Relationship between vitamin D status and deposition of bound calcium in skeletal muscle of the rat.

The effects of vitamin D on the intramuscular distribution of total and bound calcium, phosphate and on available cytosolic calcium, were investigated in skeletal muscle. Total calcium and phosphorus were measured on ashed subcellular fractions of muscles from vitamin D-repleted and vitamin D-deprived rats. The variations in available calcium were followed by determining the activities of calcium-sensitive enzymes in isolated cytosol. Bound-calcium was revealed ultra-microscopically by pyroantimonate. In vitamin D-repleted muscles, the pyroantimonate method revealed specific areas of intense bound-calcium deposition: the myofibrils, where they formed pronounced lines parallel to the Z-bands. In vitamin D-deficient muscles, the calcium-pyroantimonate deposits appeared clearly reduced. This loss was accompanied by a marked reduction in total calcium and phosphorus in all the subcellular fractions, as compared to vitamin D-repleted muscles. Unexpectedly, the activity of the Ca(+)-activated isocitrate-dehydrogenase was increased in the cytosol, while that of the Ca2(+)-inhibited pyruvate-kinase decreased. Prolonged vitamin D-administration to vitamin D-repleted rats led to an intensification of calcium-pyroantimonate deposits and a general increase in total calcium and phosphorus, but no change in the cytosolic Ca2(+)-sensitive enzyme activities. Cessation of vitamin D-administration to vitamin D-repleted rats produced a regression of calcium-pyroantimonate deposits, a general decrease of total calcium and phosphate levels, and stimulation of the Ca2(+)-activated isocitrate-dehydrogenase accompanied by lowering of the Ca2(+)-inhibited pyruvate-kinase. The results clearly indicate a correlation between vitamin D-repletion and the total and bound calcium content of skeletal muscle. In addition, they demonstrate an apparent contradiction between the decrease of total and bound calcium, and the activities of cytosolic Ca2+ sensitive enzymes during vitamin D-deprivation, which can only be explained by an increase in available calcium. It is suggested that vitamin D stimulates intramuscular mechanisms tending to lower available calcium by inactivating the cation via the formation of calcium chelates.