title: Evidence for direct vasoconstrictor activity of melatonin in "pressurized" segments of isolated caudal artery from juvenile rats.
abstract: Responses of isolated, 60 mmHg 'pressurized' segments of the distal caudal artery of adult and juvenile Wistar rats to melatonin and the selective alpha 2-adrenoceptor agonist 5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline bitartrate (UK-14304) were examined using the Halpern pressure myograph. Melatonin showed no direct vasoconstrictor activity in vessels from adult rats, whereas UK-14304 produced moderate vasoconstriction (pD2-7.43 +/- 0.09). In the presence of phenylephrine-induced tone, melatonin produced a variable but small constrictor response (less than 10 microns reduction in diameter) in some vessels; the response to 1 mumol/l UK-14304 was less than in the absence of tone. In vessels isolated from juvenile rats, melatonin caused concentration-dependent vasoconstriction with a maximum response about 70% of the maximum response elicited by UK-14304. Vessels from juvenile rats were more sensitive to melatonin (pD2-9.40 +/- 0.07) than they were to UK-14304 (pD2-8.12 +/- 0.14). In the presence of phenylephrine-induced tone, the vasoconstrictor responses to both melatonin and IK-14304 were markedly less; the sensitivity to melatonin was not different from that seen in the absence of tone. These findings indicate that 'pressurized' segments of the isolated distal caudal artery may provide a simple and convenient, functional model of melatonin receptors. The findings also appear to implicate melatonin in thermoregulatory processes in juvenile rats.
authors.affiliation: Department of Physiology and Pharmacology, University of Nottingham Medical School, Queen's Medical Centre, England.
authors.forename: B K
authors.initials: BK
authors.lastname: Evans
authors.forename: R
authors.initials: R
authors.lastname: Mason
authors.forename: V G
authors.initials: VG
authors.lastname: Wilson
mesh_terms: D000375:Aging / Q000378:metabolism; D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology; D001158:Arteries / Q000187:drug effects; D000068438:Brimonidine Tartrate; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D014662:Vasoconstrictor Agents / Q000494:pharmacology*
chemical_list: D000959:Antihypertensive Agents; D011810:Quinoxalines; D014662:Vasoconstrictor Agents; D000068438:Brimonidine Tartrate; D008550:Melatonin