Summary. Rabbits were given 50 i.u. hCG, i.v., to initiate ovulation and pseudopregnancy (Day 0) and were treated, s.c., with or without a 1-cm Silastic oestradiol implant. Serum progesterone concentrations were measured at 4-day intervals and 3-hydroxy-3-methylglutaryl–coenzyme A (HMG–CoA) reductase activity was estimated by the conversion of HMG to mevalonate in microsomes from corpora lutea removed on Days 4, 8, 12, 16 and 20 of pseudopregnancy (4 rabbits/day). Total HMG–CoA reductase activity was significantly (P < 0·05) higher in control rabbits on Days 8 and 12 (5·29 ± 0·63 and 5·5 ± 0·28 nmol/min/mg protein, respectively) compared to oestradioltreated rabbits (2·57 ± 0·25 and 4·03 ± 0·23 nmol/min/mg protein, respectively). On Days 16 and 20, total HMG–CoA reductase activity was not different in control and oestradiol-treated animals. There was no difference in the levels of the active fraction of HMG–CoA reductase, which represented <20% of the total enzyme activity, in control and oestradiol-treated rabbits (<780 pmol/min/mg protein, Day 12). These results indicate that oestradiol does not alter the active form, but can reduce the total activity of HMG–CoA reductase in the rabbit corpus luteum without a decline in serum progesterone. Therefore, neither total nor active forms of HMG–CoA reductase are directly related to progesterone secretion. This suggests that other sources of cholesterol may contribute to progesterone production in the rabbit.
Keywords: HMG–CoA reductase; corpus luteum; progesterone synthesis; oestradiol action; lipoproteins