A SELECTIVE AGONIST AFFINITY LABEL FOR A(3) ADENOSINE RECEPTORS

A newly synthesized, chemically reactive adenosine derivative, othiocy anatobenzyl)adenosine-5'-N-methyluronamide, was found to bind selectiv ely to A(3) receptors. K-i values for this isothiocyanate derivative i n competition binding at rat brain A(1), A(2a), and A(3) receptors wer e 145, 272 and 10.0 nM, respectively. A preincubation with this deriva tive resulted in irreversible inhibition of radioligand binding at rat A(3) receptors in membranes of transfected CHO cells or RBL-2H3 mast cells, but not at rat A(1) or A(2a) receptors. The loss of binding sit es for 0.1 nM N-6-(4-aminobenzyl)adenosine-5'-N-methyluronamide, a hig h affinity A(3) receptor radioligand, in transfected CHO cell membrane s was concentration-dependent with an IC50 of 50 n M. No change was ob served in the K-d value of the remaining A(3) receptor sites. The inhi bition was also insensitive to theophylline (1 mM), consistent with th e pharmacology of rat A(3) receptors. Structurally similar adenosine a nalogues lacking the chemically reactive isothiocyanate group failed t o irreversibly inhibit A(3)-binding. (C) 1994 Academic Press, Inc.