Goff J P, Liesegang A, Horst R L
Periparturient Diseases of Cattle Research Unit, National Animal Disease Center, US Department of Agriculture-Agricultural Research Service, Ames, IA 50010.
Institute of Animal Nutrition, Vetsuisse Faculty University of Zurich, Winterthurerstr. 260, 8057 Zurich, Switzerland.
J Dairy Sci. 2014 Mar;97(3):1520-8. doi: 10.3168/jds.2013-7467. Epub 2014 Jan 11.
Subclinical hypocalcemia may affect half of all multiparous cows, and clinical hypocalcemia or milk fever affects approximately 5% of dairy cows each year. This disorder of calcium homeostasis can be induced by several dietary factors. Recent studies implicate high dietary potassium and high dietary cation-anion difference (DCAD) with increased risk of milk fever. The hypothesis tested in this study was that high-DCAD diets fed to prepartum cows reduce tissue sensitivity to parathyroid hormone (PTH), inducing a pseudohypoparathyroid state that diminishes calcium homeostatic responses. Multiparous Jersey cows were fed low- or high-DCAD diets in late gestation, creating a compensated metabolic alkalosis in the high-DCAD cows and a compensated metabolic acidosis in the low-DCAD cows. They then received synthetic PTH injections at 3-h intervals for 48 h. Parathyroid hormone is expected to cause an increase in plasma calcium by increasing renal production of 1,25-dihydroxyvitamin D and increasing bone calcium resorption. Plasma calcium concentration increased at a significantly lower rate in cows fed the high-DCAD diet. Cows fed the high-DCAD diet also produced significantly less 1,25-dihydroxyvitamin D in response to the PTH injections than cows fed the low-DCAD diet. Serum concentrations of the bone resorption marker carboxyterminal telopeptide of type I collagen were numerically lower in cows fed the high-DCAD diet but this difference was not statistically significant. These data provide direct evidence that high-DCAD diets reduce tissue sensitivity to PTH. The metabolic alkalosis associated with high-DCAD diets likely induces a state of pseudohypoparathyroidism in some dairy cows at the onset of lactation, resulting in hypocalcemia and milk fever.
亚临床低钙血症可能影响一半的经产奶牛，而临床低钙血症或乳热每年影响约5%的奶牛。这种钙稳态紊乱可由多种饮食因素诱发。最近的研究表明，高膳食钾和高膳食阴阳离子差（DCAD）会增加乳热风险。本研究检验的假设是，给产前奶牛饲喂高DCAD日粮会降低组织对甲状旁腺激素（PTH）的敏感性，引发假性甲状旁腺功能减退状态，从而削弱钙稳态反应。在妊娠后期，给经产泽西奶牛饲喂低DCAD或高DCAD日粮，使高DCAD组奶牛出现代偿性代谢性碱中毒，低DCAD组奶牛出现代偿性代谢性酸中毒。然后，每隔3小时给它们注射一次合成PTH，持续48小时。甲状旁腺激素预计会通过增加肾脏1,25 - 二羟维生素D的生成和增加骨钙吸收来使血钙升高。饲喂高DCAD日粮的奶牛血浆钙浓度升高的速率显著较低。与饲喂低DCAD日粮的奶牛相比，饲喂高DCAD日粮的奶牛对PTH注射的反应中产生的1,25 - 二羟维生素D也显著减少。饲喂高DCAD日粮的奶牛血清中骨吸收标志物I型胶原羧基末端肽的浓度在数值上较低，但这种差异无统计学意义。这些数据提供了直接证据，表明高DCAD日粮会降低组织对PTH的敏感性。与高DCAD日粮相关的代谢性碱中毒可能在泌乳开始时诱发一些奶牛出现假性甲状旁腺功能减退状态，导致低钙血症和乳热。
