Monovalent cation stimulation of Ca2+ uptake by cardiac membrane vesicles

Jones, L.R.; Besch, H.R.; Watanabe, A.M.

Journal of Biological Chemistry 252(10): 3315-3323


ISSN/ISBN: 0021-9258
PMID: 140870
Accession: 068521854

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Monovalent cations increased Ca2+ uptake as well as Ca2+-ATPase activities of a cardiac membrane vesicle fraction. The velocity of Ca2+ uptake was stimulated by chloride salts of the monovalent cations with the following order of effectiveness: K+ > Na+ .gtoreq. NH4+ > Rb+ > Cs+ > Li+ .simeq. Mg2+ (alone) > choline+. Half-maximal stimulation occurred at 10-20 mM added monovalent cations and apparent saturation of stimulation occurred at concentrations of 50-100 mM. The initial velocity of Ca2+ uptake was increased 3- to 5-fold by K+ ions. The order of effectiveness of the monovalent cations for stimulation of the Ca2+-ATPase activity of the preparation was similar to that for the stimulation of Ca2+ uptake. Ca2+ uptake by the cardiac membrane vesicles was typical of that described by others for sarcoplasmic reticulum vesicles, being insensitive to ouabain or sodium azide and completely prevented by the calcium ionophore A23187. The ionophore alamethicin prevented Ca2+ uptake while stimulating the Ca2+-ATPase activities of membrane vesicles 65% above control in the presence of Mg2+ ions alone, as well as in the presence of all monovalent cations tested. The evidence suggests that alamethicin, like A23187, exerts its effects by increasing the Ca2+ permeability of the membrane vesicles. Ca2+ uptake measured in the presence of K+ ions was maximal after 10 min of incubation whereas Ca2+-ATPase activity remained linear with respect to time for at least 20 min of incubation. This suggests that considerable Ca2+ exchange is occurring in membrane vesicles after net Ca2+ uptake has ceased. The same population of membrane vesicles which exhibited stimulation of Ca2+ uptake by monovalent cations also contained considerable latent ouabain-sensitive Na+- plus K+-ATPase (Na and K ion-stimulated ATPase) activity. Mild treatment of the membrane vesicle preparation with sodium dodecyl sulfate (0.30 mg/ml) allowed expression of the latent ouabain-sensitive Na+- plus K+-ATPase activity while the Ca2+-ATPase activity and Ca2+-accumulating capacity of the preparation was destroyed. The results provide direct evidence that the rate of Ca2+ transport by intact membrane vesicles is controlled by monovalent cations. Because K+ is the most active monovalent cation capable of regulating the rate, and because the maximal rate depends on the Ca2+ concentrations, the transport enzyme was designated as a K+, Ca2+-ATPase.