Selective inhibition by barbiturates of the synthesis of endothelium‐derived hyperpolarizing factor in the rabbit carotid artery

Abstract

Several lines of evidence suggest that both volatile and intravenous anaesthetics may interfere with the synthesis and release of endothelium‐derived vasoactive factors. We have investigated the effects of three different barbiturates on the release of nitric oxide (NO) and endothelium‐derived hyperpolarizing factor (EDHF) in phenylephrine (1 μm)‐preconstricted, endothelium‐intact ring segments of the rabbit carotid artery. The segments were pretreated with the cyclo‐oxygenase inhibitor, diclofenac (1 μm), to prevent the formation of vasoactive prostanoids, such as prostacyclin (PGI₂). Acetylcholine (ACh) elicited a concentration‐dependent relaxation (EC₅₀ 0.15 μm) in control segments which was not significantly different from the relaxant responses of segments pretreated with methohexitone (0.03‐0.3 mM), phenobarbitone (0.1–0.3 mM) or thiopentone (0.1‐0.3 mM). Inhibition of NO synthesis with N^G–nitro‐L‐arginine (0.1 mM) significantly reduced the maximum relaxant response to ACh from 96 to 40%. This NO/PGI₂‐independent relaxation appeared to be mediated by the release of EDHF, since it was strongly diminished in the presence of the K⁺_ca inhibitors, tetrabutylammonium (1–3 mM) and charybdotoxin (10 nM), following preconstriction with potassium calcium (40 mM) or removal of the endothelium. Thiopentone or methohexitone markedly attenuated the EDHF‐mediated relaxant response to ACh, while phenobarbitone had no effect. The endothelium‐independent relaxation elicited by sodium nitroprusside (0.01 −10 μm), on the other hand, was only marginally affected by these anaesthetics. The cytochrome P450 inhibitor, clotrimazole (3–100 μm), mimicked the inhibitory effect of thiopentone and methohexitone on the NO/PGI₂‐independent relaxant response to ACh. Moreover the cytochrome P450‐catalyzed O‐dealkylation of 7‐ethoxycoumarin by rabbit liver microsomes was inhibited in the presence of thiopentone or methohexitone (0.3‐1 mM), while phenobarbitone was without effect. These findings suggest that thiopentone and methohexitone selectively attenuate the EDHF‐mediated relaxant response to ACh in the rabbit carotid artery, presumably by interfering with its synthesis from arachidonic acid via the cytochrome P450 epoxygenase pathway.