Differential Changes in the Milk Concentrations of Epidermal Growth Factor and Insulin-like Growth Factor-I during Lactation in the Tammar Wallaby, Macropus eugenii

Insulin-like growth factor-I (IGF-I) and epidermal growth factor (EGF) have been measured in milk during lactation of the tammar wallaby ( Macropus eugenii) and related to the total growth-promoting activity of the milk as determined in cultured L6 rat myoblasts. EGF increased throughout lactation from 10ng/ml at 99 days to 25ng/ml at 263 days. As a greater increase occurred with total soluble proteins in the milk, the EGF content per milligram of protein was decreased slightly during lactation. That EGF is secreted in tammar milk at a relatively constant amount is consistent with data from eutherian mammals, even though actual EGF concentrations either decrease during lactation in those species. A very different pattern of secretion was observed with IGF-I, which increased sixfold to a maximum of 1043 ng/ml at 205 days of lactation before falling to approximately 300 ng/ml toward the end of lactation. The protein synthesis-stimulating activity of the milk measured in myoblasts demonstrated a similar pattern to that obtained with IGF-I. The IGF-I changes are unlike the data reported in eutherian mammals in which this growth factor falls to low levels from high concentrations in initial colostrum. The highest concentration of IGF-I in tammar milk coincides with the changeover to a high fat, high protein, low hexose milk composition that is produced at an increased rate when the young leaves the pouch. IGF-I in tammar milk may be important for mammary gland maturation at this stage.