Locomotor function of skeletal muscle is regulated by vitamin D via adenosine triphosphate metabolism.

Objectives: During musculoskeletal development, the vitamin D endocrine system is crucial, because vitamin D-dependent calcium absorption is a major regulator of bone growth. Because exercise regimens depend on bone mass, the direct action of active vitamin D (1,25-dihydroxyvitamin D [1,25(OH)D]) on musculoskeletal performance should be determined.

Methods: To evaluate the effect of 1,25(OH)D on muscle tissue, the vitamin D receptor (Vdr) gene was genetically inactivated in mouse skeletal muscle and the role of 1,25(OH)D-VDR signaling on locomotor function was assessed. The direct action of 1,25(OH)D on muscle development was determined using cultured C2C12 cells with myogenic differentiation.

Results: The lack of Vdr activity in skeletal muscle decreased spontaneous locomotor activity, suggesting that the skeletal muscle performance depended on 1,25(OH)D-VDR signaling. Bone phenotypes, reduced femoral bone mineral density, and accelerated osteoclast bone resorption were confirmed in mice lacking skeletal muscle Vdr activity. In vitro study revealed that the treatment with 1,25(OH)D decreased the cellular adenosine triphosphate (ATP)-to-adenosine monophosphate ratio without reducing ATP production. Remarkably, protein expressions of connexin 43, an ATP releaser to extracellular space, and ATP metabolizing enzyme ectonucleotide pyrophosphatase phosphodiesterase 1 were increased responding to 1,25(OH)D treatment. Furthermore, the concentration of pyrophosphate in the culture medium, which inhibits tissue calcification, was increased with 1,25(OH)D treatment. In the presence of 1,25(OH)D-VDR signaling, calcium accumulation was suppressed in both muscle samples isolated from mice and in cultured C2C12 cells.

Conclusions: This study dissected the physiological functions of 1,25(OH)D-VDR signaling in muscle and revealed that regulation of ATP dynamics is involved in sustaining locomotor function.