Interstitial exclusion of IgG in rat tissues estimated by continuous infusion

Interstitial exclusion, defined as the fraction of interstitial fluid volume inaccessible to a solute, was evaluated for immunoglobulin G (IgG) in selected tissues of rats by a method previously applied to serum albumin (29). IgG distribution volumes were also measured for intestine. ¹²⁵I- labeled rat IgG was infused for 5 or 7 days (n = 4 rats each) with an implanted osmotic pump (Alzet). At the termination of infusion, the rat was anesthetized, nephrectomized, and injected with ⁵¹Cr-labeled EDTA (4 h) to label total extracellular fluid volume and ¹³¹I-labeled bovine IgG (5 min) to label plasma volume. Samples of skin, muscle, and tendon were assayed for total and extractable tracer activity. Interstitial fluid from these tissues was sampled postmortem with nylon wicks for assay of ¹²⁵I-labeled IgG and endogenous albumin and IgG. Exclusion of IgG was calculated from the difference between extravascular ¹²⁵I-labeled IgG and ⁵¹Cr-labeled EDTA distribution volumes. In contrast to our previous experience with tracer albumin, ¹²⁵I-labeled IgG was not fully extractable from minced skin, muscle, or tendon by isotonic saline; only 71-83% was recovered under conditions that eluted 92-96% of tracer albumin and 94-99% of tracer EDTA. We conclude that ~20% of extravascular ¹²⁵I-labeled IgG in these tissues is sequestered or bound in the interstitium. Calculation of IgG fractional exclusion from extractable tracer yielded the following values (means ± SE, n = 8 rats): leg muscles 0.37 ± 0.09, leg skin 0.44 ± 0.03, back skin 0.36 ± 0.04, tail skin 0.40 ± 0.08, and tail tendon 0.55 ± 0.04. None of these values is significantly different from the corresponding albumin exclusion fractions. It seems possible that the steric factors, which might be expected to increase IgG exclusion relative to albumin, are balanced by electrostatic differences, which would increase albumin exclusion relative to IgG.