Exclusion of plasma proteins in interstitium of tissues from the dog hind paw

Exclusion of plasma albumin and fibrinogen from portions of the extravascular sucrose space in tissues of the hind paw of anesthetized dogs was studied by use of radioactive traces. Thirty hours after a bolus injection of the labeled proteins, tracer concentrations were measured in plasma, prenodal popliteal lymph, and samples of hind paw skin, tendon, muscle, and toe pad. In a few experiments, constant plasma levels of tracer sucrose were maintained for 2 h before terminal sampling. In all experiments a vascular volume marker was given 15 min before termination. A linear correlation was found between the amounts of extravascular protein tracers in the tissue samples and their concentrations in lymph from the same paw, as would be expected if their actual interstitial concentrations were closely related to their lymph concentrations. For albumin, the extravascular distribution volume calculated at lymph concentration was less than the sucrose space for all tissues except toe pad. For fibrinogen, the extravascular distribution volume was less than albumin and sucrose for all tissues except skin. Differences in pattern and degree of exclusion may reflect the differences in interstitial gel-phase composition and structure.