Involvement of potassium channels in the protective effect of 17beta-estradiol on hypercholesterolemic rabbit carotid artery.

The involvement of endothelium-derived hyperpolarizing factor (EDHF) in the protective effect of 17beta-estradiol was investigated on the phenylephrine-precontracted carotid artery from cholesterol fed rabbits. Animals were fed for 8 weeks as follows: control group, standard chow; (control+estradiol) group, standard chow+17beta-estradiol; standard chow+1% cholesterol, cholesterol group; or (cholesterol+estradiol) group, 1% cholesterol chow+17beta-estradiol. Relaxations to acetylcholine (ACh) (3 nM-30 microM) were performed with N(omega) nitro-L-arginine methyl ester (300 microM) and indomethacin (10 microM). Charybdotoxin (50 nM)+apamin (50 nM), glibenclamide (10 microM) or 4-aminopyridine (1 mM) were used to block, respectively, calcium-activated-K(+), adenosine triphosphate (ATP)-sensitive-K(+) and voltage-dependent K(+) channels. In the control group, ACh induced a residual concentration-dependent relaxation. This response was impaired by hypercholesterolemia and restored by 17beta-estradiol. In control and cholesterol groups, 4-aminopyridine or glibenclamide did not affect this relaxation, but in (control+estradiol) and (cholesterol+estradiol) groups, glibenclamide suppressed it. In all groups, this persisting relaxation was completely abolished by charybdotoxin alone or with apamin, by hemoglobin (10 microM), a nitric oxide scavenger, or by LY83183 (10 microM), a guanylate cyclase inhibitor. Thus, in the rabbit carotid artery, the protective effect of 17beta-estradiol against hypercholesterolemia is probably mediated by a nitric oxide/cyclic GMP pathway which activates calcium-targeted and ATP-dependent K(+) channels.

Involvement of cyclooxygenase 2 in the protective effect of 17beta-estradiol on hypercholesterolemic rabbit aorta.

The involvement of cyclooxygenase (COX) in the effects of 17beta-estradiol was investigated on hypercholesterolemic rabbits aorta. Acetylsalycilic acid, nimesulide, or SQ22536 was used as respective antagonist of COX-1, COX-2, or adenylate cyclase using aortic rings precontracted with phenylephrine and exposed to cumulative concentrations of acetylcholine (ACh). The relaxation effect of ACh was impaired by hypercholesterolemia and restored by an 8-week 17beta-estradiol treatment. In the control group treated with estrogen, nimesulide, acetylsalycilic acid, or SQ22536 slightly reduced the response to ACh. In hypercholesterolemic rabbits treated with estrogen, nimesulide significantly reduced the maximal relaxation and shifted to the right the relaxation curve of ACh, whereas acetylsalycilic acid did not modify the maximal response to ACh but displaced slightly the concentration-response curve. SQ22536 reduced the relaxant effect of ACh down to the level obtained in the presence of nimesulide. These results suggest that the protective effect of 17beta-estradiol against hypercholesterolemia involved COX-2/adenylate cyclase pathway.

Effects of treatment with 17beta-estradiol on the hypercholesterolemic rabbit middle cerebral artery.

OBJECTIVE: The effects of acute and long-term treatment with 17beta-estradiol on the vasomotor responses of rabbit middle cerebral artery (RMCA) were investigated. METHODS: For 8 weeks, male rabbits consumed standard chow (control group), standard chow+1% cholesterol (cholesterol group) or 1% cholesterol chow+17beta-estradiol (i.m. injection 700 microg per week) (estradiol group). The RMCA was precontracted with high K(+) solution and exposed to agonists. RESULTS: Acute exposure to 17beta-estradiol strongly induced relaxation of the RMCA isolated from either control or cholesterol groups. This effect was endothelium independent. Incubation with 17beta-estradiol shifted the calcium contraction curve to the right. High cholesterol diet impaired the relaxation induced by acetylcholine and did not alter relaxation to sodium nitroprusside or to papaverine. Chronic treatment with 17beta-estradiol restored this impaired relaxation to acetylcholine. This protective effect of estradiol was significantly reduced in the presence of N(omega) nitro-L-arginine methyl ester, a constitutive nitric oxide-synthase inhibitor and was not modified in the presence of aminoguanidine, an inducible nitric oxide-synthase inhibitor. Neither tetrabutylammonium, a blocker of calcium-activated K(+) channels, nor glibenclamide, a blocker of ATP-sensitive K(+) channels, affected concentration-response to acetylcholine in the RMCA of the estradiol group, whereas 4-aminopyridine, a blocker of voltage-dependent K(+) channels strongly inhibited this relaxation. CONCLUSIONS: These results suggest that acute effects of 17beta-estradiol in the RMCA is mediated through blockade of calcium entry into vascular smooth muscle cells, while chronic treatment with this hormone seems to be mediated by release of nitric oxide which activates voltage-dependent potassium channels.

[17B-estradiol and hypercholesterolemia: involvement of nitric oxide]. / 17B-estradiol et hypercholestérolémie: implication du monoxyde d'azote.

The mechanisms of the protective effect of 17B-estradiol were investigated in the middle cerebral artery (MCA) and aorta isolated from cholesterol-fed rabbits. Three groups were assigned: control group (standard chow), cholesterol group (standard chow+1% cholesterol) and estradiol group (1% cholesterol+17B-estradiol). The MCA and the aorta were isolated, precontracted respectively with high K(+) solution or with phenylephrine and exposed to cumulative acetylcholine concentrations. In the control group, acetylcholine induced a concentration-dependent relaxation in the aorta and the MCA. Cholesterol diet for eight months reduced significantly the maximal response to acetylcholine by about 50% in the aorta and by about 30% in the MCA. The chronic treatment with 17B-estradiol restored this impaired relaxations to acetylcholine. Incubation of arteries from estradiol group with N(omega)-nitro L-arginine methyl-ester (L-NAME), a potent inhibitor of constitutive nitric oxide synthase, entirely abolished the relaxation to acetylcholine while aminoguanidine, a potent inhibitor of inducible nitric oxide synthase, did not affect this relaxation. These observations suggest that the protective effect of 17B-estradiol against hypercholesterolemia is mediated via a release of endothelial nitric oxide.

The protective effect of 17 beta-estradiol on vasomotor responses of aorta from cholesterol-fed rabbit is reduced by inhibitors of superoxide dismutase and catalase.

The involvement of endogenous antioxidant enzymes in the protective effect of 17 beta-estradiol against hypercholesterolemia was evaluated on aorta from cholesterol-fed rabbits treated with estradiol. 17 beta-Estradiol, more potent than alpha-tocopherol, restored the acetylcholine-induced relaxation, which was impaired by cholesterol chow, and enhanced the vasorelaxation induced by sodium nitroprusside. Diethyldithiocarbamate (5 mM), a superoxide dismutase (SOD) inhibitor, reduced relaxation to acetylcholine down to the level obtained in cholesterol-fed rabbits not treated with estrogen. This inhibition was dose-dependently reversed by addition of exogenous SOD. Aminotriazole (5 mM), a catalase inhibitor, reduced slightly this response, which was not reversed by exogenous catalase. A similar concentration of diethyldithiocarbamate prevented the potentiation of response to sodium nitroprusside induced by estrogen, whereas aminotriazole had no effect. These results suggest that, on aorta from cholesterol-fed rabbit, superoxide dismutase and catalase are involved in the protective effect of estrogen on the vasomotor response to acetylcholine, but only superoxide dismutase participates in response to sodium nitroprusside.

Reduction of the pressor response to nicotine in the guinea pigs by a histamine (H3) agonist is attenuated by an inhibitor of nitric oxide synthesis.

We examined whether a histamine (H3)-agonist, (R) alpha-methylhistamine, [(R) alpha-MeHA] reduced the pressor responses induced by nicotine in urethane-anesthetized guinea pigs treated by atropine. Nicotine dose-dependently increased the basal mean arterial pressure (MAP) and the heart rate (HR) of the preparation. Both effects were due to stimulation of sympathetic ganglia, since muscarinic receptors were blocked. Adrenalectomy did not affect either the hypertension or the tachycardia to nicotine. Nicotine (7 micrograms kg-1) evoked a transient hypertension of approximately 30 mm Hg and a tachycardia by approximately 20 beats/min. (R) alpha-MeHA dose-dependently inhibited the increase in mean arterial pressure and the increase in HR to nicotine but not those produced by exogenous norepinephrine (NE). The inhibitory effects of (R) alpha-MeHA were dose-dependently antagonized by the H3-antagonist thioperamide, but not by combined mepyramine/cimetidine. They were also suppressed by a nitric oxide (NO)-synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME); this suppression was reversed by L-arginine. Histamine in the presence of mepyramine and cimetidine induced a similar inhibition of the hypertension to nicotine but a less potent inhibition of the tachycardia. These findings indicate that postganglionic noradrenergic nerve fibers are endowed with presynaptic H3-receptors, the stimulation of which inhibits NE release through an NO mechanism.

Endothelium-dependent relaxation of rabbit middle cerebral artery to a histamine H3-agonist is reduced by inhibitors of nitric oxide and prostacyclin synthesis.

1. The possible involvement of prostanoids and endothelium-derived relaxing factor (EDRF) in the vasodilatation induced by a histamine H3-agonist was examined in the rabbit perfused middle cerebral artery preconstricted with K+ (50 mM). 2. The endothelium-dependent relaxation to (R)-alpha-methylhistamine [(R)-alpha-MeHA] was competitively antagonized by thioperamide (an H3-antagonist) with a pA2 of 9.05, but unaffected by propranolol, atropine, L- and D-sulpiride. This effect was stereoselective since the (S)-isomer was 100 times less potent than the (R)-isomer. 3. Two inhibitors of nitric oxide synthesis, NG-nitro-L-arginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA), inhibited the relaxation induced by (R)-alpha-methylhistamine. The inhibitory effects of 10(-5) M NG-nitro-L-arginine methyl ester and 10(-5) M NG-monomethyl-L-arginine were reversed by equimolar concentrations of L-arginine, but strongly enhanced by 10(-4) M tranylcypromine. Tranylcypromine alone (10(-5) M-10(-4) M) partially reduced the (R)-alpha-methylhistamine-induced relaxation. Both dexamethasone and indomethacin also inhibited this relaxation. 4. The results suggest that the H3-mediated relaxation of the rabbit middle cerebral artery may involve release of both a prostanoid, probably prostacyclin, and endothelium-derived relaxing factor. The relaxant effects of these two endogenous compounds appear to be synergistic.

A highly potent and selective H3 agonist relaxes rabbit middle cerebral artery, in vitro.

An H3-agonist, R alpha methylhistamine strongly relaxed isolated and perfused rabbit middle cerebral artery precontracted with K+. This vasodilatation was not antagonized by either mepyramine or cimetidine but was competitively antagonized by an H3-antagonist, a mixture of impromidine and cimetidine. Histamine activated the H3-sites in this artery since its concentration-response curve (obtained with mepyramine and cimetidine) was parallel to that of R alpha methylhistamine. Our data indicate that H3-sites could exist in the rabbit cerebral artery.

Relaxation of rabbit middle cerebral arteries in vitro by H1 histaminergic agonists is inhibited by indomethacin and tranylcypromine.

The H1-histaminergic agonists 2-pyridylethylamine (2-PEA) and 2-methylhistamine relaxed potassium-constricted, perfused, rabbit middle cerebral arteries at low concentrations (3 x 10(-11) to 3 x 10(-8) M) and constricted them at high concentrations (3 x 10(-7) to 3 x 10(-4) M). The relaxation and the contraction were not antagonized by propranolol (up to 3 x 10(-6) M) given 30 min before, suggesting that beta-adrenergic mechanisms were not involved. When 2-PEA was tested on arteries constricted with uridine triphosphate (UTP), similar results were obtained. In the UTP-constricted arteries, the 2-PEA-induced responses were competitively antagonized by 3 x 10(-9) M mepyramine. Together with previous work (Ea Kim et al., 1986), these results are compatible with the hypothesis that H1-receptors were responsible for both the relaxation and the contraction observed. When either indomethacin (10(-8), 3 x 10(-7), or 10(-5) M), dexamethasone (10(-5) M), or tranylcypromine (10(-5) or 10(-4) M) were tested on the response to 2-PEA or 2-methylhistamine, these inhibitors suppressed the relaxation or reversed it to a contraction. Furthermore, they potentiated the contraction induced by these agonists. These results favour the hypothesis that the H1-mediated relaxation in rabbit cerebral arteries may in part involve the release of prostaglandins, especially prostacyclin. The participation of such a prostanoid in histaminergic relaxation seems exclusively an H1-mediated mechanism, since the relaxation induced by the H2-agonist dimaprit (in the presence of mepyramine) was not antagonized by either indomethacin (3 x 10(-7) M) or tranylcypromine (10(-4) M).