Ontogeny of growth hormone (GH) binding in the domestic turkey: evidence of sexual dimorphism and developmental changes in relationship to plasma GH

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Abstract
The post-hatch ontogeny of hepatic GH binding and its relationship to GH plasma profile characteristics in male and female turkeys of slow- (RBC-2) and fast-growing (F; selected from RBC-2) genetic lines were determined. Specific binding of 125I-labelled recombinant chicken GH to crude hepatic membrane preparations (100 000 g pellet) was determined at 2, 4, 8, 14 and 24 weeks of age for both total (occupied plus free; 4 mol MgCl2/l pretreatment) and free (without MgCl2 pretreatment) binding sites. Characteristics of the plasma GH profile were measured at each age by serial blood sampling through indwelling jugular vein catheters. When specific binding to either free or total sites was expressed on a whole organ basis (i.e. hepatic GH-binding capacity/bird), binding increased dramatically (P < 0·0001) with increasing age over both lines and sexes. Total binding capacity (free plus occupied sites) per bird was greater for females than for males at 24 weeks of age (P < 0·04), as birds reached sexual maturity, but did not differ between fast- and slow-growing lines at any age. Available binding capacity (free sites) per bird was greater for the faster growing F than RBC-2 line at the older ages when body size was most divergent (14 and 24 weeks of age; P < 0·01, P < 0·06 respectively), but did not differ between sexes. Correlation analysis at individual ages revealed a progressive change in the nature of the relationship between hepatic GH binding, plasma GH and somatic growth. At 2 weeks of age, when growth velocity of these birds was high relative to later ages, there were no significant correlations among hepatic GH binding, plasma GH and somatic growth. Thereafter, the correlation between total binding capacity and plasma GH concentration progressed from positive (r = 0·34; P < 0·16) to no correlation (r = −0·01; P< 0·97) to increasingly negative, until at the oldest age studied (24 weeks), the correlation was strongly negative (r = −0·55) and significant (P < 0·03). At the older ages, hepatic binding became significantly correlated to measures of somatic growth. These results indicate that no consistent relationship exists between hepatic GH binding and plasma GH status, over all ages, lines and sexes. Rather different relationships exist at different ages/physiological states. In the growth-divergent lines of turkeys studied here, measures of somatic growth are most strongly correlated with hepatic GH binding at later ages, after the early period of most rapid growth is past, and as mature body size and onset of sexual maturity are approached. The phenomenon of down-regulation of hepatic GH binding as birds approach sexual maturity was suggested by a significant negative correlation between total binding sites and plasma GH at that time. Journal of Endocrinology (1990) 126, 131–139