Abstract
Hemicholinium (HC-3), a neuromuscular blocking agent which inhibits the synthesis and the release of acetylcholine (ACh) was investigated for its effects both on [14C]ACh synthesis from [2-14C]pyruvate or [6-14C]glucose and on [3H]choline uptake in purified rat striatal synaptosomes. The synthesis of the transmitter was reduced to 15% of control by 1 µM HC-3 in the presence of eserine (170 µM). The drug produced half-maximal inhibition at 0.06 µM; this effect was totally reversed by 30 µM choline. Choline concentrations as low as 10 µM stimulated significantly the synthesis of [14C]ACh in a diluted suspension of synaptosomes. The existence of a high-affinity uptake system for choline, recently observed by Yamamura and Snyder, was confirmed in our preparation: its Km was found to be 3.5 µM. Eserine (170 µM) produced only 10% inhibition of the uptake process in the presence of 1 µM choline. HC-3 was found to be a purely competitive inhibitor of the choline high-affinity uptake (Ki = 25 nM). A very good correlation was found between the inhibitory effects of HC-3 on [14C]ACh synthesis and on [3H]choline high-affinity uptake. The results suggest that the high-affinity system for the transport of choline is present in cholinergic synaptosomes and plays an important role in sustaining and perhaps regulating the synthesis of the transmitter.
ACKNOWLEDGMENT The authors are very grateful to Dr. M. Israel for constant help, advice, and stimulating discussions.
- Copyright ©, 1973, by Academic Press, Inc.
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