Changes in activity of vagal bronchopulmonary C fibres by chemical and physical stimuli in the cat.

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Abstract
In 18 anesthetized cats, action potentials in non-myelinated vagal afferent neurons were recorded in the nodose ganglion by extracellular microelectrodes. The pulmonary or bronchial origin of these C fibers was assessed in closed chest preparations by injecting phenyl diguanide into either the right atrium or the ascending aorta (bronchial circulation). This was confirmed in 2 animals by local mechanical stimulation. Of bronchopulmonary C fibers 80% increased their discharge frequency when the end-tidal CO2 concentration (FA, CO2) increased from 0.02 to 0.10. Most of these C endings showed a maximal response when FA, CO2 reached 0.04. For the others a further increase in discharge occurred when CO2 concentration reached 0.08-0.10. Continuous measurement of C fiber discharge frequency indicated that they detected preferentially changes in the inspired CO2 content, but adapted when a high CO2 level was maintained. Their activation by hypercapnia was followed by an increase in lung resistance. Lowering the O2 content of the inspired gas had no effect on the spontaneous activity of bronchopulmonary C endings. When stroke volume of the pump was doubled, spontaneous activity of bronchopulmonary C fibers decreased in intact chest preparations. Inflation of the lungs had the opposite effect after the chest was opened. In both cases hyperdeflation was a potent stimulus to these receptors. In tracheotomized cats, the tracheal temperature was 28.degree.-29.degree. C. When normal thermal conditions were restored in the tracheal lumen (33.degree.-34.degree. C) the spontaneous discharge frequency of some bronchial C fibers was greatly increased. Spontaneous activity of most of the bronchial or pulmonary C fibers was maximal when chemical and physical physiological conditions were restored in the lungs. Changes in alveolar CO2 concentration probably constitute the usual stimulus for these fibers.