LH and FSH response of Holstein heifers to fertirelin acetate, gonadorelin and buserelin.

This study was conducted to describe the changes in serum LH and FSH concentrations in Holstein heifers following intramuscular (i.m.) injection of various dosages of fertirelin acetate and other commercially available GnRH products at their labeled dosages. The design was an incomplete Latin-square which gave nine replicates of each treatment. Treatments administered on Days 8 to 16 of the estrous cycle consisted of saline; 25, 50, 100 or 200 microg fertirelin acetate; 100, 250 or 500 microg gonadorelin; and 10 or 20 microg buserelin. Blood samples were collected via jugular catheters from 1 h before to 8 h after each injection. Log (Base 2) area under the LH and FSH curves (log AUC) were used to evaluate response to fertirelin acetate dosages and to determine difference (LSD: 0.05) and bioequivalence (alpha = 0.05) between the various dosages of GnRH products tested. Significant quadratic dose response relationships were detected for the LH and FSH log AUC data. Plots of the LH log AUC but not the FSH log AUC data suggested that a response plateau was being approached at the higher dosages of fertirelin acetate. Bioequivalence (alpha = 0.05) was based on the assumption that two means are equivalent if they differ by no more than 20% of the reference log AUC mean. Using these criteria fertirelin acetate is 2.5 to 10 times more potent than gonadorelin, whereas buserelin is approximately 10 to 20 times more potent than fertirelin acetate in the bovine for LH and FSH release.