STUDIES ON THE VASOCONSTRICTOR ACTION OF MELATONIN AND PUTATIVE MELATONIN RECEPTOR LIGANDS IN THE TAIL ARTERY OF JUVENILE WISTAR RATS

1 In this study we compared the vasoconstrictor activity of melatonin in rat isolated tail artery using two different recording systems, the Halpern pressure myograph and the Halpern-Mulvany wire myograph, with the view to determining a reliable method for obtaining pharmacologic al data on vascular melatonin receptors. In addition, we characterized the melatonin receptor in this preparation, using analogues of melato nin, and examined the activity of various naphthalenic derivatives wit h biological activity in non-vascular models of melatonin receptors. 2 Using the Halpern pressure myograph, cumulative addition of melatonin (0.1 nM to 1 mu M) produced direct vasoconstriction (19.3+/-6.4% redu ction in lumen diameter, n=5) in five of 11 pressurized segments, with pEC(50) of 9.14+/-0.17. Similarly, non-cumulative application of mela tonin caused vasoconstriction (19.7+/-4.6% reduction in lumen diameter , n=7) in seven of 20 preparations examined with pEC(50) of 8.74+/-0.2 6. The selective alpha(2)-adrenoceptor agonist, UK-14304 (5-bromo-6-[2 -imidazolin-2-ylamino]-quinoxaline bitartrate), produced vasoconstrict ion in all 'melatonin-insensitive' preparations. 3 Melatonin (0.1 nM t o 1 mu M) failed to elicit isometric contractions of tail artery segme nts in the Halpern wire myograph, but produced concentration-dependent potentiation of electrically-evoked, isometric contractions (maximum effect of 150-200% enhancement) when applied either noncumulatively (s even of seven preparations) or cumulatively (four of seven preparation s). The pEC(50) value of melatonin (non-cumulative) was 8.50+/-0.10 (n =7) which was not different from that obtained in the pressure myograp h. All further experiments were conducted using a non-cumulative proto col against electrically-evoked, isometric contractions. 4 Based on th e pEC(50) values for the melatonin analogues examined, the pharmacolog ical profile for the enhancement of electrically-evoked contractions w as 2-iodomelatonin > 6-chloromelatonin greater than or equal to (-)-AM MTC greater than or equal to S21634 greater than or equal to melatonin greater than or equal to S20098 > S20242 greater than or equal to S20 304 > 6-hydroxymelatonin > S20932 > (+)-AMMTC > N-acetyl-5-HT. Our dat a suggests the vascular receptor belongs to the MELi-like subtype. All the indole-based analogues of melatonin, 2-iodomelatonin, (-)-AMMTC, (+)-AMMTC, S20932, 6-chloromelatonin, 6-hydroxymelatonin and N-acetyl- 5-HT, behaved as full agonists. All the naphthalenic derivatives exami ned, S21634, S20098, S20242 and S20304 behaved as partial agonists rel ative to melatonin. 5 The naphthalenic-based antagonists, S20928 and S 20929, did not modify electrically-evoked, isometric contractions of t he tail artery, but produced a parallel, rightward displacement of the melatonin concentration-response curve. Based upon the effect of 1 mu M S20928 and S20929, the estimated pK(B) values for these antagonists were 7.18+/-0.25 (n=4) and 7.17+/-0.25 (n=5), respectively.6 We demon strated that enhancement of electrically-evoked, isometric contraction s of the rat isolated tail artery (using the Halpern-Mulvany wire myog raph) is a simple and reproducible model for assessing the activity of putative agonists, partial agonists and antagonists at vascular melat onin receptors. Pharmacological characterization of the receptor sugge sts the presence of a MEL1-like subtype.