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Vol. 300, Issue 2, 673-680, February 2002
Autonomic Neuroscience Institute, Royal Free and University College
Medical School, Royal Free Campus, Hampstead, United Kingdom (S.G.B.,
G.B., B.F.K.); Department of Biochemistry and Molecular Biology,
University College London, London, United Kingdom (A.T.N.); and
Molecular Recognition Section, Laboratory of Bioorganic Chemistry,
National Institute of Diabetes and Digestive and Kidney Diseases,
National Institutes of Health, Bethesda, Maryland (K.A.J.)
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 rP2X1
and 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 P2X2
responses 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.
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