroduction of Ca(2+) for short periods of time (<300 s) before and during agonist application yielded consistently larger responses. Using this Ca(2+)-pulse conditioning, rP2X(5) responded to ATP and other nucleotides (ATP, 2-methylthio-ATP, adenosine-5'-O-(thiotriphosphate), 2'-&-3'-O-(4-benzoylbenzoyl)-ATP, alpha,beta-methylene-ATP, P(1)-P((4))-diadenosine-5'-phosphate, and more) with pEC(50) values within 1 log unit of respective determinations for rP2X(1). Only GTP was selective for rP2X(5), although 60-fold less potent than ATP. At rP2X(5), lowering extracellular pH reduced the potency and efficacy of ATP, whereas extracellular Zn(2+) ions (0.1-1000 microM) potentiated then inhibited ATP responses in a concentration-dependent manner. However, these modulators affected rP2X(1) receptors in subtly different ways-with increasing H(+) and Zn(2+) ion concentrations reducing agonist potency. For P2 receptor antagonists, the potency order at rP2X(5) was 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 (Ip(5)I). In contrast, the potency order at rP2X(1) was TNP-ATP = Ip(5)I > PPADS > suramin = RB-2. Thus, the Ca(2+)-sensitized homomeric rP2X(5) receptor is similar in agonist profile to homomeric rP2X(1)-although it can be distinguished from the latter by GTP agonism, antagonist profile, and the modulatory effects of H(+) and Zn(2+) ions.