Reproduction Abstracts

Growth hormone (GH) is necessary to grow to normal adult size. While GH plasma concentrations are highest during early development, there is no evidence that GH influences somatic growth until after birth when GH-receptors are upregulated in peripheral tissues. The gradual decline in plasma GH during fetal life in a range of mammals suggests that negative feedback to the developing pituitary occurs progressively to decrease pituitary GH output. We have tested this hypothesis using a marsupial model in which we can rapidly accelerate growth of the young by fostering day 60 post-partum tammar wallabies to females at day 120 of the lactation cycle, thereby significantly increasing growth rate relative to controls. Foster and control young were then killed at 120 days of chronological age (180 days of the lactation cycle in the recipient female) and plasma GH concentrations and pituitary mRNA expression compared with control young. Foster young had significantly lower plasma GH concentrations, indicative of temporally advanced growth axis maturation, but these differences were absent when calculated relative to body weight. There were no differences in pituitary GH expression or hepatic GH-binding protein (GHBP) expression between groups. Young of small mothers, which are smaller at the same chronological age, had higher plasma GH concentrations but no difference relative to body weight and no difference in pituitary GH expression or hepatic GHBP expression relative to controls and fosters. This data suggests that GH decline during early life is an effect of dilution by a greater blood volume as the animal grows.