Inhibition of acetylcholine receptor assembly by activity in primary cultures of embryonic rat muscle cells.

Silencing of contractile activity in muscle is known to increase the level of acetylcholine receptor on the cell surface. Both in vivo and in vitro studies indicate that modulation of receptor-specific mRNA levels plays a role in the activity-related regulation, but other mechanisms have not been explored. In this study, we examine the synthesis and post-translational fate of receptor alpha subunit in actively contracting and tetrodotoxin-inhibited rat muscle cultures. Using metabolic labeling and immunoprecipitation with subunit-specific monoclonal antibodies, we find that the increase of alpha subunit synthesis in tetrodotoxin-inactivated cultures is insufficient to account for the increased rate at which new receptors appear on the cell surface. In evaluating stages in the post-translational processing of alpha subunit, we find that in active and inactive cultures, newly synthesized subunit acquires the ability to bind alpha-bungarotoxin with the same kinetics. However, differences were noted at or preceding the stage where alpha subunit becomes assembled with the other subunits to form the 9 S receptor. In inactivated cultures, newly synthesized alpha subunit transits a 5 S precursor pool more rapidly and is assembled more efficiently than in contracting cultures. The possibility that these differences represent a type of post-translational regulation is discussed.