Melatonin Potentiates Phenylephrine-Stimulated Intracellular Ca2+ Transient in Smooth Muscle Cell of Large Arteries of Chick Embryo

In the current study we first characterized the properties of the 2-[I]iodomelatonin binding in pulmonary artery and aorta of chick embryo, and then determined the location of the binding site with autoradiography. Receptor binding assays were used using 2-[I]iodomelatonin as ligand. The binding was stable, saturable, specific and reversible. Scatchard anlaysis revealed an equilibrium binding constant (Kd) of 27.12 ± 1.34 p M and a maximum binding capacity (Bmax) of 1.93 ± 0.19 fmol/mg protein (n = 5). Guanosine 5´-O-(3-thiotriphosphate) (GTPγS) increased the Kd, but reduced the Bmax, indicating the binding being coupled to a G-protein. Autoradiography revealed the binding sites in the smooth muscle layer. To delineate the physiological function of melatonin in the large artery of the chick embryo, we determined the intracellular calcium ([Ca]i) in smooth muscle cells of the aorta with spectrofluorometry, using fura 2-AM as calcium indicator. Melatonin at 10 to 10M, which itself had no effect, potentiated the stimulating effect of 0.1 μM phenylephrine, a selective agonist of α1-adrenoceptor, on [Ca]i. In conclusion, specific binding of 2-[I]iodomelatonin and physiological response to melatonin suggest the existence of melatonin receptor in the smooth muscle of large arteries of the chick embryo. Melatonin potentiates the effects of α1-adrenergic stimulation.