Protein kinase C enhances Ca2+ mobilization in human platelets by activating Na+/H+ exchange.
Control of cytoplasmic pH (pHi) by a Na+/H+ antiport appears a general property of most eukaryotic cells. In human platelets activation of the Na+/H+ exchanger enhances Ca2+ mobilization and aggregation induced by low concentrations of thrombin (Siffert, W., and Akkerman, J. W. N. (1987) Nature 325, 456-458). Several observations indicate that the exchanger is regulated by protein kinase C. (i) Inhibitors of protein kinase C (trifluoperazine, sphingosine) inhibit the increase in pHi seen during thrombin stimulation as well as Ca2+ mobilization; artificially increasing pHi by monensin or NH4Cl then restores Ca2+ mobilization. (ii) Direct activation of protein kinase C by 1-oleoyl-2-acetylglycerol initiates an increase in pHi that depends on the presence of extracellular Na+ and is sensitive to inhibition by ethylisopropylamiloride. The pHi sensitivity of thrombin-induced Ca2+ mobilization is particularly evident in the range between pH 6.8 and 7.4 and at low thrombin concentrations, whereas thrombin concentrations of more than 0.2 unit/ml bypass the pH sensitivity. In the absence of thrombin an increase in pHi, either induced artificially (by addition of the ionophores nigericin or monensin) or via activation of protein kinase C (by addition of 1-oleoyl-2-acetylglycerol), does not induce Ca2+ mobilization. We conclude that activation of protein kinase C is essential for Ca2+ mobilization in platelets stimulated by low concentrations of thrombin and that protein kinase C exerts this effect via activation of the Na+/H+ exchanger.
Siffert
W
W
Institut für Physiologie, Ruhr-Universität, Bochum, Federal Republic of Germany.
Akkerman
J W
JW
D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000097:blood*; D002352:Carrier Proteins / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008970:Molecular Weight; D011493:Protein Kinase C / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers; D013110:Sphingosine / Q000494:pharmacology; D014268:Trifluoperazine / Q000494:pharmacology
D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D014268:Trifluoperazine; D011493:Protein Kinase C; D013110:Sphingosine; D002118:Calcium