The effect of vagotomy, vagal cooling and efferent vagal stimulation on breathing and lung mechanics of rabbits.

1. The effects of bilateral cervical vagotomy, of bilateral vagal cooling and of efferent vagal stimulation were studied on rabbits anaesthetized with pentobarbitone sodium. Total lung conductance, lung compliance, breathing frequency, tidal volume, end-tidal CO(2)%, systemic arterial and right atrial blood pressures and heart rate were measured. Some of the rabbits were first paralysed and artificially ventilated.2. The changes in lung conductance were consistent with the presence of a tonic efferent vagal discharge in bronchoconstrictor fibres, reflexly damped down by a tonic afferent vagal discharge dilator to the airways, probably the Hering-Breuer inflation reflex.3. Neither vagotomy nor efferent vagal stimulation significantly influenced lung compliance, right atrial pressure or end-tidal CO(2)%; vagal cooling lowered end-tidal CO(2)% in spontaneously breathing rabbits, but did not affect the other variables.4. Efferent vagal stimulation in the rabbit decreased lung conductance with no significant change in lung compliance.5. In the rabbit, vagal efferent activity affects primarily the larger (resistance) air passages with little action on the distal (compliance-influencing) airways.