Progesterone Levels in the Cyclic and Pregnant Hamster

Progesterone levels were determined for cyclic and pregnant hamsters by the competitive binding technique. On day 1 of the estrous cycle (metestrus) the corpora lutea (CL) contained progesterone despite the fact that luteinization had not yet occurred. Peak luteal and arterial plasma values of progesterone were reached on day 2 of the cycle, followed by an abrupt drop in activity on day 3. Progesterone was also low on the morning of day 4, but beginning in the afternoon (4 pm) there was a surge in ovarian progesterone culminating in the highest blood and ovarian levels at 8 PM of day 4. Several lines of evidence suggest that the progesterone was derived from nonluteal sources, with the interstitium as a major contributor. The critical stage during the estrous cycle for CL progesterone falls between days 2 and 3. In the pregnant hamster, progesterone levels are maintained on day 3 but at the same rate as the first 2 days of the cycle. Thus, the transformation of the cyclic CL to that of pregnancy was not characterized by any dramatic increase in synthesis or release of progesterone. Nidation was not marked by any sudden upsurge in plasma or luteal progesterone. Peak plasma levels in the first half of pregnancy were reached on day 8, most likely reflecting a luteotropic contribution of the placenta. The increased weight of CL during the second half of pregnancy was not reflected in increased CL concentration of progesterone. Progesterone content per CL remained relatively constant throughout pregnancy. In contrast to other species, in the pregnant hamster, luteal progesterone levels are not appreciably different from the values found during the second day of the estrous cycle. (Endocrinology86: 1, 1970)