Functional evidence of distinct ATP activation sites at the human P2X(7) receptor.
1. The effect of the agonist ATP on whole cell currents of Xenopus oocytes expressing either the wild-type human P2X(7) receptor (hP2X(7)), an N-terminally hexahistidyl-tagged hP2X(7) receptor (His-hP2X(7)), or a truncated His-hP2X(7) receptor (His-hP2X(7)DeltaC) lacking the C-terminal 156 amino acids was investigated using the two-microelectrode voltage clamp technique. 2. The activation time course of the wild-type hP2X(7) receptor can be described as the sum of an exponentially growing and an additional almost linearly activating current component. 3. The amplitude of the exponentially activating current component of the wild-type hP2X(7) receptor displayed a biphasic dependence on the agonist concentration, which could be best approximated by a model of two equal high-sensitivity and two equal low-sensitivity non-cooperative activation sites with apparent dissociation constants of about 4 and 200 microM free ATP(4-), respectively. 4. The linearly activating current was monophasically dependent on the agonist concentration with an apparent dissociation constant of about 200 microM. 5. The contribution of the low-sensitivity sites to current kinetics was reduced or almost abolished in oocytes expressing His-hP2X(7) or His-hP2X(7)DeltaC. 6. Our data indicate that the hP2X(7) receptor possesses at least two types of activation sites, which differ in ATP(4-) sensitivity by a factor of 50. The degree of occupation of these two sites influences both activation and deactivation kinetics. Both N- and C-terminal domains appear to be important determinants of the current elicited by activation of the sites with low ATP sensitivity, but not for that mediated by the highly ATP-sensitive sites.
Klapperstück
M
M
Julius-Bernstein-Institut für Physiologie, Martin-Luther-Universität Halle-Wittenberg, Magdeburger Strasse 6, D-06097 Halle/Saale, Germany.
Büttner
C
C
Schmalzing
G
G
Markwardt
F
F
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002413:Cations, Divalent / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004553:Electric Conductivity; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000502:physiology; D009865:Oocytes; D009994:Osmolar Concentration; D010446:Peptide Fragments / Q000494:pharmacology; D058910:Purinergic P2 Receptor Agonists; D011930:Reaction Time; D018048:Receptors, Purinergic P2 / Q000737:chemistry / Q000187:drug effects* / Q000502:physiology*; D058486:Receptors, Purinergic P2X7; D016324:Sequence Tagged Sites; D013997:Time Factors; D014982:Xenopus laevis
D002413:Cations, Divalent; C546468:P2RX7 protein, human; D010446:Peptide Fragments; D058910:Purinergic P2 Receptor Agonists; D018048:Receptors, Purinergic P2; D058486:Receptors, Purinergic P2X7; D000255:Adenosine Triphosphate; D008274:Magnesium