EFFECTS OF A GNRH ANALOG (BUSERELIN) INFUSED VIA OSMOTIC MINIPUMPS ONPITUITARY AND OVARIAN ACTIVITY OF PREPUBERTAL HEIFERS

This work was carried out to study the pituitary and ovarian responses of prepubertal heifers to a continuous administration of GnRH, and to evaluate the effectiveness of this treatment in stimulating ovulation . Each of eight 7-8-month-old prepubertal Holstein heifers was adminis tered with 5 mug h-1 of a GnRH analogue (buserelin) for 12 days by mea ns of an osmotic minipump implanted subcutaneously. They were bled eve ry 15 min for 6-8 h on Days - 1, 0 (pump insertion), 3, 6, 9, 12 (pump removal) and 16. Additional blood samplings were carried out for estr adiol-17beta (E2) and progesterone determination. All heifers showed a n immediate and marked pituitary response to GnRH stimulation, with co ncentrations of LH reaching the magnitude of a preovulatory surge with in 2-4 h after pump insertion. Thereafter, plasma LH concentrations re mained higher than pretreatment values, and decreased after pump remov al. E2 plasma concentrations increased to 12 pg ml-1 between Day 5 and Day 9. Undetectable concentrations of progesterone in plasma during a nd after the treatment indicated the lack of ovulation. Our data show that a continuous infusion with GnRH seems to be effective in stimulat ing a long-lasting pituitary response in prepubertal heifers, and in p romoting some ovarian activity and follicular development but not in i nducing ovulation.