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Effects of extracellular pH on agonism and antagonism at a recombinant P2X2receptor

King, B. F., Wildman, Scott S.P., Ziganshina, L. E., Pintor, J., Burnstock, G. (1997) Effects of extracellular pH on agonism and antagonism at a recombinant P2X2receptor. British Journal of Pharmacology, 121 (7). pp. 1445-1453. ISSN 0007-1188. E-ISSN 1476-5381. (doi:10.1038/sj.bjp.0701286) (Access to this publication is currently restricted. You may be able to access a copy if URLs are provided)

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Abstract

1. Under voltage-clamp conditions, the activity of agonists and antagonists at a recombinant P2X2 receptor expressed in Xenopus oocytes was examined at different levels of extracellular pH (pHe). 2. In normal Ringer (Mg2+ ions absent), the amplitude of submaximal inward currents to ATP was increased by progressively lowering pHe (8.0-5.5). ATP-responses reached a maximum at pH 6.5 with a 5 fold increase in ATP-affinity; the apparent pKa was 7.05 +/- 0.05. 3. Receptor affinity for ATP was lowered when extracellular Ca2+ ions were replaced with equimolar Mg2+ ions. However, the amplitude of the ATP-responses was still enhanced under acidic conditions, reaching maximal activity at pH 6.5 with a 5 fold increase in ATP-affinity; the apparent pKa was 7.35 +/- 0.05. 4. ATP species present in the superfusate (for the above ionic conditions and pH levels) were calculated to determine the forms of ATP which activate P2X2 receptors: possible candidates include HATP, CaHATP and MgHATP. However, levels of these protonated species increase below pH 6.5, suggesting that receptor protonation rather than agonist protonation is more important. 5. The potency order for agonists of P2X2 receptors was: ATP> 2-MeS-ATP ATPgammaS> ATPalphaS> >CTP >BzATP, while other nucleotides were inactive. EC50 and nH values for full agonists were determined at pH 7.4 and re-examined at pH 6.5. Extracellular acidification increased the affinity by approximately 5 fold for full agonists (ATP, 2-MeSATP, ATPyS and ATP alpha S), without altering the potency order. 6. The potency order for antagonists at P2X2 receptors was: Reactive blue-2 >trinitrophenol-ATP > or = Palatine fast black > or = Coomassie brilliant blue > or = PPADS>suramin (at pH 7.4). IC50 values and slopes of the inhibition curves were re-examined at different pH levels. Only blockade by suramin was affected significantly by extracellular acidification (IC50 values: 10.4 +/- 2 microM, at pH 7.4; 78 +/- 5 nM, at pH 6.5; 30 +/-6 nM, at pH 5.5). 7 In summary, a lowered pHe enhanced the activity of all agonists at P2X2 receptors but, with the exception of suramin, not antagonists. Since a lowered pHe is also known to enhance agonist activity at P2x receptors on sensory neurones containing P2X2 transcripts, the sensitization by metabolic acidosis of native P2x receptors containing P2X2 subunits may have a significant effect on purinergic cell-to-cell signalling.

Item Type: Article
DOI/Identification number: 10.1038/sj.bjp.0701286
Uncontrolled keywords: ATP receptor; P2X receptor; recombinant (P2X2) receptor; ATP; suramin; pH; acidosis; Xenopus oocyte
Subjects: R Medicine > RM Therapeutics. Pharmacology
Divisions: Faculties > Sciences > Medway School of Pharmacy
Depositing User: Scott S.P. Wildman
Date Deposited: 08 Dec 2015 22:35 UTC
Last Modified: 29 May 2019 16:40 UTC
Resource URI: https://kar.kent.ac.uk/id/eprint/52944 (The current URI for this page, for reference purposes)
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