Follicle Stimulating Hormone, Luteinizing Hormone and Estradiol-17β Response in GnRH Treated Prepuberal Holstein Heifers1

Abstract

The effect of i.v. injection of 200 µg gonadotropin releasing hormone (GnRH) on plasma LH, FSH and estradiol-17β(E2-17β) was investigated in 18 prepuberal Holstein heifers at either 3,6 or 9 months of age. GnRH caused increased levels of both LH and FSH within 20 min postinjection. Both LH and FSH plasma concentrations reached peak values by 20 min following treatment in 3- and 9-month-old heifers, but in 6-month-old animals neither hormone reached maximum concentration until 2 h postinjection. Levels of both gonadotropins were not different with age. The profiles of both gonadotropins were similar following GnRH treatment (r = 0.86), but the relative magnitude of LH response (30-fold) was greater than FSH response (8-fold). FSH response was less variable in 9-month-old heifers than in younger animals. Concentration of plasma E2-17β was not altered by GnRH treatment by 6 h postinjection. It appears that the prepuberal heifer is capable of releasing large quantities of both LH and FSH and that this response is not different at 3, 6 or 9 months of age. However, it does not appear that the GnRH-induced rise in plasma gonadotropins is sufficient stimulus to promote increased E2-17β production within 6 h after CnRH administration in the prepuberal heifer.