Estrogen effects on the renal handling of calcium in the ovariectomized perfused rat

Estrogen deficiency is a major cause of bone loss in women but the mechanism is unclear. The ovariectomized (OVX) rat is a well recognized model for post-menopausal osteoporosis. In this study we have examined the effects of OVX and estrogen replacement in the OVX rat on the renal handling of calcium in response to alterations in the calcium load in the perfused rat. The interaction of estrogen administration and parathyroid hormone (PTH) was also examined in the OVX, parathyroidectomized (PTX) rat. Calcium or EDTA was infused into sham or OVX rats to obtain a range of filtered calcium loads. The excretion of calcium, was compared to the filtered load for the data from both perfusions indicating a lower calcium (P = 0.006) and sodium (P = 0.009) excretion in the OVX rat. A similar result was seen in the OVX rat replaced with 20 micrograms of estrogen valerate 48 and 24 hours prior to perfusion with calcium excretion being greater with estrogen administration (P = 0.005) compared to vehicle alone. This was not observed in the parathyroidectomized rat. Correlations between sodium and water reabsorption and calcium and sodium reabsorption during perfusion indicate that the results of OVX were due primarily to proximal tubule effects. Prior to the perfusion experiment PTH (sham vs. OVX pmol/liter, mean +/- SD; 20 +/- 6 vs. 18 +/- 4) and calcitriol (128 +/- 85 vs. 97 +/- 74) were similar in both groups, indicating that the results were not dependent on calcitropic hormone effects. It is concluded that, in the perfused rat, OVX results in decreased excretion of calcium and sodium as a result of estrogen effects on the renal proximal tubule, an effect dependent on PTH. This effect is opposite to that found in postmenopausal women, perhaps due to the high filtered load of calcium used in the experimental design and species differences in the relative importance of proximal versus distal calcium handling.