Experiments were performed to test the hypothesis that
lamprey GnRH-III (
lGnRH-III) selectively releases FSH. Primary cultures of bovine adenohypophyseal cells were treated with mammalian
GnRH (mGnRH) and
lGnRH-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
lGnRH-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
lGnRH-III. In Experiments 2 and 3, ovariectomized cows with basal levels of
estradiol only (Experiment 2) or in combination with luteal phase levels of progester-one (Experiment 3) were injected with mGnRH and
lGnRH-III (0.055, 0.11, 0.165 and 1.1 microg/kg
body weight (BW) and saline. All doses of mGnRH released (P < 0.001) LH and FSH, but only the highest dose of
lGnRH-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
lGnRH-III (1.1 microg/kg BW; Experiment 4A),
lGnRH-III (1.1 and 4.4 microg/kg BW; Experiment 4B) and saline. As before, mGnRH released (P < 0.001) both LH and FSH at all doses. In contrast,
lGnRH-III at the highest dose released (P < 0.001) LH but not FSH. These findings suggest that
lGnRH-III may act as a weak competitor for the mGnRH receptor and do not support the hypothesis that it selectively releases FSH in cattle.