On the Uptake of Materials by the Intact Liver. THE TRANSPORT AND NET REMOVAL OF GALACTOSE

Abstract

D-galactose, a monosaccharide rapidly phosphorylated within liver cells, is irreversibly removed from the portal circulation. We have studied the kinetic relations between the hepatic cell entry process and the metabolic sequestration process, by means of the multiple indicator dilution technique. Labeled red blood cells (a vascular indicator), labeled sucrose (an extracellular reference), and labeled galactose were rapidly injected into the portal vein, and from rapidly sampled hepatic venous blood, normalized outflow-time patterns were secured. The labeled red cell curve rises to the highest and earliest peak, and decays rapidly; and that for labeled sucrose rises to a later and lower peak. Its extrapolated recovery is equivalent to that of the labeled red cells. At low blood galactose concentrations, the labeled galactose appears at the outflow with labeled sucrose, but is much reduced in magnitude, and exhibits a long tailing. Its outflow recovery is much reduced. At high blood galactose concentrations, the initial part of the profile increases towards that for labeled sucrose, the tailing becomes much larger in magnitude, and the outflow recovery becomes virtually complete. We have modeled the uptake of labeled galactose, and find two parts to the predicted outflow pattern, corresponding to our experimental observations; throughput material, which sweeps past the cell surface in the extracellular space; and returning material, which has entered the cells but escaped the sequestration process. Analysis of the data by use of this model provides estimates of both transmembrane fluxes and rates of sequestration. The capacity of the process subserving cell entry is found to be 40 times that for phosphorylation; and, whereas the K(m) value for sequestration is less than 15 mg/100 ml, that for entry is approximately 500 mg/100 ml. Both processes are relatively stereospecific; the entry of the L-stereoisomer is very slow and it undergoes no significant amount of metabolic sequestration. The sequestration process produces a lobular intracellular concentration gradient; and this gradient, in turn, produces some uncertainty in the estimate of the true K(m) value for the sequestration process.