Permeability of renal capillaries. II. Transport of neutral and charged protein molecular probes

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Abstract
The permselectivity of the renal capillaries was investigated from the transport of a series of molecular probes: (1) inulin, (2) positive (+ 2, net charge at pH 7.4), neutral (o), and negative (‐6) myoglobin, (3) neutral (0) and negative (‐14) horseradish peroxidase (HRP) and (4) two isomers of lactate dehydrogenase (LDH), namely the positively charged (+ 2) LDH‐M4 and the negatively charged (‐19) LDH‐H4. The determination of the concentration of tracer proteins necessitated gel separation of both plasma and renal hilar lymph. The reason for this is that the proteins, after filtration, will be reabsorbed and degraded by the proximal tubular cells into small molecular compounds (amino acids), which will return to both the renal interstitium and systemic plasma. Even if this degradation is of low degree, as for high‐molecular‐weight proteins, separation is still required, since the relative lymph concentration (plasma concentration put at 1) is also low, that is, even small amounts of low molecular compounds will distort the relative lymph concentration obtained. The transport from plasma to renal hilar lymph of the tracer molecules fell with increasing Stokes‐Einstein radius. The relative lymph concentration of the 11 Å inulin was 1.06 ± 0.03 (mean ± SEM), of the neutral 17.5 Å myoglobin 0.76 ± 0.05, of the neutral 32 Å HRP 0.32 ± 0.02 and of the neutral 46 Å LDH 0.12 ± 0.01. The data are compatible with a two‐pore system. The negative tracer molecules were in general proportionally more restricted than the neutral (or positive) moieties (P < 0.001) thus suggesting a negatively charged peritubular capillary membrane. The mass transport of the negative myoglobin and HRP was lower than that of the corresponding neutral moieties (P < 0.001), which indicates a negatively charged glomerular capillary membrane.

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