Gonadotropin releasing hormone induced luteinizing hormone release after prostaglandin F2alpha in heifers.

Twenty Holstein heifers in diestrus were given 30 mg of PGF2alpha Tham salt (im). Thereafter each of five heifers received no further treatment of 40 mug of GnRH (im) at 10, 30, or 50 hr after PGF2alpha. Serum progesterone decreased to less than 1 ng/ml at 24 hr after PGF2alpha and remained low throughout the remainder of the sampling period. Following PGF2alpha, estradiol concentrations increased gradually in serum but GnRH given at 30 hr after PGF2alpha abruptly decreased estradiol concentration. The magnitude of LH release induced by GnRH given at 30 or 50 hr after PGF2alpha 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 PGF2alpha 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 PGF2alpha and GnRH given at 30 hr completely inhibits and at 50 hr partially inhibits endogenous LH release.