Impact of arterial nutrient concentration on the uptake of nutrients by the mammary glands in sows

The high nursing frequency observed in sows results in a relatively constant demand for nutrients for milk synthesis, while the supply of nutrients in arterial blood is greatly affected by the time relative to feeding. Thus, characterization of the mammary uptake of nutrients at different times relative to feeding could provide a better understanding of how macronutrients are utilized for milk synthesis. The objective was to characterize the use of nutrients by the mammary glands by measuring arterial concentrations and mammary arterial-venous differences (AV-diff) of nutrients in catheterized sows at different times relative to feeding. Indwelling catheters were surgical inserted in the femoral artery and the cranial mammary vein of eight multiparous sows at day 76 ± 2 of gestation. Eight sets of blood samples were simultaneously sampled from the catheters in hourly intervals from 0.5 h before to 6.5 h after feeding at day 17 (range 15 to 19) of lactation. Plasma samples were analyzed for glucose, non-esterified fatty acids (NEFA), triglycerides (TG), lactate and essential amino acids. Arterial concentration of NEFA and TG decreased postprandially (P < 0.05), while all other studied metabolites showed a postprandial increase, although leucine (P = 0.06) and histidine (P = 0.11) did not reach significance. Mammary AV-diff of glucose (1.4 mmol/L; P =0.38) and TG (49 µmol/L; P = 0.39) were not affected by time after feeding, suggesting that the liver and other organs were capable of buffering mammary uptake after feeding. In contrast, the AV-diff for lactate, lysine, methionine and threonine (P < 0.001) increased by 20 to 40 % postprandially compared to preprandial levels, while a postprandial decrease in AV-diff was observed for NEFA (P < 0.001). Arterial concentrations were positively correlated to the AV-diff for NEFA (r = 0.88; P < 0.001), lactate (r = 0.78; P < 0.001), methionine (r = 0.61; P < 0.001), threonine (r = 0.60; P < 0.001), leucine (r = 0.56; P < 0.001), and lysine (r = 0.46; P < 0.001), suggesting that the arterial concentration of these amino acids was to some extent important for the transfer into the mammary glands. Collectively, the mammary uptake of lactate, NEFA, lysine, methionine and threonine changed with time after feeding, and were associated with arterial concentrations, while glucose, TG and the remaining essential amino acids seemed to be independent of the arterial supply.