The ATP4- receptor-operated ion channel of human lymphocytes: inhibition of ion fluxes by amiloride analogs and by extracellular sodium ions.
Extracellular ATP is known to increase the membrane permeability of a variety of cells. Addition of ATP to human leukemic lymphocytes loaded with the Ca2+ indicator, fura-2, induced a rise in cytosolic Ca2+ concentration which was attenuated or absent in NaCl media compared with KCl, choline Cl, or NMG Cl media. In contrast, anti-immunoglobulin antibody gave similar Ca2+ transients in NaCl and KCl media. A half-maximal inhibition of peak ATP-induced Ca2+ response was observed at 10-16 mM extracellular Na+. Basal 45Ca2+ influx into lymphocytes was stimulated 9.6-fold by ATP added to cells in KCl media, but the effect of ATP was greatly reduced for cells in NaCl media. Hexamethylene amiloride blocked 74% of the ATP-stimulated Ca45 uptake of cells in KCl media. Flow cytometry measurements of fluo-3-loaded cells confirmed that the ATP-induced rise in cytosolic Ca2+ was inhibited either by extracellular Na+ or by addition of hexamethylene amiloride. Extracellular ATP stimulated 86Rb efflux from lymphocytes 10-fold and this increment was inhibited by the amiloride analogs in a rank order of potency 5-(N-methyl-N-isobutyl)amiloride greater than 5-(N,N-hexamethylene)amiloride greater than 5-(N-ethyl-N-isopropyl)amiloride greater than amiloride. ATP-induced 86Rb efflux showed a sigmoid dependence on the concentration of ATP and Hill analysis gave K1/2 of 90 and 130 microM and n values of 2.5 and 2.5 for KCl and NaCl media, respectively. However, the maximal ATP-induced 86Rb efflux was 3-fold greater in KCl than in NaCl media. Raising extracellular Na+ from 10 to 100 mM increased ATP-induced Na+ influx from a mean of 2.0 to 3.7 nEq/10(7) cells/min, suggesting either saturability or self-inhibition by Na+ of its own influx. These data suggest that ATP opens a receptor-operated ion channel which allows increased Ca2+ and Na+ influx and Rb+ efflux and these fluxes are inhibited by extracellular Na+ ions as well as by the amiloride analogs.
Wiley
J S
JS
Haematology Department, Austin Hospital, Heidelberg, Victoria, Australia.
Chen
R
R
Wiley
M J
MJ
Jamieson
G P
GP
D000241:Adenosine / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000584:Amiloride / Q000031:analogs & derivatives* / Q000494:pharmacology*; D002118:Calcium / Q000097:blood*; D002463:Cell Membrane Permeability / Q000187:drug effects; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D015451:Leukemia, Lymphocytic, Chronic, B-Cell / Q000097:blood; D008214:Lymphocytes / Q000187:drug effects / Q000378:metabolism*; D012265:Ribonucleotides / Q000494:pharmacology; D012413:Rubidium / Q000097:blood; D012964:Sodium / Q000494:pharmacology*
D007473:Ion Channels; D012265:Ribonucleotides; D000584:Amiloride; D000255:Adenosine Triphosphate; D012964:Sodium; D000241:Adenosine; D012413:Rubidium; D002118:Calcium