Role of external Na+ and cytosolic pH in agonist-evoked cytosolic Ca2+ response in human platelets.
The role of external Na+ in agonist-evoked platelet Ca2+ response is poorly understood. This was explored in this study. Removal of external Na+ decreased both cytosolic Ca2+ mobilization and external Ca2+ entry, induced by thrombin but not by ADP or vasopressin. That external Na+ regulates thrombin activities was demonstrated by 1) Na+ dependency of the amidolytic activity of thrombin, 2) inhibition of thrombin binding to the high-affinity binding sites in Na(+)-free medium, and 3) attenuation of thrombin-induced inositol 1,4,5-trisphosphate production in Na(+)-free medium. Moreover, Ca2+ response to the thrombin receptor 6-amino acid peptide was independent of external Na+. The role of external Na+ in modifying agonist-evoked Ca2+ response through activation of Na+/H+ antiport and cytosolic alkalinization was then explored. Cytosolic alkalinization by monensin or NH4Cl enhanced thrombin, ADP, and thimerosal-induced external Ca2+ entry. Thimerosal-induced acceleration of external Ca2+ entry was diminished by the inhibition of Na+/H+ antiport. Thus external Na+ enhances thrombin activities, and cytosolic pH mediates store-regulated external Ca2+ entry. However, Na+/H+ antiport activation is not essential for agonist-evoked Ca2+ mobilization and external Ca2+ entry.
Kimura
M
M
Hypertension Research Center, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark 07103-2714.
Nakamura
K
K
Fenton
J W
JW
Andersen
T T
TT
Reeves
J P
JP
Aviv
A
A
D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008345:Manganese / Q000378:metabolism; D012964:Sodium / Q000502:physiology*; D017923:Sodium-Hydrogen Exchangers / Q000502:physiology; D013917:Thrombin / Q000494:pharmacology*
D017923:Sodium-Hydrogen Exchangers; D008345:Manganese; D012964:Sodium; D013917:Thrombin; D002118:Calcium