A B S T R A C T 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 transmenmbrane 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 Km value for sequestration is less than 15 mg/100 ml, that for entry is approximately 500 mg/100 ml. Both processes are relatively stereoReceived for publicationt 3 February 1971

The red cell membrane is a permeability barrier that limits the equilibration of a variety of solutes between red cell and plasma water. We utilized the multiple indicator dilution technique to investigate the effect of this barrier on the exchange in the liver of a group of tracer substances that are not removed in net fashion from the hepatic circulation: thiourea, urea, and chloride. We demonstrated that, after preequilibration of the label with red cells, a red cell carriage effect appeared (the trapping and translocation of label in the red cells), that this effect was most marked when the permeability of the red cell was relatively low for the substance under consideration (thiourea), and that the effect became small when the permeability of the red cells was large for the exchanging substance (urea and chloride). We developed a theoretical description of the retarding effect of the red cell permeability barrier on the extravascular exchange of label and were able to use this description to obtain estimates of the red cell permeability from the in vivo dilution curves. We examined the effect of plasma injection, of changing the input in such a fashion that the label was not preequilibrated with red cells, and found, both experimentally and theoretically, that for substances of low permeability the transit time from these experiments, if multiplied by the total water flow or solute flux, gave an overestimate of both the apparent total volume of distribution and the mass of traced material in the system. This last effect is of great importance for the practical design of many biological experiments. Reliable volume and mass estimates can be made only when the labeled material has been preequilibrated with red cells. KEY WORDSred cell permeability thiourea urea and chloride multiple indicator dilution studies hepatic microcirculation theoretical estimates of volume and mass dog red blood cells permeability barrier of the red cell membrane• The membrane of the red blood cell constitutes a permeability barrier that is significant for a variety of low molecular weight substances. When tracer is added to blood, this barrier limits the rate of equilibration of label between the red cells and the plasma. Through this effect, the barrier changes the manner in which the label is distributed into tissue during the blood's passage through the microcirculatory bed of an organ. In similar fashion, if there is net removal of material by the organ, the presence of the red cell permeability barrier limits the accessibility of the material within the red cells to the removal mechanism. The phenomena which result from limited permea- 328bility across the red cell membrane have not been extensively explored in a quantitative fashion in mammalian systems. Our purpose is to provide such an exploration.We selected the liver as the organ most suitable for this exploration. The hepatic parenchymal cells are rather more permeable to low molecular weight substances than are the red cells, and this permeability difference pro...

A B S T R A C T D-Glucose equilibrates within liver cells.We have studied its process of entry into and exit from these cells with the multiple indicator dilution technique. Labeled red cells (a vascular indicator), labeled sucrose (an extracellular reference), and labeled D-glucose were rapidly injected into the portal vein, and from serially sampled hepatic venous blood, normalized outflow-time patterns were obtained. The labeled red cell curve rises to an early high peak, and decays rapidly; and that for sucrose reaches a later and lower peak and decays less rapidly, but generates an equivalent area. The curve for labeled D-glucose begins with that for labeled sucrose, gradually rises to a peak which is later and substantially lower than that for sucrose, and then decreases slowly. At high glucose levels this curve assumes a squared-off shape, rises fairly quickly to its highest level, at the time of the sucrose peak, and then slowly decreases. Phlorizin and galactose infusion result in the emergence of a pronounced early peak, under the sucrose peak; and the curve for tracer L-glucose approaches that for sucrose.

Young male Sprague-Dawley rats were induced to overeat (approximately 45%) by provision of a "cafeteria" (CAF) diet of palatable human foods. Normophagic rats fed a commercial chow or a semisynthetic diet served as controls. The CAF rats exhibited (a) the reduced food efficiency and the propranolol-inhibitable elevation in resting metabolic rate (resting VO2) that are indicative of a facultative diet-induced thermogenesis (DIT) by which excess energy gain is resisted, and (b) certain changes in brown adipose tissue (BAT) that are among those taken as evidence for BAT as the effector of DIT, e.g., increased protein content and increased mitochondrial binding of GDP. To assess directly and quantitatively the contribution by BAT to the elevation in VO2 (apparent DIT) of the CAF rats, BAT O2 consumption was determined (Fick principle) from measurements of tissue blood flow (microsphere method) and the arteriovenous difference in blood O2 across interscapular BAT (IBAT). To obtain the measurements, the animals were fitted under halothane anesthesia with vascular cannulas for intraventricular injection of microspheres and sampling of arterial blood and the venous effluent of IBAT. After recovery from anesthesia and rewarming to normal body temperature the animals were placed singly in a temperature-controlled metabolic chamber and the measurements, which also included determination of resting VO2, were made 1.5-2 h later about 11:30 h. As determined from measurements made at 28 degrees C (thermoneutrality) mean values of resting VO2 for the cannulated rats were unchanged from those of intact (unoperated) CAF or control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

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