Regulations of Mammary Transcripts During Once Daily Milking in Goats and Cows

Increasing milking intervals is accompanied by a loss in milk synthesis. This phenomenon has been associated to a decrease in the activity of some enzymes implicated in milk synthesis and in the number of secretory cells. The aim of this study was to investigate the transcriptional regulations that happen in bovine and caprine mammary gland during once daily milking (ODM) and focuses on the synthetic activity and apoptosis of mammary epithelial cells (MEC). In a first experiment, 5 Holstein cows were milked once or twice daily during 7 days according to a crossover design. In the second experiment, 8 Alpines goats were milked once or twice daily for 5 weeks according to a crossover design. In both experiments, milk cells were harvested 7 days after milking frequency modification. The MEC were purified by using magnetic beads coated with anti-cytokeratin antibodies. The purified MEC were used to quantify mRNA level by quantitative RT-PCR.

The milk losses induced by ODM were in the same range (20%, P<0.001) for both cows and goats. For the protein yields, we observed an important decrease in goat’s milk (32%, P<0.001) and cow’s milk (18%, P<0.001). Similarly, casein yield was reduced 22% (P<0.001) and 24% (P<0.001) by ODM in cows and goats, respectively. Lactose yield decreased for both species (22%, P<0.001). The mRNA level of a-lactalbumin, which is the cofactor of lactose synthase, was reduced in both species (respectively 61%, P=0.02 and 69%, P<0.01) during ODM. The transcription of the ê-casein gene was also down-regulated by ODM in goats (80%, P<0.01) and in cows (58%, P<0.001). The transcriptional regulation of pro- and anti-apoptotic genes Bax and Bcl2 in MEC was evaluated. In cow, ODM induced an increase of both Bax and Bcl2 mRNA levels (respectively, 26 folds, P=0.03 and 29 folds, P=0.01). By contrast, no significant changes of either Bax, Bcl2 mRNA levels during ODM were observed in goats. Conversely, a trend toward a higher Bcl2/Bax ratio was observed during ODM in goat purified mammary epithelial cells.

These results suggest that the cell activity decrease observed  during ODM follows, at least in part, a transcriptional regulation pathway in both bovine and caprine species. However our data suggest different regulation pathways for apoptosis of MEC in these two species.