Abstract
Rat P2X1 and P2X2 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 (P2X1-like) component and a later slowly inactivating (P2X2-like) component. The maximum amplitude of P2X1-like ATP responses was increased in some cells by lowering extracellular pH (from 7.5 to 6.5), whereas P2X2-like responses and those of homomeric rP2X1 and rP2X2 receptors were not changed by this treatment. Concentration-response (C/R) curves for ATP for pH-enhanced P2X1-like responses were biphasic, and clearly distinct from monophasic ATP C/R curves for homomeric rP2X1and rP2X2 receptors. Under acidic (pH 5.5 and 6.5) and alkaline (pH 8.5) conditions, ATP C/R curves for P2X1-like responses showed increases in agonist potency and efficacy, compared with data at pH 7.5, but the same was not true of homomeric rP2X1 and rP2X2 receptors. ATP C/R curves for P2X2-like responses overlay C/R curves for homomeric rP2X2 receptors, and determinations of agonist potency and efficacy were identical for P2X2-like and P2X2responses at all pH levels tested. Our results show that P2X1-like responses possessed the kinetics of homomeric P2X1 receptors but an acid sensitivity different from homomeric P2X1 and P2X2 receptors. In contrast, the P2X2-like responses exactly matched the profile expected of homomeric P2X2 receptors. Thus, coexpression of P2X1 and P2X2 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 P2X1/2 ion channels.
Footnotes
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This work was supported by grants from the Biotechnology and Biological Sciences Research Council (UK) and British Heart Foundation (UK), as well as by funding from Gilead Sciences (Foster City, CA) and Roche Bioscience (Palo Alto, CA). S.G.B. was supported by Gilead Sciences; this work appears as part of S.G.B.'s Ph.D. thesis, entitled: Pharmacological Agents That Distinguish between P2X Receptor Subtypes, University of London, Senate House, London.
- Abbreviations:
- cRNA
- capped RNA
- C/R
- concentration-response
- Ap6A
- P1,P6-diadenosine hexaphosphate ammonium salt
- α,β-meATP
- α,β-methylene ATP lithium salt
- pHe
- extracellular pH
- Received September 12, 2001.
- Accepted November 12, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
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