The interaction of essential fatty acids and conjugated linoleic acid on regulation of milk synthesis and the formation of milk ingredients from [13C6]-glucose during early lactation in dairy cows

Long-chain fatty acids such as essential fatty acids (EFA) and CLA have the potential to affect glucose metabolism and milk synthesis in the mammary gland. The present study tested the hypothesis that EFA, CLA, or a combined EFA and CLA treatment influences the formation of milk constituents from glucose and regulation of milk synthesis during early lactation. For this purpose, the carbon flow from glucose and important enzymes and regulatory factors of milk synthesis were measured in the mammary gland. Rumen-cannulated German Holstein cows (n = 38) were investigated from wk 9 antepartum until wk 9 postpartum. The cows were abomasally infused with coconut oil (CTRL, 76 g/d; n = 9), 78 g/d linseed and 4 g/d safflower oil (EFA treatment; n = 9), Lutalin (CLA treatment, isomers cis-9,trans-11 and trans-10,cis-12 CLA, each 10 g/d; n = 10), or the combination of EFA+CLA (n = 10). In wk 3 postpartum, [¹³C₆]-glucose was infused intravenously for 4 h, and ¹³C-enrichment was measured in milk ingredients (lactose, casein, and fat) before and several times after tracer infusion. Tissue from the mammary gland was collected after tracer infusion in wk 4 by biopsy and in wk 9 after slaughtering the cows. Tissue was used for measuring gene and protein (only wk 9) expression of parameters related to milk synthesis. Milk FCM and ECM decreased with CLA treatment. After [¹³C₆]-glucose infusion, ¹³C-enrichment increased about 30-fold higher in lactose than in casein and milk fat. ¹³C-Enrichment in milk fat increased more, but in casein increased less in CLA-treated than non-CLA-treated cows after infusion of [¹³C₆]-glucose. In milk fat, ¹³C-enrichment of triglycerides was closely related to glycerol. The mRNA abundance of key factors and enzymes associated with milk fat synthesis, such as SREBF1, ACACA, FASN, ELOVL2, and EEF1A1, was reduced by CLA treatment. Protein expression of FAS and SREBP (68 kD) was reduced, but FABP4 was increased in cows infused with CLA. The reduced ¹³C-enrichment in casein of CLA-treated cows was partly combined with lower urea and protein concentrations in milk but no clear changes in gene and protein expression of parameters associated with milk protein synthesis. The EFA treatment did not affect ¹³C-enrichment of milk ingredients after [¹³C₆]-glucose infusion, but protein expression of SREBP (68 kD) was reduced. Results indicated a coordinated inhibition of parameters related to milk fat synthesis in the mammary gland in CLA and EFA+CLA cows. The inhibitory effect on milk fat synthesis was associated with an elevated carbon flux of glucose in glycerol of the triglycerides in milk fat. Reduction of glucose flux in casein by CLA treatment and less milk urea pointed at less nonessential AA synthesized from glucose, but regulation of milk protein synthesis was barely affected by CLA and EFA+CLA. The EFA treatment had no influence on glucose flux into milk ingredients and seems to barely affect milk fat and protein synthesis in the present study.