On differences between the preovulatory luteinizing hormone surges of 4- and 5-day cyclic rats

Abstract

Preovulatory LH surges were studied in rats with ovarian cycles of 4 days (4d-rats) and 5 days (5d-rats). In 5d-rats the maximal peak height was about twice that observed in 4d-rats, whereas in 4d-rats peaks occurred about 1-5 h later than in 5d-rats and were much less consistently timed. From experiments in which LH releasing hormone (LH-RH) was infused into pentobarbitone-blocked pro-oestrous rats, it was concluded that (a) differences between the two types of preovulatory LH surges originate within the central nervous system, and (b) pro-oestrous LH-RH secretion may not be restricted to the period in which increased LH levels in blood were found, but may exceed this period for a considerable time.

PIP: Preovulatory luteinizing hormone (LH) surges were studied in rats with ovarian cycles of 4 days and 5 days as well as the pituitary responses of pentobarbitone-blocked, proestrous 4- and 5-day rats to a continuous infusion of LH-releasing Hormone (LH-RH). Plasma LH was measured by double-antibody radioimmunoassay. Ovulatory response was verified by the presence of ova in the oviducts of sacrificed rats. In 5-day rats the maximum LH peak was twice that observed in 4-day rats. However, in 4-day rats the peaks occurred 1.5 hours later than in 5-day rats and were much less consistently timed. The course of plasma LH levels of individual 4- and 5-day rats during continuous LH-RH infusion, showed a sharp rise initially declining sharply within 2 hours. It is concluded that differences between the 2 types of preovulatory LH surges originate within the central nervous system and the proestrous LH-RH secretion may not be restricted to the period in which increased LH levels in blood were found, but may exceed this period for a considerable time.