Do we know the absolute values of intracellular free calcium concentration?
More than 20 years ago, it was shown that the addition of EGTA increases the affinity of the plasma membrane Ca2+ pump for Ca2+ by an order of magnitude. The left-hand shift of Ca2+-dependencies in the presence of EGTA has been also documented in studies of the sarcoplasmic reticulum Ca2+ pump, mitochondrial Ca2+-transporter as well as Ca2+-binding by calmodulin and troponin C. These data allow us to hypothesise that this effect is caused by an admixture of di- and trivalent cations possessing high affinity for EGTA and interacting with Ca2+-transporting and binding proteins. Here, we propose that polyvalent cations affect the estimation of absolute values of free intracellular Ca2+ concentration. Indeed, EGTA sharply increases the apparent affinity of the fluorescent Ca2+ indicators quin-2 and fluo-3 for Ca2+. The impact of polyvalent cations on Ca2+ measurement was further confirmed by the study showing the high sensitivity of Ca2+-induced fluo-3 fluorescence to Mn2+, Fe2+, Cu2+, and Co2+ seen in the absence of EGTA.
Research Centre, Centre Hospitalier de l'Université de Montréal, Department of Medicine, Université de Montréal, 3850 rue St-Urbain, Montreal, Quebec, Canada H2W 1T7. sergei.n.orlov@umontreal.ca
Sergei N
SN
Orlov
Ryszard
R
Grygorczyk
Sergei V
SV
Kotelevtsev
D000634:Aminoquinolines; D002118:Calcium / Q000378:metabolism / Q000592:standards*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D005456:Fluorescent Dyes; D007202:Indicators and Reagents; D007424:Intracellular Fluid / Q000737:chemistry* / Q000378:metabolism; D012015:Reference Standards; D012519:Sarcoplasmic Reticulum / Q000201:enzymology / Q000378:metabolism; D019209:Troponin C / Q000378:metabolism
D000634:Aminoquinolines; D002147:Calmodulin; D002614:Chelating Agents; D005456:Fluorescent Dyes; D007202:Indicators and Reagents; D019209:Troponin C; D004533:Egtazic Acid; D000252:Calcium-Transporting ATPases; C035739:Quin2; D002118:Calcium