Histamine enhances vagal pulmonary C-fiber responses to capsaicin and lung inflation

Effects of histamine on the base-line activity of vagal pulmonary C-fibers and their responses to chemical and mechanical stimulations were studied in anesthetized, open-chest dogs. Histamine aerosols (5 breaths, 1% solution) increased the change in tracheal pressure per breath (ΔP_t) to 53±9% (mean±SE) above its base line, whereas an accompanying increase in activity was found in only 27% of the C-fibers studied and hence the overall afferent activity was not significantly different from the control. After P_t returned toward the base line in several minutes, the receptor activity evoked by right atrial injection of capsaicin (0.8-4.0 μg/kg) increased from a base line of 0.43±0.02 imp/sec to 8.12±1.16 imp/sec (averaged over 10 sec), which was markedly greater and longer lasting than the response triggered by the same dose of capsaicin after phosphate buffer aerosols (4.72±0.71 imp/sec). Furthermore, pretreatment with histamine aerosols also enhanced the afferent responses of these receptors to static lung inflation (P_t=20 cmH₂O). We conclude that the low dose of histamine aerosols alone did not consistently stimulate the vagal pulmonary C-fibers, but it potentiated the stimulatory effects of both capsaicin and lung inflation on these receptors.