Effect of two different mitchondrial targeted antioxidants, mitochondrial vitamin E (mtVitE) and MitoQ, on mitochondrial Ca2+ signaling in HeLa cells. HeLa cells were transfected with a mitochondrially targeted low-affinity aequorin (mtAeqmut) probe, treated with (A) 5 μmol/L MitoE2, (B) 10 μmol/L MitoQ10, or (C) 20 μmol/L CGP37157 for 10 min, and then stimulated with histamine 10 μmol/L. Agonist stimulation induced a rapid Ca2+ uptake into mitochondria and a consequent release, which is strongly inhibited in MitoE2- and CGP37157-pretreated cells. MitoQ10 has a milder effect. These and the following traces are representative of more than five independent experiments that gave similar results.