Abstract
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.
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