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 higher than 1.8 mmol/l scopolamine failed to enhance transmitter release markedly. A reduction of the calcium concentration (less than 1.8 mmol/l) significantly enhanced the effect of scopolamine, when acetylcholine release was evoked at 0.1 Hz. Oxotremorine (10 mumol/l) completely suppressed acetylcholine release at 1 Hz (120 pulses). When 120 pulses were applied at 10 Hz the maximal effect was only a 64% inhibition and the concentration-response curve was significantly shifted to the right. However, after a reduction of both the train length or the calcium concentration oxotremorine produced a complete inhibition of acetylcholine release evoked at 10 Hz. In contrast to the effect of oxotremorine, the concentration-response curves for morphine and noradrenaline were similar at 1 Hz and 10 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)