Heteromultimeric P2X(1/2) receptors show a novel sensitivity to extracellular pH.
Rat P2X(1) and P2X(2) subunits were coexpressed in defolliculated Xenopus oocytes and the resultant P2X receptors studied under voltage-clamp conditions. Extracellular ATP elicited biphasic inward currents, involving an initial rapidly inactivating (P2X(1)-like) component and a later slowly inactivating (P2X(2)-like) component. The maximum amplitude of P2X(1)-like ATP responses was increased in some cells by lowering extracellular pH (from 7.5 to 6.5), whereas P2X(2)-like responses and those of homomeric rP2X(1) and rP2X(2) receptors were not changed by this treatment. Concentration-response (C/R) curves for ATP for pH-enhanced P2X(1)-like responses were biphasic, and clearly distinct from monophasic ATP C/R curves for homomeric rP2X(1) and rP2X(2) receptors. Under acidic (pH 5.5 and 6.5) and alkaline (pH 8.5) conditions, ATP C/R curves for P2X(1)-like responses showed increases in agonist potency and efficacy, compared with data at pH 7.5, but the same was not true of homomeric rP2X(1) and rP2X(2) receptors. ATP C/R curves for P2X(2)-like responses overlay C/R curves for homomeric rP2X(2) receptors, and determinations of agonist potency and efficacy were identical for P2X(2)-like and P2X(2) responses at all pH levels tested. Our results show that P2X(1)-like responses possessed the kinetics of homomeric P2X(1) receptors but an acid sensitivity different from homomeric P2X(1) and P2X(2) receptors. In contrast, the P2X(2)-like responses exactly matched the profile expected of homomeric P2X(2) receptors. Thus, coexpression of P2X(1) and P2X(2) subunits yielded a mixed population of homomeric and heteromeric P2X receptors, with a subpopulation of novel pH-sensitive P2X receptors showing identifiably unique properties that indicated the formation of heteromeric P2X(1/2) ion channels.
Brown
Sean G
SG
Autonomic Neuroscience Institute, Royal Free and University College Medical School, Royal Free Campus, Hampstead, United Kingdom.
Townsend-Nicholson
Andrea
A
Jacobson
Kenneth A
KA
Burnstock
Geoffrey
G
King
Brian F
BF
D000255:Adenosine Triphosphate / Q000502:physiology; D000818:Animals; D004594:Electrophysiology; D005110:Extracellular Space / Q000187:drug effects / Q000502:physiology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010641:Phenotype; D058910:Purinergic P2 Receptor Agonists; D018048:Receptors, Purinergic P2 / Q000187:drug effects / Q000502:physiology*; D058469:Receptors, Purinergic P2X; D058476:Receptors, Purinergic P2X2; D011994:Recombinant Proteins / Q000187:drug effects; D009435:Synaptic Transmission / Q000187:drug effects; D014981:Xenopus
C546401:P2RX2 protein, human; C546403:P2rx2 protein, rat; D058910:Purinergic P2 Receptor Agonists; D018048:Receptors, Purinergic P2; D058469:Receptors, Purinergic P2X; D058476:Receptors, Purinergic P2X2; D011994:Recombinant Proteins; D000255:Adenosine Triphosphate