Energy-dependent contraction of swollen heart mitochondria.

Isolated beef heart mitochondria, swollen in 100 mM Nat or K+ nitrate, contract and extrude the accumulated salt in a respiration-dependent reaction. The reaction is more rapid and extensive in Na+ than in K+ nitrate. Contraction is inhibited by uncouplers and by reagents, such as valinomytin and gramicidin, which increase transmembrane permeability to cations. The rate and extent of contraction are strongly dependent on pH, with lower rates and decreased efficiency under the alkaline conditions which favor high electrophoretic permeability to cations. The contraction reaction is characterized by high rates of respiration which return to controlled values when ion extrusion is complete. The rate of contraction is directly proportional to the rate of respiration at neutral pH. Estimates of the efficiency of contraction show that, depending on the pH, anywhere from 2 to 6 Na+ ions may be extruded per succinate oxidized. Both the rate and the efficiency of contraction are increased by the addition of nigericin at pH values above 7.2 in Na+ nitrate and an even greater stimulation of the reaction in K+ is produced by addition of nigericin when the pH is above 7.0. Contraction is inhibited as extramitochondrial Na+ or K+ is increased and, at unfavorable salt concentration, the reaction is stimulated by decreased pH. Less net H+ extrusion is detected by a glass electrode during contraction than when respiration is initiated in unswollen mitochondria. Contraction is also activated by mersalyl at concentrations just over that required to abolish phosphate transport. The results are consistent with the suggestion that respiration-dependent contraction is an osmotic response to the extrusion of accumulated cations on an endogenous cation/H+ exchanger.