1. The reductions in arterial pressure and preganglionic sympathetic activity evoked by aortic nerve stimulation in the rabbit were studied before and during administration of constant inspired concentrations of the inhalation anaesthetics
cyclopropane,
halothane, and
ether. The background anaesthetic was
pentobarbitone,
gallamine triethiodide was given, and pulmonary ventilation was with 100%
oxygen.2. During light
pentobarbitone anaesthesia, aortic nerve stimulation usually induced similar reductions in arterial pressure and preganglionic discharge, expressed as the maximum percentage reduction from prestimulation levels. There were two components in the sympathetic responses, attributable to A and C fibre excitation in the aortic nerve, which was also shown to contain a third fibre group with properties similar to those of B fibres.3. The arterial pressure, heart rate, and preganglionic sympathetic responses to aortic nerve stimulation were rapidly and profoundly inhibited by 50%
cyclopropane, which also produced arterial
hypertension.4.
Halothane (3%) significantly inhibited the depressor responses, but even in the presence of severe
hypotension the arterial pressure could usually be further reduced by aortic nerve stimulation. The inhibitory effects of 2%
halothane were slow in onset and not pronounced. In the concentrations used, these actions of
halothane were significantly less than those of
cyclopropane.5. The inhibitory effects of
ether on the depressor responses were roughly intermediate between those of
cyclopropane and
halothane; complete suppression of the responses occurred with high
ether concentrations, which were also liable to cause
circulatory collapse.6. It is concluded that the three anaesthetics significantly inhibit impulse transmission through central baroreceptor pathways; the implications of the findings are discussed in relation to the different circulatory actions of these anaesthetics.