Enhanced inhibition by melatonin of alpha-adrenoceptor-induced aortic contraction and inositol phosphate production in vascular smooth muscle cells from spontaneously hypertensive rats.
OBJECTIVE
To test the hypothesis that the enhanced inhibition by melatonin of the norepinephrine-induced vasoconstriction and formation of inositol phosphate in spontaneously hypertensive rats are mediated by its antioxidant effect


METHODS
Aortic rings from spontaneously hypertensive rats and age-matched Wistar-Kyoto rats were used for measuring vascular contraction forces. Cultured aortic smooth muscle cells were prelabelled with myo-[2-3H]-inositol for evaluation of formation of inositol phosphate after exposure to agonist or antagonist Basal or forskolin-induced formation of cyclic AMP was evaluated using a [3H]-cyclic AMP assay system. Oxygen-derived free radicals were generated with a hypoxanthine and xanthine oxidase system.


RESULTS
The inhibition of the norepinephrine-induced aortic contraction by melatonin was more potent in spontaneously hypertensive than it was in Wistar-Kyoto rats. The inhibition of the norepinephrine-induced formation of inositol phosphate by 0.3-300 micromol/l melatonin was also greater in smooth muscle cells from spontaneously hypertensive rats than it was in those cells from Wistar-Kyoto rats. In contrast, the inhibition of the norepinephrine-induced formation of inositol phosphate in smooth muscle cells from spontaneously hypertensive and Wistar-Kyoto rats by 2-iodomelatonin, an agonist of melatonin receptors, was not different. Prazosin, but not yohimbine, eliminated or partially inhibited the norepinephrine-induced formation of inositol phosphate in smooth muscle cells from Wistar-Kyoto rats or from spontaneously hypertensive rats, respectively. In the presence both of prazosin and of melatonin, the norepinephrine-induced production of inositol phosphate was abolished in smooth muscle cells from spontaneously hypertensive rats. Furthermore, superoxide dismutase significantly inhibited the norepinephrine-induced aortic contraction and formation of inositol phosphate in smooth muscle cells from spontaneously hypertensive rats, but not in those cells from Wistar-Kyoto rats. In contrast, catalase had no effect on the norepinephrine-induced formation of inositol phosphate and vascular contraction either in cells from spontaneously hypertensive rats or in cells from Wistar-Kyoto rats. Hypoxanthine-xanthine oxidase induced formation of more inositol phosphate in smooth muscle cells from spontaneously hypertensive rats than it did in those from Wistar-Kyoto rats. Melatonin and superoxide dismutase similarly inhibited the hypoxanthine-xanthine oxidase-induced formation of inositol phosphate more in cells from spontaneously hypertensive rats than it did in those from Wistar-Kyoto rats. However, melatonin had no effect either on basal or on the forskolin-induced formation of cyclic AMP in smooth muscle cells from rats of both strains.


CONCLUSIONS
The enhanced inhibitory effect of melatonin on the norepinephrine-induced cellular production of inositol phosphate in spontaneously hypertensive rats was not mediated by melatonin receptors or alpha-adrenoceptors. Rather, the antioxidant effect of melatonin could become important in spontaneously hypertensive rats, which are suspected to have a lower cellular content of antioxidants or a greater sensitivity to superoxide anions, or both, of the vascular tissue.
Wu
L
L
Department of Physiology, Faculté de Médecine, Université de Montréal, Québec, Canada.
Wang
R
R
de Champlain
J
J
D000818:Animals; D000975:Antioxidants / Q000494:pharmacology; D001011:Aorta / Q000187:drug effects / Q000503:physiopathology; D000242:Cyclic AMP / Q000096:biosynthesis; D006973:Hypertension / Q000503:physiopathology*; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000096:biosynthesis*; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000503:physiopathology; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D011918:Rats, Inbred SHR; D011921:Rats, Inbred WKY; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D013482:Superoxide Dismutase / Q000494:pharmacology; D013481:Superoxides / Q000378:metabolism; D014661:Vasoconstriction / Q000187:drug effects* / Q000502:physiology*
D000975:Antioxidants; D007295:Inositol Phosphates; D011942:Receptors, Adrenergic, alpha; D013481:Superoxides; D000242:Cyclic AMP; D013482:Superoxide Dismutase; D008550:Melatonin; D009638:Norepinephrine