Interstitial exclusion of positively and negatively charged IgG in rat skin and muscle

Volume exclusion, i. e., the space not available for a specific probe, may be dependant on the probe charge. Therefore, interstitial exclusion was measured for positively and negatively charged immunoglobulin (IgG) in skin and muscle of rats by using a continuous infusion method (30). Steadystate concentration of I-125-labeled IgG 1 (pI = 8.7) and I-131-labeled IgG 4 (pI = 6.6) was maintained by infusion of tracer for 120- 168 h with an implanted osmotic pump. At the end of the infusion period and before tissue sampling, the rat was anesthetized and nephrectomized, and Cr-51-labeled EDTA was injected and allowed 4 h for equilibration to measure interstitial fluid volume (V-i). Interstitial fluid was isolated from skin and muscle by using nylon wicks implanted post mortem. The relative IgG available space was measured as the ratio between labeled IgG and Cr-51-labeled EDTA wick fluid equivalent spaces, and relative excluded volume fraction (V-e/V-i) was calculated as 1 - V-a /V-i. V-e/V-i in hindlimb skin averaged 0.37 +/- 0.05 (SE) and 0.65 +/- 0.06 (P < 0.01) for IgG 1 and 4, respectively, with corresponding figures of 0.24 +/- 0.05 and 0.51 +/- 0.04 (P < 0.01) in hindlimb muscle (n = 9 for both tissues). These experiments suggest that fixed negative charges, most likely glycosaminoglycans, influence distribution of macromolecules in the interstitium and therefore affect interstitial fluid balance.