MT2 melatonin receptors are present and functional in rat caudal artery

In rat caudal artery, contraction to melatonin results primarily from activation of MT1 melatonin receptors; however, the role of MT2 melatonin receptors in vascular responses is controversial. We examined and compared the expression and function of MT2 receptors with that of MT1 receptors in male rat caudal artery. MT1 and MT2 melatonin receptor mRNA was amplified by reverse transcription-polymerase chain reaction from caudal arteries of three rat strains (i.e., Fisher, Sprague-Dawley, and Wistar). Antisense (but not sense) P-33-labeled oligonucleotide probes specific for MT1 or MT2 receptor mRNA hybridized to smooth muscle, as well as intimal and adventitial layers, of caudal artery. In male Fisher rat caudal artery denuded of endothelium, melatonin was 10 times more potent than 6-chloromelatonin to potentiate contraction to phenylephrine, suggesting activation of smooth muscle MT1 melatonin receptors. The MT1/MT2 competitive melatonin receptor antagonist luzindole (3 muM), blocked melatonin-mediated contraction (0.1-100 nM) with an affinity constant (K-B value of 157 nM) similar to that for the human MT1 receptor. However, at melatonin concentrations above 100 nM, luzindole potentiated the contractile response, suggesting blockade of MT2 receptors mediating vasorelaxation and/or an inverse agonist effect at MT1 constitutively active receptors. The involvement of MT2 receptors in vasorelaxation is supported by the finding that the competitive antagonists 4-phenyl 2-acetamidotetraline and 4-phenyl-2-propionamidotetraline, at MT2-selective concentrations (10 nM), significantly enhanced contractile responses to all melatonin concentrations tested (0.1 nM-10 muM). We conclude that MT2 melatonin receptors expressed in vascular smooth muscle mediate vasodilation in contrast to vascular MT1 receptors mediating vasoconstriction.