Asymmetric effects of divalent cations and protons on active Ca2+ efflux and Ca2+-ATPase in intact red blood cells

The Journal of Membrane Biology
Y H Xu, B D Roufogalis

Abstract

The influence of the asymmetric addition of various divalent cations and protons on the properties of active Ca2+ transport have been examined in intact human red blood cells. Active Ca2+ efflux was determined from the initial rate of 45Ca2+ loss after CoCl2 was added to block Ca2+ loading via the ionophore A23187. Ca2+-ATPase activity was measured as phosphate production over 5 min in cells equilibrated with EGTA-buffered free Ca2+ in the presence of A23187. The apparent Ca affinity of active Ca2+ efflux (K0.5 = 30-40 mumol/liter cells) was significantly lower than that measured by the Ca2+-ATPase assay (K0.5 = 0.4 microM). Possible reasons for this apparent difference are considered. Both active Ca2+ efflux and Ca2+-ATPase activity were reduced to less than 5% of maximal levels (20 mmol/liter cells.hr) in Mg2+-depleted cells, and completely restored by reintroduction of intracellular Mg2+. Active Ca2+ efflux was inhibited almost completely by raising external CaCl2 (but not MgCl2) to 20 mM, probably by interaction of Ca2+ at the externally oriented E2P conformation of the pump. Cd2+ was more potent than Ca2+ in this inhibition, while Mn2+ was less potent and 10 mM Ba2+ was without effect. A Ca2+: proton exchange mechanism for...Continue Reading

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Citations

Aug 1, 1991·Journal of Cellular Biochemistry·Y H XuB D Roufogalis
Oct 3, 2002·The Journal of General Physiology·M A Milanick, Krista L Arnett
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Related Concepts

Ca(2+)-Transporting ATPase
Calcium
Cations, Divalent
Erythrocytes
Hydrogen-Ion Concentration
Magnesium
Resting Potentials
Protons

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