Reversal of decreased phosphorylation of sarcoplasmic reticulum calcium transport ATPase by 1,25-dihydroxycholecalciferol in experimental uremia

Vitamin D Receptor Ablation and Vitamin D Deficiency Result in Reduced Grip Strength, Altered Muscle Fibers, and Increased Myostatin in Mice

Vitamin D deficiency is associated with muscle weakness, pain, and atrophy. Serum vitamin D predicts muscle strength and age-related muscle changes. However, precise mechanisms by which vitamin D affects skeletal muscle are unclear. To address this question, this study characterizes the muscle phenotype and gene expression of mice with deletion of vitamin D receptor (VDRKO) or diet-induced vitamin D deficiency. VDRKO and vitamin D-deficient mice had significantly weaker grip strength than their controls. Weakness progressed with age and duration of vitamin D deficiency, respectively. Histological assessment showed that VDRKO mice had muscle fibers that were significantly smaller in size and displayed hyper-nuclearity. Real-time PCR also indicated muscle developmental changes in VDRKO mice with dysregulation of myogenic regulatory factors (MRFs) and increased myostatin in quadriceps muscle (>2-fold). Vitamin D-deficient mice also showed increases in myostatin and the atrophy marker E3-ubiqutin ligase MuRF1. As a potential explanation for grip strength weakness, both groups of mice had down-regulation of genes encoding calcium-handling and sarco-endoplasmic reticulum calcium transport ATPase (Serca) channels. This is the first report of reduced strength, morphological, and gene expression changes in VDRKO and vitamin D-deficient mice where confounding by calcium, magnesium, and phosphate have been excluded by direct testing. Although suggested in earlier in vitro work, this study is the first to report an in vivo association between vitamin D, myostatin, and the regulation of muscle mass. These findings support a direct role for vitamin D in muscle function and corroborate earlier work on the presence of VDR in this tissue.

The roles of vitamin D in skeletal muscle: form, function, and metabolism

Beyond its established role in bone and mineral homeostasis, there is emerging evidence that vitamin D exerts a range of effects in skeletal muscle. Reports of profound muscle weakness and changes in the muscle morphology of adults with vitamin D deficiency have long been described. These reports have been supplemented by numerous trials assessing the impact of vitamin D on muscle strength and mass and falls in predominantly elderly and deficient populations. At a basic level, animal models have confirmed that vitamin D deficiency and congenital aberrations in the vitamin D endocrine system may result in muscle weakness. To explain these effects, some molecular mechanisms by which vitamin D impacts on muscle cell differentiation, intracellular calcium handling, and genomic activity have been elucidated. There are also suggestions that vitamin D alters muscle metabolism, specifically its sensitivity to insulin, which is a pertinent feature in the pathophysiology of insulin resistance and type 2 diabetes. We will review the range of human clinical, animal, and cell studies that address the impact of vitamin D in skeletal muscle, and discuss the controversial issues. This is a vibrant field of research and one that continues to extend the frontiers of knowledge of vitamin D's broad functional repertoire.

Changes in muscle lipid metabolism induced in vitro by 1,25-dihydroxy-vitamin D-3

1,25-Dihydroxy-vitamin D-3 has been shown to increase phosphatidylcholine and decrease phosphatidylethanolamine levels in skeletal muscle in vitro. To elucidate the metabolic pathway and mechanism involved in the effect of the sterol, chick embryo myoblast and vitamin D-deficient chick soleus muscle cultures were treated with 1,25-dihydroxy-vitamin D-3 (5.0 X 10(-10)-3.6 X 10(-11) M, total concentration) for 12-14 h and 1 h, respectively, in the absence and presence of protein and RNA synthesis inhibitors. Lipids were then labelled with [3H]glycerol and [14C]acetate. A significant increase in phosphatidylcholine and triacylglycerol labelling and a decrease in phosphatidylethanolamine labelling were observed in response to the hormone. Cycloheximide suppressed these changes in both types of preparations. Puromycin and actinomycin D were also effective blockers in cultured muscle cells. In addition, double-labelling of control and 1,25-dihydroxy-vitamin D-3-treated myoblasts with [3H]choline and [14C]ethanolamine were performed. The sterol did not affect [3H]choline labelling of total cell lipid extracts and phosphatidylcholine. However, the total lipid fraction of treated cells was labelled to a greater extent with [14C]ethanolamine. In addition, an increased incorporation of this precursor into phosphatidylcholine accompanied by a proportional decrease in phosphatidylethanolamine could be shown in cells pretreated with the metabolite. These changes were abolished by cycloheximide and actinomycin D. The results suggest that 1,25-dihydroxy-vitamin D-3 stimulates methylation of phosphatidylethanolamine into phosphatidylcholine in myoblasts by a nuclear mechanism. The data are consistent with the presence in the cells of a receptor specific for the hormone.

Presence of a 1,25-dihydroxy-vitamin D3 receptor in chick skeletal muscle myoblasts

The presence of a specific receptor for 1,25-dihydroxy-vitamin D3 was investigated in myoblasts released from chick embryo skeletal muscle by trypsin and collagenase treatment. Density gradient analysis of the cytosol obtained from these muscle cell preparations showed that 1,25-dihydroxy-vitamin D3 binds specifically to a 3.7 S macromolecule. Scatchard analysis yielded an equilibrium dissociation constant of 2.46 x 10(-10) M and a Nmax of 74 fmol/mg of cytosol protein. The data is in agreement with previous evidence which indicates that the action of the vitamin D metabolite on muscle Ca uptake is mediated by de novo protein and RNA synthesis, and supports the concept that muscle is a target organ for 1,25-dihydroxy-vitamin D3.

Effects of vitamin D3 metabolites on calcium fluxes in intact chicken skeletal muscle and myoblasts cultured in vitro

25-hydroxycholecalciferol (25OHD3) and 1,25-dihydroxycholecalciferol (1,25(OH)2D3) at physiological concentrations exerted direct effects on Ca fluxes in cultured vitamin D-deficient chick soleus muscle and myoblasts. Isotopic desaturation curves of soleus muscle prelabeled with 45Ca indicated that the action of 25OHD3 is localized in a slow-exchangeable Ca pool where it stimulates net Ca uptake. On the other hand, the predominant effects of 1,25(OH)2D3 consist in an increase of the rate constant of Ca efflux of this pool and in an increase of net Ca uptake in a fast-exchangeable pool. 24,25-dihydroxycholecalciferal proved to be inactive on both Ca uptake and efflux. In addition, 1,25(OH)2D3 significantly increased 45Ca labeling of cultured chick myoblasts. These effects were accompanied by changes in the growth and differentiation of the cultures. The results suggest a direct involvement in vivo of 25OHD3 and 1,25(OH)2D3 on muscle cellular calcium.

Effects of vitamin D-3 on phosphate and calcium transport across and composition of skeletal muscle plasma cell membranes

The effects of vitamin D-3 on calcium and phosphate transport in skeletal muscle plasma membranes were studied. Sarcolemma vesicles were isolated from vitamin D-deficient and vitamin D-treated (one week) chicks by sucrose density gradient centrifugation of a crude muscle plasma membrane fraction. Measurement of (Na+ + K+)-ATPase activity, cholesterol to phospholipid molar ratios and levels of intracellular marker enzymes showed a high degree of purification of the preparations. Administration of vitamin D-3 significantly increased active Ca2+ and phosphate uptake into the vesicles. The efflux of both ions from preloaded vesicles was only slightly altered by the sterol. Ca2+-ATPase activity was higher in sarcolemma from treated animals. This confirms that the effects of vitamin D-3 on calcium transport are related to the Ca2+ pump and not to the passive permeability properties of the membrane. No changes in the protein composition of vesicles from both experimental groups were observed. However, treatment with vitamin D-3 increased sphingomyelin and phosphatidylcholine concentrations. These changes in lipid structure may play a role in the effects of vitamin D-3 on transport characteristics of sarcolemma.