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Sequential effects of chronic human PTH (1‐84) treatment of estrogen‐deficiency osteopenia in the rat

Although daily injections of parathyroid hormone (PTH) can rapidly reverse estrogen‐deficiency bone loss in rats, PTH treatment of osteoporotic humans has to date produced more modest increases in bone mass. To explore the reasons for this important difference, we evaluated the dose‐ and time‐dependence of human PTH 1–84 treatment effects on bone mass and biochemical markers of bone metabolism in rats with estrogen‐deficiency bone loss. The highest doses of PTH increased spinal, femoral, and total skeletal mass to supra‐normal levels and stimulated cortical endosteal bone formation. Spine and whole skeleton mass and density increased rapidly at first, but then increased more slowly; the rate of change decreased significantly (p < 0.01) during continued treatment with the highest doses of PTH. The effects of PTH treatment on biochemical markers also were both dose‐dependent and time‐dependent. Serum osteocalcin, a marker of osteoblast function, increased with the highest doses of PTH (p < 0.001), but reached an early plateau and later returned toward baseline. Urinary excretion of pyridinolines, a marker of osteoclast function, increased in a time‐dependent fashion throughout treatment (p < 0.001). Serum 1,25(OH)2 vitamin D levels increased in a dose‐related fashion, but then decreased toward control levels despite continued treatment. We demonstrate that both osteoblast and osteoclast function are increased during daily PTH therapy in the rat. The pattern of response depends on both the dose of PTH and the duration of therapy. These dose‐ and time‐related effects should be taken into account when designing experimental PTH treatments for osteoporosis, and they deserve intensive study.