Pituitary Refractoriness to Luteinizing Hormone Releasing Hormone: Its Importance in Ending the Luteinizing Hormone Surge in the Cyclic Rat

Abstract

A study was conducted to assess the importance of pituitary refractoriness to luteinizing hormone releasing hormone (LHRH) in ending the preovulatory surge of luteinizing hormone (LH) and in modulating the associated surge of follicle-stimulating hormone (FSH) in rat plasma. Phenobarbital-blocked proestrous rats were infused iv with LHRH at a constant rate of 50 ng/hr to restore the rising and plateau phases of the spontaneous surges of LH and FSH in plasma. Refractoriness to LHRH was demonstrated for LH but not FSH release when LHRH was infused beyond 2 hr. The plasma LH declined from high levels from 2 to 4.5 hr of infusion while the plasma FSH remained at an elevated plateau. The decline in plasma LH was not as rapid as that observed when the infusion was ended. Injection of a large dose (1 μg) of LHRH at 2 or 3.5 hr after the start of infusion caused substantial increases in plasma LH and FSH but only the LH response after the later injection was less than that observed in other rats given the injection earlier. The results suggest that pituitary refractoriness to LHRH plays a minor role in expediting the decline in the plasma LH during the latter portion of the LH surge and that it has no appreciable effect on the pattern of the plasma FSH at this time.