Calcium transport and inner mitochondrial membrane damage in renal cortical mitochondria

Ca2+ uptake and efflux processes, as they are manifested during procedures used for isolation of renal cortical mitochondria, were characterized in order to provide a better basis for making inferences from isolated mitochondria about the in vivo state of mitochondrial Ca2+ homeostasis in both normal and injured tissues and to better define the mechanisms by which Ca2+ mediates injury to renal cortical mitochondria. Mitochondrial Ca2+ uptake predictably occurred when the capacity of the Ca2+ chelator added to the isolating medium to maintain free Ca2+ in the submicromolar range was exhausted unless ruthenium red was present to specifically inhibit the Ca2+ uniport. Ca2+ uptake during isolation ultimately led to loss of accumulated Ca2+ and intramitochondrial K+ as well as to deterioration of respiratory function. Extramitochondrial Ca2+ also evoked the latter two events in the absence of Ca2+ uptake but only at much higher medium Ca2+ levels than were required when Ca2+ uptake was allowed to occur. Studies using mitochondria loaded with known amounts of Ca2+ at 4 degrees C and then subjected to a reisolation procedure including all the steps of normal isolation demonstrated that phosphate markedly potentiated Ca2+-induced alterations of mitochondrial membrane permeability properties. Of several agents studied singly, fatty acid-free albumin was most effective in blocking Ca2+ + phosphate-induced alterations of mitochondrial membrane permeability. The protective effect of fatty acid-free albumin was further enhanced by combining it with Mg2+, dibucaine, or oligomycin + ADP. This study thus quantitatively defined conditions under which Ca2+ uptake can be expected to occur during mitochondrial isolation, demonstrated that the effects of this Ca2+ uptake on mitochondrial properties are similar to those previously elucidated in mitochondria studied at warmer temperatures, and defined methods suitable for blocking such Ca2+ movements and their deleterious effects.