Alkalinization transients in single mitochondria. HeLa cells expressing mito-SypHer were recorded on a spinning disc confocal microscope at a frequency of 1.2 Hz. Ratio F480/F430 images from two cells exhibiting spontaneous alkalinization transients are shown, with warm colors denoting high ratio values. The pH_mito elevations occurred either in different regions of the mitochondrial network or repeatedly at the same location, but they always remained restricted to a particular mitochondrial cluster.