CAPSAICIN-SENSITIVE STRETCH RESPONSES IN FERRET TRACHEALIS MUSCLE

1. Stretch-induced electrical and mechanical responses in segments of ferret trachealis muscle were studied. Stretches and post-stretch leng th changes were quantified by measuring distances between two marker s pheres placed on the muscle surface. Electrical responses were determi ned by measuring membrane potential in the muscle cell syncytium. 2. S mooth muscle mechanical and electrical responses to the stretch manoeu vre were characterized by an initial shortening and depolarization pha se and a reversal-repolarization phase. Both phases were resistant to atropine and tetrodotoxin. During the initial phase, the membrane depo larized to potentials as low as -20 mV. For stretches to 1.0 L(max), f rom a holding length of 0.75 L(max), 50% repolarization occurred at 6. 8 +/- 0.4 min post-stretch; 50% reversal of shortening of the stretche d segment occurred at 6.9 +/- 0.8 min post-stretch. 3. Depolarizing cu rrents generated within muscle cells in the stretched segment spread i nto cells in non-stretched muscle. Space constants in the transverse a nd longitudinal directions averaged 480 + 46 and 146 +/- 50 mum, respe ctively. 4. During infusion of capsaicin (10 muM), muscle cells depola rized by 5.5 +/- 2.3 mV. Maximal depolarization was achieved after 15- 20 min. After inhibition of neutral enkephalinase, capsaicin-evoked de polarization occurred more rapidly. Muscles depolarized by 11.2 +/- 2. 1 mV after about 10 min of capsaicin and then slowly repolarized durin g continued treatment. When muscle segments were stretched during admi nistration of capsaicin, the initial phase was similar to that observe d before capsaicin, but the reversal-repolarization phase was prolonge d. Following wash exposure to capsaicin, maximal stretch-induced depol arization was unchanged, but the time for 50% repolarization (t50-repo larization) decreased from the pre-capsaicin value of 8.4 +/- 1.3 to 4 .1 +/- 0.5 min. The t50-reversal of stretch-evoked muscle shortening d ecreased to 54% of control values. 5. Short exposures (< 2 min) to sub stance P (SP, 2-7.5 muM) depolarized smooth muscle cells. Maximal depo larization was delayed, and occurred after [SP] had decreased to < 10 nm. Repolarization was delayed as long as 6 min following wash-out of SP. Stretches performed when SP-induced depolarization had nearly reve rsed showed no changes in the initial mechanical or electrical respons es, but t50-repolarization increased to 162% of control values. 6. Imm unochemical studies showed networks of neurones which react with SP an tibodies. 7. These findings suggest that stretch induces SP release fr om capsaicin-sensitive C fibres, and that released SP affects smooth m uscle ionic mechanisms which control and delay the reversal of stretch -induced membrane depolarization and shortening.