REGULATION OF PROGESTERONE DURING PREGNANCY IN THE CAT - STUDIES ON THE ROLES OF CORPORA-LUTEA, PLACENTA AND PROLACTIN SECRETION

The regulation of progesterone secretion and release by the corpus lut eum and/or by the placenta in cats during pregnancy has been poorly st udied and the relative roles of the corpus luteum and the placenta in the release of progesterone into the plasma is not well defined. It is generally assumed that after 40-45 days of pregnancy, circulating pro gesterone is mainly produced by the placenta and that after 45-50 days the corpus luteum is no longer necessary to maintain pregnancy, even if remnants of corpora lutea are detected in the ovaries during the fo llowing interoestrous period. In this study, the relative roles of the placenta and ovary in progesterone production after 30 days of pregna ncy have been analysed in an attempt to define the role of progesteron e in pregnant cats. Different experiments involved (1) analysis of the effects of an ovariectomy performed at 30 days of pregnancy, and of m egestrol acetate supplementation on plasma progesterone during the sec ond half of pregnancy, (2) analysis of the effects of an ovariectomy p erformed at 45 days on plasma progesterone production and on pregnancy and (3) analysis of the effects of an antiprolactin agent (cabergolin e) at days 30 and 40 of pregnancy on progesterone secretion and pregna ncy. These results were all compared with the normal plasma progestero ne profiles during (a) pseudopregnancy and (b) pregnancy. The results suggest that the main source of progesterone during the second part of pregnancy is the corpus luteum, whereas production by the placenta is of minor importance or non-existent and may be limited to a local par acrine effect that is not responsible for the difference in plasma pro gesterone observed between pregnant and pseudopregnant animals. The pl acenta either does not secrete progesterone or does not do so in suffi cient amounts to maintain pregnancy. The results suggest that the corp ora lutea remain functional throughout pregnancy and regress before pa rturition. Pregnancy in the cat, therefore, probably involves a pregna ncy-specific secretion of luteotrophic hormones of placental or pituit ary origin.