Lung mechanics and activity of slowly adapting airway stretch receptors

Transpulmonary pressure is thought to be closely associated with slowly adapting mechanoreceptor activity. The transpulmonary pressure required to inflate the lung to a given volume depends on pulmonary compliance: for equal tidal volumes less pressure will be required if compliance is higher. Therefore, an inverse relationship is expected between receptor activity and lung compliance for equal changes in lung volume. We have studied 33 slowly adapting airway stretch receptors (SARs) in anaesthetized, vagotomized, paralysed and artificially ventilated dogs, with the chest open, at constant tidal volume and frequency. After lung compliance had been increased by hyperinflation, all of the ten tracheal receptors studied and fourteen of the 23 intrapulmonary receptors reduced their activity. Of the remaining intrapulmonary receptors five increased their activity and four were unaffected. Our results indicate that, although airway stretch receptor discharge is usually related to transpulmonary pressure, this relationship is not always present in the case of peripherally located endings; this is possibly due to a discrepancy between local transmural pressure and overall transpulmonary pressure. Thus, in addition to the well described independence of SAR discharge frequency from lung volume, the activity of SARs having peripheral locations is not always predictable on the basis of changes in transpulmonary pressure.