An isotope dilution model for partitioning leucine uptake by the liver of the lactating dairy cow

An isotope dilution model for partitioning leucine uptake by the liver of t he lactating dairy cow is constructed and solved in the steady state. If as sumptions are made, model solution permits calculation of the rate of leuci ne uptake from portal and hepatic arterial blood supply, leucine export int o the hepatic vein, leucine oxidation and transamination, and synthesis and degradation of hepatic constitutive and export proteins. The model require s the measurement of plasma flow rate through the liver in combination with leucine concentrations and plateau isotopic enrichments in arterial, porta l and hepatic plasma during a constant infusion of [1-C-13]leucine tracer. The model can be applied to other amino acids with similar metabolic fates and will provide a means for assessing the impact of hepatic metabolism on amino acid availability to peripheral tissues. This is of particular import ance when considering the dairy cow and the requirements of the mammary gla nd for milk protein synthesis. (C) 1999 Academic Press.