CONTROL OF BRONCHOMOTOR TONE DURING PERINATAL-DEVELOPMENT IN SHEEP

To characterize the perinatal maturation of the cholinergic control of the airways, we compared the effects of bilateral cervical vagotomy and supramaximal vagal stimulation on the airway resistances of 7 premature (130-133 days of gestation), 11 term newborn, and 9 9-wk-old lambs anesthetized with pentobarbital. Resistances were partitioned between central and peripheral airways with catheters placed retrogradely into peripheral bronchi and capsules attached to the pleural surface in communication with subpleural air spaces. The central and peripheral airway resistances of premature and term newborn lambs and the peripheral resistance of 9-wk-old lambs decreased after vagotomy but only when the lungs were ventilated with small tidal volumes at mean transpulmonary pressures less-than-or-equal-to 7.5 cmH2O. Vagotomy caused smaller absolute changes in central airway resistance than vagal stimulation. In contrast, both vagotomy and vagal stimulation produced similar absolute changes in peripheral airway resistance, indicating that resting cholinergic outflow is preferentially distributed to the peripheral airways even at the earlier ages. The effects of vagal stimulation on airway resistance were prevented by atropine. Our results prove that a resting bronchomotor tone can be present in both central and peripheral airways before the term of gestation in sheep. This tone depends on the activity of cholinergic fibers in the vagus and can be regionally controlled.