Growing evidence suggests that intracellular vitamin D receptors are present in skeletal muscle tissue mediating vitamin D hormone response. The aim of the work reported here was to investigate the in situ expression of 1,25-dihydroxy vitamin D-3 receptor in human skeletal muscle tissue. Intraoperative periarticular muscle biopsies were taken from 20 female orthopaedic patients (17 middle-aged and elderly patients receiving total hip arthroplasty due to osteoarthritis of the hip or an osteoporotic hip fracture and 3 young patients who received back surgery). The immunohistological distribution of the vitamin D-3 receptor was investigated using a monoclonal rat antibody to the receptor (Clone Nr. 9A7). The receptor-positive nuclei were quantified by counting 500 nuclei per biopsy. Strong intranuclear immunostaining of the vitamin D receptor was detected in human muscle cells. Biopsies of hip patients had significantly fewer receptor-positive nuclei compared to those of back surgery patients (Mann-Whitney U-test: p = 0.0025). VDR expression (number of antigen-positive nuclei) was significantly correlated with age (coefficient of correlation = 0.46; p = 0.005), but not with 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D levels. The data clearly demonstrate presence of nuclear 1,25-dihydroxyvitamin D-3 receptor in human skeletal muscle. To our knowledge this is the first in situ detection of the receptor in human skeletal muscle. The difference in the expression of the receptor between hip and spinal muscle biopsies might be explained by age or location. Further research is needed in order to evaluate whether vitamin D-3 receptor expression in human skeletal muscle is age-dependent and varies between different muscles.
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