1. The release of acetylcholine (ACh) from Auerbach's plexus of guinea‐pig ileum has been measured in eserinized Krebs solution using longitudinal muscle strip preparations. 2. Removal of the external K ions enhanced both the resting and stimulated release of ACh from the plexus. This effect was not affected by tetrodotoxin. 3. On readmission of K+ to tissues which had been suspended in K‐free Krebs solution the release of ACh was promptly reduced in both stimulated and unstimulated tissues. The extent of the reduction of ACh release depended on the exposure time to K‐free solution, the recovery being delayed by longer exposure. 4. The ACh releasing effect of (1,1‐dimethyl‐4‐phenyl‐piperazinium iodide (DMPP) was completely inhibited by the readmission of K ions to tissue which had been kept in K‐free Krebs solution. 5. Rb+ substitution for K+ produced no change in ACh release and addition of 5‐9 mM‐Rb after K removal reduced the release of ACh as K did readmission. When the K ions were substituted by Cs+, both the resting and stimulated release were enhanced. The amount of ACh released by a stimulus was enhanced both at low and high frequency of sustained stimulation. 6. Removal of the external K ions increased the release of tritiated noradrenaline (NA), from isolated rat iris; however, when K+ (5‐9 mM) was readmitted the release was reduced even below the control value. 7. It is concluded that the stimulation of (Na+‐K+)‐activated ATP‐ase in the membrane inhibits the release of transmitter, and under physiological condition Ca‐fluxes and the subsequent inhibition of membrane ATP‐ase may be involved in triggering the release of transmitter.
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