Lactogenic Content of Pituitaries of Pseudopregnant Rabbits. ∗ †

It has been suggested that copious lactation can be initiated only when (a) there is a well developed mammary gland present and (b) when there is a high lactogen content in the anterior pituitary. 1 After the middle of pregnancy one of these two factors is present, namely, a well developed mammary gland, but the production of lactogenic hormone by the pituitary remains at too low a level to initiate abundant lactation. Thus in the rat, 2 3 guinea pig, 4 and rabbit, 1 4 the lactogen content of the pituitary remains as low during pregnancy as in the non-pregnant state. Almost immediately following parturition, however, there is a great increase in the lactogenic hormone of the pituitary (from 200 to over 400%), and full lactation is initiated. 2 3 4 An increased lactogen content in the urine has also been demonstrated in postpartum women. 5

Inasmuch as the mammary glands of the rabbit become well developed during pseudopregnancy, it was of interest to determine whether the content of lactogenic hormone in the pituitary at the end of pseudopregnancy would account for the absence of any marked degree of milk secretion in such animals. Seven normal, mature New Zealand white rabbits served as control animals, while 11 similar rabbits were each injected intravenously with 100 I.U. chorionic gonadotropin (Korotrin)‡ in order to induce pseudopregnancy. Twenty days later both the control and pseudopregnant rabbits were killed, and their pituitaries were removed for assay. In 6 of the 11 pseudopregnant rabbits it was found possible to express either a slight amount of milk or serous secretion from the teats. The pituitaries of each group were assayed quantitatively for lactogen by the Reece-Turner pigeon method. 2