Pulmonary rapidly adapting receptors reflexly increase airway secretion in dogs

We attempted to determine whether stimulation of pulmonary rapidly adapting receptors (RARs) increase tracheal submucosal gland secretion in anesthetized open-chest dogs. Electroneurographic studies of pulmonary afferents established that RARs but not lung C-fibers were stimulated by intermittent lung collapse during deflation, collapse being produced by removing positive end-expiratory pressure (PEEP, 4 cmH₂O) or by applying negative end-expiratory pressure (NEEP, -4 cmH₂O). We measured tracheal secretion by the 'hillocks' method. Removing PEEP or applying NEEP for 1 min increased secretion from a base line of 6.0 ± 1.1 to 11.8 ± 1.7 and 22.0 ± 2.8 hillocks · cm^-2 · min^-1, respectively (P < 0.005). After PEEP was restored, dynamic lung compliance (Cdyn) was 37% below control, and secretion remained elevated (P < 0.05). A decrease in Cdyn stimulates RARs but not other pulmonary afferents. Hyperinflation, which restored Cdyn and RAR activity to control, returned secretion rate to base line. Secretory responses to lung collapse were abolished by vagal cooling (6°C), by pulmonary vagal section, or by atropine. We conclude that RAR stimulation reflexly increases airway secretion. We cannot exclude the possibility that reduced input from slowly adapting stretch receptors contributed to the secretory response.