The influence of 1,25-dihydroxycholecalciferol on the ultrastructural organization of skeletal muscle fibres. Morphometric analyses on vitamin D deficient or calcium deficient growing rats.

Ultrastructural evidence was obtained from vitamin D deficient and calcium deficient rats that the sarcoplasmic reticulum (SR) is involved in the intracellular calcium transport of the muscle cell. In the absence of vitamin D there was a relative decrease in SR volume and a compensatory increase in mitochondrial volume. Administration of exogenous doses of 1,25-dihydroxycholecalciferol significantly increased SR volume, while mitochondrial volume correspondingly decreased. This supports the hypothesis that the production of calcium binding proteins within the SR is influenced by active metabolites of vitamin D.