Evidence that lamprey GnRH-III does not release FSH selectively in cattle

Abstract

Experiments were performed to test the hypothesis that lamprey GnRH-III (IGnRH-III) selectively releases FSH. Primary cultures of bovine adenohypophyseal cells were treated with mammalian GnRH (mGnRH) and IGnRH-III (10-9, 10-8, 10-7 and 10-6 M) or control media in Experiment 1. All doses of mGnRH and the two highest doses of IGnRH-III stimulated (P < 0.001) a non-selective release of LH and FSH. In Experiments 2-4, Latin Square designs were utilized in vivo to examine whether physiological and hormonal milieu regulate putative selective effects of IGnRH-III. In Experiments 2 and 3, ovariectomized cows with basal levels of estradiol only (Experiment 2) or in combination with luteal phase levels of progesterone (Experiment 3) were injected with mGnRH and IGnRH-III (0.055, 0.11, 0.165 and 1.1 μg/kg body weight (BW) and saline. All doses of mGnRH released (P < 0.001) LH and FSH, but only the highest dose of IGnRH-III stimulated (P < 0.001) a non-selective release of both LH and FSH (Experiment 3). For Experiments 4A and 4B, intact, mid-luteal phase cows were injected with mGnRH and IGnRH-III (1.1 μg/kg BW; Experiment 4A), IGnRH-III (1.1 and 4.4 μg/kg BW; Experiment 4B) and saline. As before, mGnRH released (P < 0.001) both LH and FSH at all doses. In contrast, IGnRH-III at the highest dose released (P < 0.001) LH but not FSH. These findings suggest that IGnRH-III may act as a weak competitor for the mGnRH receptor and do not support the hypothesis that it selectively releases FSH in cattle.