The modulation of rat brain Na(+)-Ca2+ exchange by K+.
The involvement of potassium ions in the Na(+)-Ca2+ exchange process was studied in rat brain synaptic plasma membrane (SPM) vesicles. Addition of equimolar [K+] to the intravesicular and the extravesicular medium led to a stimulation of the Na+ gradient-dependent Ca2+ influx; this stimulation was noticeable already at 0.5 mM and reached its maximum at 2 mM K+. The magnitude of the K+ stimulation was between 1.3-2.5-fold in different SPM preparations. K+ ions also stimulated the Na(+)-dependent Ca2+ efflux. K+ stimulation of Na(+)-Ca2+ exchange is of considerable specificity, since it is not mimicked by either Li+ or H+. The following lines of evidence suggest that K+ modulation of Na(+)-Ca2+ exchange involves the catalytic moiety of the transporter itself and not an unrelated K+ channel which modulates the membrane potential. 1) K+ stimulation of the transport process was conserved following reconstitution of the transporter into phospholipid-rich liposomes, an experimental condition which presumably separates the native membrane proteins among different vesicular structures. 2) K+ stimulation of Na+ gradient-dependent Ca2+ influx persists also when the build up of negative inside membrane potential is prevented by addition of carbonyl cyanide p-trifluoromethoxy phenylhydrazone which renders the membrane highly permeable to protons both in the native and the reconstituted preparation. 3) K+ stimulation of Na+ gradient-dependent Ca2+ influx is obtained also when tetraethylammonium chloride, 2,3-diaminopyridine and Cs+ are added to the Ca2+ uptake medium. Reconstituted SPM vesicles take up 86Rb+ in response to activation of Na+ gradient-dependent Ca2+ influx. The ratio of Ca2+ taken up by SPM vesicles in a Na+ gradient-dependent manner to the corresponding amounts of Rb+ taken up varies between 8-5 in different SPM preparations. If the stoichiometry of the process is 1 Rb+/1 Ca2+, then Rb+ cotransport is mediated by 10-20% of the transporters present in the preparation.
Dahan
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D
Department of Biochemistry, Hebrew University Hadassah Medical School, Jerusalem, Israel.
Spanier
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R
Rahamimoff
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H
D000818:Animals; D001921:Brain / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007474:Ion Exchange; D007700:Kinetics; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger; D013570:Synaptic Membranes / Q000378:metabolism; D014634:Valinomycin / Q000494:pharmacology
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D014634:Valinomycin; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D012964:Sodium; D012413:Rubidium; D011188:Potassium; D002118:Calcium