We have examined the effects of microtubule- and microfilament-disrupting drugs on the stability, formation, and removal of acetylcholine (ACh) receptors and ACh receptor clusters on the surface of aneurally cultured chick embryonic myotubes. (a) In muscle cell cultures, cytochalasin D (0.2 microgram/ml) or B (2.0 micrograms/ml) causes the dispersal of 50-60% of the existing clusters over a 24-h period (visualized with rhodamine-conjugated alpha-bungarotoxin); Colcemid (0.5 micrograms/ml) has no affect on these clusters. The total number of cell surface ACh receptors does not decline during this period (measured by [125I]alpha-bungarotoxin binding) in the presence of either drug. (b) When cells are treated with biotinylated alpha-bungarotoxin and fluorescent avidin, ACh receptors are cross-linked and rapidly internalized (Axelrod, D., 1980, Proc. Natl. Acad. Sci. USA., 77: 4823-4827). Within 6 h, I have found that 0-15% of the existing large clusters remain. Cytochalasin D or B had no effect on this removal of clusters; however, Colcemid completely prevented the removal of clusters from the cell surface. (c) Addition of chick brain extract to chick myotubes causes an increase in the synthesis and clustering of ACh receptors (Jessell et al., 1979, Proc. Natl. Acad. Sci. USA. 76: 5397-5401). Cytochalasin D caused a slight increase in the number of receptors synthesized in the presence of brain extract whereas Colcemid had no effect on the synthesis and insertion of new receptors into the plasma membrane induced by the brain extract. However, both drugs prevented the increase in the number of receptor clusters. These results are consistent with the hypothesis that receptor clusters are stabilized by actin-containing filaments, but that the movement of receptors in the plane of the membrane requires Colcemid-sensitive microtubules.

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