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

Plasma and pituitary GH concentrations and liver GH receptor (GHR), IGF-I and IGF-binding protein-3 (IGFBP-3) mRNA expression were determined in brushtail possum (Trichosurus vulpecula) pouch young aged 12-150 days post-partum and in adults. Mean plasma GH concentrations were highest, measuring around 150 ng/ml, from 12 to 100 days post-partum, and thereafter declined so that by 150 days post-partum levels were not significantly different from those in adults (10.8+/-1.8 ng/ml (S.E.M.)). In contrast to plasma levels, pituitary GH content increased markedly throughout pouch life, with an 87-fold increase between 12 and 150 days post-partum. However, when expressed per gram body weight, pituitary content was relatively constant between 25 and 150 days post-partum, indicating that the decline in plasma GH after 100 days post-partum was not due to decreased synthesis and/or storage of GH in the pituitary gland. Expression of GHR, IGF-I and IGFBP-3 mRNAs was determined by semi-quantitative RT-PCR. Liver GHR and IGF-I mRNA expression were low at 12 and 25 days post-partum and did not show sustained and significant increases (P<0.05) until 125 and 150 days post-partum. IGFBP-3 expression was also low at 12 days post-partum but then increased rapidly to a maximum at 50 days post-partum and thereafter declined. For all three mRNAs, liver expression at day 150 was not significantly different from that in adults. These patterns of gene expression for GHR and IGF-I suggest that the possum liver is resistant to the high plasma GH concentrations during early pouch life and in this way is similar to the fetal liver of some eutherian mammals.