Interstitial exclusion of albumin in rat tissues measured by a continuous infusion method.

Steady-state 125I-labeled rat serum albumin (125I-labeled RSA) concentration in plasma was maintained by intravenous infusion of tracer for 72-168 h with an implanted osmotic pump. At the end of the infusion period, the rat was anesthetized and nephrectomized, and extracellular fluid was equilibrated with intravenous 51Cr-labeled EDTA for 4 h. Five minutes before final plasma and tissue sampling, 131I-labeled bovine serum albumin (131I-labeled BSA) was injected intravenously as a plasma volume marker. Samples of skin, muscle, tendon, and intestine were assayed for all three tracers. Apparent distribution volumes were calculated as tissue tracer content/plasma tracer concentration. Interstitial fluid volume (Vi) was calculated as V51Cr-EDTA-V131I-BSA. Steady-state extravascular distribution of 125I-labeled RSA as plasma equivalent volume (Va,p) was calculated as V125I-RSA-V131I-BSA. Steady-state interstitial fluid concentrations of 125I-labeled RSA in skin, muscles, and tendon were measured with nylon wicks implanted postmortem, and steady-state interstitial albumin distribution volumes were recalculated as wick-fluid equivalent volumes (Va,w). Relative albumin exclusion fraction (Ve/Vi) was calculated as 1-Va,w/Vi. For skin and muscle, steady-state 125I-labeled RSA tissue concentrations were reached at 72 h. Ve/Vi for albumin averaged 26% in hindlimb muscle, 41% in hindlimb skin, 30% in back skin, 39% in tail skin, and 54% in tail tendon. For muscle, Ve/Vi corresponds to expectation if all tissue collagen and hyaluronan is dispersed in the interstitium. However, for skin and tendon, albumin exclusion is considerably lower than expected on this basis, suggesting that much of their collagen is organized into dense bundles of fibers containing no fluid accessible to 51Cr-labeled EDTA or 125I-labeled RSA.