Evaluation of the mechanism of action of conjugated linoleic acid (CLA) isomers on reproduction - uterine release of PGF2A.

The objective of this study was to evaluate uterine PGF2α release (PGFM) in response to an oxytocin challenge after dietary supplementation with different CLA isomers. Lactating Holstein cows (n = 45, 20 ± 1 DIM) were assigned to one of 3 treatments: 70 g/d of Ca salts of tallow (CTL); 63 g/d of lipid-encapsulated CLA providing 7.1 g/d of cis 9, trans 11 CLA and 2.4 g/d of trans 10, cis 12 CLA (CLA 75:25), or ; 76 g/d of lipid-encapsulated CLA providing 7.1 g/d each of cis 9, trans 11 and trans 10, cis 12 CLA (CLA 50:50). Supplements were top-dressed for 37 d, milk production and DMI were recorded daily, and milk components measured weekly. Blood samples were taken 3x per wk for NEFA and 1x per wk for IGF-I analyses. At 30 ± 3 DIM ovulation was synchronized in all cows with a modified Ovsynch protocol. On d 15 of the cycle cows received an i.v. injection of 3 mg of estradiol 17β and 4 hr later an i.v. challenge with 100 IU of oxytocin; blood samples were obtained frequently over a 4 hr period. PGFM was determined by RIA and area under the curve was calculated. CLA 50:50 caused 16.6% reduction in milk fat content after wk 4 (P < 0.03), but milk fat content for CLA75:25 was not different from CTL. Milk fat yield was similar among treatments due to a trend (P < 0.07, wk 4) to increase milk yield of cows treated with CLA50:50 (43.5, 42.6 and 41.9 kg/d for CLA50:50, CLA75:25 and CTL, respectively). The oxytocin challenge resulted in an immediate elevation of circulating PGFM which gradually returned to basal levels over the next 4 h Area under the curve was not different among treatments (P = 0.2); likewise DMI and NEFA were similar among treatments (P > 0.05). Plasma IGF-I tended to be higher (P < 0.07) in cows treated with CLA 50:50 compared to CTL (81.4, 76.3 and 72.5 ng/ml for CLA50:50, CLA75:25 and CTL, respectively). Overall, results suggest that short periods of CLA supplementation do not impact reproductive performance via affects on uterine release of PGF2α . [ABSTRACT FROM AUTHOR]