Rapid shallow breathing evoked by capsaicin from isolated pulmonary circulation

Recently Green et al. (J. Appl. Physiol. 57:562-567, 1984) reported that pulmonary C-fibers initiate the prompt apnea evoked by pulmonary arterial injections of capsaicin; however, their role in the subsequent rapid shallow breathing of the pulmonary chemoreflex is still in dispute. To determine whether this reflex tachypnea is triggered by pulmonary C-fibers rather than by afferents further downstream, we separately perfused the pulmonary and systemic circulations in dogs anesthetized with either halothane or alpha-chloralose as the lungs were ventilated with a servo-controlled ventilator driven by phrenic nerve activity. Injection of capsaicin (10 micrograms/kg) into the pulmonary artery of the isolated pulmonary circulation evoked an immediate apnea followed by rapid shallow breathing. Injection of the same dose of capsaicin into the left atrium of the isolated pulmonary circulation had no effect. By contrast, when capsaicin was administered at a slower rate into the pulmonary artery (10-20 micrograms X kg-1 X min-1) rapid shallow breathing occurred but without apnea. Our results are consistent with the hypothesis that in spontaneously breathing animals, stimulation of pulmonary C-fibers can evoke rapid shallow breathing.