The localization of membrane-associated specializations (basal lamina and cytoplasmic density) at sites of acetylcholine receptor (AChR) aggregation is consistent with an involvement of these structures in receptor stabilization. We investigated the occurrence of these specializations in association with AChR aggregates that develop at the cathode-facing edge of Xenopus muscle cells during exposure to a DC electric field. The cultures were labeled with a fluorescent conjugate of alpha-bungarotoxin and the receptor distribution on selected cells was determined before and after exposure to the field. In thin sections taken from the same cells, the cathode-facing edge was characterized by plaques of basal lamina and cytoplasmic density co-extensive with sarcolemma of increased density. In sections cut in a plane similar to the fluorescence image, it was possible to demonstrate that the specializations were concentrated at areas of field-induced AChR aggregation, and at receptor clusters existing on control cells. This finding further indicates that these structures participate in AChR stabilization, and that the mechanisms involved in AChR aggregation that result from field exposure and nerve contact may be similar.

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