Makoto Nishiwada

Mechanisms of inhibition of endothelium-dependent relaxation by halothane, isoflurane, and sevoflurane.

Volatile anaesthetics inhibit endothelium-dependent relaxation, but the underlying mechanism(s) have not been clarified. In an attempt to elucidate the mechanism(s), we determined the effects of halothane, isoflurane and sevoflurane on relaxation induced by acetylcholine and sodium nitro-prusside (SNP) and the cGMP formation elicited by exogenous nitric oxide (NO) and SNP in rat aortas. Acetylcholine (10−7−10−5M) - induced relaxation was attenuated by halothane (2%), isoflurane (2%) and sevoflurane (4%). SNP (10−8 M) - induced relaxation was reduced by halothane (2%), but not by isoflurane (2%) or sevoflurane (4%). The cGMP level of NO-stimulated aorta was reduced by halothane (2%) and sevoflurane (4%), but not by isoflurane (2%). The cGMP level of SNP (10−7 M) - stimulated aorta was reduced by halothane (2%), but not by isoflurane (2%) and sevoflurane (4%). We conclude that the mechanisms responsible for the inhibition of endothelium-dependent relaxation differ among anaesthetics. Isoflurane inhibits the relaxation mainly by inhibiting the formation of NO in the endothelium. In contrast, the effect of halothane on endotheliumdependent relaxation may be largely due to the inhibition of action of NO in the vascular smooth muscle and the effect of sevoflurane may be to inactivate NO or to inhibit the action of NO.RésuméLes agents anesthésiques volatils inhibent la relaxation d’origine endothéliale dont le mécanisme sous-jacent n’a pas été éclairci. Dans le but d’en élucider le(s) mécanisme(s), nous avons déterminé sur des aortes de rats les effets de l’halothane, de l’isoflurane et du sévoflurane sur la relaxation induite par l’acétylcholine et le nitroprussiate de sodium (SNP), et la synthèse de cGMP élicitée par l’oxyde nitrique (NO) et le SNP La relaxation induite par l’acétylcholine (10−7−10−5 M) est atténuée par l’halothane 2%, l’isoflurane 2% et le sévoflurane 4%. La relaxation induite par le SNP (10−8 M) est diminuée par l’halothane 2%, mais non par l’isoflurane 2% ou le sévoflurane 4%. Le niveau de cGMP de l’aorte stimulée par le NO est diminué par l’halothane 2% et le sévoflurane 4% mais non par l’isoflurane 2%. Le niveau de cGMP de l’aorte stimulée par le SNP (10−7) est diminué par l’halothane 2%, mais non par l’isoflurane 2% et le sévoflurane 4%. Nous concluons que les mécanismes responsables de l’inhibition de la relaxation d’origine endothéliale different selon l’anesthésique. L’isoflurane inhibe la relaxation principalement en inhibiant la synthèse endothéliale de NO. Par contre, l’effet de l’halothane sur la relaxation d’origine endothéliale peut être en grande partie due à l’inhibition de l’activité du NO sur le muscle vasculaire lisse et l’effet du sévoflurane peut être dû à l’inactivation du NO ou à l’inhibition de l’activité du NO.

Halothane and isoflurane inhibit endothelium-dependent relaxation elicited by acetylcholine

The purpose of this study was to determine whether volatile anesthetics modify the release of endothelium-derived relaxing factor. We examined the effects of halothane and isoflurane on endothelium-dependent relaxation and 3′,5′-cyclic guanosine monophosphate formation elicited by acetylcholine and ionophore A23187 in isolated rat aorta. Halothane and isoflurane (1%--2%) significantly attenuated acetylcholine-induced relaxation of the phenylephrine-contracted aorta but had no significant effect on relaxation induced by A23187, nitroprusside, and nitroglycerin. Basal and A23187 (10−7 M)-stimulated levels of 3′,5′-cyclic guanosine monophosphate were slightly lowered by halothane and isoflurane (2%). In contrast, the increase of 3′, 5′-cyclic guanosine monophosphate elicited by acetylcholine (10−5 M) was significantly attenuated by halothane (2%) and abolished by isoflurane (2%). These findings indicate that halothane and isoflurane strongly inhibit the release of endothelium-derived relaxing factor elicited by acetylcholine.

Comparison of the Direct Effects of Halothane and Isoflurane on Large and Small Coronary Arteries Isolated from Dogs

Relaxant responses to halothane and isoflurane were compared in helical strips of dog epicardial coronary arteries of different sizes: proximal large coronary arteries with outside diameters (OD) larger than 2.5 mm and distal small arteries with 0.7-0.9 mm OD. Responses to pharmacologic vasodilators, including nitroglycerin (NTG) and adenosine, were also studied for comparison. The relaxation induced by halothane in concentrations of 0.8-2.3% and by NTG (10(-9)-10(-5) M) was greater in proximal large coronary arteries than in distal small ones contracted with 20 mM KCl. In contrast, the relaxation by isoflurane (1.2-3.5%) and by adenosine (10(-8)-10(-4) M) was greater in small coronary arteries than in large ones. These results suggest that isoflurane is, like adenosine, preferentially a small artery dilator.

Halothane and Enflurane Constrict Canine Mesenteric Arteries by Releasing Ca2+ from Intracellular Ca2+ Stores

Background:Recent studies suggest that volatile anesthetics cause not only vasodilation but also vasoconstriction, depending on the experimental conditions. However, the mechanism of the constrictive effect of volatile anesthetics has not been clarified. The aim of this study was to evaluate the vasoconstrictor effects of halothane, enflurane, and isoflurane and to elucidate the underlying mechanism. Methods:Vascular rings of canine mesenteric arteries were mounted in organ baths, and isometric tension changes were recorded. Changes in intracellular free Ca2+ concentration of vascular smooth muscle were examined by using the fluorescent Ca2+ indicator fura 2 and a dual-wavelength fluorometer. Results:Halothane (0.75–2.3%) and enflurane (1.7–3.4%), but not isoflurane (1.2–3.5%), induced a concentration-dependent transient contraction, followed by a slight, sustained contraction. Halothane (1.5%)- and enflurane (3.4%)-induced contractions were reduced by endothelial denudation and enhanced by indomethacin (10-5 M) treatment but were not affected by L-NG-nitroarginine (10-5 m) or nifedipine (2 X 10-7 M) treatment. Ryanodine (2 X 10-5 M) treatment completely abolished the transient increases in tension and Ca2+ concentration. Even in ryanodine-treated arteries, however, both anesthetics induced a slowly developing sustained contraction, and the sustained contraction induced by enflurane (3.4%) was not accompanied by a significant increase in Ca2+ concentration. Conclusions:Halothane and enflurane, but not isoflurane, induce vasoconstriction by releasing Ca2+ from intracellular stores. Release of a vasodilating prostanoid and endothelium-derived constricting factor may also be involved in the vasoconstrictor effect. Furthermore, increased Ca2+ sensitivity of contractile machinery may be involved in the effect of enflurane.

Effects of Thiobarbiturates on Smooth Muscle Reactivity in Isolated Aortas From Spontaneously Hypertensive Rats

The direct effects of thiobarbiturates on helical strips of aortas from spontaneously hypertensive (SH) rats were compared with those from Wistar-Kyoto (WKY) rats. At 5–6 wk of age, the arterial pressure of SH and WKY rats did not differ, and the effects of thiobarbiturates on aortic strips from SH and WKY rats were similar. By contrast, at the age of 10–12 or 20–21 wk arterial pressure was higher in SH than in WKY rats, and responses to thiobarbiturates differed in aortic strips from SH and WKY rats: contractile responses were greater in WKY than in SH rats, and relaxing effects were greater in SH than in WKY rats. Responses to sodium nitroprusside did not differ in the aortas of SH and WKY rats, but the effects of nifedipine were greater in strips from SH rats than from WKY rats at the age of 10–12 wk. Ca2+ -induced contractions of strips exposed to Ca2+ -free media and depolarized by high K+ were inhibited by treatment with thiamylal; the inhibition was greater in SH than in WKY rats. The increase in smooth-muscle relaxation induced by thiobarbiturates in strips from SH rats may be due to increased sensitivity to the Ca2+ -channel blocking action of thiobarbiturates.

Halothane and isoflurane preferentially inhibit prostanoid-induced vasoconstriction of rat aorta

In a previous study, we demonstrated that halothane and isoflurane inhibit binding of thromboxane A2 to its receptors on human platelets and thus inhibit prostanoid-induced aggregation strongly. The aim of this study was to determine whether volatile anaesthetics inhibit prostanoid-induced vasoconstriction preferentially. Rat isolated aortic rings were mounted in organ baths and their isometric tension was measured. They were contracted with STA2 (a stable thromboxane A2 analogue), prostaglandin F2α (PGF2α), phenylephrine, and 20 mM KCl, and then exposed to halothane (0.5–3%), isoflurane (0.5–3%), and sodium nitroprusside (SNP, 10−9−3 × 10−7 M). Halothane (2–3%) and isoflurane (2–3%) induced greater relaxation of aortic rings precontracted with STA2 and PGF2α than of those precontracted with phenylephrine (P < 0.01). Halothane induced greater relaxation of rings precontracted with KCl than phenylephrine only at 3%, whereas isoflurance relaxed rings precontracted with KCl more than those with phenylephrine at 0.5, 2 and 3% (P < 0.05). In contrast, SNP relaxed rings precontracted with PGF2α, KCl and phenylephrine equally, but induced smaller relaxations of those precontracted with STA2 (P < 0.05). We conclude that halothane and isoflurane inhibit prostanoid-induced vasoconstriction preferentially, possibly by interacting with prostanoid receptors.RésuméLors d’une étude antérieure, nous avons démontré que l’halothane et l’isoflurane inhibaient la liaison de la thomboxane A2 avec ses récepteurs situés sur les plaquettes humaines et inhibaient fortement ainsi l’agrégation induite par les prostanoïdes. Cette étude vise à déterminer si les anesthésiques volatils inhibent la vasoconstriction induite par les prostanoïdes de façon préférentielle. Des anneaux isolés d’aorte de rat sont introduits dans des bains organiques et leur tension isométrique est mesurée. On les fait a’abord contracter avec du STA2 (un analogue stable de la thomboxane A2), de la prostaglandine F2α (PGF2α), de la phényléphrine, et 20 mM de KCl, et on les expose ensuite à l’halothane (0,5% à 3,0%), l’isoflurane (0,5%–0,3%) et au nitroprussiate de soude (SNP, 10−9−3 × 10−7 M). Vhalothane (2–3%) et l’isoflurane (2–3%) produisent une plus grande relaxation des anneaux aortiques précontractés avec la phényléphrine (P < 0,01). L’halothane produit, mais seulement à 3%, te plus grande relaxation des anneaux précontractés avec le KCL qu’avec la phényléphrine, alors que l’isoflurane aux concentrations de 0,5 2 et 3% relaxe les anneaux précontractés avec le KCl plus que ceux contractés avec la phényléphrine (P < 0,05). Par contre, le SNP relaxe également les anneaux précontractés avec la PGF2α, le KCl et la phényléphrine, mais produit une relaxation moins grande sur ceux qui ont été précontractés avec du STA2 (P < 0,05). Nous concluons que lhalothane et l’isoflurane inhibent la vasoconstriction induite par les prostanoïdes de façon préférentielle, possiblement par interaction avec les récepteurs prostanoïdes.

The relaxing effects of barbiturates in vascular smooth muscle of rat aorta.

The effects of thiamylal and pentobarbital on contractions mediated through the influx of extracellular Ca++ and the release of intracellularly stored Ca++ were compared in rat aortic strips. Thiamylal (3×10−5M to 10−3M) and pentobarbital (10−4 to 10−3M) significantly attenuated the contraction induced by KCl (20 mM), and shifted the dose-response curve for Ca++ of KCl (20 mM)-treated strips downwards and to the right. Caffeine (l0−2M)-induced contraction was significantly attenuated by thiamylal at concentrations greater than 10−4M and by pentobarbital at 3×10−4M. Only a high concentration (l0−3M) of these barbiturates significantly inhibited the contractions induced by norepinephrine (NE, 10−6M) in Ca++-free medium. Contraction of strips without endothelium by a Ca++ ionophore, A23187 (5×10−6M), in the presence of a Ca channel blocker, was relaxed by high concentrations of thiamylal (3×10−4M to 10−3M) and pentobarbital (l0−3M). It is concluded that thiamylal inhibits contraction through an intracellular action as well as a Ca channel-blocking action in vascular smooth muscle of rat aorta. However, the intracellular action of pentobarbital is less potent than that of thiamylal.

Age-dependent Alterations in the Response of Isolated Rat Aortas to Thiobarbiturates

Responses to thiamylal and thiopental were compared in helical strips of rat thoracic aortas of different ages (5–6, 10–12 and 20–22 weeks old), precontracted partially with phenylephrine or prostaglandin F2α (PGF2α). Thiamylal and thiopental, in concentrations of 3 × 10−5 to 10−4M, produced a dosedependent relaxation in aortas at 5–6 weeks of age, no significant change of tension in those at 10–12 weeks of age, and a marked constriction in those at 20–22 weeks of age. These thiobarbiturates, in a high concentration of 10−3M, produced a profound relaxation in aortas at any age studied. It is concluded that the responses to thiobarbiturates of thoracic aorta precontracted with phenylephrine or PGF2α alter with age.