LOCALIZATION AND GENE-EXPRESSION OF GLUCOSE TRANSPORTERS IN BOVINE MAMMARY-GLAND

Glucose uptake in the mammary gland is a rate-limiting step in milk sy nthesis. To study glucose transporters in the bovine mammary gland, th e erythrocyte-type glucose transporter (GLUT1) and the insulin-respons ive glucose transporter (GLUT4) proteins were assessed by Western blot ting and immunohistochemical staining, using polyclonal antibodies aga inst the C-terminal peptide of GLUT1 and GLUT4. Our results demonstrat ed that the bovine mammary gland expressed a relatively high level of GLUT1 protein, whereas GLUT4 protein was not detected in the mammary g land of either lactating or dry cows. The absence of GLUT4 may indicat e that glucose transport is not regulated by insulin in the lactating and dry bovine mammary gland. The anti-GLUT1 antibody strongly stained the single layer of epithelial cells of mammary alveoli. The expressi on of GLUT1 mRNA was similar in the mammary gland of late lactation an d non-lactating cows. However, a smaller molecular weight species (38 kDa) of GLUT1 protein was detected in the mammary gland of nonlactatin g cows where its abundance in crude membrane preparation was 80% highe r than in lactating animals. There were no significant differences in GLUT1 mRNA in bovine mammary gland at 118 d and 181 d postpartum, howe ver, GLUT1 protein expression tended to be greater at 118 d postpartum .