[Effects of exogenous acetylcholine on the tension of smooth muscle in the isolated guinea pig trachea].
Effects of ACh and 6 related drugs on smooth muscle tension of isolated spirally-cut guinea pig trachea were studied. ACh (0.02-0.4 mmol/L) induced a slight, transient and dose-dependent relaxation before contraction of guinea pig trachea in the presence of atropine (0.2μmoI/L). This relaxant effect was blocked by propranolol (1.7μmol/L) and in vivo pretreatment of guinea pig with reserpine (5 mg/kg, im) 24 h before making preparations,This suggested the involvement of adrenergic nerves in it. It was mimicked by DMPP (1,1-dimethyl-4-phenyl-piperazinium)，a selective N agonist (ED=38.0μmol/L，for upper segments). However, the relaxant effect induced by TMPA (trimethyl phenylammonium 1 mmol/L) a selective agonist for N receptors, was only 3.83% of the maximum of DMPP. Tetradotoxin (0.8μmol/L) inhibited the relaxant effect of DMPP by 87.98% but not that of norepinephrine. All these results showed that exogenous ACh relaxed guinea pig trachea by inducing the release of norepinephrine from adrenergic terminals via stimulating Nreceptors on them. It was strongly suggested that cholinergic nerves modulate the release of neurotransmitters from adrenergic nerves and relax the smooth muscle tension of guinea pig trachea.