Journal of Applied Physiology, Vol 60, Issue 3 949-958, Copyright © 1986 by American Physiological Society
Reflex cardiovascular responses caused by stimulation of pulmonary C-fibers with capsaicin in dogs
S. S. Cassidy, J. H. Ashton, W. B. Wead, M. P. Kaufman, Y. Monsereenusorn and J. A. Whiteside
The purpose of these studies was to determine quantitatively the reflex
cardiovascular responses to stimulation of the pulmonary C-fibers in dogs.
We used a preparation in which the airway, pulmonary artery, and the
pulmonary veins to the left lung were cannulated in situ. Ventilation and
perfusion of the right lung maintained the animal in relatively normal
homeostasis. Capsaicin, a decylenic acid amide of vanillylamine that
selectively stimulates nerve endings of unmyelinated fibers (C-fibers), was
injected into the left pulmonary artery in 5-ml boluses. Maximal reflex
responses were obtained with concentrations as low as 0.8-1.6 X
micrograms-1 X kg-1. Heart rate, hindlimb resistance, and left ventricular
contractility were lowered transiently (the maximal responses showing
declines of 40, 13, and 15.2%, respectively). As a result of these changes,
combined with vasodilation in other resistance vessels, cardiac output fell
28% and blood pressure fell 35%. Interrupting the afferent neural pathway
by severing the ipsilateral cervical vagus nerve eliminated these
responses, confirming the distribution of their reflex origin. Although the
role of these reflexes in homeostasis has not been decided, the results of
this study suggest that the lungs of dogs, if appropriately stimulated,
potentially can exert a major inhibitory influence on the neural regulation
of cardiovascular function.
