PHYSIOLOGICAL CYTOSOLIC CA2+ TRANSIENTS EVOKE CONCURRENT MITOCHONDRIAL DEPOLARIZATIONS

Calcium, a ubiquitous second messenger, stimulates the activity of sev eral mitochondrial dehydrogenases. This has led to the suggestion that the same messenger that signals cell activation could also activate m itochondrial electron/proton transport, thereby meeting demands for in creased cellular energy. To test this in live cells, quantitative thre e-dimensional microscopy and ratio imaging were used to measure membra ne potential of individual mitochondria and cytosolic calcium distribu tion. Mitochondria reversibly depolarized as cytosolic calcium rose an d then fell following physiological stimulation. Thus, the dominant re sponse of the mitochondrion to a rise in cytosolic [Ca2+] is to draw o n the electrochemical potential, possibly to accelerate processes dire ctly involved in ATP synthesis and calcium homeostasis.