The reversible Ca2+-induced permeabilization of rat liver mitochondria

Rat liver mitochondria became permeabilized to sucrose according to an apparent first-order process after accumulating 35 nmol of Ca2+/mg of protein in the presence of 2.5 mM-Pi, but not in its absence. A fraction (24-32%) of the internal space remains sucrose-inaccessible. The rate constant for permeabilization to sucrose decreases slightly when the pH is decreased from 7.5 to 6.5, whereas the rate of inner-membrane potential (delta psi) dissipation is markedly increased, which indicates that H+ permeation precedes sucrose permeation. Permeabilization does not release mitochondrial proteins. 14CSucrose appears to enter permeabilized mitochondria instantaneously. Chelation of Ca2+ with EGTA restores delta psi and entraps sucrose in the matrix space. With 20 mM-sucrose at the instant of resealing, about 21 nmol of sucrose/mg of protein becomes entrapped. The amount of sucrose entrapped is proportional to the degree of permeabilization. Entrapped sucrose is not removed by dilution of the mitochondrial suspension. Resealed mitochondria washed three times retain about 74% of the entrapped sucrose. In the presence of Ruthenium Red and Ca2+ buffers permeabilized mitochondria reseal only partially with free Ca2+ greater than 3 microM. 14CSucrose enters partially resealed mitochondria continuously with time, despite maintenance of delta psi, in accordance with continued interconversion of permeable and impermeable forms. Kinetic analyses of 14Csucrose entry indicate two Ca2+-sensitive reactions in permeabilization. This conclusion is supported by the biphasic time courses of resealing and repolarization of permeabilized mitochondria and the acute dependence of the rapid repolarization on the free Ca2+. A hypothetical model of permeabilization and resealing is suggested and the potential of the procedure for matrix entrapment of substances is discussed.