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The hydroxy-carboxylic acid 2 (HCA2) receptors have attracted new interests as the target of antidyslipidemic drug niacin. The presence of mRNA for these receptors has been previously described in rats’ femur. The present study aimed at detection of cell types that express HCA2 receptor, their relative receptor density and effect of niacin administration in proximal femur of rats. Growing male rats kept untreated or received niacin (200 mg/kg/day orally). After 4 weeks, proximal femur was evaluated. Despite a minor decrease in histomorphometric parameters in niacin group, no deleterious effect was observed. In immunohistochemical evaluation, many different cells showed HCA2 receptor expression with the highest receptor density in marrow cells followed by endothelial cells, adipocytes, fibroblasts, striated muscle, osteoblasts, osteocytes, chondroblasts and finally chondrocytes. Osteoclasts were moderately positive. Niacin reduced receptor density in adipocytes, fibroblasts, striated muscle, osteoblasts and osteocytes. Receptor density of marrow cells, chondroblasts and endothelial cells remained statistically the same and in chondrocytes appreciably increased. Immunohistochemical reactivity of the total slide (marrow and trabeculae) as well as mRNA level of HCA2 receptor in qPCR evaluation remained statistically the same. In conclusion, the study showed the presence of HCA2 receptors in bone and cartilage cells with higher expression in osteoblasts and chondroblasts than their mature forms. Moreover, niacin administration resulted in cell type-dependent changes in receptor density of the cells present in the proximal femur of rats.

Localization, Cellular Density and Gene Expression of HCA2 Receptors in Femoral Epiphysis and Metaphysis of Normal and Niacin-Treated Rats