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J. K. KULSKI, MARGARET SMITH and P. E. HARTMANN

Departments of Biochemistry and *Obstetrics and Gynaecology, University of Western Australia, Nedlands, Australia

(Received 25 April 1977)

The hormonal signal for lactogenesis in a number of different species is considered to be a precipitous fall in the concentration of progesterone in the blood during late pregnancy (see Hartmann, Trevethan & Shelton, 1973). In women, the major fall in the level of blood progesterone just after delivery (Yannone, McCurdy & Goldfien, 1968) precedes lactogenesis by 2-3 days (Reynolds, 1972). However, the effects of this fall may be modified to some extent by progesterone, which accumulates in the milk (Heap, Gwyn, Laing & Walters, 1973). Progesterone could be the 'lipid-soluble substance' in the mammary secretion which Linzell & Peaker (1974) suggested might control the final stage of lactogenesis. The purpose of the present investigation was to compare changes in the concentrations of progesterone, lactose and α-lactalbumin in the mammary secretion of women

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K. R. Nicholas, L. Sankaran, J. K. Kulski, P. Chomczynski and P. Qasba

ABSTRACT

Commercial preparations of serum albumin from six species can markedly enhance the prolactin-independent induction of α-lactalbumin in mammary explants from pregnant rats, and evoke such induction in the tissue from virgin rats. These effects are similar to those of epidermal growth factor (EGF) reported previously. The stimulatory activity of bovine serum albumin (BSA) resides in a putative impurity in the albumin. Charcoal extraction and gel filtration of the BSA results in complete loss of activity. Of the five milk protein mRNAs studied, only α-lactalbumin mRNA is induced by insulin, glucocorticoid and serum albumin in the absence of prolactin. Despite these similarities, the biological effects of serum albumin and EGF on mammary tissue diverge in some respects. They appear to operate by different mechanisms since their effects on the rat system are additive. Furthermore, while both inhibit prolactin-mediated induction of α-lactalbumin in rabbit mammary explants, cortisol converts EGF into a stimulatory agent, but merely blocks the inhibitory effect of serum albumin. The results emphasize that commercial serum albumin is not to be regarded simply as an inert protein additive to culture media.

J. Endocr. (1988) 119, 133–139