Autoregulation of acetylcholine release from vagus nerve terminals through activation of muscarinic receptors in the dog trachea.

1. The effects of pirenzepine and gallamine on the membrane and contractile properties of smooth muscle cells and on excitatory neuro-effector transmission in the dog trachea were investigated by means of microelectrode, double sucrose gap and tension recording methods. 2. Pirenzepine (10(-7) M) and gallamine (10(-5) M) had no effect on the resting membrane potential or the input resistance of the smooth muscle cells. 3. Pirenzepine (10(-10)-10(-9) M) and gallamine (10(-7) M) enhanced the amplitude of twitch contractions evoked by field stimulation in the combined presence of indomethacin (10(-5) M) and propranolol (10(-6) M). At higher concentrations pirenzepine (10(-8) M) inhibited the twitch contractions in a dose-dependent manner. Both pirenzepine and gallamine in doses over 10(-7) and 10(-5) M, respectively, reduced muscle tone. 4. Pirenzepine (10(-10)-10(-9) M) and gallamine (10(-7) M) enhanced the amplitude of excitatory junction potentials (e.j.ps) evoked by field stimulation (single or repetitive stimulation). However, a high concentration of pirenzepine (10(-8) M) reduced the amplitude of e.j.ps. In parallel with its action on e.j.ps, pirenzepine (over 10(-9) M) reduced the response of smooth muscle cells to acetylcholine (ACh), in a dose-dependent manner. Gallamine (5 X 10(-5) M) markedly enhanced the amplitude of e.j.ps but also reduced the response of muscle cells to ACh. 5. ACh (10(-10)-10(-9) M) inhibited twitch contractions evoked by field stimulation, with a slight increase of resting tension. 6. Gallamine enhanced the summation of e.j.ps during repetitive field stimulation at a high frequency (20 Hz), but was without effect on the depression phenomena of e.j.ps observed during double stimulus experiments at different time intervals (5-60 s). 7. These results indicate that both pirenzepine and gallamine have dual actions on pre- and post-junctional muscarinic receptors in dog tracheal tissue. At low concentrations both agents potentiate excitatory neuro-effector transmission, presumably due to enhancement of release of ACh from vagal nerve terminals through blockade of a negative auto-regulatory process activated by endogenous ACh. At higher concentrations, these agents inhibit the response of smooth muscle cells to ACh through post-junctional muscarinic receptors and relaxation of the muscle tissue occurs.