The relationships of the electrical to the mechanical responses of the canine trachealis muscle during stimulation of its cholinergic nerves or exposure to exogenous acetylcholine were recorded in the single or the double sucrose gap. At 27 °C, the responses to a train of stimuli consisted of a transient depolarization excitatory junction potential of 10–30 mV followed by fading oscillations and contractions. When stimulus parameters were varied in the single sucrose gap, contractions were more closely associated with the occurrence of and varied in duration with the oscillations rather than with the amplitude of the EJP. Acetylcholine superfused at a concentration of 10−6 M for 30 s caused a prolonged depolarization of 10–20 mV, but a much larger contraction than could be elicited by nerve stimulation. None of the responses to acetylcholine was significantly affected by the Ca channel antagonists, nifedipine, nitrendipine, or verapamil in Ca channel blocking concentrations. When tissues were exposed to a Ca-free medium, the excitatory junction potentials and oscillations rapidly disappeared, but the electrical and mechanical responses to acetylcholine persisted and only gradually disappeared with repetitive exposures. Furthermore, in a medium with normal Ca2+ in the double sucrose gap, depolarization by 10–15 mV with an applied current caused no contraction, and repolarization to the normal membrane potential during acetylcholine-induced contraction caused no relaxation. Tetraethylammonium ion (20 mM) depolarized the membrane, increased membrane resistance, and enhanced the secondary oscillations and contractions after field stimulation. No other K+-channel blocker tested (Ba2+, apamin, 4-aminopyridine, glibenclamide, charybdotoxin) had the effect of prolonging secondary oscillations. We concluded that acetylcholine under our conditions acted to release internal Ca2+ and that the depolarization was secondary to that release or an associated event. Moreover, contractile responses to released acetylcholine during field stimulation do not appear to depend significantly on the opening of voltage-dependent Ca channels. The secondary oscillations were closely related to contraction; they seemed to be modulated by the opening of tetraethylammonium-sensitive, possibly Ca2+-activated K+ channels. The physiological functions, if any, of the initial depolarization associated with acetylcholine released (excitatory junction potential) or added remain unclear.Key words: acetylcholine, tracheal smooth muscle, trachea, chloride channels, sucrose gap, potassium channels.
Post-tetanic decay of evoked and spontaneous transmitter release and a residual-calcium model of synaptic facilitation at crayfish neuromuscular junctions.