Spontaneous inhalation of acrolein vapor (350 ppm, 1 ml/100 g body wt) elicited an immediate and transient inhibitory effect on breathing in anesthetized rats, characterized by a prolongation of expiratory duration and accompanied by a bradycardia; ventilation was reduced by 47 +/- 6%, which returned to baseline after three to seven breaths. When both vagi were cooled to 6.6 +/- 0.1 degrees C, the reflex apneic response to lung inflation was completely abolished but the bradypneic response to acrolein was not affected. After perineural capsaicin treatment of both cervical vagi to selectively block the capsaicin-sensitive C-fiber afferents, acrolein no longer evoked an inhibitory effect on breathing; conversely, an augmented inspiration was consistently elicited with the first breath of acrolein inhalation, which was subsequently abolished by cooling both vagi to 6.5 degrees C. The inhibitory effect of inhaling acrolein at a lower concentration (200 ppm) was not detectable, whereas that of a higher concentration (600 ppm) was more intense and prolonged. All these responses were completely eliminated by bilateral vagotomy. These results suggest that inhaled acrolein activated both vagal C-fiber endings and rapidly adapting irritant receptors in the airways, but the acrolein-induced inhibitory effect on breathing was elicited primarily by the C-fiber afferent stimulation.

Experiments were carried out to characterize the cardiorespiratory reflex responses to intravenous injection of lactic acid and to determine the involvement of vagal bronchopulmonary C-fiber afferents in eliciting these responses in anesthetized rats. Bolus injection of lactic acid (0.2 mmol/kg i.v.) immediately elicited apnea, bradycardia, and hypotension, which were then followed by a sustained hyperpnea. The immediate apneic and bradycardiac responses to lactic acid were completely abolished by bilateral vagotomy and were absent when the same dose of lactic acid was injected into the left ventricle. The subsequent hyperpneic response was substantially attenuated by denervation of carotid body chemoreceptors. After a perineural capsaicin treatment of both vagus nerves to block the conduction of C fibers, lactic acid no longer evoked the immediate apnea and bradycardia, whereas the hyperpneic response became more pronounced and sustained, presumably because of the removal of the inhibitory effect on breathing mediated by pulmonary C-fiber activation. Single-unit electrophysiological recording showed that intravenous injection of lactic acid consistently evoked an abrupt and intense burst of discharge from the vagal C-fiber afferent endings in the lungs. In conclusion, the cardiorespiratory depressor responses induced by lactic acid are predominantly elicited by activation of vagal pulmonary C fibers.

No abstract