GH, GH-RELEASING FACTOR AND SOMATOSTATIN IN THE GROWING LAMB - SEX-DIFFERENCES AND MECHANISMS FOR SEX-DIFFERENCES

Factors contributing to sex differences in the somatotrophic axis were investigated in growing lambs. In the first experiment, circulating p atterns of GH in venous blood, pituitary content of GH and GH mRNA, an d median eminence (ME) contents of GH-releasing factor (GRF) and somat ostatin (SRIF) were characterized in prepubertal ram and ewe lambs whi ch were pair-fed to remove sex differences in feed intake. Mean and ba seline plasma GH concentrations, GH pulse amplitude, and integrated pl asma GH were greater in ram lambs than in ewe lambs, but GH interpulse interval did not differ between sexes. The pituitary GH content and M E contents of GRF and SRIF were greater in rams than in ewes, but stea dy-state levels of mRNA for GH in the pituitary gland did not differ b etween sexes. A second experiment investigated sex effects on the leve ls of SRIF in hypophysial portal blood, and found that these did not d iffer between sexes. We concluded that the presence of sexually dimorp hic patterns of GH secretion in the growing lamb is independent of fee d-intake differences between sexes. The lack of sex differences in cir culating patterns of SRIF in portal plasma implies that there may be a difference in GRF secretion which may produce sexually dimorphic patt erns of GH secretion in lambs.