In vitro calcium transport properties of skeletal muscle mitochondria from vitamin D-deficient and 1,25-dihydroxy-vitamin D3-treated chicks

Summary Previous work has shown that vitamin D₃ or 1,25-dihydroxy-vitamin D₃ affect calcium content and fluxes in mitochondria of chick skeletal musclein situ. Studies were performed to investigate whether these effects are related to variations in the Ca²⁺ transport properties of mitochondrial membranes. Mitochondria isolated from skeletal muscle of vitamin D-deficient chicks and chicks dosed with 1,25(OH)₂D₃ for 3 or 7 days (50 ng/day) were employed. No changes in the rate and affinity for calcium of the Ruthenium Red-sensitive Ca²⁺ uptake system were detected after treatment with 1,25(OH)₂D₃. The metabolite did not cause either modifications in Ca²⁺ efflux from mitochondria preloaded with the cation induced by Na⁺ or blockage of mitochondria energy supply. Prior treatment of animals with vitamin D₃ was also without effects. However, a significant stimulation of Ca²⁺ uptake by intact muscle preparations from the same experimental animals was observed in response to treatment with 1,25(OH)₂D₃ in vivo (50 ng/day, 3 days) orin vitro (10⁻¹⁰ M, 60 minutes). In addition, the Ca content of muscle mitochondria was markedly diminished in chicks treated with the sterol. It is suggested that the effects of 1,25(OH)₂D₃ on muscle mitochondrial Ca metabolism may be secondary to changes in cytoplasmic Ca²⁺.