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Am J Physiol Cell Physiol 278: C965-C972, 2000;
0363-6143/00 $5.00
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Vol. 278, Issue 5, C965-C972, May 2000

Probing the extracellular release site of the plasma membrane calcium pump

Wanyan Xu, Betty Jo Wilson, Lin Huang, Emma L. Parkinson, Brent J. F. Hill, and Mark A. Milanick

Department of Physiology and Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri 65212

The plasma membrane Ca2+ pump is known to mediate Ca2+/H+ exchange. Extracellular protons activated 45Ca2+ efflux from human red blood cells with a half-maximal inhibition constant of 2 nM when the intracellular pH was fixed. An increase in pH from 7.2 to 8.2 decreased the IC50 for extracellular Ca2+ from ~33 to ~6 mM. Changing the membrane potential by >54 mV had no effect on the IC50 for extracellular Ca2+. This argues against Ca2+ release through a high-field access channel. Extracellular Ni2+ inhibited Ca2+ efflux with an IC50 of 11 mM. Extracellular Cd2+ inhibited with an IC50 of 1.5 mM, >10 times better than Ca2+. The Cd2+ IC50 also decreased when the pH was raised from 7.1 to 8.2, consistent with Ca2+, Cd2+, and H+ competing for the same site. The higher affinity for inhibition by Ni2+ and Cd2+ is consistent with a histidine or cysteine as part of the release site. The cysteine reagent 2-(trimethylammonium)ethyl methanethiosulfonate did not inhibit Ca2+ efflux. Our results are consistent with the notion that the release site contains a histidine.

calcium ion; red blood cell; calcium ion adenosinetriphosphatase; membrane potential


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