Manganese ions inhibit acetylcholine receptor synthesis in cultured mouse soleus muscles

Thin bundles dissected from mouse soleus muscles were kept in culture media containing 10% calf serum. After 3 days of culture at 37 degrees C the muscles were tested for acetylcholine (ACh) sensitivity by measuring the force of the contracture produced by 10 microM ACh. The ACh-receptor density was measured by the [125I]alpha-bungarotoxin method. When the [Ca2+] of the culture medium was 0.02 mM (instead of the normal 2 mM), ACh sensitivity and ACh-receptor density were not different from control. Replacement of 99% Ca2+ by Sr2+ or by Mg2+ was also without effect. The muscles did not survive in solutions containing Ni ions. ACh sensitivity and ACh-receptor density were at levels characteristic of non-cultured muscles when the culture medium contained 0.4-1.8 mM Mn2+. Since the incorporation of ACh receptors into the membrane was not affected by Mn ions these results indicate that Mn ions inhibit the synthesis of ACh receptors.