Volume 4, Number 4, pp. 201-207
Calmidazolium causes mitochondrial uncoupling by futile cycling: membrane potential-driven uptake and efflux as an electroneutral chloride complex
N.-E.L. Saris1, V.V. Teplova1,2 and P.A. Grigoriev1,3
1
Department of Applied Chemistry and Microbiology, Viikki Biocentre 1, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland
2
Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia
3
Institute of Cell Biophysics, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia
The use of calmidazolium (CMZ) as a calmodulin antagonist in mitochondrial studies has been discouraged because of its unspecific effects. We found it to stimulate State 2 respiration in rat liver mitochondria respiring on glutamate + malate, but not on succinate. State 3 respiration was strongly inhibited with both substrates. CMZ inhibited the reduction of pyridine nucleotides oxidized in State 3. These effects were chloride-specific and indepenÂÂdent of the monovalent cation present. With nitrate, some time-dependent inhibition of ADP and uncoupler-stimulated respiration was seen at higher CMZ concentrations. These data are interpreted as promotion by lowering the membrane potential of the mitochondrial permeability transition, leading to loss of nucleotides (in the presence of Ca2+), but there is also a direct effect on oxidative phosphorylation (in the presence of EGTA). It was found by black lipid membrane studies that CMZ penetrates phospholipid membranes as a dimer with two positive charges. It promotes chloride permeation as an electroneutral complex, but not nitrate when that is the medium anion. Calmidazolium thus acts as a chloride ionophore. The uncoupling is suggested to be due to cycling of calmidazolium through influx as a cation, driven by the membrane potential, and efflux as an electroneutral chloride complex.
Keywords: calcium, calmidazolium, chloride, ionophore, mitochondria, oxidative phosphorylation, respiration, uncoupling