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

Abstract

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 ®eld 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 77 M) to reduce the eects 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 76 to 10 74 M; P50.01 ± 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, 5 The onset of relaxation to EFS was more prompt and the rate of relaxation more rapid in foetal bronchi than in adult bronchi (P50.0005). Maximum relaxation and recovery times were the same. 6 Foetal and adult bronchi were relaxed by sodium nitroprusside (SNP) with similar sensitivity and maximum eect. The rate of relaxation to SNP was not dierent 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 74 M) or L-NAME (10 74 M) in the adult bronchus but were modestly increased by L-NOARG in the foetal bronchus (P50.01). 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 eects of cholinergic neurotransmission in the foetal bronchus but not in the adult.

Potent bronchodilation and reduced stiffness by relaxant stimuli under dynamic conditions

“…2). Previous studies suggested that the relaxant responses to EFS under the described conditions were neural in origin since they are abolished by the nerve blocker tetrodotoxin [9]. This was confirmed in the present study (data not shown).…”

“…Thus the combined relaxant effect of two interventions (oscillation and evoked relaxation) was the same or similar to the multiple of the fractional changes to the two. EFS induced relaxation was blocked by L-NOARG, a blocker that has been used previously to identify nitric oxide-mediated neural relaxation [8,9]. The multiplicative model of response to EFS or isoprenaline and oscillation could indicate that the interventions act at separate, noninteracting sites, as previously suggested by GUMP et al [4] for isoprenaline.…”

“…The protocol for recording inhibitory electrical field stimulation (EFS) responses in pig airways was taken from KANNAN and JOHNSON [8] and CONNELLAN and MITCHELL [9]. Briefly, bronchi were exposed to a low concentration (10 -7 M) of atropine for 20 min, which blocks contributing effects from cholinergic nerves on the EFS-induced response.…”

“…Inhibitory input to ASM in vivo typically involves both pharmacological (e.g. b 2 adrenoreceptor agonists) and neural input, the latter principally via nitrergic neurotransmission [7][8][9]. Nitric oxide from neural and non-neural sources…”

Airway narrowing in porcine bronchi with and without lung parenchyma

“…Since ASM activation by agonists is dependent on the route of drug delivery [8], airway narrowing was induced by direct vagal stimulation in vivo and by EFS in vitro. There is no significant nonadrenergic or noncholinergic innervation to pig bronchi and propranolol was used both in vivo and in vitro to inhibit effects produced by stimulation of sympathetic nerves [14]. Hence, EFS in vitro stimulated the same post-ganglionic parasympathetic neurons that are under vagal control in vivo.…”