Rate constants of acetylcholine receptor internalization and degradation in mouse muscles

The rate constants for internalization and subsequent extrusion of acetylcholine receptors (AChRs) during degradation in adult innervated and denervated mouse diaphragm muscles were determined using proteinase K (PK) digestion. This procedure separated (125)I-alpha-bungarotoxin (Bgt)-labeled AChRs into PK-sensitive and PK-resistant compartments. The time course of the residual radioactivity in these two compartments suggested that they represented surface membrane and internalized compartments, respectively. The data were compatible with a mathematical model based on the assumption that during degradation of AChRs a surface compartment, A, fed an internal compartment, B, with an internalization rate constant (k(i)), and that B is drained from the cell with an extrusion rate constant (k(o)). Using the mathematical model, we were able to determine that k(i) and k(o) were, respectively, 0.068 (t(1/2) approximately 10.2 days) and 0.69-0.55 (t(1/2) approximately 1.0- 1.25 days) for innervated muscle and were, respectively, 0.69 (t(1/2) approximately 1.0 day) and 6.93 (t(1/2) approximately 0.1 day) for denervated muscle. Thus, the rate for internalization was about 8-10 times slower than that for extrusion from the cell for both the slowly degrading innervated (Rs) AChRs and for the rapidly degrading denervated (Rr) AChRs. This inequality between k(i) and k(o) therefore allows the combined quantity of A(t) + B(t), usually measured in AChR degradation studies, to approximate a single exponential.