MELATONIN MEDIATES 2 DISTINCT RESPONSES IN VASCULAR SMOOTH-MUSCLE

The pineal hormone melatonin was found to produce two distinct contrac tile responses in vascular smooth muscle. In isolated rat caudal arter y segments, denuded of endothelium, melatonin (10(-10)-10(-7) M) poten tiated 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 melat onin 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 t wo receptor subtypes in vascular smooth muscle; MT2 receptors may indu ce relaxation, while a second receptor subtype mediates vasoconstricti on. (C) 1998 Elsevier Science B.V.