Vitamin D₃ and 1,25-dihydroxyvitamin D₃ stimulate the skeletal muscle-calcium mobilization in rachitic chicks

The Ca content in skeletal muscle relative to vitamin D₃ intake was studied in chicks. It was found that the Ca content in rachitic chick muscle was significantly higher than normal and it decreased with vitamin D₃ treatment. In 4-week-old chicks fed a vitamin D-deficient diet, the Ca content in leg muscle reached 9.86 ± 1.07 mg/100 g wet wt, although in chicks receiving vitamin D₃ in doses of 100 and 500 IU/kg diet, it was 7.80 ± 0.72 and 6.08 ± 0.61 mg/100 g wet wt, respectively. A single i.m. dose of 0.50 μg of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) or vitamin D₃ caused a dramatic decrease in the muscle Ca content by 3 to 6 h after the injection. A simultaneous rise in the Ca level in blood serum was observed. However, at this time the Ca binding protein content in duodenal mucosa and the stimulation of Ca absorption were negligible. These findings allow the conclusion that the vitamin D deficiency in chicks leads to a surplus Ca accumulation in skeletal muscle. The administration of vitamin D₃ or its metabolites causes rapid Ca release during the first 6 h. This may be the source of the Ca level increase in blood serum. In this respect 1,25(OH)₂D₃ was much more effective than vitamin D₃.