Effect of Exogenous Progesterone on the Preovulatory Progesterone Secretion in the Rat

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A subcutaneous single injection of progesterone at certain early hours on the day of proestrus caused an advanced increase in ovarian progesterone secretion 2 to 3 hrs. before the time of its spontaneous preovulatory increase in 4-day cyclic rats. The secretion of 20α-hydroxypregn-4-en-3-one (20α-OH-P), however, was not significantly altered by exogenous progesterone. Such an effect of the exogenous progesterone on the preovulatory secretion of progesterone was not seen when rats were hypophysectomized or when treated with phenobarbital just before progesterone injection or they were at the stage of diestrus. The results indicate that the progesterone effect is mediated through neural factors controlling luteinizing hormone (LH) release from the pituitary. Our previous works have demonstrated that a preovulatory increase in progesterone secretion occurs on the evening of the day of proestrus and is entirely dependent upon the release of LH from the pituitary. Other endogenous steroids such as 20α-OH-P, estradiol, estriol, testosterone, and 5-androstenediol produced no such a promoting effect on the preovulatory progesterone secretion. It is suggested, therefore, that 20α-OH-P plays no physiological role in the regulation of LH release in the rat. The interdependency between LH release and progesterone secretion was discussed, and neither negative nor positive feed-back control mechanism was accepted for the preovulatory secretions of LH and progesterone in the cyclic rat.
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Effect of Exogenous Progesterone on the Preovulatory Progesterone Secretion in the Rat

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