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Rat liver mitochondria essentially free of endogenous Ca 2÷ show low initial rate of energy-dependent Ca 2+ uptake. Preincubation of mitochondria under de-energized conditions in the presence of small amounts of external Ca 2+ results in a 8-10-fold time-dependent increase of energy-dependent Ca 2+ uptake. 2+ Ca -dependent activation of the Ca2+-transporting system follows first-order kinetics (tu2 ~" 1 min in the presence of 5IxM Ca 2+ at 20°C). Ca2+-activated mitochondria demonstrate a simple hyperbolic initial rate-Ca 2+ concentration dependence, whereas strong apparent cooperativity is observed in the velocity-substrate curves for Ca2÷-depleted mitochondria. It is concluded that apparent cooperativity of the energy-dependent Ca 2+ uptake is due to slow (as compared with the 'turnover number') activation of a Ca2+-specific uniporter which is inactive in the absence of external Ca 2+.

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