The Relationship between Declining Plasma Progesterone Levels and Increasing Luteinizing Hormone Pulse Frequency in Late Gestation in the Rat

The object of this study was to determine whether the increase in LH pulse frequency and mean blood LH levels on day 22 of pregnancy in the rat is due to the precipitous fall in plasma progesterone (P) levels that occurs late in gestation. On day 20 of pregnancy two groups of animals with indwelling jugular cannulae were implanted sc with empty or Pcontaining Silastic capsules. Blood samples were withdrawn 0.5 h before and 5.5 h postimplantation on day 20 (0800 and 1400 h), at 1400 h on day 21, and at the end of the study between 1200–1300 h on day 22 to follow the time course of changes in plasma P levels over this 2-day period in both groups. These groups were bled on day 22 for 3 h between 0900–1200 h for analysis of pulsatile LH release. A third group not implanted with Silastic capsules was bled on day 20 for 3 h; plasma P levels in these rats bled on day 20 did not differ from the preimplantation values observed in either group of capsule-implanted rats. In empty capsule-implanted animals, plasma P ...