Temporal Changes in Serum Progesterone in Aging Female Rats*

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Abstract
The effect of age on serum progesterone was measured in female Long-Evans rats. Progesterone concentrations were measured in young (3 months old) and aged (12–24 months old) rats at proestrous, estrous, and diestrous stages of their ovarian cycles. In addition, progesterone was measured in aged noncycling rats with constant estrous, pseudopregnant, or constant diestrous vaginal cytological patterns. Serum progesterone was increased on the afternoon of proestrus in both young and aged rats. Both the magnitude of increase and the proportional increase over basal progesterone were greater in young than in aged animals. The increase in progesterone in young animals compared to aged groups was still present during estrus. Both young and aged groups had a diurnal surge in serum progesterone concentration on diestrous day 2. This increase in progesterone on diestrous day 2 was of greater mganitude and of longer duration in young compared to aged rats. Diurnal patterns of progesterone secretion were absent in aged constant estrous and repetitive pseudopregnant groups. Aged constant estrous rats had low serum progesterone concentrations which were not different between 12- and 24-month-old constant estrous groups. Serum progesterone concentrations were similar in aged repetitive pseudopregnant and constant diestrous groups and were elevated above values in aged constant estrous rats. These data suggest that differences in serum progesterone concentrations during the ovarian cycle could contribute to decreases in gonadotropin secretion, ovulatory rates, and postovulatory reproductive efficiency in the aging laboratory rat. (Endocrinology106: 1579, 1980)