INTERMITTENT PARATHYROID-HORMONE TREATMENT INCREASES OSTEOBLAST NUMBER, STEADY-STATE MESSENGER-RIBONUCLEIC-ACID LEVELS FOR OSTEOCALCIN, ANDBONE-FORMATION IN TIBIAL METAPHYSIS OF HYPOPHYSECTOMIZED FEMALE RATS

The effects of GH and PTH on cancellous histomorphometry were determin ed in the proximal tibial metaphysis of hypophysectomized (HYPOX) sexu ally mature female rats. HYPOX resulted in uterine atrophy and a loss in body weight. Longitudinal bone growth ceased and bone formation was greatly reduced. There were decreases in cancellous bone area, trabec ular number, and trabecular thickness. Intermittent treatment with GH did not influence uterine weight in HYPOX rats. However, GH resulted i n resumption of whole body weight gain, as well as maintenance of norm al longitudinal bone growth. Additionally, GH partially maintained bon e formation in HYPOX rats and did not have a significant effect on ste ady state messenger RNA levels for osteocalcin. Intermittent treatment with PTH had no effect on whole body weight gain, uterine weight, or longitudinal bone growth. In contrast, PTH increased bone formation co mpared with the baseline, HYPOX, and GH-treated HYPOX rats, and dramat ically increased osteocalcin messenger RNA levels compared with the la tter two groups. The increased bone formation was primarily due to an increase in osteoblast number; the mineral apposition rate, an index o f osteoblast activity, was increased compared with control and GH-trea ted rats but not compared with baseline values. Interestingly, neither treatment influenced indices of bone resorption.