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

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Abstract
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. 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. 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. 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. 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. 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. Prevention of transmitter release with high‐Mg or low‐Ca medium also prevented K‐depolarization from stimulating [3H]‐ACh synthesis. 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. 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. 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.