Renal tubular protein absorption in the rat

Radioiodinated protein solutions, 20 nl in volume, were injected into surface proximal and distal convoluted tubules of the rat kidney. The unabsorbed fraction was collected in ureteral urine, and the percentage recovered was expressed as a function of the injection site. Recovery increased progressively from more distal sites of injection indicating absorption along the proximal tubule of human serum albumin, insulin, and ribonuclease. Fractional absorption of albumin along the proximal tubule varied from 0 to 20% of the injected load, and was similar when the injectate concentration was 20, 40, or 100 mg/100 ml. Fractional absorption along the proximal tubule of insulin and ribonuclease, smaller proteins, was 30 to 50% of the injected load, and was similar with insulin concentrations of 0.09 mg/100 ml and 40 mg/100 ml and ribonuclease concentration of 40 mg/100 ml. In addition to this constant fractional absorption of each protein in the proximal tubule, smaller amounts were absorbed when injections were made in distal convoluted tubules.