Non-adrenergic inhibitory nerves and putative transmitters in the smooth muscle of cat trachea

1. Effects of non-adrenergic non-cholinergic inhibitory nerve stimulation on the smooth muscle of the cat trachea were studied in vitro using micro-electrode, double sucrose-gap and tension recording methods.2. Electrical field stimulation with a short pulse (50 mus) evoked an excitatory junction potential (e.j.p.) followed by a twitch, with or without a subsequent small, long-lasting relaxation. This response was abolished by tetrodotoxin (5 x 10(-7) M).3. After pre-treatment with atropine (10(-7) to 5 x 10(-6) M) both e.j.p. and twitch tension were selectively blocked and repetitive stimuli at 20 Hz evoked in some preparations slow membrane hyperpolarization followed by long-lasting relaxation. These effects were abolished by propranolol (2 x 10(-6) M). In the presence of atropine (10(-6) M) and propranolol (2 x 10(-6) M) field stimulation caused no detectable change in tension, membrane potential or membrane resistance.4. 5-Hydroxytryptamine (5-HT) (10(-6) to 5 x 10(-6) M) produced a tonic contracture of the cat trachea. In the presence of atropine, field stimulation induced a marked reduction in the amplitude of the contracture which was only partly suppressed by propranolol (5 x 10(-6) M). During this muscle relaxation, produced by field stimulation in the presence of 5-HT, atropine and propranolol, there was no apparent change in the membrane potential or in membrane resistance. The magnitude of the relaxation was proportional to the number of stimuli applied.5. ATP (< 10(-3) M) and VIP (< 10(-9) M) had no effect on the membrane potential or resistance of the smooth muscle cells of the cat trachea, although higher concentrations of VIP (> 10(-8) M) hyperpolarized the membrane and reduced the membrane resistance. Low concentrations, i.e. ATP 5 x 10(-6) M or VIP 10(-12) M, which had no effects in the electrical membrane properties, caused relaxation of the muscle contracture evoked by 5-HT.6. After desensitization to exogenous ATP or adenosine, in the presence or absence of dipyridamole (2 x 10(-7) M), the amplitude of the muscle relaxation evoked by the activation of non-adrenergic inhibitory nerves was not affected. It was, however, much smaller during partial desensitization to VIP.7. These results indicate that cat tracheal smooth muscles are innervated by non-adrenergic, non-cholinergic inhibitory nerves, which cause muscle relaxation without affecting the electrical membrane properties. The possible involvements of ATP or VIP in the inhibitory nervous transmission is discussed.