AN ISOTOPE-DILUTION MODEL FOR PARTITIONING LEUCINE UPTAKE BY THE BOVINE MAMMARY-GLAND

A model of leucine uptake and partition by the mammary gland (i.e. the udder) of the lactating dairy cow is constructed and solved in the steady state. Model solution permits-if assumptions are made-calculation of leucine uptake from the arterial blood supply, leucine export into the venous drainage, leucine oxidation, and synthesis and degradation of milk protein and constitutive tissue protein within the gland. The experimental measurements required for model solution are blood flow and milk secretion, arterial and venous concentration and plateau enrichment of leucine, and concentration and plateau enrichment of free and protein-bound leucine in milk. The model, at least in theory, is also applicable to other amino acids and provides a means of ranking the nutritional importance of individual amino acids and identifying the most limiting in the gland.