Effects of Indirect Selection for Pituitary Responsiveness to Gonadotropin-Releasing Hormone on the Storage and Release of Luteinizing Hormone and Follicle-Stimulating

Divergent selection based on the LH response to a 5-g9 dose of GnRH in 10-wk-old Finn Dorset ram lambs has created two lines of sheep that differ significantly in their response to both physiological and pharmacological dosages of GnRH. The aims of this study were to investigate whether there are between-line differences in endogenous LH and FSH secretion in ram lambs, as in the adult ewes, and to determine whether these differences are related to differences in storage of gonadotropins within the pituitary glands of the two lines. To address the first aim, endogenous LH and FSH secretion was monitored in 20 ram lambs from each line for a 6-h period at 2, 6, and 10 wk of age. To investigate potential between-line differences in gonadotropin storage, the LH responses to two acute challenges with either 0.5, 5.0, or 50.0 pg of GnRH were examined in 10-wk-old ram lambs from each line. The results demonstrated that the between-line difference in the LH response to a 5-pg GnRH challenge in ram lambs is accompanied by significant between-line differences in the regulation of endogenous gonadotropin secretion. Specifically, lambs from the High line secreted LH pulses of significantly greater amplitude than did lambs from the Low line at 2 and 10 wk of age. In addition, the results demonstrated that the readily releasable and releasable pools of LH were larger in the lambs from the High line than in those from the Low line but that there were no differences in the dynamics of LH secretion from these two pools between the two lines. In conclusion, the observed differences in the amount of releasable gonadotropin stored within the pituitary glands of lambs from the two lines explain the between-line differences in LH responsiveness to pharmacological dosages of GnRH but not the observed between-line difference in pituitary sensitivity