Figure 1. Radioiodine labeling detects total amount of ¹²⁵I contained in muscle. Scheme illustrating the working principle of the radioiodine approach. (A) Pre-pulse situation. CHRN channels (white barrels) are present at high amounts in the postsynaptic membranes of muscle fibers. Small amounts of CHRN are constantly internalized by endocytic retrieval. From there, CHRN might undergo recycling back to the plasma membrane or follow a degradative pathway. (B) Pulse labeling with ¹²⁵I-BGT (black dots). About 10% of the ¹²⁵I-BGT binds to CHRN in the injected hind limb, the rest of the ²⁵I-BGT is washed out and binds to other muscles or is excreted. (C and D) CHRN labeled with ¹²⁵I-BGT undergo either recycling (C) or are degraded (D). Only in the latter case, ¹²⁵I is removed from the muscle (gray cloud), i.e., the radioiodine approach measures the rate of ¹²⁵I removal from the muscle as a consequence of CHRN degradation.