Because a single intramuscular or intravenous injection of ACTH induced a short-term increase in cortisol concentrations (2 to 3 h duration) [7,8], we performed 12-h continuous administration of ACTH solution to reproduce the periparturient fluctuations in plasma cortisol concentrations in dairy cows and observed a sustained increase in plasma cortisol concentrations for
18 h. The present ACTH dose (0.5 mg, 50 IU) seemed appropriate, because the peak plasma cortisol concentration increased about 21-fold compared with the baseline level, which was consistent with levels in parturient cows reported elsewhere [2,3]. Cortisol secreted as a result of infusion of ACTH for 12 h was functional, based on the significantly increased plasma glucose
concentrations, as observed in a previous study [7]. Moreover, we used ovariectomized cows to exclude the effects of gonadal steroid hormones. Therefore, the experimental protocol used in the present study will be useful for inducing the sustained increase in plasma cortisol concentration observed in periparturient cows. However, dairy cows experience abrupt physiological changes and hormonal fluctuations during the periparturient period. In particular, increases in the levels of calcitropic hormones such as parathyroid hormone