Cervical, intestinal and hepatic lymphatics are cannulated in Nembutalized dogs and I131-labeled serum albumin and dextran fractions of average molecular weights from 10,600 to 412,000 are simultaneously infused. The concentrations of the test molecules are periodically determined in plasma and lymph. ‘Steady state’ concentration lymph/plasma ratios for each test molecule are related to those for albumin and a coefficient obtained which relates the permeability of a capillary bed to a given molecular weight of dextran to its permeability to albumin. This makes it possible to accurately compare permeabilities to different size dextrans from dog to dog. The data indicate the presence of two discrete sets of ‘pores:’ 1) a set of ‘small pores’ allowing passage of molecules not greater than 250,000 M; 2) ‘large pores’ permitting passage of molecules of at least 412,000 M. An alternate suggestion is made that molecules of greater than 250,000 M are transferred by the process of ‘cytopemphis’ rather than passage through pores.
Radioactive iodinated serum albumin and dextran fractions of average molecular weights of 51,000– 255,000 are injected into nembutalized dogs, and concentration changes of these substances are followed in plasma and thoracic and right duct lymphs for 4–6 hours. At this time, when a ‘steady state’ has been established between plasma and lymph, the plasma volumes of the dogs are increased by the infusion of 40 ml/kg of 5% serum albumin in 0.9% saline solution. This results in a significant and striking increase in the concentration of the injected radioactive iodinated albumin and dextrans in right duct and thoracic duct lymph in spite of increased lymph flows. The increase in dextran lymph/plasma concentration ratios occurs with all molecular weight fractions. These results are interpreted as reaffirming our previously formulated concept that infusions producing plasma volume expansion decrease the resistance of the capillary wall to the passage of macromolecules or increase the size of the capillary ‘pores.’ The concept of capillary permeability as a function of ‘pore’ size must, therefore. be modified to include a labile capillary ‘pore’ size, subject to change with variations in plasma volume as well as other factors.
Small infusions of dextran fractions having average molecular weights ranging from 10,600 to 412,000 yield plasma to lymph concentration ratios which are directly proportional to molecular weight. The concentration gradient for a specific molecular weight, however, decreases as the volume of infusion is increased. This volume effect, explained in terms of stretching of capillary pores, consequently provides less resistance to the passage of macromolecules through the capillary wall. The significance of these results in terms of the conventional pore theory of capillary permeability is discussed.
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