Acetylcholine receptor turnover in clonal muscle cells: Role of plasmin and effects of protease inhibitors

Abstract

Characteristics of acetylcholine receptors were evaluated in G8‐1, a continuous skeletal muscle line. Peak binding of ¹²⁵I‐α‐bungarotoxin was in 10‐day‐old contractile myotubes at 4‐8 nm. Turnover was studied using two different methods; both indicated half‐times as little as half as long as previously reported for primary cultures. The effects of a variety of protease inhibitors on receptor turnover were assessed to determine if G8‐1 receptors were less stable or turned over faster because of increased neutral protease activity. Leupeptin, antipain, and chloroquine markedly slowed receptor degradation. Inhibitors of plasmin or plasminogen activator had definite but less dramatic effects on receptor turnover. Results from studies in which plasmin was increased in the tissue culture media indicated that a small but definite acceleration of receptor turnover occurred. In clonal G8‐1 cells, total number of acetylcholine receptors is controlled by negative feedback and although the major pathway for receptor degradation is lysosomal, plasmin may play a role in initiating receptor internalization.