Glucose transporter and hypoxia-associated gene expression in the mammary gland of transition dairy cattle

Glucose is an important energy substrate, especially needed by dairy cows postpartum to support the onset of lactation. The prioritization and regulation of glucose uptake is accomplished, in part, by changes in expression of cellular glucose transport molecules (GLUT) within the mammary gland. The objectives of this study were to (1) evaluate the expression and cell-type specific localization of GLUT and hypoxia-associated genes that may regulate GLUT expression over the transition period and through lactation in bovine mammary tissue and (2) determine functionality of GLUT on primary bovine mammary endothelial cells (BMEC). Mammary tissue biopsies were taken from cows at 15 d before calving and again at 1, 15, 30, 60, 120, and 240 d post-parturition for quantitative real-time PCR analysis of GLUT and hypoxia-associated genes. Additional mammary tissue samples were used to localize GLUT within the cells of the lobulo-alveolar system via fluorescence microscopy. Cultures of primary bovine mammary endothelial cells were used to confirm the functionality of GLUT with a fluorescent glucose analog uptake assay. Significant increases in GLUT1 gene expression were observed during early lactation, whereas both GLUT3 and GLUT4 gene expression increased during late lactation. The gene expression for 2 receptors of vascular endothelial growth factor increased significantly during early lactation and remained increased throughout lactation when compared with gene expression during the transition period. All GLUT were detected on cultured BMEC and were capable of internalizing glucose through GLUT-mediated mechanisms. These data suggest mammary vascular tissues express GLUT during lactation and BMEC express functional glucose transporters. A better understanding of glucose uptake at the endothelial level may prove to be critical to improve glucose absorption from the blood for utilization by mammary epithelial cells.