Hormonal Mechanisms Involved in the Luteotropic Action of LH During the Estrous Cycle in the Rat

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Abstract
The mechanisms involved in LH luteotropic action were studied in 4 day cyclic female rats. A rapid increase in blood progesterone concentration was observed within 3 h in females injected with LH on diestrus I at 0800-0900 h. Females treated with LU also displayed a higher level of blood progesterone than did controls on diestrus II morning. Estradiol benzoate (10 µg EB), when injected on diestrus I morning, also caused an increase in blood progesterone concentration on the morning of diestrus II. An increase in blood prolactin concentration was observed on diestrus I afternoon and on diestrus II morning following either LH or EB injection on the morning of diestrus I. A decrease in blood estradiol-17β concentration occurred on the morning and in the afternoon of diestrus I following LH injection in the morning. A dose of progesterone (2 mg) capable of increasing circulating progesterone within 3 h, when injected on diestrus I morning, increased the blood prolactin concentration on diestrus II morning above that of the uninjected control females. It is concluded that estrogens did not mediate the changes in blood progesterone concentration caused by LH injection during the diestrus period of the cycle in the rat. The possibility that progesterone might be involved in LH luteotropic action is discussed in the light of these observations.