Effect of Protein Supply on Hepatic Synthesis of Plasma and Constitutive Proteins in Lactating Dairy Cows

The effects of metabolizable protein (MP) supply on the synthesis of plasma total proteins and albumin, as well as total hepatic protein synthesis, were determined in 6 multicatheterized lactating Holstein cows. Three TMR formulated to supply the same amount of energy but different amounts of MP, 1,922 (low), 2,264 (medium), and 2,517 g of MP/d (high), were fed every 2 h according to a double 3 × 3 Latin square design. For the low and high MP treatments, the cows were continuously infused with [2H5]Phe (d5-Phe) into a jugular vein for 8 h (1.3 mmol/h) on d 21 of each period. Concentration and isotopic enrichment of d5-Phe were measured for free plasma Phe, plasma total proteins, and albumin on hourly samples collected between 3 and 8 h. Low MP decreased the plasma albumin concentration (32.3 vs. 33.7 ± 0.11 g/L) but the plasma total protein concentration was unchanged (74.1 vs. 75.6 ± 1.13 g/L). Incorporation of d5-Phe over time into both plasma total proteins and albumin was linear (R2 > 0.98). Neither fractional nor absolute synthesis rates of plasma total proteins (6.8 vs. 6.5 ± 0.65%/d; 168 vs. 154 ± 19.9 g/d) or albumin (3.4 vs. 3.4 ± 0.10%/d; 36.3 vs. 36.5 ± 1.11 g/d) were affected by the MP supply. Net hepatic removal of Phe was lower with the low-MP diet (−12.3 vs. −20.2 ± 1.98 mmol/h). As a result, net hepatic Phe removal used for total export protein synthesis (17.9 vs. 11.1 ± 1.83%) and albumin synthesis (4.6 vs. 2.9 ± 0.54%) tended to be greater at low MP. These results suggest that hepatic synthesis of plasma proteins, including albumin, is maintained in lactating dairy cows even when the protein supply is reduced.