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-micrograms 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 micrograms 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-micrograms 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.(ABSTRACT TRUNCATED AT 250 WORDS)