Age and gender effects on 1,25-dihydroxyvitamin D-3-regulated gene expression

Several factors involved in regulation of bone mineral metabolism were compared in male and female Fischer 344 rats of different ages (1, 2.5, 6, and 18 months). Plasma 1,25-(OH)(2)D-3 concentrations decreased with age in rats of both genders. Abundance of calbindin-D-28K and its mRNA in kidney and calbindin-D-9K and its mRNA in duodenum also decreased with age in both male and female rats, Renal 24-hydroxylase activity and 24-hydroxylase mRNA content were elevated significantly in 18-month-old males and females, compared with younger ages. These data suggest that increased renal catabolism of 1,25-(OH)(2)D-3 may be responsible for low plasma 1,25-(OH)(2)D-3 concentrations observed in older animals. Plasma PTH and 1,25-(OH)(2)D-3 concentrations, renal 24-hydroxylase enzyme activity and 24-hydroxylase mRNA content, duodenal 24-hydroxylase mRNA abundance, and duodenal calbindin-D-9K and calbindin-D-9K mRNA content were greater in males than in females at 2.5 months of age. Lower plasma 1,25-(OH)(2)D-3 concentrations in females seem to explain observed gender differences in expression of 1,25-(OH)(2)D-3-stimulated genes. The combined effects of these gender differences at ages when peak bone density is being developed may contribute to the greater incidence of osteoporosis in females than in males.