Prolongation of the Bovine Estrous Cycle with a Gonadotropin-Releasing Hormone Analog

A series of in vitro and in vivo experiments was conducted to determine the effects of gonadotropin-releasing hormone (GnRH) on bovine luteal function. Biosynthesis of progesterone by bovine luteal cells during a 2-h incubation was determined following addition of 0, 10, 20, 40, 80 and 100 ng GnRH. Synthesis of progesterone was significantly depressed only by the 100-ng dose of GnRH. Luteal cells were incubated with 0.1 and 1.0 µg GnRH in the presence and absence of 0, 1, 2 and 5 ng bovine luteinizing hormone (LH). Again, only the highest dose of GnRH significantly depressed LH-stimulated production of progesterone. Three experiments were conducted to assess the effects of repetitively administered GnRH. In the first, twice daily intrauterine infusions of 100 µg GnRH on Days 12, 13 and 14 of the bovine estrous cycle was without effect on plasma concentrations of progesterone and the functional life span of the corpus luteum (CL). In the second experiment, 10 µg of a highly potent GnRH analog (GnRH-A) was injected subcutaneously four times daily on Days 9–12 of the estrous cycle. GnRH-A-treated heifers had longer (P<0.05) mean estrous cycle lengths (26.2 ± 0.72 days) when compared to control heifers (20.25 ± 0.25 days). Plasma concentrations of progesterone were higher in the GnRH-A-treated group on Days 9–13 and Days 15–22 of the estrous cycle. Although GnRH-A treatment resulted in an extension in the life span of the CL and higher concentratmons of progesterone during the experimental estrous cycle, concentrations of progesterone were lower (P<0.05) than in control heifers during the subsequent estrous cycle. In a third experiment, 100 µg GnRH was administered four times daily on Days 9–12 of the cycle. This treatment had no effect on concentrations of progesterone or on estrous cycle length. These data indicate that GnRH has an inhibitory effect on bovine luteal cells in vitro when added at extremely high doses. Repetitive in vivo administration of large amounts of a potent GnRH-A had a luteotropic effect and prolonged the cycle length. Thus, it appears that the actions of GnRH on the bovine CL are unlike the luteolytic effects reported for the rat and primate.