Effects of physostigmine and electrical stimulation on the acetylcholine content of the guinea-pig ileum.

Incubation with physostigmine (7.7 muM) caused an approximately 2 fold increase in the acetylcholine content of the myenteric plexus--longitudinal muscle preparation of the guinea-pig ileum. This effect was due mainly to an increase in 'free' acetylcholine, which was directly assayable in either the homogenate after removal of cell debris or the supernatant fraction (100,000 g for 60 min) after subcellular fractionation. Acetylcholine output during stimulation at 0.017, 0.1 or 1 Hz was maintained for 60 min at a rate 2--4 times greater than the non-stimulated output; there was no change in content. At 10 HZ, output was high at the start of stimulation and then decreased continuously; there was a proportionate loss of mainly 'free' acetylcholine from the tissue. Mn2+, hexamethonium, morphine and noradrenaline, which depressed acetylcholine output during stimulation at 0.1 HZ, had no effect on the acetylcholine content nor did they affect the increase in acetylcholine content during incubation with physostigmine.