Capsaicin injected into the right heart of dogs causes reflex bronchoconstriction by stimulating pulmonary C-fibers, but injected into the left heart it is said to have little effect even though it stimulates bronchial C-fibers, which are known to cause contraction of airway smooth muscle. Attempting to resolve this apparent contradiction, we recorded smooth muscle tension in an innervated tracheal segment in anesthetized dogs and examined the reflex effects of injecting capsaicin intravascularly at different sites. Right atrial injection of capsaicin (10 μg/kg) caused tracheal contraction, as did bronchial arterial injection (0.15-5.0 μg); left atrial injection (10 μg/kg), however, caused relaxation or slight contraction, or a combination of the two. Contraction but not relaxation was abolished by cutting or cooling (0°C) the cervical vagus nerves. Femoral arterial injection (10-100 μg) caused tracheal relaxation, which was abolished by cutting hindlimb nerves. We conclude that both 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.
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