Abstract

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)³³P-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 MT₁melatonin receptors. The MT₁/MT₂ competitive melatonin receptor antagonist luzindole (3 μM), 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 MT₁constitutively active receptors. The involvement of MT₂receptors 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 μM). We conclude that MT₂ melatonin receptors expressed in vascular smooth muscle mediate vasodilation in contrast to vascular MT₁ receptors mediating vasoconstriction.