EFFECTS OF INDIRECT SELECTION FOR PITUITARY-RESPONSIVENESS TO GONADOTROPIN-RELEASING-HORMONE ON THE STORAGE AND RELEASE OF LUTEINIZING-HORMONE AND FOLLICLE-STIMULATING-HORMONE IN PREPUBERTAL MALE LAMBS

Divergent selection based on the LH response to a 5-mu g 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 phar macological dosages of GnRH, The aims of this study were to investigat e 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 gonadotrop ins within the pituitary glands of the two lines. To address the first aim, endogenous LH and FSH secretion was monitored in 20 ram lambs fr om each line for a 6-h period at 2, 6, and 10 wk of age, To investigat e potential between-line differences in gonadotropin storage, the FH r esponses to two acute challenges with either 0.5, 5.0, or 50.0 mu g 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-mu g GnRH challenge in ram lambs is accompanied by significant betw een-line differences in the regulation of endogenous gonadotropin secr etion. Specifically, lambs from the High line secreted LH pulses of si gnificantly greater amplitude than did lambs from the Low line at 2 an d 10 wk of age, In addition, the results demonstrated that the readily releasable and releasable pools of LH were larger in the lambs from t he High line than in those from the Low line but that there were no di fferences 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 respons iveness to pharmacological dosages of GnRH but not the observed betwee n-line difference in pituitary sensitivity.