Abstract
Plasma prolactin concentration of serial blood samples taken from cows and goats bearing indwelling jugular venous catheter was assayed by a radioimmunologic procedure. Using 9 lactating cows, the response of prolactin release to milking was investigated at different stages of lactation. In all cases, plasma prolactin levels rapidly increased and reached a peak 4-20 min after the start of milking. The magnitude of the response was considerably different with the stage of lactation in the same cow. The peak of plasma prolactin released by milking was highest in the early stage of lactation (189±34ng/ml, SE), and decreased thereafter with the advance of lactation. Conspicuous response of prolactin release with a 40-to 100-told increase by milking was also obtained in the lactating goats. Feeding could induce a rapid elevation of plasma prolactin in the lactating cow. Mimic-or sham-milking in heifers and in virgin goats in which teats were pulled and the udder was massaged by hands for 5 min, induced an acute prolactin release within 2 min after the start of stimulation, although there was no milk flow. Stressful stimulus associated with venipuncture also elevated the plasma prolactin in the lactating and non-lactating animals. These findings lead to the conclusion that prolactin is a hormone which can be released by various stimuli and that milking is the most powerful of the stimuli for the acute prolactin discharge from the anterior pituitary gland in the cow as well as in the goat at least in the early stage of lactation. At the same time, a variety of non-specific stimuli can also cause a rapid rise in plasma prolactin concentration.
