Inhibitory action of acetylcholine on the smooth muscle from the lower esophageal sphincter.

The effect of acetylcholine (Ach) on smooth-muscle strips isolated along the transversal axis of cat lower esophageal sphincter (LES) is studied. Ach in low concentrations (10(-11)--10(-9) g/ml) causes contraction of the muscle strips. Increase of the concentration to 10(-8) g/ml leads to biphasic effect: contraction with relaxation. Inhibitory response predominates at Ach 10(-6) and 10(-5) g/ml. Atropine (10(-6) M) eliminates the excitatory phase but it has no effect on the second relaxation phase. Propranolol (10(-6), 2 X 10(-6) M) as well as phentolamine turn the inhibitory response to Ach into contraction. Noradrenaline leads to LES contraction while isoprenaline induces relaxation. In smooth-muscle LES strips from cats pretreated with reserpine (1 mg/kg for 3 days), Ach in the concentrations used (10(-5), 10(-6) g/ml) leads to contraction. The changes observed are membrane-dependent -- the contraction is accompanied by depolarization, relaxation by hyperpolarization. The inhibitory effect of Ach on LES smooth muscle is discussed in the light of the hypothesis of Burn and Rand (1960) about the release of noradrenaline under the effect of Ach.