Correlation between Degree of Inhibition of Parasympathetic Reflex Vasodilation and MAC Value for Various Inhalation Anesthetics

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The effects of various types of general anesthetics (isoflurane, propofol1 ketamine, pen-tobarbitone) on parasympathetic reflex vasodilatation were compared to investigate the involvement in their actions of GABA-mediated synaptic inhibition, enhancement of which is thought to underlie the action of many anesthetics agents. In urethane plus alpha-chloral-ose anaesthetized cats, parasympathetic reflex vasodilatation in the ipsilateral lower lip was elicited by electrical stimulation of the central cut end of the lingual nerve. Isofluraneand pentobarbitone both produced a marked dose-dependent inhibition of this vasodila-tor response. By contrast, propofol and ketamine had no effect on the parasympathetic reflex vasodilatation. The administration of a GABA-antagonist (picrotoxin) reversed the inhibitions produced by isoflurane (previous results) and pentobarbitone (the presentstudy). A dose-related inhibition of this response was induced by the inhalation anesthetics isoflurane, halothane, sevoflurane, and enflurane, the ID_<50> values being 0.94%, 0.82%, 1.74%, and 2.0%, respectively. These results indicate that the ID_<50> value isapproximately two-thirds of the published MAC value for such anesthetics, suggesting that parasympathetic reflex vasodilation is more susceptible than somato-somatic reflexes to inhibition by inhalation anesthetics.