Expression of amino acid transporters in porcine mammary gland during lactation

During lactation, adequate protein nutrition of the mother is crucial to provide amino acids essential for the synthesis of milk proteins, a process unique to the mammary gland, hence unique to mammals. Conversely, excessive intake of dietary protein may be detrimental to milk protein production as revealed by depressed neonatal growth. Dietary amino acids are transported via the circulatory system to organs and enter these tissues in a regulated fashion via transporters specific to amino acids. The transport processes are tightly regulated by mechanisms that are still largely undefined. In this study, a porcine model was used to begin addressing whether protein nutrition and stage of lactation modulate amino acid transport in mammary tissue. In particular, it is unknown whether the porcine mammary gland possesses amino acid transporters and whether the uptake of amino acids is regulated similarly in other tissues studied to date. The objectives of this dissertation are to determine the presence of amino acid transporters in porcine mammary tissue, and investigate if dietary amino acid availability and milk demand play a role in regulating the abundance of these transporters during lactation. A non-invasive mammary biopsy technique was developed to collect large quantities of mammary tissue from lactating sows at different stages of lactation. Mammary tissue was used for quantification of RNA and amino acid transporter proteins in response to dietary protein intake and lactation demand. The results of this study indicate that the porcine mammary gland in fact express the amino acid transporters CAT-1, CAT-2B, ASCT1, and B0,+ and that the abundance of CAT-2B is depressed by excess dietary protein intake and amino acid availability during early lactation. The changes in mammary tissue expression of CAT-2B mRNA paralleled the response in piglet growth and milk casein yield, suggesting that CAT-2B may play a crucial role in regulating milk protein synthesis in response to changes in protein concentration of sow's diets. These results indicate that amino acid transport into the porcine mammary gland is, in part, regulated by amino acid transporters. Depressed abundance of amino acid transporters and hence, a depressed uptake of amino acids by the mammary gland, may explain in part the effect of excess maternal protein intake on the depression in milk protein yield and on piglet neonatal growth.