Plasma growth hormone and growth hormone-binding protein during development in the marsupial brushtail possum (Trichosurus vulpecula)

Abstract

Plasma concentrations of growth hormone (GH) were measured in the brushtail possum (Trichosurus vulpecula) pouch young from 25 through to 198 days post-partum (n=71). GH concentrations were highest early in pouch life (around 100 ng/ml), and thereafter declined in an exponential fashion to reach adult concentrations (10·8 1·8 ng/ml; n=21) by approximately 121-145 days post-partum, one to two months before the young is weaned. Growth hormone-binding protein (GHBP), which has been shown to modify the cellular actions of GH in eutherian mammals, was identified for the first time in a marsupial. Based on size exclusion gel filtration, possum GHBP had an estimated molecular mass of ]65 kDa, similar to that identified in other mammalian species, and binding of 125 I-labelled human GH (hGH) was displaced by excess hGH (20 µg). An immunoprecipitation method, in which plasma GHBP was rendered polyethylene glycol precipitable with a monoclonal antibody to the rabbit GHBP/GH receptor (MAb 43) and labelled with 125 I-hGH, was used to quantitate plasma GHBP by Scatchard analysis in the developing (pooled plasma samples) and adult (individual animals) possums. Binding affinity (K a ) values in pouch young aged between 45 and 54 and 144 and 153 days post-partum varied between 1·0 and 2·4 10 9 /M, which was slightly higher than that in adult plasma (0·96 0·2 10 9 /M, n=6). Binding capacity (B max ) values increased from nondetectable levels in animals aged 25-38 days post-partum to reach concentrations around half that seen in the adult (1·4 0·2 10 -9 M) by about 117 days post-partum and remained at this level until 153 days post-partum. Therefore, in early pouch life when plasma GH concentrations are highest, the very low concentrations of GHBP are unlikely to be important in terms of competing with GH-receptor for ligand or altering the half-life of circulating GH.

The GH/IGF-I axis in the brushtail possum (Trichosurus vulpecula) pouch young

“…We have previously reported that plasma GH concentrations in the possum pouch young are very high early in life but then decline to reach adult levels by about 96-120 days post-partum (Saunders et al 2002). In the present study on precisely aged pouch young, plasma GH concentrations remained elevated above those in the adult until 125 days post-partum, even though they had declined from those seen in younger animals.…”

“…Our results for GHR and IGF-I, which are negatively correlated with plasma GH, suggest that the possum liver is resistant to GH early in pouch life, possibly because GHR expression is low. In support of this explanation, plasma GH-binding protein concentration, which correlates with liver GHR levels in eutherian mammals (Mulumba et al 1991, Ambler et al 1992, is also low early in pouch life (days 45-87) but then increases more than 3-fold by about 120 days post-partum (Saunders et al 2002). In eutherian mammals, the fetal liver is relatively insensitive to GH and only post-natally does GH become an important regulator of hepatic IGF-I production.…”

“…A breeding colony of brushtail possums was maintained as previously described (Saunders et al 2002) with food and water freely provided. Blood, pituitary gland and liver samples were obtained from brushtail possum adults and pouch young aged 12, 25, 50, 75, 100, 125 and 150 days post-partum (nd3 at each age).…”

“…Plasma and pituitary gland GH concentrations were determined in a heterologous RIA (Saunders et al 2002). This assay uses an antiserum against wallaby GH (wGH) (G5/3; Curlewis & McNeilly (1992)), brushtail possum GH purified from pituitary glands as standard and wGH (cWB-9) as the radioiodinated ligand.…”

“…We have recently shown that for two species of marsupial, the bandicoot and brushtail possum (Saunders et al 2002), plasma growth hormone (GH) concentrations are very high for about the first half of pouch life before gradually declining to adult levels. Growth rate is slower during the first half of pouch life (Gemmell & Hendrikz 1993) when plasma GH concentrations are at their highest (Saunders et al 2002), which suggests that GH does not play a major role in stimulating growth at this time. One possible explanation for this apparent paradox would be if GHstimulated insulin-like growth factor-I (IGF-I) production from the liver were low during early pouch life and then subsequently increased around the time of increasing growth rate after about 96 days post-partum.…”