Effects of high frequency administration of GnRH or kisspeptin on LH pulse and surge profiles in ovariectomized Bos taurus and Bos indicus heifers†.

We hypothesized that high-frequency stimulation with kisspeptin or GnRH, mimicking increased GnRH pulsatility, would amplify the GnRH-induced LH surge in heifers. Additionally, LH pulse and surge parameters were compared between Bos taurus and Bos indicus heifers. Ovariectomized estradiol-replaced Hereford (n=6) and Brahman (n=8) heifers received two intravaginal progesterone devices on d -5. On d -0.5, jugular catheters were placed and on d 0 blood samples were collected every 15 min for 12 h. In a crossover design, each heifer received all three treatments in separate replicates, administered hourly during the first 8 h: Control = saline; Kisspeptin = 0.4 μg/kg of kisspeptin; or GnRH = 0.005 μg/kg of gonadorelin acetate. At 8 h, 100 μg of gonadorelin acetate was administered to induce an LH surge. Both kisspeptin and GnRH treatments effectively induced high-frequency LH pulses. Regardless of the genetic group, 75% of kisspeptin and 87.5% of GnRH injections resulted in detectable LH pulses. LH pulsatility was greater (P<0.01) in kisspeptin- and GnRH-treated heifers compared to controls (6.1±0.6, 7.0±0.4, 2.3±0.5 pulses/8 h; respectively). Nevertheless, neither endogenous nor treatment-induced LH pulse parameters differed between breeds. The GnRH-induced LH surge was not enhanced by the high-frequency stimulation, however, it was markedly reduced in Brahman compared to Hereford heifers (P<0.01). In conclusion, the high-frequency GnRH stimulation did not enhance LH surge response to a GnRH challenge. Although LH pulsatile secretion was similar, differential pituitary responsiveness to a GnRH ovulatory stimulus may contribute to differences in reproductive function between Bos taurus and Bos indicus heifers.