Plasma levels of parathyroid hormone that induce anabolic effects in bone of ovariectomized rats can be achieved by stimulation of endogenous hormone secretion

Abstract

Parathyroid hormone (PTH) administration increases bone mass in normal and osteopenic animals. However, this treatment currently requires the daily injection of large amounts of PTH, and the relationship of these doses to plasma levels of PTH that are achievable physiologically is unknown. We determined in ovariectomized (ovx) rats: 1) the plasma PTH levels that occur after the subcutaneous injection of graded doses of rat PTH, 2) whether similar PTH levels can be achieved by stimulation of endogenous PTH secretion, and 3) whether a plasma PTH profile that is achievable physiologically is anabolic on bone. Injection of 1, 5, or 25 μg/kg rat PTH-(1–34) increased plasma PTH by 46, 164, or 520 pg/mL, respectively, above basal levels within 60 min. Infusion of ethylene glycol- bis(β-aminoethyl ether)- N,N,N′,N′-tetraacetic acid for 2 h reduced plasma Ca 2+ by 0.36 mmol/L and produced a total plasma PTH response (area under the plasma PTH curve) similar to that with the 5 μg/kg rat PTH injection. Then, 1, 5, or 25 μg/kg doses or rat PTH-(1–34) were injected subcutaneously daily for 28 days in 19-week-old rats that were ovx 7 weeks earlier. The 5 and 25 μg/kg doses significantly increased bone mineral density in the distal femur and trabecular bone area and average trabecular thickness in the proximal tibia. All doses of PTH significantly increased indices of trabecular connectivity and cancellous bone formation, including double-labeled surface, mineralizing surface, and surface-referent bone formation rate. In conclusion, anabolic effects on bone can be achieved with a plasma PTH profile similar to that attained following stimulation of the parathyroid gland by induced hypocalcemia. These data suggest that agents that transiently increase endogenous PTH secretion may represent a novel means to promote anabolic effects in skeletal tissues.