Suppression of parasympathetic reflex vasodilatation in the lower lip of the cat by isoflurane, propofol, ketamine and pentobarbital: implications for mechanisms underlying the production of anaesthesia.

We have compared the effects of isoflurane, propofol, ketamine and pentobarbital on parasympathetic reflex vasodilatation to investigate their involvement in GABA-mediated synaptic inhibition, enhancement of which is thought to underlie the action of many anaesthetic agents. In cats anaesthetized with urethane-alpha-chloralose, parasympathetic reflex vasodilation in the ipsilateral lower lip was elicited by electrical stimulation of the central cut end of the lingual nerve. Isoflurane and pentobarbital both produced marked dose-dependent inhibition of this vasodilator response. In contrast, propofol and ketamine had no effect on parasympathetic reflex vasodilation. Administration of a GABA antagonist (picrotoxin) reversed the inhibition produced by isoflurane (previous results) and pentobarbital (present study). Our results suggest that isoflurane and pentobarbitone inhibit parasympathetic reflex vasodilatation via a GABA-mediated mechanism, but that propofol and ketamine have no such effect. Our results with propofol cast doubt on its presumed mechanism of action as an anaesthetic.