RESUMO
Feed restriction and once-daily milking (ODM) reduce milk yield in dairy cows and the amount of glucose taken up by the mammary gland. The modulation of mammary glucose uptake may be the consequence of modifications to glucose transport, capacity for lactose synthesis, and cell death in mammary epithelial cells (MEC). The aim was to demonstrate the usefulness of a new method to purify MEC from milk somatic cells and to examine the effects of feed restriction and ODM on mammary transcripts. Five Holstein cows were subjected to a 2 x 2 factorial arrangement of 2 milking frequencies and 2 feeding levels, during which the cows were milked once or twice daily while fed a diet providing either 98 or 70% of requirements. The cows were equipped to study net mammary balance of glucose. On d 7 of each experimental week, milk and lactose yields and mammary glucose uptake were measured. Cells were isolated from fresh milk by centrifugation to generate total milk cell samples. Mammary epithelial cells were separated from total milk cells by using magnetic beads associated with anticytokeratin 8 antibodies. Total RNA was extracted from both total milk cells and purified MEC samples. Real-time reverse transcription PCR was performed to determine mRNA levels in purified MEC under feed restriction and under ODM. Purified MEC samples revealed higher total RNA quality (RNA integrity number = 8) and were better suited to the measurement of mammary transcripts than total milk cell samples (RNA integrity number = 4). Significant correlations were obtained between mRNA levels and net glucose balance data (0.465 < r < 0.680), demonstrating the validity of results obtained by using purified MEC. Feed restriction induced a significant reduction (by half) in type 1 glucose transporter mRNA levels without any effect on alpha-lactalbumin (alpha-LA), galactosyltransferase, kappa-casein, bcl2, or bax mRNA levels. When compared with twice daily milking, ODM reduced kappa-casein (-86%) and alphaLA (-73%) mRNA levels and up-regulated bax and bcl2 mRNA levels (7- and 9-fold). The results suggest that the regulation of glucose uptake and milk yield is dependent on the transcription of glucose transporters under feed restriction and on the transcription of alphaLA under ODM. Further studies are required to con-firm the suggested onset of cell death after 7 d of ODM.
