Comparison between sodium-hydrogen ion and lithium-hydrogen ion exchange in human platelets.
The membrane-coupled exchange of Li+ and/or Na+ for H+ was studied in human platelets measuring intracellular pH(pHi) with a fluorescent indicator. A Li(+)-containing medium restored the internal pH of preacidified platelets to their prior pHi control value. When Na+ was replaced by Li+, similar steady-state values were attained in this system, although it was transported more slowly. The Km and Vmax were both higher with Na+ than with Li+. Exchanges of Li+ or Na+ with H+ were both blocked by ethylisopropylamiloride (EIPA) achieving half-maximal inhibition at submicromolar concentrations. The efflux Li+ or Na+ and resuspended in a choline medium exhibited an influx of H+ sensitive to EIPA. Thrombin, an activator of Na+/H+ exchange, induced a rapid increase in platelet internal pH in the presence of exogenous Li+. Thus: (1) Li+ can be substituted for Na+ in both the forward and the reverse exchange reaction; (2) Li+, while having a higher affinity than Na+ for the external site of the membrane carrier, has a lower Km and (3) Li+ as well as Na+ exchange are activated by thrombin.
Gende
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Centro de Investigaciones Cardiovasculares, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, Argentina.
Cingolani
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HE
D000584:Amiloride / Q000031:analogs & derivatives; D017920:Antiporters / Q000378:metabolism; D001711:Biotransformation / Q000187:drug effects; D001792:Blood Platelets / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008094:Lithium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D013917:Thrombin / Q000494:pharmacology
D017920:Antiporters; D017923:Sodium-Hydrogen Exchangers; C045885:sodium-lithium countertransporter; D000584:Amiloride; D008094:Lithium; D012964:Sodium; D013917:Thrombin; C039614:ethylisopropylamiloride