Separation of Recycling and Reserve Synaptic Vesicles from Cholinergic Nerve Terminals of the Myenteric Plexus of Guinea Pig Ileum

Acetylcholine‐rich synaptic vesicles were isolated from myenteric plexus‐longitudinal muscle strips derived from the guinea pig ileum by the method of Dowe, Kilbinger, and Whittaker [J. Neurochem.35, 993–1003 (1980)] using either unstimulated preparations or preparations field‐stimulated at 1 Hz for 10 min using pulses of 1 ms duration and 10 V ˙ cm⁻¹ intensity. The organ bath contained either tetradeuterated (d₄) choline (50 μM) or [³H]acetate (2 μCi ˙ ml⁻¹); d₄ acetylcholine was measured by gas chromatography‐mass spectrometry. As with Torpedo electromotor cholinergic vesicle preparations made under similar conditions the distribution of newly synthesized (d₄ or [³H]) acetylcholine in the zonal gradient from stimulated preparations was not identical with that of endogenous (d₀, [¹H]) acetylcholine, but corresponded to a subpopulation of denser vesicles (equivalent to the VP₂ fraction from Torpedo) that had preferentially taken up newly synthesized transmitter. The density difference between the reserve (VP₁) and recycling (VP₂) vesicles was less than that observed in Torpedo but this smaller difference can be accounted for theoretically by the difference in size between the vesicles of the two tissues. At rest, a lesser incorporation of labelled acetylcholine into the vesicle fraction was observed, and the peaks of endogenous and newly synthesized acetylcholine coincided. Stimulation in the absence of label followed by addition of label did not lead to incorporation of labelled acetylcholine, suggesting that the synthesis and storage of acetylcholine in this preparation and its recovery from stimulation is much more rapid than in Torpedo.