Abstract
ABSTRACT In order to explain the increased mammotrophic activity of pregnant rat serum, some hormones known to have an in vitro effect on the mammary gland were added to non-pregnant rat serum and their effect compared with that of pregnant rat serum. The addition of insulin to the culture medium increased the mammotrophic effect of pregnant rat serum, but a difference between non-pregnant and pregnant rat serum could be demonstrated either in the presence or absence of a high concentration of insulin. While pregnant rat serum had a mammogenic (i. e. growth-promoting) effect, cortisol had no such effect and antagonized the mitotic activity induced by the pregnant serum. Pregnant rat serum was active at such low concentrations, that the progesterone content of pregnant rat serum could not explain the mammogenic activity of the latter. Moreover, the combination of progesterone with non-pregnant rat serum did not produce other changes produced by pregnant rat serum alone such as cytoplasmic opalescence and the localized development of vacuolization and secretion. Prolactin with non-pregnant rat serum produced a mammotrophic effect which was in all respects similar to that of pregnant rat serum. An effect could be demonstrated at 0.08 μg prolactin/ml medium. However, again pregnant rat serum was active at such low concentrations that the prolactin content of pregnant rat serum could not explain the mammotrophic effect of pregnant rat serum. Moreover, the effect of pregnant rat serum could not be blocked with rabbit anti-rat prolactin serum, but the anti-serum blocked the effect of rat prolactin added to non-pregnant rat serum. The results exclude the possibility that insulin, cortisol, progesterone or prolactin can account for the mammotrophic activity in pregnant rat serum. They provide indirect evidence for the importance of rat chorionic mammotrophin.
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