GnRH-induced gonadotrophin secretion in ovariectomized Booroola ewes with hypothalamic--pituitary disconnection
To test whether the F gene-specific differences in the plasma concentrations of FSH and LH are due to differences in the pituitary responsiveness to exogenous GnRH, ovariectomized Booroola ewes with hypothalamic\p=n-\pituitary disconnection (HPD\p=n-\ovx) were treated with GnRH (250 ng i.v.) once every 2 h for up to 5 weeks. In Exp. 1, jugular venous blood was collected once weekly from 13 FF and 14 + + HPD\p=n-\ovxewes for 6 weeks before GnRH treatment and every 2nd, 3rd or 6th day for 5 weeks
... during treatment. In Exp. 2, jugular venous blood was collected from another 8 FF and 7 + + HPD\p=n-\ovx ewes at 5or 10-min intervals over 4 GnRH pulses (250 ng i.v. once every 2 h) on 3 separate occasions after the animals had been subjected to the GnRH pulse regimen for \m=~\7 days beforehand. Also in Exp. 2, the animals were extensively sampled around a larger (10\g=m\g) i.v. injection of GnRH and the pituitary FSH and LH contents assessed after the animals had been re-exposed to the once every 2 h GnRH (250 ng i.v.) pulse regimen for several days following the larger GnRH bolus. In Exp. 3 the distributions of mean plasma concentrations of FSH and LH in individual GnRH-treated HPD\p=n-\ovxewes were compared with those in ovariectomized and ovary-intact FF and + + ewes. During the 6 weeks before GnRH treatment (Exp. 1), the plasma concentrations of FSH (\m=~\ 1 ng/ml) and LH (\ m=l e\ 0\ m=. \ 8ng/ml) were not different between the genotypes. After GnRH treatment both the mean FSH and LH concentrations increased significantly (P < 0\m=.\01)above basal values after 2 days with F gene-specific differences being noted for FSH but not LH (FSH; FF > + + ; P < 0\m=.\05).Thereafter, the mean FSH but not LH concentrations increased at a faster rate in FF than in + + ewes with the overall mean FSH concentrations between the genotypes being significantly different (P < 0\m=.\05). In Exp. 2 considerable between-animal variation in the pulsatile pattern of FSH but not LH concentrations was seen in ewes of both genotypes during GnRH treatment. The overall mean FSH concentrations were higher in FF than in + + ewes (P < 0\m=.\05) and the mean FSH response to each GnRH pulse was significantly higher in FF than in + + ewes (P < 0\m=.\05). For LH a trend towards higher mean peak amplitudes and peak areas was noted in FF than in + + ewes but no significant differences were noted. Also, no gene differences were observed in the LH or FSH responses to the 10 \g=m\g i.v. GnRH bolus or the pituitary contents of FSH and LH. In Exp. 3, \m=ge\75% oft he FFewes had higher mean FSH concentrations than in 50% of the + + animals irrespective of treatment (i.e. GnRH\p=n-\HPD\p=n-\ovx, ovariectomized or ovary-intact controls; P < 0\m=.\05).Likewise, \m=~\50%of the FF ewes had higher mean concentrations of LH than did 75% of the + + ewes for all treatments. These findings suggest that an association between the Booroola F gene and GnRH-induced FSH secretion exists, but whether such an association also exists for LH remains uncertain. These data support the hypothesis that at least part of the expression of the F gene is at the level of the pituitary gland to affect its responsiveness to physiological concentrations of GnRH.