Abstract
The metabolic heterogeneity of synaptic vesicles in the cholinergic nerve terminals of the electromotor neurons ofTorpedp marmoratta has been studied in resting tissue by evaluating the molecular acetylcholine content (MAC) of synaptic vesicles after vesicles extraction from frozen and crushed tissue and high-resolution centrifugal density gradient separation in a zonal rotor. Although vesicular acetylcholine was distributed in the gradient as a single, more or less symmetrical peak, 3 subpopulations of synaptic vesicles could be identified: a small, relatively light subpopulation of low MAC on the ascending limb of the acetylcholine peak, designated V0, a main population of fully charged vesicles designated V1, and a small, denser subpopulation also of low MAC on the descending limb of the acetylcholine peak, designated V2. The mean proportions and MACs of the 3 pools were: V0, 13%, 58,00; V1, 53%, 246,000; V2, 34%, 79,000. When triated acetate was perfused through excised blocks of electric organ for 1–2 h before vesicle isolation, the specific radioactivity of thr acetylcholine in the V0 and V2 pools was 10–30 times higher than in the V1 pool. This suggest that both the V0 and V2 pools are not generated by the isolation procedure but are present in the intact endings and are functionally active. On the basis of their density and uptake of newly synthesized acetylcholine, the V0 and V2 pools were identified with the previously described VP0 pool of axonal vesicles and the VP2 pool of recycling vesicles in stimulated nerve terminals respectively. Since stimulation of electromotor nerve terminals is known to generate large proportions VP2 vesicles, variations in the proportion of V2 vesicles in unstimulated tissue are attributed to varying amounts of adventitous stimulation of the tissue during dissection and perfussion.
