Studies on the vasoconstrictor action of melatonin and putative melatonin receptor ligands in the tail artery of juvenile Wistar rats

Abstract

1 In this study we compared the vasoconstrictor activity of melatonin in rat isolated tail artery using two dierent recording systems, the Halpern pressure myograph and the Halpern-Mulvany wire myograph, with the view to determining a reliable method for obtaining pharmacological data on vascular melatonin receptors. In addition, we characterized the melatonin receptor in this preparation, using analogues of melatonin, and examined the activity of various naphthalenic derivatives with biological activity in non-vascular models of melatonin receptors. 2 Using the Halpern pressure myograph, cumulative addition of melatonin (0.1 nM to 1 mM) produced direct vasoconstriction (19.3+6.4% reduction in lumen diameter, n=5) in ®ve of 11 pressurized segments, with pEC 50 of 9.14+0.17. Similarly, non-cumulative application of melatonin caused vasoconstriction (19.7+4.6% reduction in lumen diameter, n=7) in seven of 20 preparations examined with pEC 50 of 8.74+0.26. The selective alpha 2 -adrenoceptor agonist, UK-14304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline bitartrate), produced vasoconstriction in all`melatonin-insensitive' preparations.3 Melatonin (0.1 nM to 1 mM) failed to elicit isometric contractions of tail artery segments in the Halpern wire myograph, but produced concentration-dependent potentiation of electrically-evoked, isometric contractions (maximum eect of 150 ± 200% enhancement) when applied either noncumulatively (seven of seven preparations) or cumulatively (four of seven preparations). The pEC 50 value of melatonin (non-cumulative) was 8.50+0.10 (n=7) which was not dierent from that obtained in the pressure myograph. All further experiments were conducted using a non-cumulative protocol against electrically-evoked, isometric contractions. 4 Based on the pEC 50 values for the melatonin analogues examined, the pharmacological pro®le for the enhancement of electrically-evoked contractions was 2-iodomelatonin46-chloromelatonin5(7)-AMMTC 5 S216345melatonin5S200984S202425S2030446-hydroxymelatonin4S209324 (+) -AM-MTC4N-acetyl-5-HT. Our data suggests the vascular receptor belongs to the MEL 1 -like subtype. All the indole-based analogues of melatonin, 2-iodomelatonin, (7)-AMMTC, (+)-AMMTC, S20932, 6-chloromelatonin, 6-hydroxymelatonin and N-acetyl-5-HT, behaved as full agonists. All the naphthalenic derivatives examined, S21634, S20098, S20242 and S20304 behaved as partial agonists relative to melatonin. 5 The naphthalenic-based antagonists, S20928 and S20929, did not modify electrically-evoked, isometric contractions of the tail artery, but produced a parallel, rightward displacement of the melatonin concentration-response curve. Based upon the eect of 1 mM 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 demonstrated that enhancement of electrically-evoked, isometric contractions of the rat isolated tail artery (using the Halpern-Mulvany wire myograph) is a simple and reproducible model for assessing the activity of putative a...

MT2Melatonin Receptors Are Present and Functional in Rat Caudal Artery

“…MT 1 melatonin receptor mRNA from Wistar rat caudal arteries was previously detected by RT-PCR (Ting et al, 1997), but these authors were unable to amplify mRNA for the MT 2 melatonin receptor. The discrepancy with our results could be attributed to differences in experimental conditions.…”

“…First, our RT-PCR studies used RNA isolated from caudal arteries pooled from six to eight rats, providing a higher number of MT 2 mRNA copies. In the previous study, RNA was isolated and reverse transcribed from arteries of individual animals (Ting et al, 1997). Second, in the present study, the sensitivity of mRNA detection was increased by using the OneStep PCR technique (Limbach et al, 1999).…”

“…Both vasoconstrictor and vasodilator responses have been reported; however, data supporting the presence of melatonin receptors is found in some, but not all, vascular beds (Mahle et al, 1997). In isolated rat caudal arteries, nanomolar concentrations of melatonin potentiate contraction induced by either endogenous or exogenous vasoconstrictors (Viswanathan et al, 1990;Evans et al, 1992;Krause et al, 1995;Mahle et al, 1997;Ting et al, 1997;Geary et al, 1998). In rat cerebral arteries and arterioles, melatonin is a direct vasoconstrictor (Geary et al, 1997;Viswanathan et al, 1997).…”

“…It is now well accepted that activation of MT 1 melatonin receptors in caudal arteries facilitates adrenergic vasoconstriction (Krause et al, 1995;Ting et al, 1997Ting et al, , 1999Bucher et al, 1999;Lew and Flanders, 1999). However, definitive demonstration of vascular MT 2 receptors has been hampered by technical difficulties and discrepancies in technical approaches, rat strains and gender, drug types, and concentrations.…”

“…In rat cerebral arteries and arterioles, melatonin is a direct vasoconstrictor (Geary et al, 1997;Viswanathan et al, 1997). Melatonin-mediated contraction in caudal and cerebral arteries is blocked by melatonin receptor antagonists (Viswanathan et al, 1992;Krause et al, 1995;Geary et al, 1997;Ting et al, 1997Ting et al, , 1999.…”