Role of calcium in the regulation of acetylcholine receptor synthesis in cultured muscle cells

1. Embryonic muscles differentiated in vitro were used to study the effects of intracellular Ca²⁺ ([Ca²⁺]_i) variations on the amount of acetylcholine receptors ([AChR]) in the cell membrane. 2. Increased Ca²⁺ concentration in the growth medium ([Ca²⁺]_o) caused a marked elevation of AChR levels, apparently through de novo synthesis. 3. Agents known to increase [Ca²⁺]_i and its accumulation in the sarcoplasmic reticulum (SR), such as ionophore A23187, sodium dantrolene (DaNa), or high [Mg²⁺]_o all enhanced α-bungarotoxin (α-BGT) binding after 48 h of treatment. 4. Electrical stimulation or caffeine, both affectors of SR calcium release, brought about a decrease in [AChR] probably by suppressing its synthesis. 5. The effects of simultaneous treatment with two AChR-inducing agents, namely, high [Ca²⁺]_o in the presence of tetrodotoxin (TTX) or high [Mg²⁺]_o were not additive, thus suggesting action via a common saturable mediator. 6. Intermediate AChR levels obtained following simultaneous treatments with opposing effects, e. g., electrical stimulation in the presence of high [Ca²⁺]_o or DaNa, suggest contradictory actions on a common mediator. 7. All these observations indicate a strong correlation between SR calcium levels and [AChR] on myotubes; while calcium accumulation in the SR was followed by increased AChR synthesis, calcium release was accompanied by suppression of receptor synthesis.