Dynamic recordings of ΔpH_m during cell activation. Simultaneous recordings of pH_mito (black trace, mito-SypHer) and pH_cyto (red trace, SNARF) in HeLa cells repeatedly stimulated with 100 µM histamine to elicit Ca²⁺ elevations. ΔpH_m can be calculated online as pH_mito − pH_cyto (green trace). Both pH_cyto and pH_mito decrease during Ca²⁺ elevations as large quantities of cytosolic acid are generated by the activity of plasma membrane Ca²⁺ pumps. The larger decrease in pH_mito reflects the lower H⁺-buffering capacity of mitochondria and causes ΔpH_m to collapse at the peak of the Ca²⁺ elevations. pH_mito and ΔpH_mito increased upon histamine removal, reflecting the Ca²⁺-dependent activation of matrix dehydrogenases. See also Poburko et al. (2011).