A transient Ca
2+ release from preloaded mitochondria can be induced by a sudden decrease in the pH of the outer medium from 8.0 or 7.4 to 6.8. In the presence of inorganic phosphate the released Ca
2+ is not taken up again. Upon Ca
2+ addition to respiring mitochondria the mitochondrial membrane potential (Δ♀) decreases to a new resting level. A further decrease in Δ♀ occurs after the decrease in pH from 7.4 to 6.8, concomitant with the reuptake phase of the Ca
2+ release. Phosphate, EGTA, and ruthenium red restore Δ♀ to its initial level. If phosphate is present initially, only transient changes in Δ♀ occur upon addition of Ca
2+ or H
+ ions. Only a small transient change in Δ♀ upon H
+ ion addition is seen in the absence of accumulated Ca
2+. La
3+, a competitive inhibitor of Ca
2+ transport, prevents the H
+ ion-induced Ca
2+ efflux, whereas this is not the case in the presence of the noncompetitive inhibitor ruthenium red. Ruthenium red, however, prevents the reuptake phase. Mg
2+, an inhibitor of the surface binding of Ca
2+, has no or only a slight effect on the H
+ ion-induced Ca
2+ release. Mitochondria preloaded with Ca
2+ release a small fraction of Ca
2+ during the subsequent uptake of another pulse of Ca
2+. The results indicate that at least one pool of mitochondrial Ca
2+ exists in a mobile state. The possible existence of a
exchanger in the mitochondrial membrane is discussed.
相似文献