Excitation and contraction in bovine tracheal smooth muscle

1. The smooth muscle layer of the bovine trachea was studied in vitro with the micro-electrode and sucrose-gap techniques. The membrane potential was stable at--47-6 plus or minus 0-98 (S.E. of mean) mV, and there was no spontaneous electrical or mechanical activity. 2. The cell membrane had strong rectifying properties, making it impossible to elicit action potentials by electrical stimulation in normal Krebs Solution. The rectification was abolished by TEA (30 mmol/l), which depolarized the membrane and produced plateau-type action potentials. 3. The spontaneous repetitive action potentials produced by TEA were associated with rhythmic oscillatory contractions of the muscle strips. 4. Histamine caused an increased tone, with superimposed rhythmic fluctuations in tension. The electrical response consisted of depolarization, with rhythmic slow oscillations in potential (slow waves) which were synchronous with the fluctuations in tension. 5. Acetylcholine produced smooth, tonic contractures of tracheal muscle strips, and caused simple depolarization of the membrane. No action potentials were recorded. 6. In calcium-free solutions containing EGTA, the mechanical response to TEA was completely abolished; the response to histamine was greatly reduced; the response to acetylcholine was reduced to a lesser extent. All responses reverted to normal when normal concentrations of extracellular calcium were restored. 7. Lanthanum added to the bathing solution abolished the contraction due to TEA even though the solution contained calcium. It reduced the histamine-induced contraction to 26% of control, and reduced the acetylcholine-induced contraction to 58% of control; extracellular calcium was present throughout. 8. It is suggested that TEA produces contraction by promoting influx of calcium ions into the cytoplasm. Acetylcholine, and to a smaller extent histamine, are less dependent upon the presence of extracellular calcium, and may be capable of releasing calcium sequestered within the cell; acetylcholine appears to be more effective in releasing sequestered calcium.