Relationships between Ca2+ release, Ca2+ cycling, and Ca2+-mediated permeability changes in mitochondria.

Ruthenium red and/or EGTA prevent cyclic uptake and release of Ca2+ in mitochondria. These compounds inhibit but do not prevent the swelling of liver mitochondria induced by Ca2+ plus t-butyl hydroperoxide or Ca2+ plus N-ethylmaleimide. Ruthenium red and/or EGTA have complex effects on the release rate of Ca2+ and other cations induced by t-butyl hydroperoxide or N-ethylmaleimide. To determine the relationship between permeability changes and Ca2+ release in the absence of Ca2+ cycling, a novel method of data collection and analysis is developed which allows the relative time courses of Ca2+ release and Mg2+ release or swelling to be accurately and quantitatively compared. This method eliminates errors in time course comparisons which arise from the aging of mitochondrial preparations and allows data from different preparations to be directly contrasted. Using the method, it is shown that permeability changes caused by Ca2+-releasing agents are not secondary effects arising from Ca2+ cycling between uptake and release carriers. In the absence of Ca2+-cycling inhibitors, Ca2+ release induced by t-butyl hydroperoxide or N-ethylmaleimide is, in part, carrier-mediated. In the presence of EGTA and ruthenium red, Ca2+ release induced by either agent is mediated solely by the permeability pathway. No differences are apparent in the solute selectivity of the inner membrane permeability defect induced by Ca2+ plus t-butyl hydroperoxide or Ca2+ plus N-ethylmaleimide. A novel type of Ca2+ release from energized liver mitochondria is reported. This release is induced by EGTA, occurs in the absence of other releasing agents or nonspecific permeability changes, and is rapid (greater than or equal to 50 nmol/min/mg protein).