Multireceptor activation of the pulmonary chemoreflex

Abstract

Schertel et al. (J. Appl. Physiol. 61: 1237–1240, 1984) reported that pulmonary C fibers initiate the prompt apnea followed by rapid shallow breathing evoked by pulmonary arterial injections of capsaicin. However, doubt has remained as to whether these changes in breathing pattern are induced exclusively by direct stimulation of pulmonary C fibers or whether secondary stimulation of slowly adapting pulmonary stretch receptors by capsaicin-induced reflex bronchoconstriction also contributes to the response. To determine the contribution of this secondary mechanism to changes in breathing pattern, we evoked the pulmonary chemoreflex in spontaneously breathing dogs before and after blockade of muscarinic receptors with atropine. Right atrial injections of capsaicin before the administration of atropine induced a classical pulmonary chemoreflex, i.e., apnea, hypotension, and bradycardia followed by rapid shallow breathing and bronchoconstriction. After atropine, all components of the pulmonary chemoreflex induced by right atrial injections of capsaicin remained intact except bronchoconstriction. However, the absolute magnitude of the change in each component of the reflex except apnea was significantly attenuated. We conclude that the classic pulmonary chemoreflex is a complex phenomenon initiated primarily by stimulation of pulmonary C fibers but significantly influenced by secondary stimulation of slowly adapting pulmonary stretch receptors.