Glomerular filtration and tubular absorption of the basic polypeptide aprotinin

The basic polypeptide aprotinin (Ap), mol. wt 6500, pI 10.5, is filtered in the glomeruli, virtually completely taken up by the proximal tubular cells and retained there for many hours. This process was studied in rats by determining the renal plasma clearance (CAp) as the amount of [125I]Ap accumulated in the kidney plus that excreted in the urine per unit of time divided by the integrated plasma concentration. In periods lasting 4-20 min after i.v. bolus injection or infusion to constant plasma concentration, CAp was 65% of glomerular filtration rate (GFR) estimated as kidney plus urinary clearance of [51Cr]EDTA (Ccr-EDTA). Less than 0.8% of the filtered Ap appeared in the urine. CAp varied inversely with plasma protein concentration in mg ml-1: CAp/Ccr-EDTA = 0.98-0.0058 x Ppr, corresponding to a glomerular Gibbs-Donnan distribution for a net molecular charge of +6, in agreement with the amino acid composition of Ap. CAp (kidney + urinary) was not altered by inhibiting tubular uptake of [125I]Ap by maleate or by exceeding the uptake capacity with large doses of unlabelled Ap. Neutralized Ap (malonylated) did not accumulate in the kidney, but showed a urinary clearance indistinguishable from that of [51Cr]EDTA. Both CAp and Ccr-EDTA were reduced to 0.04 ml min-1 when glomerular filtration pressure was lowered by ureteral stasis and increased Ppr (80-90 mg ml-1). These findings indicate: (1) no steric or charge restriction to filtration of Ap in the glomerular membrane, (2) the Gibbs-Donnan equilibrium should be considered when estimating glomerular sieving of charged polypeptides in intact animals (3) charge dependent tubular uptake, (4) little or no transtubular transport of intact Ap, (5) no appreciable tubular uptake of Ap from the peritubular side and (6) local renal accumulation of Ap in a period of up to 20 min may be used to estimate local glomerular filtration and/or local proximal tubular reabsorption rates. Model analysis based on the appearance of 125I in plasma, the time course of renal Ap content, and literature data on subcellular Ap distribution are consistent with two populations of endosomes, transporting Ap at widely different rates from the proximal tubular brush border to the lysosomes where breakdown occurs at a high rate.