Brain mitochondria do not accumulate, but rather lose, Ca²⁺ in the presence of ruthenium red.

Fluorimetric measurement of extramitochondrial Ca²⁺ with Ca²⁺green. Brain mitochondria were incubated in EGTA-free medium with 10 mM glutamate and 2 mM malate. Additions: BM, 0.25 mg/ml brain mitochondria; RR, 250 nM ruthenium red (RR); Ca²⁺₁₀, 10 µM Ca²⁺_free, Insertion: Control experiment without RR demonstrating normal Ca²⁺ accumulation of brain mitochondria after repeated Ca²⁺ additions.