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

Previous work has shown that vitamin D3 or 1,25-dihydroxy-vitamin D3 affect calcium content and fluxes in mitochondria of chick skeletal muscle in 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) or in 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+.