Effect of somatotropin and insulin-like growth factor-I on milk lipid and protein synthesis in vitro.
The objective of this research was to test the hypothesis that bST stimulates milk secretion through the action of IGF-I. Cocultures of bovine mammary, adipose, and liver tissues were incubated with increasing concentrations (0 to 1000 ng/ml) of bST, IGF-I, prolactin, or bST plus prolactin. In addition, cocultures of mammary and adipose tissues without liver tissue were incubated with IGF-I. The synthesis of milk lipids and proteins and the concentration of cellular DNA were measured. The addition of liver tissue depressed DNA concentration and the synthesis of lipids and proteins in mammary tissue. In mammary tissue, increasing bST concentration resulted in greater synthesis of FFA, total lipids, 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 bST, and the effect was linear. The results do not support the hypothesis 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.
Keys
J E
JE
Agricultural Research Service, USDA, Beltsville, MD 20705, USA.
Van Zyl
J P
JP
Farrell
H M
HM
D000273:Adipose Tissue / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D002417:Cattle; D018920:Coculture Techniques; D046508:Culture Techniques; D004247:DNA / Q000032:analysis; D005230:Fatty Acids, Nonesterified / Q000096:biosynthesis; D005260:Female; D005434:Flow Cytometry; D013006:Growth Hormone / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008055:Lipids / Q000096:biosynthesis*; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008321:Mammary Glands, Animal / Q000187:drug effects / Q000378:metabolism; D008892:Milk / Q000737:chemistry*; D008894:Milk Proteins / Q000096:biosynthesis*; D012313:RNA / Q000032:analysis
D005230:Fatty Acids, Nonesterified; D008055:Lipids; D008894:Milk Proteins; D012313:RNA; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D004247:DNA