To investigate whether melatonin has a direct action on the cardiovascular system, putative melatonin receptors were studied in quail heart membrane preparations using the specific melatonin agonist 2-[125I]iodomelatonin (125I]Mel, as the radioligand. The [125I]mel binding demonstrated in the mature quail heart was saturable, highly 5.2 pM; Bmax = 1.32 +/- 0.25 fmol/mg protein; n = 8). The linear Scatchard plots and the close to unity Hill coefficient indicated a single class of binding sites. The pharmacological profile was in the affinity order of 2-iodomelatonin = 2-phenylmelatonin > melatonin > 6-chloromelatonin >> 6-hydroxymelatonin > 6-sulphatoxymelatonin >> N-acetylserotonin>>>5-hydroxytryptamine. Guanosine 5'-triphosphate and guanosine 5'-O-(3-thiotriphosphate) (GTP gammaS) dose dependently inhibited the binding. Ten microM GTPgammaS lowered the binding affinity by 50% in saturation studies. The order of potency of inhibition by cations was: Ca2+ > Mg2+ > Li+ > Na+ > K+ > choline chloride. Contrary to most other melatonin binding sites, millimolar concentrations of Ca2+ and Mg2+ did not promote binding in the quail heart membranes. In vitro autoradiography indicated homogenous labeling in the heart. Our results demonstrated [125I]Mel binding sites in the quail heart. That guanine nucleotides and Na+ inhibited the binding indicated that these putative melatonin receptors are coupled to guanine nucleotide-binding proteins (G-proteins).