EFFECT OF SOMATOTROPIN AND INSULIN-LIKE GROWTH-FACTOR-I ON MILK LIPIDAND PROTEIN-SYNTHESIS IN-VITRO

The objective of this research was to test the hypothesis that bST sti mulats milk secretion through the action of IGF-I. Cocultures of bovin e mammary, adipose, and liver tissues were incubated with increasing c oncentrations (0 to 1000 ng/ml) of bST, IGF-I, prolactin, or bST plus prolactin. In addition, cocultures of mammary and adipose tissues with out liver tissue were incubated with IGF-I. The synthesis of milk lipi ds and proteins and the concentration of cellular DNA were measured. T he addition of liver tissue depressed DNA concentration and the synthe sis of lipids and proteins in mammary tissue. In mammary tissue, incre asing bST concentration resulted in greater synthesis of FFA, total li pids, and proteins than that produced by increasing concentrations of IGF-I, and the effect on protein synthesis was linear. Conversely, in adipose tissue, increasing the concentration of IGF-I in the presence of liver resulted in greater synthesis of FFA than that obtained by bS T, and the effect was linear. The results do not support the hypothesi s that bST acts through the actions of IGF-I on mammary cells. Rather, bST alters the delivery of nutrients to the mammary cells from other tissues and affects milk component synthesis through a mechanism that may not involve IGF-I.