Blood-derived proteins in milk at start of lactation: Indicators of active or passive transfer

Colostrum has a different composition compared with milk in established lactation. This difference is in part due to the partially open blood-milk barrier, which, when closed, is designed to prevent the interdiffusion of blood and milk components. In the first days of lactation, α-lactalbumin (α-LA), a milk protein, is typically present in blood and several blood-derived proteins are also present in milk, such as IgG₁, IgG₂, serum albumin (SA), and lactate dehydrogenase (LDH). With the exception of IgG₁, which is known to be transferred by active transcellular transport, the other proteins are thought to pass paracellularly through the temporarily open barrier. Along with an exchange of blood and milk components, somatic cell count (SCC) is typically high in colostrum. The decline of these proteins and SCC can be used as indicators to determine transcellular or paracellular transport. Two hypotheses were tested. The first hypothesis was that the decline curve for a protein or SCC would be the same as IgG₁, indicating transcellular transport, or the decline curve would be different than IgG₁, indicating paracellular transport. The second hypothesis was that the decline curves of SCC and all proteins that are thought to have paracellular transport would be the same. Ten Holstein cows were milked at 4 h after parturition, the next 5 consecutive milkings, and the afternoon milking on d 5, 8, 10, and 14 of lactation for a total of 10 milking time points, and sequential jugular blood samples were also taken. Blood and milk samples were analyzed for the concentrations of LDH, SA, IgG₁, IgG₂, and α-LA and milk samples were measured for SCC. Protein concentration and SCC curves were generated from all 10 time points and were evaluated using the tau time constant model to determine the rate of decline of the slope of each protein. When examining the first hypothesis, the concentration of IgG₁ declined significantly faster in the milk than the proteins IgG₂ and LDH, but declined at the same rate as SA. Immunoglobulin G₁ also declined significantly faster than SCC and α-LA in plasma. The second hypothesis showed that IgG₂, LDH, and SA in milk were declining at the same rate, but were declining significantly faster than SCC and α-LA in plasma. These results indicate that only active transcellular transport of IgG₁ occurred, with a sharp decline at parturition, compared with IgG₂, SA, LDH, α-LA, and SCC, which are likely following paracellular transport.