Ovarian Secretion of Progesterone and 20α-Hydroxypregn-4-en-3-one during Rat Estrous Cycle in Chronological Relation to Pituitary Release of Luteinizing Hormone

Abstract

The secretory rates of progesterone and 20α-hydroxypregn-4-en-3-one (20α-OHP) from the ovary and the concentration of both progestins in the ovarian tissues were determined during the estrous cycle in 4-day cyclic rats illuminated 12 hrs. a day (8a.m. to 8 p.m.). The progesterone secretion increased twice a cycle, markedly on the evening (peak at 7p.m.) of the day of proestrus and slightly on the afternoon of the day of early diestrus. The prompt and tremendous increase of progesterone secretion at the preovulatory stage almost synchronized with the release of luteinizing hormone (LH). It was also demonstrated that a marked preovulatory rise of progesterone secretion depended upon pituitary LH discharge, and a slight increase in both progestin secretions-at early diestrus was not dependent on the pituitary function at that stage. These facts together with the following fmdings, (a) an enormous increase in the ratio of progesterone over total progestin (progesterone plus 20α-OH-P) in the ovarian venous blood at the preovulatory stage, (b) the secretory pattern of 20α-OH-P which roughly paralleled with that of progesterone at the diestrous phase, and (c) a higher concentration of progesterone in the ovary as compared with that of 20α-OH-P at the diestrous phase and its reverse relationship of the concentrations at the stages of proestrus and estrus, suggest that a drastic shift occurs at the sites of progesterone synthesis from the involuting corpora lutea to the LH sensitive ripening follicles or interstitial tissues under the influence of released LH which exerts both luteinizing effect and steroidogenic action on the ovary, and the present data suggest that the newly developed corpora lutea after ovulation gain the automaticity of progestin synthesis, storage and secretion independently of the pituitary function.