Selective effects of ascorbic acid on acetylcholine receptor number and distribution.

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Abstract
Ascorbic acid in soluble extracts of neural tissue can account for the increase in surface acetylcholine receptors (AChR's) seen on L5 myogenic cells treated with crude brain extract (Knaack, D., and T. R. Podleski, 1985, Proc. Natl. Acad. Sci. USA., 82:575-579). The present study further elucidates the nature of the response of L5 cells to ascorbic acid. Light autoradiography showed that ascorbic acid treatment affects both the number and distribution of surface AChR's. Ascorbic acid, like crude brain extracts, caused a three- to fourfold increase in average AChR site density. However, the number of AChR clusters induced by ascorbic acid was only one-fifth that observed with crude brain extract. The rate constant for degradation of AChR in ascorbic acid-treated cells of 0.037 +/- 0.006 h-1 (t1/2 = 19 h) was not significantly different from that in untreated controls of 0.050 +/- 0.001 h-1 (t1/2 = 14 h). The increase in AChR site density is primarily due to a 2.8-fold increase in the average rate of AChR incorporation. Ascorbic acid also stimulates thymidine incorporation and increases the total number of nuclei per culture. However, cellular proliferation is not responsible for the increase in AChR's since 10 microM cytosine arabinofuranoside blocks the mitogenic effect without affecting the AChR increase. The specificity of ascorbic acid on AChR expression was established by showing that (a) ascorbic acid produced only a slight increase in total protein, which can be accounted for by the mitogenic effect, and (b) the normal increase seen in creatine kinase activity during muscle differentiation was not altered by the addition of ascorbic acid. We conclude that the action of ascorbic acid on AChR number cannot be explained by changes in cell growth, survival, differentiation, or protein synthesis. Therefore, in addition to a minor stimulation of AChR clustering, ascorbic acid specifically affects some aspect of the AChR biosynthetic pathway.