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Relative importance of the role of the mammary gland in milk fat CLA synthesis

Milk, together with ruminant meat, represent the major dietary source of CLA. Milk CLA has two origins,
- Ruminal biohydrogenation of linoleic acid into CLA followed by its uptake and secretion by the mammary gland,

- Mammary synthesis by action of the enzyme stearoyl-CoA desaturase (SCD) on circulating vaccenic acid (trans-11-18:1) coming from ruminal biohydrogenation of C18 unsaturated FA.

The latter pathway contributes to more than 90% of milk CLA. Then, it is of major importance to study the regulation of genes involved in lipogenesis and CLA synthesis (particularly SCD gene expression and activity) in ruminant mammary gland, both in vitro and in vivo, in response to nutritional factors.

In this context, the effect of diets supplemented with lipids rich in C18:2 (n-6) and differing by the forage:concentrate ratio and/or degradable starch content in concentrate, on milk fatty acid composition and in vivo mammary metabolism, was examined in dairy goats. Regarding milk composition, the high starchy concentrate diets led to
- An increase in the trans-10 isomers at the expense of the trans-11,
- A decrease in milk cis-9, trans-11 CLA and
- An increase in the level of medium-chain fatty acids at the expense of the long-chain fatty acids.

These effects were more pronounced with the concentrate rich in rapidly degradable starch. Regarding mammary metabolism, the responses of the four studied lipogenic genes (LPL, ACC, FAS, SCD) were not always related to the correspondent milk fatty acid secretion.

ACC activity was not significantly different between the dietary treatments, due to the individual variability, but was related to the variation of secretion of medium-chain fatty acids. SCD mRNA and activity did not vary under these dietary conditions, which is in accordance with milk delta-9 desaturation ratios that represent a proxy for SCD activity.

This suggests that the amount of lipogenic enzymes or mRNA is only one regulator of lipogenesis together with the availability of substrates and cofactors. Furthermore, the trans-10 C18:1 secretion was not related in this goat experiment to lipogenic enzymes and fat yield responses, conversely to what was observed in cows.

In vitro trial was performed in parallel by the development and characterization of a mammary explant culture technique on tissue of lactating goats. Mammary explants were incubated for 20 hours in 9 media supplemented with three different C18 FAs (18:1 n-9, 18:2 n-6 and 18:3 n-3). These conditions led to convergent data using different approaches (14C-acetate incorporation representing total lipogenic activity, FAS and G6PDH activities, ACC, FAS and SCD mRNA levels) to estimate mammary lipid synthesis.

Then, was investigated on 5 lactating goats, the effect of 2 CLA isomers (cis9, trans11-18:2, trans10, cis12-18:2), 2 major trans-18:1 (trans11-18:1, trans10-18:1) and of unsaturated C18 FAs (18:1 n-9, 18:2 n-6 and 18:3 n-3), on mammary lipid metabolism. The 7 FAs had less effect on goat mammary FAS and G6PDH activities and on ACC, FAS and SCD mRNA levels than previously published in cows.

These results suggest that in goats the nature of FA brought to mammary tissue is not as important as in cow to regulate mammary lipogenesis and CLA synthesis, in accordance with the determinant role of the availability of the substrates suggested by the in vivo experiment. Whole results from this study are still under analysis.

Informazioni correlate

Reported by

Institut National de la Recherche Agronomique
Clermont-Theix Research Centre, Herbivore Research Unit
63122 Saint-Genès Champanelle