Integrating cytosolic calcium signals into mitochondrial metabolic responses

Stimulation of hepatocytes with vasopressin evokes increases in cytosolic free Ca2+ ([Ca2+](c)) that are relayed into the mitochondria, where the resulting mitochondrial Ca2+ ([Ca2+](m)) increase regulates intramitochondrial Ca2+-sensitive targets. To understand how mitochondria integrate the [Ca2+](c) signals into a final metabolic response, we stimulated hepatocytes with high vasopressin doses that generate a sustained increase in [Ca2+](c). This elicited a synchronous, single spike of [Ca2+](m) and consequent NAD(P)H formation, which could be related to changes in the activity state of pyruvate dehydrogenase (PDH) measured in parallel. The vasopressin-induced [Ca2+](m) spike evoked a transient increase in NAD(P)H that persisted longer than the [Ca2+](m) increase. In contrast, PDH activity increased biphasically, with an initial rapid phase accompanying the rise in [Ca2+](m), followed by a sustained secondary activation phase associated with a decline in cellular ATP. The decline of NAD(P)H in the face of elevated PDH activity occurred as a result of respiratory chain activation, which was also manifest in a calcium-dependent increase in the membrane potential and pH gradient components of the proton motive force (PMF). This is the first direct demonstration that Ca2+-mobilizing hormones increase the PMF in intact cells, Thus, Ca2+ plays an important role in signal transduction from cytosol to mitochondria, with a single [Ca2+](m) spike evoking a complex series of changes to activate mitochondrial oxidative metabolism.