Filtration and reabsorption of endogenous low-molecular-weight protein in the rat kidney

Abstract

Low-molecular-weight proteins (LMW; mol wt <2 mg protein) into rats prepared for micropuncture of surface proximal tubules and collection of bladder urine. Rat IgG was similarly labeled and infused. Fractional fluid reabsorption was simultaneously assessed from inulin values. From specific activity of capsular fluid and plasma, the sieving coefficients (SC) of protein fractions were determined, and fractional reabsorption was calculated from tubular fluid and urine values. SC varied from 0.30 (mol wt, 35,000 daltons) to 1.0 (mol wt, 3500 daltons) for LMW fractions. Reabsorption of filtered protein was virtually complete (84 to 97%) at these endogenous plasma levels. The results are consistent with conservation of filtered protein and the regulation of LMW plasma levels by the kidney.