Changes in luteinizing hormone bioactivity associated with gonadotrophin pulses in the cycling mare.

The research article investigates the changes in bioactivity of luteinizing hormone (LH) associated with its release pulses in the cycling mare. It examines the role of pituitary secretion and peripheral metabolism by measuring LH ratio under various conditions.

Overview of the Research Experiment

• The experiment involved analyzes of pituitary venous effluent and the circulating plasma collected from 5 dioestrous (between two periods of sexual receptivity) and 2 oestrous (period of sexual receptivity) mares using both an in vitro bioassay and a radioimmunoassay.
• These two assay methods provide two different measures of luteinizing hormone (LH) to get the biological and immunological activities, respectively.
• By measuring the ratio of biological to immunological activity (B:I ratio), the researchers can estimate the contribution made by pituitary secretion and peripheral metabolism in LH changes.

Experiment Findings for Dioestrous Mares

• During dioestrus, LH was found to be released in pulses at a frequency of 1.4 pulses/24 hours, and each pulse lasted for 20-40 minutes (when measured centrally) or 2-4 hours (when measured peripherally).
• Between these LH pulses, no further secretion of LH from the pituitary could be detected.
• The B:I ratio increased 2- to 3-fold during spontaneous pulses and remained elevated for at least 1 hour.
• A low dose (10 ng/kg bodyweight) of the GnRH agonist Buserelin, a hormone that stimulates LH release, resulted in similar B:I ratio changes, unless the administration of this hormone was close to an endogenous LH discharge.
• A higher dose (50 micrograms) of Buserelin led to a significant decrease in the B:I ratio, despite causing a significant release of LH.

Experiment Findings for Oestrous Mares

• For the mares in oestrus, the B:I ratio was consistently elevated. This was due to a much higher frequency of LH pulses.

Conclusion

• The research indicates that both the pituitary secretion and peripheral metabolism contribute to changes in the biological and immunological activities of luteinizing hormone. The luteinizing hormone’s release is pulsatile, and its B:I ratio fluctuates accordingly.