Effects of intravenous infusion of trans-10, cis-12 18:2 on mammary lipid metabolism in lactating dairy cows.

It has previously been established that supplementation of trans-10, cis-12 18:2 reduces milk fat content and fat deposition in several species. The objectives of the study were 1) to examine whether potential mechanisms by which trans-10, cis-12 18:2 is reported to affect lipid metabolism in adipose tissue of different species could be partly responsible for the inhibition in milk fat synthesis in lactating dairy cows; and 2) to investigate the effects of trans-10, cis-12 18:2 on the expression of a newly identified isoform of stearoyl-coenzyme A desaturase (SCD) in bovine mammary tissue. Four primiparous Holstein cows in established lactation, fitted with indwelling jugular catheters, were used in a balanced 2 x 2 crossover design. For the first 5 d of each period, cows were infused intravenously with a 15% lipid emulsion providing 10 g/d of either cis-9, cis-12 18:2 (control) or trans-10, cis-12 18:2 (conjugated linoleic acid; CLA). On d 5 of infusion, mammary gland biopsies were performed and tissues were analyzed for mRNA expression of acetyl-coenzyme A carboxylase, fatty acid synthetase, lipoprotein lipase, SCD1, SCD5, sterol regulatory element-binding protein-1, IL6, IL8, and tumor necrosis factor-alpha by real-time PCR. Compared with the control treatment, CLA reduced milk fat concentration and yield by 46 and 38%, respectively, and increased the trans-10, cis-12 18:2 content in milk fat from 0.05 to 3.54 mg/g. Milk yield, milk protein, and dry matter intake were unaffected by treatment. Infusion of the CLA treatment reduced the mRNA expression of acetyl-coenzyme A carboxylase and fatty acid synthetase by 46 and 57%, respectively, and tended to reduce the expression of SCD1 and lipoprotein lipase. Abundance of mRNA for sterol regulatory element-binding protein-1 was reduced by 59% in the CLA treatment group. However, infusing trans-10, cis-12 18:2 did not affect the expression of transcripts for SCD5, tumor necrosis factor-alpha, IL6, and IL8. Results from the current study corroborate the idea that effects of trans-10, cis-12 18:2 reported on adipose tissue in animal models and humans are not part of the response in the inhibition of milk fat synthesis in lactating dairy cows. They also support the hypothesis that SCD1 and SCD5 present important differences in their regulation and physiological roles.