RÉSUMÉ
Nine Holstein-Friesian cows in the same stage of lactation were randomly given IM injections of saline solution, 100 IU of ACTH, or 200 IU of ACTH for a period of 3 days at 0800 and 1600 hours on one day and at 0100 hours the next day. A dose of 100 IU of ACTH represented the amount of ACTH released during physiologic stress and 200 IU of ACTH represented a therapeutic dose. Blood samples were obtained from indwelling jugular catheters for total circulating leukocyte and total plasma corticosteroid determinations at -1, 0, 0.25, 0.50, 1, 1.50, 2, 4, 6, and 8 hours after the 0800-hour ACTH injections. Concentrations of blood leukocytes and total plasma corticosteroids were affected significantly (P < 0.05) by treatment, cow within treatment, day of injection, treatment by day interaction, time, treatment by time interaction, and day by time interaction. Total plasma corticosteroids were 5.6 ng/ml (saline), 38.9 ng/ml (100 IU of ACTH), and 59.8 ng/ml (200 IU of ACTH). Mean corticosteroid concentrations from 0.5 to 8 hours were 29.2 ng/ml on day 1, 42.5 ng/ml on day 2, and 32.7 ng/ml on day 3. Blood leukocytes were 8,545 cells/mm3 (saline, 11,241 cells/mm3 (100 IU of ACTH) and 9,777 cells/mm3 (200 IU of ACTH). When compared with basal circulating leukocyte concentrations (within cow), this represented increases of 20.7% for 100 IU of ACTH and 24.5% for 200 IU. Mean circulating leukocyte concentrations from 0.5 to 8 hours were 8,900 cells/mm3 on day 1, 10,500 cells/mm3 on day 2, and 10,000 cells/mm3 on day 3. These data indicate that the adrenal gland responded less on day 3 and that continuous stimulation of bovine adrenal glands by ACTH at physiologic and therapeutic concentration could lead to reduced synthesis or release of adrenal corticosteroids (or both) with concomitant reduction in the concentration of total circulating leukocytes.
