Antisense oligonucleotides in neuroendocrinology: enthusiasm and frustration

A model of amino acid (AA) uptake and metabolism by the mammary gland (udder) of a high producing dairy cow in mid-lactation is presented. Two cell types, milk-secreting alveolar epithelial cells, and constitutive stromal cells are represented separately. Solution of the model at steady state, with appropriate assumptions, allows calculation of amino acid (AA) pool sizes in tissue bound AA and milk protein bound AA, and concentrations of intracellular free AA. Bidirectional flux rates of amino acids between each of these pools is also estimated. The flow of two amino acids, phenylalanine (Phe) and tyrosine (Tyr), is described using uptake kinetics of the L-system AA transporter. Model results suggest that AA uptake capacity of the alveolar cells, and the amount/activity of protein translation components limit the rate of milk protein synthesis in the high-producing dairy cow. A considerable amount of experimental work is necessary to provide data, if a comprehensive quantitative understanding of milk protein synthesis is to be achieved.