Presynaptic effects of scopolamine, oxotremorine, noradrenaline and morphine on [3H] acetylcholine release from the myenteric plexus at different stimulation frequencies and calcium concentrations

The inhibition by three modulators (oxotremorine, noradrenaline, morphine) of acetylcholine release from the myenteric plexus preincubated with [3H]choline was investigated at different stimulation frequencies and calcium concentrations. Moreover, [3H]acetylcholine release evoked by a low (0.1 Hz) or a high (10 Hz) stimulation rate was investigated at different calcium concentrations either in the absence or presence of scopolamine. A reduced calcium concentration (0.6 mmol/l) inhibited acetylcholine release more at 0.1 Hz (74% +/- 3%) than at 10 Hz (44% +/- 8%). Scopolamine enhanced the stimulated acetylcholine release at a calcium concentration of 1.8 mmol/l. At calcium concentrations hig…