Longitudinal muscle strips of the guineapig small intestine were incubated with Tyrode solution containing the choline analogue pyrrolidinecholine. At the end of the incubation the concentrations of acetylcholine and its analogue acetylpyrrolidinecholine were determined in the strips by gas chromatography.
After 120 min of incubation with 1 mM pyrrolidinecholine, acetylpyrrolidinecholine comprised about 15% of the total amount of acetylcholine plus acetylpyrrolidinecholine. Electrical stimulation (1 Hz) of the strips during the incubation slightly increased the proportion of acetylpyrrolidinecholine to 21%.
The acetylcholine content of control strips increased significantly during incubation in Tyrode solution. It is suggested that this increase results from the presence of large amounts of endogenous choline that were found in the medium and in the strips. Pyrrolidinecholine in concentrations between 1 and 10 mM inhibited the increased acetylcholine synthesis. The missing acetylcholine was fully replaced by acetylpyrrolidinecholine.
In denervated strips the formation of acetylpyrrolidinecholine was reduced to 21±5% of that of innervated strips. In both, denervated and innervated strips the amounts of acetylpyrrolidinecholine were significantly correlated to the acetylcholine contents. This indicates that acetylpyrrolidinecholine is synthesized at the same site as acetylcholine, i.e. in the myenteric plexus attached to the strips.
Electrical stimulation (10 Hz; 1 ms; 5 min) of the strips at the end of the incubation with pyrrolidinecholine caused an output of acetylcholine and acetylpyrrolidinecholine into the bath fluid. The release experiments were carried out in the presence of eserine. The release of acetylcholine was significantly diminished, but the false transmitter released fully balanced the decrease in acetylcholine release. The proportion of acetylpyrrolidinecholine to acetylcholine was higher in the medium than in the tissue indicating a preferential release of the false transmitter.