Cholesterol Storage and Progestin Secretion During Pregnancy and Pseudopregnancy in the Rabbit

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Abstract
The basal and LH [luteinizing hormone]-stimulated levels of progesterone (P) and 20[alpha]-hydroxy-pregn-4-en-3-one (20[alpha]-OH) released from the rabbit ovary in vivo during pregnancy and pseudopregnancy have been correlated in this study with luteal weights and the relative amounts of cholesterol stored in the corpora lutea (CL) and interstitial tissue (IST). P output and luteal weight were greatest between days 15-20 of pregnancy when the ratio of P/20[alpha]]-OH was 1.34. At this time the release of P was estimated to be 3.06 mg/24 hr. After day 20, P output declined gradually to 0.46 mg/24 hr in the final week of pregnancy and the ratio of P/20[alpha]-OH dropped to 0.27. Throughout gestation, the ratio of CL-cholesterol/lST-cholesterol remained low, indicating that this sterol was being continuously mobilized by the CL. In the pseudo-pregnant animals, luteal weight, P output and cholesterol storage by the IST were greatest between days 8-14, but the release of P did not exceed that of 20[alpha]-OH. As pseudopregnancy ended, luteal weight and P output dropped sharply and cholesterol accumulated rapidly in the CL. The shift in the mobilization of cholesterol from the CL to the IST which occurred at the close of pseudopregnancy and the beginning of estrus was also reflected in marked elevations of both the basal and LH-stimulated release of 20[alpha] -OH. The differences in cholesterol storage and progestin secretion in these 2 states indicates that some additional hormonal stimulus is necessary to maintain the morphology and prolong the function of the corpora lutea of pregnancy beyond that of pseudopregnancy.