Increased Sensitivity of Bone to Parathyroid Hormone in Ovariectomized Rats

The inhibitory effect of endogenous estrogens on the parathyroid hormone (PTH) induced bone changes in rats was studied through the analysis of calcium and hydroxyproline content of the femur and tibia, and the measurement of cortical thickness of the femurs. Administration of 30 USP units of PTH for 3 weeks caused a significant fall in calcium content of the right femur and hydroxyproline content of the right tibia in both intact and ovariectomized rats. However, the magnitude of PTH induced bone changes was significantly greater in ovariectomized rat than in intact ones. Furthermore, decrease in cortical thickness of the femur caused by PTH was significantly greater in ovariectomized rat than in intact ones. It is concluded that the sensitivity of bone to PTH is increased in the absence of endogenous estrogens. (Endocrinology90: 760, 1972)