Differences between blood and plasma concentrations of acetate, beta-hydroxybutyrate, glucose, ammonia and urea: implications for measurement of portal net fluxes in ewes.

The aim of this study was to determine the differences in concentrations of various metabolites between blood and plasma, and their implications for the measurement of portal net fluxes in ewes. A wide range of concentrations of metabolites in arterial and portal blood was induced by submitting three ewes to three different levels of intake. Plasma measurements underestimated the portal drained viscera net flux for acetate, glucose, ammonia and urea but not for beta-hydroxybutyrate. The underestimation of net release was low and largely constant for acetate (7%). The net flux of glucose tended to be overestimated by plasma measurements when it was low, but was underestimated by 25% when high net uptake occurred. The underestimation of net fluxes of ammonia and urea increased with their magnitude, up to 17% for ammonia release and 41% for urea uptake.