The stimulation of calcium-dependent processes by chelating agents

Cell Calcium

5: 559, 1984

THE STIMULATION OF CALCIUM-DEPENDENT T.J.B.Simons,

Department

PROCESSES BY CHELATING AGENTS

of Physiology,

King's College,

Strand,

London

WC2R 2LS, United Kingdom A recent chelators

paper

EGTA,EDTA

sarcolemmal

111 demonstrated and CDTA

vesicles.

a stimulatory

on Na+-dependent

The Km for Ca2+ was

effect

of calcium

Ca2+ uptake

by cardiac

lowered

by the addition

of

chelators, while the Vmax was unchanged. The effect was seen by measuring free Ca2+ concentrations with an Orion Ca electrode and comparing 45Ca uptake

rates

chelators.

measured

One possible

electrode

gives

unbuffered

either

in the

explanation

absence

or

in the

for the effect

presence

of

may be that the Ca

an over-estimate of the free Ca2+ concentration in I have shown that the at low Ca 2t concentration.

solutions

Orion Ca electrode gives a reading corresponding to 2uM-Ca2+ in a solution containing

only 1 PM-Ca,

measured

electrode

was

buffered

solutions,

but only down to 5uM-Ca2+

Overestimation high value

linear

by atomic absorption spectrophotometry

[21. The

in log

[Ca2'l

of the free Ca2+ in unbuffered

for the Km of a Ca-dependent

down

to 0.1 pM-Ca2+

in unbuffered solutions

would

in

solutions. lead to a

process. This difficulty

would

probably not occur if Ca2+ was measured with an indicator dye. This may also account for another report of a stimulatory effect of EGTA on Ca transport not be explained

[31, but the stimulation

of Ca2+-ATPases

[4,53 can

in this way as it occurs when the FGTA concentration is

increased in buffered solutions at constant free Ca 2t . References 1. Trosper, T.L.,Phi1ipsonK.D. (1984) Stimulatory effect of calcium chelators on Nat-Ca2+ exchange in cardiac sarcolemmal vesicles. Cell Calcium 5, 211-222. 2. Simons,T.J.B. (1982) Amethod for estimating freecawithin human red blood cells, with an application to the study of their Ca-dependent K permeability. J. Membrane Biol. 66, 235-247. 3. Berman, M.C. (1982) Stimulation of calcium transport of sarcoplasmic reticulum vesicles by the calcium complex of ethylene glycol bis(r-aminoethylether) N,N,N',N'-tetraacetic acid.J.Biol.Chem.257,1953-1957. 4. Al-Jabore,A., Roufogalis, B.D. (1981) Influence of EGTA on the apparentCa2+ affinity of Mg2+-dependent, Ca2+-stimulated ATPase in the human erythrocyte membrane. B&him. Biophys. Acta 645, l-9. 5. Kotagal, N., Colca, J.R. &McDaniel, M.L. (1983) Activation of an islet cell 1 ATPase by calmodulin and Ca-FGTA. J. Biol. Chem. plasma membrane (Ca2++Mg2+258, 4808-4813. Received

31.7.84

559