Melatonin mediates two distinct responses in vascular smooth muscle

The pineal hormone melatonin was found to produce two distinct contractile responses in vascular smooth muscle. In isolated rat caudal artery segments, denuded of endothelium, melatonin (10^-10-10^-7 M) potentiated phenylephrine-induced contractions in a concentration-dependent manner. At higher melatonin concentrations (10^-7-10^-5 M), however, the potentiating effect was attenuated. In the presence of the melatonin MT₂ receptor antagonist, 4-phenyl-2-acetamidotetraline (4P-ADOT), the attenuated constrictor responses were selectively enhanced. These results are consistent with the hypothesis that melatonin activates two receptor subtypes in vascular smooth muscle; MT₂ receptors may induce relaxation, while a second receptor subtype mediates vasoconstriction.