Glomerular capillary wall permeability to albumin and horseradish peroxidase

Tritium labelled albumin present in urine fractions from in vivo rat filtration studies and isolated perfused rat kidneys (IPK) has been shown by gel chromatograph ic analysis to be severely degraded (approximately 90%) to small peptides. The in vivo degradation of rat serum albumin and bovine serum albumin has been shown to be similar. The degradation has a marked influence on the fractional clearance analysis. The fractional clearance for albumin in the isolated perfused kidney as determined by Lowry assay was 0.00180 +/- 0.0007, 0.0060 +/- 0.0033 by Biuret assay and 0.0075 +/- 0.0039 as measured by total radioactivity in the urine. Specific analysis of intact albumin by gel chromatography gave a fractional clearance of 0.00039 +/- 0.00016, which is comparable to in vivo values. Degradation of albumin in the urine from IPK experiments was inhibited by 42% by including 150 mmol/L lysine, an inhibitor of tubular cell uptake, in the perfusate. Inhibition of degradation of anionic horseradish peroxidase was also observed. These studies demonstrate that albumin is degraded during renal passage probably by tubular cells. In a series of isolated perfused kidney systems using different inhibitors of protein tubular uptake, namely lysine, ammonium chloride, chloroquine and cytochalasin B, we estimate the glomerular sieving coefficient of albumin to be approximately 0.074 as determined using [H-3]-albumin. This glomerular sieving coefficient is considerably higher than originally thought, yet it is in accord with the size selectivity characteristics of the glomerular capillary wall as determined by transport probes calibrated for hydrodynamic size by gel exclusion chromatography. The tubular inhibitors also destroyed charge selectivity of the anionic and neutral forms of albumin and horseradish peroxidase. The tubular inhibitors were characterized in terms of their effect on tubular lysozyme uptake and glomerular size selectivity as determined using polydisperse dextran fractions. By accounting for the destination of all the tritium labelled albumin in the urine, perfusate, whole kidney and isolated glomeruli we conclude that a major rapid transtubular cell pathway must exist that directs albumin from the tubular lumen to either the renal lymphatics or peritubular capillaries or both. This pathway appears to be an overload pathway because since the albumin concentration in the perfusate was lowered from 50 mg/mL to 2.0 mg/mL it was not active. The pathway is also inactive for other proteins, such as horseradish peroxidase, that are at relatively low concentration.