Mammalian LH (250 μg of NIH-LH-B9) and FSH (500 μg of FSH-P, Armour-Baldwin) elicit increases in serum estradiol (E) and progesterone (P) of laying turkeys within 30 min after a single iv injection. Prolactin (NIH-P-S11) at doses of 250 and 500 μg had no effect on the E or P levels when given in a single iv injection. One thousand micrograms of prolactin caused a significant decrease in serum E within 30 min of injection. Prolactin (500 and 1000 μg), given 30 min before LH, blocked the LH induced rise in P and at doses of 250, 500 and 1000 μg it blocked the LH induced rise in E. Five hundred and 1000 μg of prolactin blocked the FSH induced rise in P and doses of 250, 500 and 1000 μg of prolactin blocked the FSH induced rise in E. It is concluded that prolactin's antigonadotropic effect is exerted, at least in part, by interfering with gonadotropin induced increases in ovarian steroid levels in the blood. © 1977.
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