Metabolic Clearance Rates (MCR) and Production Rates (PR) of Plasma Progesterone in Pregnant and Pseudopregnant Rabbits

Abstract

Our studies were designed to determine whether changing peripheral progesterone levels in rabbits reflected changing metabolic clearance rates (MCR) or changing production rates (PR), or both. Plasma progesterone concentrations rise from nonpregnancy levels to peak values at the end of the first third of gestation and at midpseudopregnancy. In the pregnant rabbit, these decline slowly during the second third of gestation and then more rapidly until near nonpregnancy values are reached at term. Progesterone levels decline sharply during the second half of pseudopregnancy. During pregnancy and pseudopregnancy, we found only small variations in MCRs which cannot account for the approximately 10-fold increase in plasma progesterone concentrations. The increases can, however, be accounted for by changes in PRs which rose sharply after conception of hCG injection to 14-fold the nonpregnancy level on day 16 of gestation and 11-fold on day 7 of pseudopregnancy. These results indicate that changes in ovarian PRs are the major factor for the variations in peripheral progesterone levels during pregnancy and pseudopregnancy. The rabbit differs in this respect from the guinea pig, in which changing progesterone concentrations during pregnancy were shown to reflect sharply reduced MCRs. After a single injection of progesterone in 20-day pregnant rabbits, the disappearance of the steroid from the circulation consisted of two components; an initial phase during which progesterone disappeared rapidly (t1/2 = 2.4 +/- 0.2 min) followed by a slower rate of disappearance (t1/2 = 21.5 +/- 2.2 min).

Steroid concentrations in rabbit oviducal fluid during oestrus and pseudopregnancy

“…The peripheral serum levels of progesterone observed in this study are similar to those reported previously for pregnant animals (Challis et al, 1973;Fuchs & Beling, 1974;Thau & Lanman, 1975); but are slightly higher than those reported for pseudopregnant animals (Fuchs & Beling, 1974;Thau & Lanman, 1975;Spilman & Wilks, 1976). The few studies of serum oestradiol levels do not report values for the same days as the present investigation.…”

“…There have been numerous studies measuring the steroid concentrations in the serum of rabbits before and after ovulation and one report of the progesterone content of uterine flushings (Challis, Davies & Ryan, 1973;Fuchs & Beling, 1974;Thau & Lanman, 1975;Fowler, Johnson, Walters & Pratt, 1976;Spilman & Wilks, 1976;Waterson & Mills, 1976;Wu, Blasco, Flickinger & Mikhail, 1977a). There have been no reports, however, of the levels of steroids in rabbit oviducal fluid.…”

Gender and the injured brain

“…The pharmacokinetics of progesterone indicates that the half-life of this neurosteroid in serum is approximately 15 min, and it is fully metabolized by 24 h. [96][97][98] This results in a spiking effect that is attenuated by subcutaneous delivery, as the bolus of drug seeps into tissues at a slower rate. [99][100][101] al.…”

Conditions for establishment of reflex ovulation in light estrous rats.

“…Pseudopregnancy following copulatory stimulation which was characterized by a typical onepeak change in peripheral progesterone levels covering a continuous diestrous period lasting 10 to 11 days, proceeded in concert with that of the rabbit after infertile mating (Hilliard and Eaton, 1971;Hilliard et al, 1968;Thau and Lanman, 1975).…”