MT2 Receptors

We recently re-examined responses to melatonin in precontracted tail artery segments, denuded of endothelium, and found that luzindole not only shifted the melatonin concentration-response curve to the right, as expected, but also enhanced the maximal constrictor effect of melatonin (36, Figure 4). Because this antagonist acts on all three melatonin receptor subtypes, we tested 5-MCA-NAT to see if MT3 receptors were involved (17). No significant effects (potentiation, constriction nor dilation) of this MT3 selective agonist were observed in the tail artery (36). Selective MT2 antagonists, 4-P-ADOT and 4-P-PDOT (4-phenyl-2-propionamidotetralin), however, enhanced the vasoconstriction elicited by melatonin (14,16,36). Using concentrations designed to maximally block MT2 receptors without affecting mti receptors, these antagonists not only increased the maximal effect of melatonin, but shifted the melatonin concentration-response curve to the left (14,36, Figure 4). These data are consistent

Figure 4. The effect of melatonin antagonists in endothelial-denuded segments of rat tail artery, precon-stricted with phenylphrine. Concentration-response curves are shown for melatonin alone (control) or in the presence of either the mt1/MT2 antagonist, luzindole (3 x 10-6M), or the selective MT2 antagonist, 4-P-ADOT (10-8M).

Figure 4. The effect of melatonin antagonists in endothelial-denuded segments of rat tail artery, precon-stricted with phenylphrine. Concentration-response curves are shown for melatonin alone (control) or in the presence of either the mt1/MT2 antagonist, luzindole (3 x 10-6M), or the selective MT2 antagonist, 4-P-ADOT (10-8M).

with the activation of two melatonin receptor subtypes in the rat tail artery. We hypothesize that melatonin acts on both mt1-like receptors to elicit constrictor responses and MT2 receptors to cause dilation (14). When the latter effect is blocked by 4-PADOT or 4-P-PDOT, vascular tone is increased. The presence of both mt1 and MT2 receptor mRNA in tail artery smooth muscle (40) further supports this hypothesis.

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