Effect of adrenocorticotropin on milk and plasma cortisol and prolactin concentrations.

Milk cortisol and prolactin concentrations were measured in 12 lactating Holstein cows for 3 days (Experiment 1). On day 3, cows were divided randomly into groups: IV1, IV4, IM, and control. Group IV1 received one intravenous injection of 49 IU adrenocorticotropin, group IV4 the same dosage four times at 2-h intervals, group IM 240 IU adrenocorticotropin intramuscularly, and control cows were injected with saline. Cortisol concentrations in blood plasma were increased in all groups following adrenocorticotropin injection, while milk cortisol concentrations increased fourfold in groups IV4 and IM and remained unchanged in group IV1. In Experiment 2, infusion of 18% saline into the mammary gland of three cows increased the somatic cell count in the infused quarter but had no effect on prolactin and cortisol of milk or plasma. In Experiment 3, six cows each were assigned to treatment groups of saline controls, IV2-0 (40 IU adrenocorticotropin intravenously at 0 and 2 h post-milking) or IV2-8 (40 IU adrenocorticotropin intravenously at 8 and 10 h post-milking). Half udders of each cow were milked before treatment and 4 h later. Increases of cortisol in plasma increased cortisol concentrations of milk 4 h after each treatment (IV2-0 and IV2-8). However, by 12 h after treatment in IV2-0 cortisol concentrations of milk had returned to normal. Increases in adrenal cortisol secretion are followed rapidly (within 4 h) by increased cortisol concentrations in milk. However, these decline rapidly in the absence of sustained increments in blood cortisol. Increased cortisol concentrations in milk most likely represent sustained elevation in plasma cortisol.