The role of calcium in the regulation of mitochondrial metabolism.

presence of rotenone and oligomycin (Scott & Nicholls, 1980). This combination of inhibitors should prevent any Ca’+ uptake into the matrices of the mitochondria. As is shown in Table 1, the pelleted Ca2+ was greatly diminished under these conditions, whereas the solubilized CaZ+ actually increases. This clearly indicates that the major proportion of the synaptosomal CaZ+ is located in a compartment that is responsive to the mitochondrial membrane potential, and can therefore be identified with the mitochondrial matrix. Abolition of the mitochondrial membrane potential of necessity inhibits oxidative phosphorylation. Thus it could be argued that the diminished pellet Ca2+ resulted from the inhibition of the ATP supply to a Ca2+dependent ATPase accumulating Ca2+ within digitonin-resistant vesicles. However glycolysis allows significant ATP concentrations to be maintained under these conditions (Scott & Nicholls, 1980), whereas the presence of oligomycin alone, which inhibits the mitochondrial ATP synthase without lowering the membrane potential of the internal mitochondria (Scott & Nicholls, 1980), does not diminish the Ca2+ in the pellet (Table 1). The conclusion is therefore that the mitochondrial matrix represents the major site of Ca2+ accumulation within isolated synaptosomes amounting to some 85nmol of Ca2+/mg of mitochondrial protein under these conditions. This is well within the capacity of brain mitochondria to accumulate Ca2+, but is sufficient to saturate the efRux pathway (Nicholls & Scott, 1980). It is therefore feasible that the mitochondria regulate cytosolic Ca2+ concentrations under these conditions. There is, however, one proviso to make: the total Ca2+ content of the synaptosome is still increasing at 16min (Fig. l), implying that the internal mitochondria are capable of decreasing the cytosolic free Ca2+ concentration sufficiently to impose a net inward Ca2+ flux across the plasma membrane. It remains to be established therefore whether this represents an approach towards a steady-state, or whether a proportion of synaptosomes with a defective plasma membrane Ca2+-eWux pathway contributes disproportionately to the observed Ca2+ uptake by the total synaptosomal population. Previous studies (Kendrick et al., 1977; Blaustein et al.. 1978; Rahamimoff & Abramovitz, 1978) emphasized the role of ATP-dependent Ca*+ accumulation by intra-synaptosomal reticular membranes. However, the present study demonstrates that the mitochondria, with their elaborate Ca2+-regulatory mechanism, play the predominant role within the cytosol of the isolated synaptosome.