ROLE OF EXTERNAL NA-EVOKED CYTOSOLIC CA2+ RESPONSE IN HUMAN PLATELETS( AND CYTOSOLIC PH IN AGONIST)

The role of external Na+ in agonist-evoked platelet Ca2+ response is p oorly understood. This was explored in this study. Removal of external Na+ decreased both cytosolic Ca2+ mobilization and external Ca2+ entr y, 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 bind ing to the high-affinity binding sites in Na+-free medium, and 3) atte nuation of thrombin-induced inositol 1,4,5-trisphosphate production in Na+-free medium. Moreover, Ca2+ response to the thrombin receptor 6-a mino acid peptide was independent of external Na+. The role of externa l Na+ in modifying agonist-evoked Ca2+ response through activation of Na+/H+ antiport and cytosolic alkalinization was then explored. Cytoso lic alkalinization by monensin or NH4Cl enhanced thrombin, ADP, and th imerosal-induced external Ca2+ entry. Thimerosal-induced acceleration of external Ca2+ entry was diminished by the inhibition of Na+/H+ anti port. Thus external Na+ enhances thrombin activities, and cytosolic pH mediates store-regulated external Ca2+ entry. However, Na+/H+ antipor t activation is not essential for agonist-evoked Ca2+ mobilization and external Ca2+ entry.