Gonadotropin Releasing Hormone Induced Luteinizing Hormone Release after Prostaglandin F 2α in Heifers 1

Summary

Twenty Holstein heifers in diestrus were given 30 mg of PGF_2α Tham salt (im). Thereafter each of five heifers received no further treatment or 40 μg of GnRH (im) at 10, 30, or 50 hr after PGF_2α.

Serum progesterone decreased to less than 1 ng/ml at 24 hr after PGF_2α and remained low throughout the remainder of the sampling period. Following PGF_2α, estradiol concentrations increased gradually in serum but GnRH given at 30 hr after PGF_2α abruptly decreased estradiol concentration. The magnitude of LH release induced by GnRH given at 30 or 50 hr after PGF_2α was greater than that at 10 hr. All heifers exhibited behavioral estrus except those in group GnRH-50. Spontaneous LH peaks, which usually occur after PGF_2α were observed in 5/5 (control), 4/5 (GnRH-10), 0/5 (GnRH-30), and 2/5 (GnRH-50) heifers.

We conclude that pituitary responsiveness to GnRH increases with time after PGF_2α and GnRH given at 30 hr completely inhibits and at 50 hr partially inhibits endogenous LH release.