title: Na(+)-Ca2+ exchange modulates Ca2+ handling of human platelets by altering intracellular Ca2+ store size.
abstract: OBJECTIVE
  In order to elucidate the role of Na(+)-Ca2+ exchange in regulating cytosolic free Ca2+ concentration in human platelets, we investigated the relationship between cytosolic free Na+ and Ca2+ concentrations in human platelets.
  
  
  METHODS
  Sodium-binding benzofuran isophthalate and fura-2 were used to monitor cytosolic free Na+ and Ca2+ concentrations, respectively.
  
  
  RESULTS
  Ouabain at a concentration of 100 mumol/l induced an increase in cytosolic free Na+ concentration within 5 min, followed by increases in resting cytosolic free Ca2+ concentration and intracellular Ca2+ store. These three parameters were increased in a time-dependent manner significantly above the timed control over a period of 60 min. Pre-incubation of platelets with 100 mumol/l ouabain for 30 min significantly enhanced the cytosolic free Ca2+ response to thrombin and arginine vasopressin in the absence of extracellular Ca2+. The decrease from peak cytosolic free Ca2+ concentration in response to ionomycin in the absence of extracellular Ca2+ was significantly slower in low-Na+ buffer than in standard buffer. In addition, 5 mumol/ionomycin increased the cytosolic free Na+ concentration markedly in the presence of 0.1 mmol/l extracellular Ca2+, but the rise in cytosolic free Na+ concentration was suppressed by 2 mmol/l Ni2+ (NiCl2) or by removal of extracellular Ca2+.
  
  
  CONCLUSIONS
  These results suggest that Na(+)-Ca2+ exchange is important in extruding Ca2+ from the cytosol in human platelets, and inhibition of this exchange leads to the accumulation of intracellular Ca2+ store, which may be responsible for the enhanced responsiveness of cytosolic free Ca2+ to agonists.
authors.affiliation: First Department of Internal Medicine, Hiroshima University School of Medicine, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Ishida
authors.forename: H
authors.initials: H
authors.lastname: Matsuura
authors.forename: M
authors.initials: M
authors.lastname: Ishida-Kainouchi
authors.forename: R
authors.initials: R
authors.lastname: Ozono
authors.forename: M
authors.initials: M
authors.lastname: Watanabe
authors.forename: G
authors.initials: G
authors.lastname: Kajiyama
authors.forename: T
authors.initials: T
authors.lastname: Oshima
mesh_terms: D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000097:blood*; D002352:Carrier Proteins / Q000097:blood*; D003600:Cytosol / Q000378:metabolism; D005110:Extracellular Space / Q000378:metabolism; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D009994:Osmolar Concentration; D010042:Ouabain / Q000494:pharmacology; D012146:Rest; D012964:Sodium / Q000097:blood; D019831:Sodium-Calcium Exchanger
chemical_list: D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D010042:Ouabain; D012964:Sodium; D002118:Calcium