Abstract
The lack of a radiolabeled selective A(3) adenosine receptor antagonist
is a major drawback for an adequate characterization of this receptor
subtype. This paper describes the pharmacological and biochemical
characterization of the tritiated form of a new potent A(3) adenosine
receptor antagonist, the pyrazolo triazolo pyrimidine derivative
(3)H5N-(4-methoxyphenylcarbamoyl)amino-8-propyl-2-(2-furyl )pyrazolo
4,3-e -1,2,4- triazolo1,5-cpyrimidine ((3)HMRE 3008F20). (3)HMRE
3008F20 bound specifically to the human adenosine A(3) receptor expressed
in CHO cells (hA(3)CHO), and saturation analysis revealed a single
high affinity binding site, K(D) = 0.80 +/- 0.06 nM, with a B(max)
= 300 +/- 33 fmol/mg protein. This new ligand displayed high selectivity
(1294-, 165-, and 2471-fold) in binding assay to human A(3) versus
A(1), A(2A), and A(2B) receptors, respectively, and binds to the
rat A(3) receptors with a K(i) > 10 microM. The pharmacological profile
of (3)HMRE 3008F20 binding to hA(3)CHO cells was evaluated using
known adenosine receptor agonists and antagonists with a rank order
of potency consistent with that typically found for interactions
with the A(3) adenosine receptors. In the adenylyl cyclase assay
the same compounds exhibited a rank order of potency identical with
that observed in binding experiments. Thermodynamic data indicated
that (3)HMRE 3008F20 binding to hA(3)CHO is entropy- and enthalpy-driven
in agreement with the typical behavior of other adenosine antagonists
to A(1) and A(2A) receptors. These results show that (3)HMRE 3008F20
is the first antagonist radioligand with high affinity and selectivity
for the human A(3) adenosine receptor and may be used to investigate
the physiopathological role of A(3) adenosine receptors.
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