Extra- and Intracellular Effects of Divergent Selection for Pituitary Responsiveness to Gonadotropin-Releasing Hormone in Prepubertal Ram Lambs1

Divergent selection based on the response of 10-wk-old male lambs to a GnRH challenge has produced two lines of sheep, referred to as high and low lines, that differ in their ability to release LH in response to pharmacological and physiological doses of GnRH. The aim of this study was to determine whether the between-line differences in pituitary sensitivity were related to differences in GnRH receptor number and/or the transduction of the intracellular signal following GnRH receptor activation. Pituitary glands were collected from fourteen 20-wk-old ram lambs from each line, weighed, and sampled for GnRH receptor analysis. The remaining tissue from 9 lambs from each line was dispersed. Of the resultant cell suspension, a sample was stored for measurement of GnRH receptor content and the remainder was plated and cultured for 24 h. The LH responses of cultured cells were measured after exposure to GnRH, A23187, or the phorbol ester phorbol 12,13 dibutyrate (PDB). The results indicated that the pituitary glands of the high line contained significantly higher concentrations of GnRH receptors than did those of the low line and released significantly more LH after stimulation with either GnRH or the Ca2(+)-calmodulin or protein kinase C intracellular second messenger systems. Therefore, the between-line difference in the regulation of pituitary LH secretion occurs at a step distal to the stimulatory sites of action of A23187 and PDB.