Attenuation of contraction of isolated canine coronary arteries by enflurane and halothane

[Effects of anesthetic agents on arterial reactivity]

OBJECTIVE

To review the effects of halogenated and intravenous anaesthetics on arterial vasoreactivity.

DATA SOURCE

Articles were obtained from a MEDLINE review (search terms: 'vascular smooth muscle, endothelium' used separately or associated with following anaesthetic agents: 'halothane, isoflurane, enflurane, desflurane, sevoflurane, thiopentone, propofol, ketamine, etomidate'. Other sources included review articles and textbooks.

STUDY SELECTION AND DATA EXTRACTION

All experimental studies published since 1975 were analysed and pertinent data extracted.

DATA SYNTHESIS

Within the vascular wall, arterial vasoreactivity involves the endothelium and the vascular smooth muscle. In vivo, arterial vasoreactivity is regulated by neuronal, hormonal, and metabolic factors. In vitro, the direct action of anaesthetic agents on the vessel can be studied in the absence of such factors. In vitro studies with arterial rings have shown that inhalational anaesthetics directly decrease endothelium-independent contraction induced by various pharmacological agents. This direct effect of anaesthetics results from a decrease in intracellular calcium, mainly caused by an inhibition of transsarcoplasmic calcium influx. Volatile anaesthetics decrease endothelium-dependent vasorelaxation at a site(s) within the nitric oxide (NO) signalling pathway, located downstream from the NO-related receptors and upstream from guanylyl cyclase. They may also decrease endothelium-independent vasorelaxation by inhibiting NO activation of guanylate cyclase. Intravenous anaesthetics, such as propofol, barbiturates, ketamine and etomidate also decrease vasoconstriction by various degrees. Propofol is the most potent inhibitor of vasoconstriction and thiopental the least one. All these IV anaesthetics have been shown to inhibit in some circumstances both endothelium-dependent and -independent vasorelaxation. Further studies are required to enable a better understanding of the mechanism and the site of action of these vascular effects of anaesthetics. For example, the investigation of the effects of anaesthetic agents on vascular reactivity in diseases associated with endothelial dysfunction may indirectly provide insight into the role of endothelium.

Effects of volatile anesthetics on the calcium ionophore A23187-mediated alterations in hepatic flow and metabolism in the perfused liver in fasted rats

Alterations in intracellular calcium homeostasis have been implicated in heptic injury. Volatile anesthetics modulate the homeostasis of intracellular calcium. The effects of volatile anesthetics on the hemodynamic and metabolic alterations induced by the calcium ionophore A23187 were studied using isolated liver perfusion in fasted rats. The liver was isolated from 24 hr-fasted male Sprague-Dawley rats, and perfused through the portal vein at a constant pressure of 1.2 kPa in a recirculating perfusion-aeration system. Halothane, isoflurane and sevoflurane were administered at 2%, 3% and 4.4%, respectively. All volatile anesthetics maintained basal hepatic flow, reduced oxygen consumption, and transiently enhanced net lactate production. A23187 at initial concentrations of 0.8 to 3.2 microM decreased hepatic flow and oxygen consumption in a dose-dependent manner, and enhanced lactate production. All anesthetics significantly attenuated the decreases in hepatic flow and oxygen consumption after administration of A23187 at 1.6 microM. None of the anesthetics significantly influenced the A23187-induced enhancement of net lactate production. Volatile anesthetics may attenuate the hepatic vasoconstriction and oxygen debt induced by intracellular calcium overload.

Isoflurane reduces K+ current in single smooth muscle cells of guinea pig portal vein

Volatile anesthetics vasodilate in part by direct action on vascular smooth muscle. Isoflurane-induced relaxation of portal vein smooth muscle involves alteration of membrane ionic currents that control cell excitability and contraction. Whole cell voltage clamp technique was used to examine outward Ca(2+)-activated K+ current (IK,Ca) in guinea pig portal vein cells. Isoflurane caused a concentration-dependent reduction in IK,Ca at steady-state conditions but had no significant effect on resting potential. Isoflurane transiently potentiated IK,Ca by a mechanism that may partly involve Ca2+ release from intracellular storage sites. The depression of IK,Ca by isoflurane may occur by direct action on the channel protein or on the lipid environment of the channel to alter conductance or kinetic properties. Since isoflurane reduces IK,Ca coincident with suppression of Ca2+ channel current, it was concluded that the depression of IK,Ca by isoflurane is of secondary importance to reduction in inward Ca2+ channel current. Potentiation of IK,Ca may preclude significant membrane activation during the onset of isoflurane's action.

Effects of halothane, isoflurane and sevoflurane on calcium-related contraction in porcine coronary arteries

BACKGROUND

Volatile anaesthetics have a direct inhibitory effect upon epicardial coronary arterial smooth muscles (1-4). The site and mode of their action at the cellular level need to be clarified, which was the purpose of our study. The present investigation attempted to answer the question in what way volatile anaesthetics influence Ca(2+)-related contraction in isolated porcine epicardial coronary to understand their intracellular mechanism.

METHODS

Isolated helical strips of porcine epicardial coronary artery without endothelium were suspended for isotonic contraction recordings in Krebs-Ringer's solution. 9.4 x 10(-2) MK+, 2.5 x 10(-3) M Ca(2+)-induced shortening of the strips was regarded as the reference value (100%). After incubation in Ca(2+)-free solution with 10(-3) M ethlene glycol bis (beta-aminoethyl ether)-N, N-tetraacetic acid (EGTA) for 60 minutes, the muscle strips were exposed to increasing Ca2+ concentrations (10(-4)-10(-2)) either in the presence or absence of 1.5 or 2.5 minimum alveolar concentration (MAC) halothane, isoflurane or sevoflurane, with 9.4 x 10(-2) M K+ bath solution.

RESULTS

All three drug groups produced apparent biphasic effects with a cumulative increase of Ca2+ concentration compared with control groups. An initial increase at low Ca2+ concentration was followed by a decrease of Ca(2+)-activated contractions. Isoflurane affected Ca(2+)-induced contraction significantly more than halothane and sevoflurane.

CONCLUSIONS

The results imply that volatile anaesthetics influence Ca(2+)-dependent activity of coronary smooth muscle by complex mechanisms, which involve promotion of intracellular Ca2+ release and other mechanisms that alter sensitivity to calcium.

Effects of enflurane on adrenergic function in canine mesenteric artery and vein

We examined the pre- and postsynaptic effects of enflurane on the superior mesenteric artery and vein. We measured the release of norepinephrine (NE) from sympathetic nerve terminals caused by electrical stimulation (ES) or potassium and changes in vascular smooth muscle tension. The effect of enflurane was also examined in the presence of alpha 1- and alpha 2-adrenoceptor antagonists. Enflurane (1.4 to 1.6 minimum alveolar anesthetic concentration [MAC] in the dog) did not alter basal tension, but it significantly inhibited tension development caused by 40 mM KCl and ES in the canine mesenteric artery and vein. Enflurane inhibited the KCl- and ES-induced release of NE from sympathetic nerve terminals in the canine mesenteric vein. The alpha 1- and alpha 2-adrenoceptor antagonists, corynanthine or yohimbine, did not modify the effect of enflurane on the ES-induced frequency response curve of the mesenteric artery and vein. These data suggest that enflurane has at least two sites of action in mesenteric vascular beds. One site is presynaptic where it inhibits NE release, and the other is a postsynaptic site distal to the adrenergic receptor where it inhibits the smooth muscle contraction process.

Direct effects of propofol (2,6-diisopropylphenol) on canine coronary artery ring tension

1. The direct effect of the intravenous general anesthetic propofol (2,6-diisopropylphenol) on epicardial coronary artery tone was studied in unstimulated canine coronary artery rings. 2. Rings stretched to a basal tension of 5 g and exposed to increasing concentrations of propofol generated a characteristic dose-dependent biphasic change in tension-concentrations from 10(-7) to 10(-5) M resulted in constriction (+477 +/- 118 mg) while concentrations 10(-4) and 10(-3) M resulted in relaxation (-682 +/- 75 mg). 3. The response to propofol was reversible and was not modified by the presence of endothelium, alpha 1 and beta adrenoceptor blockade or TEA sensitive potassium channel blockade. 4. The propofol induced changes in tension were altered by changes in extracellular calcium concentrations as well as by blockade of the voltage dependent calcium channel by verapamil. 5. These experiments suggest that clinical concentrations (human) of propofol may have direct vasoactive effects on epicardial coronary artery smooth muscle and that these effects may be due to actions on voltage dependent calcium channels of vascular smooth muscle.