PULMONARY CHEMOREFLEXES ELICITED BY INTRAVENOUS-INJECTION OF LACTIC-ACID IN ANESTHETIZED RATS

Experiments were carried out to characterize the cardiorespiratory ref lex responses to intravenous injection of lactic acid and to determine the involvement of vagal bronchopulmonary C-fiber afferents in elicit ing these responses in anesthetized rats. Bolus injection of lactic ac id (0.2 mmol/kg iv) immediately elicited apnea, bradycardia, and hypot ension, which were then followed by a sustained hyperpnea. The immedia te apneic and bradycardiac responses to lactic acid were completely ab olished by bilateral vagotomy and were absent when the same dose of la ctic acid was injected into the left ventricle. The subsequent hyperpn eic response was substantially attenuated by denervation of carotid bo dy chemoreceptors. After a perineural capsaicin treatment of both vagu s nerves to block the conduction of C fibers, lactic acid no longer ev oked the immediate apnea and bradycardia, whereas the hyperpneic respo nse became more pronounced and sustained, presumably because of the re moval of the inhibitory effect on breathing mediated by pulmonary C-fi ber activation. Single-unit electrophysiological recording showed that intravenous injection of lactic acid consistently evoked an abrupt an d intense burst of discharge from the vagal C-fiber afferent endings i n the lungs. In conclusion, the cardiorespiratory depressor responses induced by lactic acid are predominantly elicited by activation of vag al pulmonary C fibers.