'Slow' K+-stimulated Ca2+ influx is mediated by Na+-Ca2+ exchange: a pharmacological study.
K+-stimulated 45Ca2+ influx was measured in rat brain presynaptic nerve terminals that were predepolarized in a K+-rich solution for 15 s prior to addition of 45Ca2+. This 'slow' Ca2+ influx was compared to influx stimulated by Na+ removal, presumably mediated by Na+-Ca2+ exchange. The K+-stimulated Ca2+ influx in predepolarized synaptosomes, and the Na+-removal-dependent Ca2+ influx were both saturating functions of the external Ca2+ concentration; and both were half-saturated at 0.3 mM Ca2+. Both were reduced about 50% by 20 microM Hg2+, 20 microM Cu2+ or 0.45 mM Mn2+. Neither the K+-stimulated nor the Na+-removal-dependent Ca2+ influx was inhibited by 1 microM Cd2+, La3+ or Pb2+, treatments that almost completely inhibited K+-stimulated Ca2+ influx in synaptosomes that were not predepolarized. The relative permeabilities of K+-stimulated Ca2+, Sr2+ or Ba2+ influx in predepolarized synaptosomes (10:3:1) and the corresponding selectivity ratio for Na+-removal-dependent divalent cation uptake (10:2:1) were similar. These results strongly suggest that the K+-stimulated 'slow' Ca2+ influx in predepolarized synaptosomes and the Na+-removal-dependent Ca2+ influx are mediated by a common mechanism, the Na+-Ca2+ exchanger.
Nachshen
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DA
Department of Physiology, Cornell University Medical College, New York, NY.
Kongsamut
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S
D000818:Animals; D001464:Barium / Q000378:metabolism; D001921:Brain / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D002352:Carrier Proteins / Q000378:metabolism; D002413:Cations, Divalent; D003300:Copper / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D008564:Membrane Potentials; D008628:Mercury / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger; D013324:Strontium / Q000378:metabolism; D013574:Synaptosomes / Q000378:metabolism*
D002132:Calcium Radioisotopes; D002352:Carrier Proteins; D002413:Cations, Divalent; D019831:Sodium-Calcium Exchanger; D001464:Barium; D008345:Manganese; D003300:Copper; D012964:Sodium; D008628:Mercury; D011188:Potassium; D002118:Calcium; D013324:Strontium