Influence of maturation on constrictive response to stimulation of C-fiber afferents in isolated guinea pig airways

We investigated whether the airway constrictive response to stimulation of bronchopulmonary C-fiber afferents is altered during the maturation process. Isometric tension was measured in airway rings isolated from three tracheobronchial locations (intrathoracic trachea and main and hilar bronchi) and compared in mature [M, 407 ± 10 (SE) g body wt, n = 36] and immature (IM, 161 ± 5 g body wt, n = 35) guinea pigs. Our results showed no difference in the ACh (10−5 M)- or KCl (40 mM)-induced contraction between M and IM groups, regardless of the airway location. In sharp contrast, the concentration-response curves of 10−8–10−6 M capsaicin were distinctly lower in IM hilar bronchi; for example, response to the same concentration of capsaicin (10−6 M) was 89.2 ± 15.3% of the response to 10−5 M ACh in IM and 284.7 ± 43.2% in M animals. Similar, but smaller, differences in the bronchoconstrictive response to capsaicin between IM and M groups were also observed in the trachea and main bronchus. Electrical field stimulation induced airway constriction in all three locations in M and IM groups. However, after administration of 10−6 M atropine and 10−6 M propranolol, electrical field stimulation-induced contraction was significantly smaller in the hilar bronchus of IM than M animals, and this difference was not prevented by pretreatment with 5 × 10−5 M indomethacin. Although radioimmunoassay showed no difference in the tissue content of substance P between M and IM airways, the constrictive responses to exogenous substance P and neurokinin A were markedly greater in M airways at all three locations. In conclusion, the constriction of isolated airways evoked by C-fiber stimulation was significantly weaker in the IM guinea pigs, probably because of a less potent effect of tachykinins on the airway smooth muscle.