Binding of an adenosine A(1) receptor agonist and adenosine A(1) receptor antagonist to sheep pineal membranes

The pineal organ of vertebrates produces melatonin and adenosine. In lower vertebrates, adenosine modulates melatonin production. We report herein that 2-chloro-cyclopentyl-[H-3]-adenosine ([H-3]CCPA: adenosine A(1) receptor agonist) and [H-3]-cyclopentyl-1,3-dipropylxanthine ([H-3]DPCPX: adenosine A(1) receptor antagonist), bind specifically to sheep pineal membranes. Binding of [H-3]CCPA reached equilibrium at 90 min and dissociation revealed the presence of two components. Saturation analysis suggested the presence of a single population of binding sites (K-d = 1.67 +/- 0.06 nM, B-max = 2386 fmol/mg protein:). Binding was sensitive to GTP and GTP gamma S. Binding of [H-3]DPCPX reached equilibrium at 60 min and dissociation was monophasic. Saturation analysis revealed a single population of binding sites (K-d = 5.8 +/- 1.12 nM, B-max = 1116 fmol/mg protein). The specificity of the [H-3]-analogues used and the rank order potency of the competitors tested in the competition experiments suggested the presence of A(1) receptors. Future investigations are necessary to elucidate the significance of the differences observed between the binding properties of the adenosine A(1) receptor agonist and adenosine A(1) receptor antagonist. (C) 1997 Elsevier Science B.V.