Estimation of degradation rate of acetylcholine receptor by external gamma counting in vivo

We describe a novel for estimating the rate of destruction of the acetylcholine receptor (AChR) associated with the motor end-plate in the living animal. After the intramuscular injection of a nontoxic dose of α-bungarotoxin that had been monoiodinated with ¹²⁵I (¹²⁵I-BGT), the loss of radioactivity from the injected site is monitored by an externally positioned gamma counter. Two days after labeling, the decline of radioactivity in the injected muscle parallels the decline of end-plate specific radioactivity and can be used for the estimation of the degradation rate and half-life of end-plate AChR. Using this method, we measured AChR half-life in normal and in myasthenic rats and found a 2.5-fold decrease of AChR half-life in the myasthenic animals. This in vivo method has potential clinical applications.