Diaphragmatic responses to graded stimulation of pulmonary C-fibers with capsaicin

It has been suggested that pulmonary C-fiber stimulation is responsible for the rapid shallow breathing that accompanies pulmonary edema. However, pulmonary C-fiber stimulation also causes apnea. To determine whether it was possible for both responses to occur from one stimulus, we infused varying concentrations of capsaicin (a compound that selectively stimulates C-fiber receptors in the dog) into an in situ vascularly isolated dog lung and measured rates and strengths of diaphragmatic contractions with a strain gauge sutured to the diaphragm and electromyogram electrodes implanted in the diaphragm. There was a dose response to capsaicin in that increased doses were related directly with the duration of cessation of diaphragmatic contractions (2-100 s) and inversely with the latency from the start of stimulation to the beginnning of the cessation of diaphragmatic contractions (100-5 s). There was no evidence, however, of rapid shallow breathing in this set of experiments. Either a gradual return to normal rate from prolonged contraction intervals or no change in contraction rate was seen, depending on capsaicin concentration. We conclude that the primary diaphragmatic response to pulmonary C-fiber stimulation is a cessation of diaphragmatic contractions rather than rapid shallow contractions.