ESTROGEN DOSE REQUIRED TO MAINTAIN PARATHYROID-HORMONE MEDIATED BONE GAIN IN OSTEOPENIC OVARIECTOMIZED RATS IS PROBABLY HIGHER THAN IN PREVENTIVE TREATMENT WITH ESTROGEN

Successful prevention of bone loss in ovariectomized rats with 17 beta-estradiol (E(2)) at 10 mu g/kg/d has been reported. Here we show that E(2) dose twice that much is not enough to maintain PTH(1-34) mediated bone gain. Three-month-old female Wistar rats were ovariectomized and fed with regular rodent chow and water ad libitum. Three months later they were divided into 9 groups (5-8 per group) and treated cyclically with PTH(1-34) (20 mu g/kg/d sc, 5d/w for 3w) and E(2) (20 mu g/kg/d sc, 5d/w for 4w). There were also a baseline group and five vehicle control groups. Cancellous bone volume (Cn.BV/TV) of distal femoral metaphyses was measured by computer-aided histomorphometry on trichrome stained thin sections, and 24-hour fasted urine was analyzed for pyridinoline/creatinine (PYR/CREA) by immunoassay. Histomorphometric results showed that PTH(1-34) progressively increased Cn.BV/TV but E(2) failed to maintain them. Urinary PYR/CREA results showed that E(2)-treated groups had lower values. We conclude that E(2) dose >20 mu g/kg/d is required to maintain the PTH mediated bone gain.