Transition of functional innervation in the developing porcine airway from nitrergic to catecholaminergic

1 We determined the distribution and chemical nature of the inhibitory neurotransmitter(s) to the airway smooth muscle (ASM) before and after birth.2 Relaxation responses to electrical held stimulation (EFS) were studied in isovolumic bronchial segments from foetal (approximately 100/115 days gestation) and adult (25 kg) pigs, and in isovolumic tracheal segments from the foetus, and tracheal smooth muscle strips from the adult pig. Preparations were conditioned in low doses of atropine (10(-7) M) to reduce the effects of excitatory neurotransmission and then exposed to carbachol to produce submaximal muscle tone. Some studies were also carried out on bronchial segments from 4 week old pigs.3 EFS (65 V, 2 ms, 5-20 Hz for 5 s) produced a TTX-sensitive relaxation in epithelium-intact and epithelium-denuded preparations. In foetal bronchial and tracheal preparations, EFS-induced relaxation was strongly inhibited by N-G-nitro-L-arginine (L-NOARG, 10(-6) to 10(-4) M; P