Measurement of equine follicle stimulating hormone and luteinizing hormone: response of anestrous mares to gonadotropin releasing hormone

Heterologous radioimmunoassays using human systems which were developed for the measurement of serum concentrations of equine follicle-stimulating hormone (eFSH) and luteinizing hormone (eLH) are described. These radioimmunoassays meet the criteria of specificity, precision and repeatability. The equine FSH assay is the first described for the measurement of this hormone. Serum levels of FSH and LH following the injection (im) of 1 mg of gonadotropin releasing hormone (GnRH) into 7 anestrous mares were measured using these methods and LH was also measured using an ovine system. Within 0.5 h following the administration of the GnRH, serum FSH reached a peak of 3.7 times the mean pretreatment concentration. The concentration of FSH then declined towards baseline for 12 h posttreatment. These changes in FSH are consistent with those occurring in normal cycling mares. Serum LH concentrations were not significantly elevated above baseline during the 12 h after injection of GnRH as measured in the human system but increased to 2.35 times the pretreatment level by 0.5 h after GnRH when measured in the ovine system. We believe this is the first species to show a significantly greater release of FSH than LH following administration of GnRH, and gives support for the existence of a single hypothalamic gonadotropin releasing hormone.