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 (Ma cropus eugenii) and related to the total growth-promoting activity of the milk as determined in cultured L6 rat myoblasts. EGF increased thr oughout lactation from 10 ng/ml at 99 days to 25 ng/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 consta nt amount is consistent with data from eutherian mammals, even though actual EGF concentrations either decrease or increase 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 m ilk measured in myoblasts demonstrated a similar pattern to that obtai ned with IGF-I. The IGF-I changes are unlike the data reported in euth erian mammals in which this growth factor falls to low levels from hig h concentrations in initial colostrum. The highest concentration of IG F-I in tammar milk coincides with the changeover to a high fat, high p rotein, low hexose milk composition that is produced at an increased r ate when the young leaves the pouch. IGF-I in tammar milk may be impor tant for mammary gland maturation at this stage. (C) 1995 Academic Pre ss, Inc.