Effects of Corticosterone on Ca²⁺ Uptake and Myofibrillar Disassembly in Primary Muscle Cell Culture

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  This study was done to examine the effects of corticosterone, a glucocorticoid, on Ca²⁺ uptake, proteolysis, and Ca²⁺ channels in primary cultures of chick muscle cells, to clarify the mechanism of glucocorticoid action on muscle proteolysis. Chick muscle cells were incubated for 24 h in a medium containing corticosterone (30 ng/ml) when the cells were confluent (6 days). To examine the contribution of Ca²⁺ channels, nifedipine, a Ca²⁺ channels antagonist, was used. Ca²⁺ uptake measured with ⁴⁵CaCl₂ was increased three-fold by corticosterone, with a peak at 12 h after the treatment started. The growth of the cells estimated from the protein content and creatine kinase activity was not affected by corticosterone. Proteolysis, evaluated with [³H]tyrosine as a label of the protein and N^τ-methylhistidine release, was unchanged by corticosterone. However, the amount of easily releasable myofilament as a measure of myofibrillar disassembly in the muscle cells was increased by corticosterone, and prevented by nifedipine. These results show that corticosterone increases Ca²⁺ uptake and starts myofibrillar protein breakdown.