Middle molecules in uremic serum, urine and dialysis fluid

Gel filtration through Sephadex (g 75 and 15) and ultrafiltration and diafiltration through selective membrances have been carried out on 172 uremic sera, 89 normal sera, uremic and normal urines and hemodialysis fluid. The accumulation in uremic sera of substances wwith molecular weights between 500 and 3,500 (so called "middle molecules") was demonstrated. Molecular weight evaluation was verified on single effluent fractions using different added isotopes. Evaporation of serum to dry weight revealed a 2-3 fold increase in solids compared to normal values. Estimation of the fractional content of individual elements and quantitative amino acid analysis (before and after acid hydrolysis) did not show any difference between normal and uremic subjects, but there was a significant increase of peptides in uremic serum. The accumulation of peptides was confirmed by high voltage electrophoresis. Urinary excretion of substances with comparable molecular weights to those found in uremic serum was demonstrated, but there was no significant difference between urine from normal and from uremic subjects. A steady state of chronic uremia with high urinary volume is therefore consistent with a normal urinary excretion of middle molecules with increased concentrations in serum and glomerular filtrate. Tubular reabsorption may also be decreased because the urinary excretion of middle molecules increases with the development of tubular proteinuria in patients with pyelonephritis. Dialysis treatment removes moderate amounts of middle molecules; their serum concentration decreases slightly after dialysis and they are detectable in dialysis fluid. The identification, metabolism and biological effects of middle molecules are discussed in relationship to uremic toxicity and the effects of different forms of dialysis treatment.