Fig 1.

(a) Active 1,25(OH)₂D₃ diffuses across the muscle cell membrane and binds the intracellular VDR. This ligand‐receptor complex then drives either (b) rapid, nongenomic effects on intracellular calcium signaling pathways; or (c) transcriptional activity of genes involved in the differentiation of myocytes into mature contractile myotubes, maintenance of skeletal muscle quality, and protection from muscle atrophy. (d) Circulating 25(OH)D₃ diffuses across the muscle cell membrane, while (e) VDBP requires endocytosis via megalin activity at the cell surface. (f) VDBP attaches to a binding site on intracellular actin filaments and binds the 25(OH)D₃, aiding in the maintenance of circulating levels of this metabolite during winter. VDBP = vitamin D binding protein; VDR = vitamin D receptor.