The Effect of Sympathetic Nerve Stimulation on Ventilation and Upper Airway Resistance in the Anaesthetized Rat

Abstract

We have previously demonstrated that moderate cooling of the isolated upper airway (UA) causes a substantial fall in UA resistance in anaesthetized rats (O’Halloran et al, 1994). This effect is partly of reflex origin but a large fall in resistance still persists following elimination of reflex effects. We have proposed that this residual effect is due to a direct effect of cooling on the UA mucosal vasculature, causing a reduction in mucosal blood flow, a thinning of the mucosa and therefore, an increase in UA cross-sectional area and a fall in resistance. If such a mechanism were responsible for the fall in resistance, then we might expect that activation of sympathetic vasoconstrictor fibres in the cervical sympathetic trunk would also reduce UA resistance in the same preparation. The present experiments examine the effects of electrical stimulation of the cervical sympathetic trunk on the airflow resistance of the isolated UA in anaesthetized rats breathing spontaneously through a low-cervical tracheostomy. Spontaneously breathing animals were used because we also wished to examine the effects of sympathetic stimulation on ventilation. The influence of the cervical sympathetic nerves on ventilation are complex and controversial and very little is known about their effects in the rat.