Abstract
Using voltage-clamp procedures on Xenopus oocytes, agonist-evoked ionic currents by P2X receptors resulting from the coexpression of P2X2 and P2X3 subunits were compared against the agonist responses of homomeric P2X2and P2X3 receptors. With the quantity of P2X3mRNA kept constant and quantity of P2X2 mRNA progressively increased, expressed P2X receptors changed from a P2X3-like receptor to a P2X2-like receptor. In all cases, however, agonist-evoked responses comprised biphasic (fast and slow) currents—the former showing the properties of P2X3receptors and latter consistent with the presence of P2X2and P2X2/3 receptors. Using desensitization procedures, the P2X3-like fast current was selectively removed to allow the slow current to be studied in isolation. P2X2/3 receptors were then characterized by slowly inactivating inward currents that were reproducible within 30 s of washout and whose pharmacological profile [selective agonists, Ap5A > α,β-methylene ATP ≫ β,γ-methylene ATP > UTP; antagonists, TNP-ATP ≫ suramin ≥ Reactive blue-2 (RB-2)] contrasted with the profile of P2X2 receptors (Ap5A, α,β-methylene ATP, β,γ-methylene ATP, and UTP inactive; antagonists, RB-2 > TNP-ATP > suramin). Thus, our experiments reveal that coexpression of two P2X subunits, which of themselves can generate functional homomeric receptors, results in a complex population of heterogeneous P2X receptors—in this case P2X2, P2X3, and P2X2/3 receptors. Depending on the relative levels of P2X subunit coexpression, the operational profile of the resultant P2X receptors can change from one phenotype to another. This spectrum may explain the variability of agonist responses in small sensory neurons that also express P2X2 and P2X3 subunits in different amounts.
Footnotes
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Send reprint requests to: Brian F. King. Ph.D., Autonomic Neuroscience Institute, Royal Free and University College Medical School, Royal Free Campus, Rowland Hill St., Hampstead, London NW3 2PF, UK. E-mail: b.king{at}ucl.ac.uk
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This work was supported by Roche Bioscience (Palo Alto, CA) and grants from the British Heart Foundation.
- Abbreviations:
- DRG
- dorsal root ganglion
- ATPγS
- adenosine-5′-O-(3-thio)triphosphate
- Ap5A
- P1,P5-diadenosine pentaphosphate
- cRNA
- capped ribonucleic acid
- Ip5I
- diinosine pentaphosphate
- α,β-meATP
- α,β-methylene ATP
- β,γ-meATP
- β,γ-methylene ATP
- 2-MeSATP
- 2-methylthioATP
- RB-2
- Reactive blue 2
- PPADS
- pyridoxal 5-phosphate 6-azophenyl-2′,4′-disulfonic acid
- TNP-ATP
- 2′,3′-O-(2,4,6-trinitrophenyl)-ATP
- Vh
- holding potential
- C/R
- concentration/response curve
- Received September 6, 2000.
- Accepted November 14, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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