The roles of stored calcium in contractions of cat tracheal smooth muscle produced by electrical stimulation, acetylcholine and high K+

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Abstract
1 Effects of direct or indirect (nerve-mediated) muscle stimulation, acetylcholine (ACh), caffeine and procaine on the membrane and mechanical properties of smooth muscle cells of the cat trachea were investigated by means of double sucrose-gap and isometric tension recording methods. 2 Outward current pulses (2 s in duration) applied to the muscle tissue in the presence of tetrodotoxin (10−7m), atropine (10−6m) and propranolol (10−6m) evoked no action potential (spike); however, when the depolarization exceeded 9 mV, a contraction was evoked. 3 The spike and contraction evoked by outward current pulses in the presence of tetraethylam-monium (TEA, 10 mm) were suppressed by treatment of the tissue with either Ca2+-free EGTA (2 mm) containing solution or Mn2+ (5 mm). In the presence of procaine (10 mm), outward current pulses evoked an action potential but no contraction. 4 Field stimulation of short duration (50 μs) applied to the whole tissue produced an excitation of the intrinsic nerves and evoked excitatory junction potentials (e.j.ps), and when the amplitude of e.j.ps exceeded 4 mV, a twitch contraction occurred. E.j.p. was more effective in producing a contraction than was the membrane depolarization evoked by outward current pulses. 5 Amplitudes of contractions evoked by exogenous ACh (10−5m) were much larger than those evoked by 128 mm-[K]o or caffeine (10 mm), in normal Krebs solution. When the amplitudes of the contractions produced by 128 mm [K]o were defined as a relative amplitude of 1.0, the mean amplitudes of contraction produced by ACh (10−5m) or caffeine were 2.5 ± 0.20 or 1.2 ± 0.26, respectively. 6 In Ca2+-free EGTA (2 mm)-containing solution, the contraction induced by 128 mm-[K]o was rapidly abolished, whereas the contractions evoked by caffeine (10 mm) or the initial phasic contraction produced by ACh (10−5m) were largely unaffected. 7 When the amount of Ca2+ stored in the muscle cell was estimated from the amplitude of caffeine-induced contraction evoked in Ca2+-free solution, procaine (10 mm) applied simultaneously with Ca2+, after depletion of Ca2+ from the cells by means of caffeine, increased the amount of Ca2+ stored to 1.31±0.14 (n = 6) times the control value. However, ACh (10−7m) or excess concentrations of [K]o applied with Ca2+ did not increase the amount of Ca2+ stored in the caffeine-sensitive intracellular compartment. 8 These results indicate that the amount of Ca2+ stored in the smooth muscle cells of the cat trachea may be larger than other visceral smooth muscle and plays an important role in the initiation of contraction, in response to endogenous or exogenous ACh.