Vitamin D independent anabolic action of PTHin vivo

Abstract

We examined the involvement of endogenous calcium-regulating hormones in the anabolic action of PTH, especially in relation to vitamin D. When normal rats (4 weeks old) were given a subcutaneous injection of hPTH(1-34) everyday or one, two or three times a week for 4 weeks adjusting the total weekly dose to 350µg/kg, femoral dry weight increased in an injection frequency-dependent manner. Daily injection of hPTH(1-34) into normal rats for 12 days caused a dose-dependent increase in serum concentration of 1,25(OH)2D but not calcitonin, associated with a dose-dependent increase in femoral dry weight. Therefore, the effect of hPTH(1-34) was examined under the condition of vitamin D (VD)-deficiency. In VD-deficient rats (13 weeks old), the change in serum immunoreactive PTH after the injection of hPTH(1-34) was the same as that observed in normal rats, although the basal level of serum PTH was significantly higher. Furthermore, VD-deficient rats treated with hPTH(1-34) for 4 weeks showed a dose-dependent increase in femoral dry weight without affecting circulating 1,25(OH)2D. These results suggest the involvement of systemic or local factor(s) independent of the action of VD in the anabolic action of daily PTH treatment.