FILTRATION AND REABSORPTION OF ENDOGENOUS LOW-MOLECULAR-WEIGHT PROTEIN IN THE RAT-KIDNEY

Low-molecular-weight proteins (LMW; mol wt<albumin) constitute the major fraction of urinary proteins in the rat. Using gel filtration technique followed by gradient gel electrophoresis, the authors have identified the major protein fractions in normal urine of mature male rats of four strains: Brown Norway (BN), Wistar (W), Wistar-Kyoto (WK), and spontaneously hypertensive rats (SHR). In a sephadex G-75 column, albumin and globulins appear as one peak followed by five additional fractionation groups, with average mol wtx10-3 of 3.5, 12, 23, 35, and 45 daltons. These LMW fractions were labeled (131I) and injected i.v. (<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.