Effects of endogenous estrogen depletion and its induced short-term rise on bone and Ca metabolism in ovariectomized dairy cows

Abstract

Estrogen, a potent inhibitor of bone resorption, rises in the final days prior to parturition and deplete precipitously after parturition in cows. However, the effect of estrogen fluctuation on bone and calcium (Ca) metabolism is unclear in dairy cows. We prepared an ovariectomized model of dairy cows to study the effects of endogenous estrogen depletion and its induced short-term rise on bone and Ca metabolism. Four multiparous nonpregnant Holstein cows were bilaterally ovariectomized. Jugular blood samples were collected prior to ovariectomy and on weeks 7 and 17 post-ovariectomy. On week 17, the cows were administered estradiol benzoate (EB) once intramuscularly and blood samples were collected prior to EB injection followed by on days 1, 2, 3, 4, 5, 7, 21 and 42 post-injection. Plasma levels of estradiol-17β (E₂), bone resorption markers tartrate-resistant acid phosphatase 5b and hydroxyproline, bone formation marker bone-specific alkaline phosphatase and Ca were measured. As compared to the pre-ovariectomy levels, plasma E₂ level decreased significantly whereas levels of both the bone resorption markers increased without altering Ca level during post-ovariectomy. Each plasma bone marker showed a strong nonlinear inverse correlation with plasma E₂ level. An increased E₂ level for a few days induced by EB injection did not alter the plasma levels of bone markers. The result showed that depletion of endogenous estrogen accelerates bone metabolism without altering plasma Ca level and a short-term estrogen rise for a few days may not alter bone metabolism in dairy cows. These findings enhance the understanding of bone metabolism and Ca homeostasis in periparturient dairy cows.