Serum Progesterone During Pregnancy and Pseudopregnancy and Gestation Length in the Aging Rat1, 2

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Abstract
The hypothesis that progesterone [P] secretion by corpora lutea contributes to reproductive failure in aged mammals was tested in aging Long-Evans rats. The effect of age on serum P during pregnancy and pseudopregnancy was studied in separate groups of rats with normal ovarian cycles at 4, 7, 9, 11, 13, 15, 20 and 22 mo. of age. P was also measured in constant estrous rats made pseudopregnant by mating at 11, 15, 20 and 22 mo. of age. Mated rats were divided into subgroups based on whether they produced litters. Successful pregnancy was reduced to 50% of the 11 mo. old and 25% of the 13 mo. old rats with no successful pregnancies occurring in older rats. P was assayed from serial blood samples collected on Days 1, 6, 11 and 16 after mating. Average P concentrations on any particular day of pregnancy or pseudopregnancy were not affected by increased age. P was increased on postcoital Days 6 and 11 in all groups. Average P concentrations on postcoital Days 1, 6 and 11 were similar in the groups with ovarian cycles which produced litters, the groups with ovarian cycles which did not produce litters and in the mated constant estrous groups which did not produce litters. P concentration was higher in the pregnant groups than in the pseudopregnant groups on postcoital Day 16. The length of gestation was progressively increased from 22.2 days in the 4 mo. old group to 23.7 days in the 13 mo. old group. A 2nd experiment considered the effect of age on serum P during early and late gestation in 4 mo. old compared with 13-16 mo. old rats. P was measured from serial blood samples collected from Days 1-5 and from Days 19-22 of pregnancy. P was similarly increased on postcoital Days 1 through 5 in young and aged groups. Average P concentration was progressively decreased from Days 19-22 of pregnancy in groups producing litters. The aged group had higher P on Day 22 of pregnancy than did the young group. The higher P concentration on Day 22 of pregnancy in the aged group was consistent with the increased length of gestation in this group (22.9 days) compared with that in the young rats (22.1 days). Apparently there is no significant effect of age on P secretion by corpora lutea during early pregnancy or pseudopregnancy in the rat; uterine failure may contribute to the loss of fertility in the aging rat.