Abstract
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.
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