Genesis of the uraemic syndrome: role of uraemic toxins.

A variety of signs and symptoms constituting the uraemic syndrome may be related to the retention and accumulation of uraemic toxins. Several identified (and yet unidentified) uraemic toxins of low molecular weight are removed at least in part by dialysis therapy resulting in marked improvement of multiple organ dysfunctions and clinical symptoms. However, many abnormalities persist due to the high protein binding of several uraemic toxins or their high molecular weight associated with inadequate dialysis clearance. Moreover, carbamoylation of amino acids and proteins in uraemia as well as metabolic acidosis contribute to the functional and metabolic abnormalities of the uraemic state. Uraemia interferes with the function of polymor-phonuclear leukocytes by deranging their cellular biochemistry and biology. P-cresol and several newly identified granulocyte inhibitory proteins are responsible for reduced chemotaxis, oxidative activity, intracellular killing of bacteria, and glucose consumption by polymorphonuclear leukocytes. Hyperhomocysteinaemia is an independent risk factor for vascular disease in end-stage renal disease patients. Uraemic toxins interfere with calcitriol synthesis and concentration or activity of the calcitriol receptor. Advanced glycolysation end-products (AGEs) accumulate as a result of impaired renal excretion. AGE peptides may represent a modern-day version of "middle molecule" toxicity or uraemia. Of potential clinical importance are pentosidine-, imidazolone- and carboxymethyllysine-modifications of beta 2-microglobulin with respect to the development of uraemia associated amyloidosis. Several uraemic toxins also affect nitric oxide pathway. Particularly, dimethyl-L-arginine (ADMA) is a potent inhibitor of nitric oxide synthesis. Parathyroid hormone satisfies the strict criteria of an uraemic toxin. Many uraemic symptoms can be attributed to the excess of parathyroid hormone in patients with chronic renal failure. Finally, recent investigations indicate, that one or more dialyzable uraemic toxin(s) suppress(es) appetite and may contribute to malnutrition in uraemia.