Tracheal contraction and relaxation initiated by lung and somatic afferents in dogs

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Abstract
Capsaicin injected into the right heart of dogs causes reflex bronchoconstriction by stimulating pulmonary C-fibers; but injected into the left heart it apparently has little effect even though it stimulates bronchial C-fibers which cause contraction of airway smooth muscle. Attempting to resolve this apparent contradiction, smooth muscle tension was recorded in an innervated tracheal segment in anesthetized dogs and the reflex effects of injecting capsaicin intravascularly at different sites was examined. Right atrial injection of capsaicin (10 .mu.g/kg) caused tracheal contraction, as did bronchial arterial injection (0.15-5.0 .mu.g); left atrial injection (10 .mu.g/kg), caused relaxation and/or slight contraction. Contraction, but not relaxation, was abolished by cutting or cooling (0.degree. C) the cervical vagus nerves. Femoral arterial injection (10-100 .mu.g) caused tracheal relaxation, which was abolished by cutting hindlimb nerves. Pulmonary and bronchial C-fibers evoke tracheal contraction, but when capsaicin is injected into the left atrium any effects of stimulating bronchial C-fibers are masked by the reflex action of somatic afferents, which cause tracheal relaxation.