title: Stimulation of suicidal erythrocyte death by increased extracellular phosphate concentrations.
abstract: OBJECTIVE
  Anemia in renal insufficiency results in part from impaired erythrocyte formation due to erythropoietin and iron deficiency. Beyond that, renal insufficiency enhances eryptosis, the suicidal erythrocyte death characterized by phosphatidylserine-exposure at the erythrocyte surface. Eryptosis may be stimulated by increase of cytosolic Ca(2+)-activity ([Ca(2+)]i). Several uremic toxins have previously been shown to stimulate eryptosis. Renal insufficiency is further paralleled by increase of plasma phosphate concentration. The present study thus explored the effect of phosphate on erythrocyte death.
  
  
  METHODS
  Cell volume was estimated from forward scatter, phosphatidylserine-exposure from annexin V binding, and [Ca(2+)]i from Fluo3-fluorescence.
  
  
  RESULTS
  Following a 48 hours incubation, the percentage of phosphatidylserine exposing erythrocytes markedly increased as a function of extracellular phosphate concentration (from 0-5 mM). The exposure to 2 mM or 5 mM phosphate was followed by slight but significant hemolysis. [Ca(2+)]i did not change significantly up to 2 mM phosphate but significantly decreased at 5 mM phosphate. The effect of 2 mM phosphate on phosphatidylserine exposure was significantly augmented by increase of extracellular Ca(2+) to 1.7 mM, and significantly blunted by nominal absence of extracellular Ca(2+), by additional presence of pyrophosphate as well as by presence of p38 inhibitor SB203580.
  
  
  CONCLUSIONS
  Increasing phosphate concentration stimulates erythrocyte membrane scrambling, an effect depending on extracellular but not intracellular Ca(2+) concentration. It is hypothesized that suicidal erythrocyte death is triggered by complexed CaHPO4.
authors.affiliation: Departments of Physiology, University of Tuebingen, Germany.
authors.forename: Jakob
authors.initials: J
authors.lastname: Voelkl
authors.forename: Kousi
authors.initials: K
authors.lastname: Alzoubi
authors.forename: Abdel-Karim
authors.initials: AK
authors.lastname: Mamar
authors.forename: Mohamed Siyabeldin E
authors.initials: MS
authors.lastname: Ahmed
authors.forename: Majed
authors.initials: M
authors.lastname: Abed
authors.forename: Florian
authors.initials: F
authors.lastname: Lang
mesh_terms: D002118:Calcium / Q000097:blood / Q000494:pharmacology; D016923:Cell Death / Q000187:drug effects*; D048429:Cell Size / Q000187:drug effects; D011756:Diphosphates / Q000494:pharmacology; D004912:Erythrocytes / Q000187:drug effects*; D045604:Extracellular Fluid / Q000378:metabolism; D006461:Hemolysis / Q000187:drug effects; D006801:Humans; D066298:In Vitro Techniques; D010710:Phosphates / Q000494:pharmacology*; D010718:Phosphatidylserines / Q000494:pharmacology; D013268:Stimulation, Chemical
chemical_list: D011756:Diphosphates; D010710:Phosphates; D010718:Phosphatidylserines; D002118:Calcium