Intersitial exluded volumes: the effect of charge.

The paper from Gyenge et al. published in volume 552(3) of The Journal of Physiology entitled ‘In vivo determination of steric and electrostatic exclusion of albumin in rat skin and skeletal muscle’ is carefully done and the data agree very well with several papers from our laboratory conducted in lung tissue (Gilchrist & Parker, 1985; Ishibishi et al. 1991; Parker et al. 1979, 1985). Gyenge et al. used implanted ‘wicks’ to measure interstitial concentrations of negatively charged (pI = 5.0) native human serum albumin (HSA), which was excluded from a greater interstitial volume than a more positive (pI = 7.6) charge-modified human serum albumin probe. Gyenge's study clearly shows that after a 5–7 day equilibration period the albumin excluded fractions were 16 and 26 % in skeletal muscle and 30 and 40 % in skin for the charge-altered HSA and native HSA, respectively, i.e. the more negative albumin has a greater excluded volume. The long infusion times required for equilibration of albumin in skin and muscle may account for the lower excluded volume fractions obtained by Gyenge et al. for native albumin compared to the 50 % reported by Bell & Mullins (1982a, b) for both skin and muscle in rabbits. Mullins & Bell (1982) reported comparable excluded volume fractions for albumin and IgG in both skin and muscle in spite of a size difference (albumin, 3.6 nm radius; IgG, 5.6 nm radius) which would predict a greater excluded volume for IgG based only on steric hindrance. The less negative charge of IgG and lower rates of restricted diffusion for IgG through the interstitium and equilibration with sampled interstitial fluid may also have affected their excluded volume values. Gilchrist & Parker (1985) also estimated the excluded volume fraction of an anionic lactate dehydrogenase (LDH)-1 (pI = 5.0), and a cationic LDH5 (pI = 7.9) (Fig. 1). The excluded volume fraction for the more negative LDH was 0.5, compared to 0.2 for the positive LDH, similar to the values reported by Gyenge et al. for negative serum albumin and a charge-modified more positively charged serum albumin. Figure 1 LDH isozyme exclusion in lung Parker et al. (1979) showed that the albumin concentration in lymph (CL) and that calculated from a separately measured interstitial volume (CAPP) are very different in normally hydrated lung, i.e. the concentration of the albumin in lymph was 60 % higher than in the interstitial fluid volume. As lung tissue became oedematous and the swollen matrix was less able to exclude protein, the lymph concentration approached the tissue concentration calculated using the total interstitial fluid volume (Fig. 2). This effect leads to more effective osmotic buffering of filtration across the microvascular wall. However, Bell & Mullins (1982b) observed that excluded volumes did not appear to change during interstitial volume increases in skin and muscle and so did not contribute to buffering of the change in tissue oncotic pressure. Reed et al. (1989) reported albumin excluded from 44 % and 58 % of subcutis and dermis, respectively, and attribute two-thirds of the exclusion to collagen and one-third to hyaluronan in skin. The decrease in interstitial albumin exclusion during interstitial volume expansion can be partly attributed to the wash-out of hyaluronan from skin and lung (Reed et al. 1990; Townsley et al. 1994). Figure 2 Albumin excluded volume in the lung Interestingly, it appears that the excluded volumes of protein with different charges in both lung, skeletal muscle and skin are very similar, and the greater exclusion of more negative proteins is a common phenomena occurring in several tissues. Different exclusion volumes could complicate the estimation of microvascular permeability using lymph concentrations at normal interstitial volumes because of more rapid transit through the tissue of the protein with the highest excluded volume (Parker et al. 1985; Ishibishi et al. 1991).