Regulation of acetylcholine receptor density in membranes of denervated mouse muscles.

The ACh sensitivity of denervated muscles of rats stimulated in situ or in culture for several days decreases to low levels characteristic of normal muscles. Possible causes are the electrical activity, depolarization per se, contraction and intracellular [Ca2+] changes. To test the last three hypotheses, isolated bundles of denervated mouse soleus muscles were placed in narrow chambers (internal diameter 2 mm) which were periodically perfused with standard mixtures of Minimal Essential Medium (MEM) and calf serum. At 10-15-min intervals the chambers were filled for 15-43 s with a Ringer solution containing 55 microM ACh or 8-10 mM caffeine or with a solution containing 200 mM KCl. After 3-7 days the muscles were tested for ACh sensitivity by comparing the force developed in response to 110 microM ACh and that to 400 mM K methanesulphonate. In addition, the ACh receptor density was measured with 125I alpha-bungarotoxin. The results showed no difference in ACh sensitivity or receptor density between treated and untreated muscles. Small differences produced by caffeine were probably caused by muscle damage. Ca influx, but not efflux, was strongly elevated during exposure to ACh. It was concluded that neither depolarization per se nor contractile activity or the associated calcium movements have any effect on ACh receptors.