A novel partial agonist of the A1-adenosine receptor and evidence of receptor homogeneity in adipocytes

This study characterizes the receptor binding and functional effects of CVT-3619 [2-{6-[((1R,2R)-2-hydroxycyclopentyl)amino]purin-9-yl}(4S,5S,2R,3R)-5-[(2-fluorophenylthio)methyl]oxolane-3,4-diol], a novel N-6-5'-substituted adenosine analog and A(1)-adenosine receptor (A(1)AdoR) agonist, on rat epididymal and inguinal adipocytes and on the isolated heart and compares these effects with those caused by the full agonist N-6-cyclopentyladenosine (CPA). In addition, the hypothesis that adipocyte A(1)AdoR are a heterogeneous population with regard to their affinities for ligands was tested. CVT-3619 was 10-100-fold selective for A(1)AdoR versus other AdoR and bound to adipocyte membranes with high (K-H = 14 nM) and low (K-L = 5.4 mu M) affinities. CVT-3619 reduced cyclic AMP content and release of nonesterified fatty acids from epididymal adipocytes with IC50 values of 6 and 44 nM, respectively. CVT-3619 was a partial agonist relative to CPA to reduce lipolysis in epididymal and inguinal adipocytes. CVT-3619 did not change atrial rate in rat heart and caused a small (6-ms) prolongation of the stimulus-to-His bundle interval without causing atrioventricular block in guinea pig heart (effects mediated by A(1)AdoR), whereas CPA caused atrioventricular block and near cessation of atrial electrical activity. CVT-3619 increased coronary conductance ( effect mediated by A(2A)AdoR) only at concentrations >= 10 mu M. Rat epididymal adipocyte A(1)AdoR had similar affinities for the antagonist 8-cyclopentyl-1,3-dipropylxanthine in the presence of three dissimilar A(1)AdoR agonists (2-chloro-N-6-cyclopentyladenosine, N-6-sulfophenyladenosine, and N-5'-ethylcarboxamidoadenosine) as determined by Schild analysis. It was concluded that rat epididymal adipocyte A(1)AdoR are a homogeneous receptor population with regard to affinities for ligands and that CVT-3619 is a partial agonist with selectivity for A(1)AdoR and inhibition of lipolysis.