We evaluated the interrelationship between, and regulation of, the hepatic growth hormone receptor and serum GH binding protein (GH BP) in pigs treated with recombinant porcine growth hormone (rpGH). Infant and pubertal male pigs (N = 5 per group) received either rpGH 0.15 mg/kg daily or diluent intramuscularly for 12 days. Somatic growth, serum IGF-I and GH BP and [125I]bovine GH (bGH) binding to MgCl2-treated hepatic membrane homogenates were examined. Marked age-related increases were seen in serum GH BP (p<0.001) and [125I]bGH binding to hepatic membranes (p<0.001). GH BP was increased in rpGH treated animals (p = 0.03), from 13.8±1.2 (mean±1 x sem) (controls) to 17.8±2.0% in infants, and from 35.2±2.6 (controls) to 41.8±3.4% in pubertal animals. [125I]bGH binding to hepatic membranes was also increased by rpGH treatment (p<0.05), from 7.0±1.6 (controls) to 15.4±3.6% in infants and from 53.7±7.1 (controls) to 65.1±11.8% in pubertal animals. No significant interaction between age and treatment was seen. Overall, serum GH BP correlated significantly with [125I]bGH membrane capacity (r=0.82, p<0.001), with a correlation of r= 0.83 in the infant animals but no significant correlation in the pubertal animals considered alone (r=0.13). Serum IGF-I correlated significantly with serum GH BP (r=0.93, p<0.001) and bGH membrane binding capacity (r = 0.91, p< 0.001). These observations suggest that serum GH BP levels reflect major changes of hepatic GH receptor status. In addition, the present study demonstrates that the hepatic GH receptor can be induced by GH in the infant pig, despite a developmentally low GH receptor population at this age, suggesting potential efficacy of GH at earlier ages than generally considered.
Geoffrey R Ambler, Bernhard H Breier, Andrzej Surus, Hugh T Blair, Stuart N McCutcheon, Aviva Silbergeld and Peter D Gluckman
Rafat A. Siddiqui, Stuart N. McCutcheon, Duncan D. S. Mackenzie, Hugh T. Blair, J. Eldon Ormsby, Peter D. Gluckman and Bernhard H. Breier
A study was undertaken to investigate the role of testosterone in regulating growth and circulating levels of insulin-like growth factor-I in male mice from lines divergently selected on the basis of plasma IGF-I. Controls of each lines were sham-operated at 10 days of age and treated with peanut oil from day 14 to day 70. A second group, which was castrated at 10 days and treated with testosterone enanthate (0.5 μg · (g body weight) −1 · day−1) from day 14 to 70, did not differ from controls in body weight but had higher plasma IGF-I concentrations. Delaying testosterone therapy until day 42 in a third group retarded growth, with body weights being significantly lower than those of other two groups from days 35 to 56. However, plasma IGF-I levels in this group were not different from those of controls. Effects of line and treatment were additive. It is concluded that the greater pubertal growth of high-line compared to low-line males is not due to greater stimulation of circulating IGF-I by testosterone. Furthermore, testosterone does not appear to influence pubertal growth by acting on circulating levels of IGF-I.