EFFECT OF POTASSIUM DEPOLARIZATION AND PREGANGLIONIC NERVE STIMULATION ON THE METABOLISM OF [3H]‐CHOLINE IN RAT ISOLATED SYMPATHETIC GANGLIA

Abstract

1 The effects of potassium depolarization and preganglionic nerve stimulation on the metabolism of [³H]-choline in the isolated superior sympathetic ganglion of the rat have been studied. 2 When unstimulated (resting) ganglia were incubated for 10 min with a low concentration (0.1 μm) of [³H]-choline (high affinity uptake), approximately 75% of the accumulated radioactivity was present as [³H]-phosphorylcholine, 11% was [³H]-acetylcholine ([³H]-ACh) and the remainder was unchanged [³H]-choline. 3 Depolarization of the ganglia with K (46 mm) before their incubation with [³H]-choline, increased [³H]-choline uptake by 70% and increased [³H]-ACh synthesis by more than 700%, so that [³H]-ACh represented almost 50% of the total radioactivity recovered. In contrast, the proportion of [³H]-phosphorylcholine fell to 36% of the total radioactivity recovered. 4 The striking effect of K-depolarization on [³H]-ACh synthesis in ganglia occurred at a concentration of 30 mm or above, and the maximum effect was seen at 45–50 mm. 5 Chronic denervation of the ganglia abolished all the effects of high-K on [³H]-choline metabolism. In resting ganglia, [³H]-ACh formation was reduced by over 80% but [³H]-phosphorylcholine synthesis and the level of unchanged [³H]-Ch were not affected by denervation. 6 Exposure of the ganglia to low-Na or hemicholinium-3 (HC-3) greatly reduced [³H]-ACh synthesis in control resting ganglia and almost abolished the effects of high-K on [³H]-ACh synthesis. 7 Prevention of transmitter release with high-Mg or low-Ca medium also prevented K-depolarization from stimulating [³H]-ACh synthesis. 8 Preganglionic nerve stimulation had an effect on [³H]-choline metabolism similar to that of K-depolarization. Thus, at all the frequencies studied (1–30 Hz), [³H]-ACh synthesis was greatly increased and [³H]-phosphorylcholine was reduced, the maximum effects occurring at 3 Hz. 9 When ganglia were incubated with a high concentration (100 μm) of [³H]-choline (low affinity uptake), a different pattern of metabolism was observed. Most of the radioactivity in resting ganglia was present as unchanged [³H]-choline (70%) with [³H]-phosphorylcholine and [³H]-ACh representing 23% and 6% of the total radioactivity respectively. K-depolarization decreased [³H]-choline uptake but increased the proportions of [³H]-phosphorylcholine and [³H]-ACh to 32% and 24% of the total radioactivity respectively. 10 It is concluded that in unstimulated (resting) rat sympathetic ganglia most of the [³H]-choline transport and metabolism occurs in postsynaptic structures. However, depolarization of the presynaptic nerve terminals appears to trigger a sodium-dependent, HC-3 sensitive, high-affinity uptake process, and causes a dramatic increase in presynaptic [³H]-ACh synthesis together with a fall in postsynaptic [³H]-phosphorylcholine synthesis. These changes in choline metabolism cannot be due to the depolarization of the nerve terminals per se, because they were abolished by high-Mg or low-Ca, i.e. when transmitter release was prevented. Thus, the increase in ACh synthesis may be triggered by a fall in the intraterminal concentration of ACh or by the changes in Ca flux induced by depolarization. Our experiments do not provide evidence on these possible mechanisms.