INTEGRATING CYTOSOLIC CALCIUM SIGNALS INTO MITOCHONDRIAL METABOLIC RESPONSES

Stimulation of hepatocytes with vasopressin evokes increases in cytoso lic free Ca2+ ([Ca2+](c)) that are relayed into the mitochondria, wher e the resulting mitochondrial Ca2+ ([Ca2+](m)) increase regulates intr amitochondrial Ca2+-sensitive targets. To understand how mitochondria integrate the [Ca2+](c) signals into a final metabolic response, we st imulated hepatocytes with high vasopressin doses that generate a susta ined 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) measu red in parallel. The vasopressin-induced [Ca2+](m) spike evoked a tran sient increase in NAD(P)H that persisted longer than the [Ca2+](m) inc rease. In contrast, PDH activity increased biphasically, with an initi al rapid phase accompanying the rise in [Ca2+](m), followed by a susta ined secondary activation phase associated with a decline in cellular ATP. The decline of NAD(P)H in the face of elevated PDH activity occur red as a result of respiratory chain activation, which was also manife st in a calcium-dependent increase in the membrane potential and pH gr adient 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 transducti on from cytosol to mitochondria, with a single [Ca2+](m) spike evoking a complex series of changes to activate mitochondrial oxidative metab olism.