The relationship between declining plasma progesterone levels and increasing luteinizing hormone pulse frequency in late gestation in the rat.

Abstract

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 P-containing 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 Blood samples were withdrawn 0.5 h before and 5.5 h postimplantation on day 20 (0800 and 1400 h), at 1400 h on day 21, and at the end of the study between 1200-1300 h on day 22 to follow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 values declined slightly from days 20 to 21 and were dramatically reduced between days 21 and 22. In contrast, after implantation of P capsules, plasma P levels were elevated on day 20 and remained elevated on day 21 compared with preimplantation values. Although these increased plasma P values declined between days 21 and 22, reflecting a decrease in endogenous P secretion, they were nonetheless comparable to day 20 values due to the presence of the P-containing capsules. Plasma estradiol values did not differ significantly between any of the experimental groups. In the empty capsule group bled on day 22, mean blood LH levels and LH pulse frequency were significantly higher compared to day 20 values, at a time when plasma P levels had fallen significantly from day 20 values. However, in the P capsule group, mean blood LH levels and LH pulse frequency on day 22 were significantly lower than values in the empty capsule group and were not different from the low values on day 20. Thus, preventing a decline in plasma P values to the low levels normally found on day 22 prevented the increase in LH pulse frequency and mean blood LH levels normally seen at this time of pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)