Circulatory accessibility of nervous receptors localized in the tracheobronchial tree

We studied the accessibility through the systemic and the pulmonary circulations of slowly and rapidly adapting mechanoreceptors having known location within the tracheobronchial tree of the dog. The accessibility was determined by injecting a challenging substance (veratridine and benzonatate) into the two atria and comparing the delays of the receptor response. Eighteen receptors localized in extrapulmonary airways were more accessible from the systemic circulation; they were affected with a shorter delay following left atrium injection. Six receptors in lobar bronchi and another 44 located in more distal intrapulmonary airways were preferentially accessible from the pulmonary circulation (i.e., shorter delays with right atrium injections). Only 4 receptors localized in lobar bronchi were found to be more accessible from the systemic circulation. Therefore the assumption that airway receptors that are more accessible from the pulmonary circulation are located distal to the terminal bronchioles should be re-evaluated. These results may also have important physiological implications since several endogenous substances capable of affecting airway structures have different concentrations in the mixed venous and arterial blood.