Na(+)-Ca2+ exchange modulates Ca2+ handling of human platelets by altering intracellular Ca2+ store size.
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.
Ishida
T
T
First Department of Internal Medicine, Hiroshima University School of Medicine, Japan.
Matsuura
H
H
Ishida-Kainouchi
M
M
Ozono
R
R
Watanabe
M
M
Kajiyama
G
G
Oshima
T
T
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
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D010042:Ouabain; D012964:Sodium; D002118:Calcium