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 field 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<0.01-0.001). However, in the adult, only relaxations of the trachea were inhibited by L-NOARG; bronchi were resistant to L-NOARG and also to N-nitro-L-arginine methyl ester (L-NAME, 10(-4) M). The inhibitory actions of L-NOARG (10(-6) to 10(-4) M) were substantially reversed by 10(-2) M L-arginine. Experiments with bronchial segments from 4 week old pigs showed partial inhibition of relaxations by L-NOARG. 4. The L-NOARG-insensitive relaxations recorded in the adult bronchus were blocked by propranolol (10(-6) M). 5. The onset of relaxation to EFS was more prompt and the rate of relaxation more rapid in foetal bronchi than in adult bronchi (P<0.0005). Maximum relaxation and recovery times were the same. 6. Foetal and adult bronchi were relaxed by sodium nitroprusside (SNP) with similar sensitivity and maximum effect. The rate of relaxation to SNP was not different in the two ages. 7. In the absence of atropine and carbachol, excitatory cholinergic responses to EFS (65 V, 2 ms, 5 Hz for 20 s) were not altered by L-NOARG (10(-4) M) or L-NAME (10(-4) M) in the adult bronchus but were modestly increased by L-NOARG in the foetal bronchus (P<0.01). 8. The tracheobronchial tree appears functionally innervated by nitrergic input to the smooth muscle before birth. However, at or after 4 weeks of age the inhibitory neural input to the bronchi is catecholaminergic, but it remains nitrergic in the trachea. There is also a weak nitrergic pre- or postsynaptic inhibition of the effects of cholinergic neurotransmission in the foetal bronchus but not in the adult.