Mode of Action of Pharmacological Doses of Cholecalciferol during Parturient Hypocalcemia in Dairy Cows

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Abstract
A high-calcium diet (130 g calcium, 35 g phosphorus/day) and a high-phosphorus diet (45 g calcium, 70 g phosphorus/day) with and without an injection of 10 × 106IU of cholecalciferol were studied in pregnant cows. Cholecalciferol injections 7 days prepartum resulted in a 150% rise in circulating 1,25-dihydroxycholecalciferol 24 hours post-injection. By day 1 prepartum the 1,25-dihydroxycholecalciferol levels returned to pre-injection levels. The levels of 1,25-dihydroxycholecalciferol in the plasma of control cows did not change until parturition at which time the hormone increased from 100–200 pg/ml to 600 pg/ml. This was followed by a decrease to 200 pg/ml by day 2 postpartum. The injection of cholecalciferol in cows eating high-phosphorus diets inhibited the rise of plasma 1,25-dihydroxycholecalciferol seen in noninjected cows. Cholecalciferol injections reduced the mean concentration of 1,25-dihydroxycholecalciferol during the 7-day sampling period. Changes in plasma calcium, phosphorus, magnesium and hydroxyproline levels were within the range reported by other investigators. The initial rise in plasma 1,25-dihydroxycholecalciferol following cholecalciferol injections and the general reduction in circulating levels of this hormone during the prepartal and parturition periods suggest that these changes are primary factors in the preventive effects of cholecalciferol injections on parturient hypocalcemia.