Calcium concentration and Ca(2+)-ATPase activity under basal conditions and after maximal stimulation with calmodulin were measured in erythrocytes from 32 patients with end-stage renal failure on haemodialysis and from 27 healthy subjects. 2. In patients with renal failure the Ca2+ concentration in erythrocytes was elevated compared with healthy subjects (4.27 +/- 1.02 versus 2.86 +/- 0.57 mumol/l, P less than 0.05). 3. Basal Ca(2+)-ATPase activity was lower in the patients with renal failure than in healthy subjects (4.62 +/- 1.34 versus 5.43 +/- 1.23 pmol of phosphate min-1 10(-6) erythrocytes). After maximal stimulation, Ca(2+)-ATPase activity reached 6.93 +/- 2.81 pmol of phosphate min-1 10(-6) erythrocytes in the patients with renal failure, whereas in healthy subjects stimulation yielded a Ca(2+)-ATPase activity of 32.54 +/- 8.48 pmol of phosphate min-1 10(-6) erythrocytes. 4. Incubation of erythrocytes from healthy subjects with plasma from uraemic patients caused inhibition of Ca(2+)-ATPase. Likewise, the ultrafiltrate from plasma obtained by haemofiltration treatment inhibited Ca(2+)-ATPase. 5. Gel chromatography of the ultrafiltrate and laser desorption/ionization mass spectroscopy revealed that a fraction containing substances with a molecular mass of about 300 Da inhibited Ca(2+)-ATPase. 6. It is concluded that, in uraemia, a Ca(2+)-ATPase inhibitor accumulates in the plasma, and this could contribute to the toxicity of uraemia by inhibiting cellular Ca2+ transport in erythrocytes and possibly other tissues.
