Ion transport by heart mitochondria. Retention and loss of energy coupling in aged heart mitochondria

The retention and loss of energy-coupling reactions in isolated beef heart mitochondria have been examined under anaerobic conditions using suspending media chosen to mimic the intracellular milieu. In long-term incubations at 37 °C, a loose coupling develops which can be controlled by adding serum albumin. This lesion closely resembles that produced by addition of free fatty acids which has been described in previous studies. Shorter incubation times produce an increased susceptibility to hydrogen peroxide which is characterized by elevated ATPase activity, increased permeability to monovalent cations, and increased proton ejection on transition from the anaerobic to the aerobic state. This peroxide sensitivity is prevented by chelators such as EGTA and appears to involve a time-dependent release of metal ions. Of the metabolites which are known to increase in concentration in the ischemic heart cell, Na⁺, P₁, lactate, and H⁺ all promote swelling of isolated heart mitochondria and contribute to a decline in energy coupling. The relationship of these results to the pathological deterioration of mitochondria in ischemic heart tissue is discussed.     

Ion transport by heart mitochondria: XXVII. The relation of mercurial-dependent adenosine triphosphatase activity to ion movements

The properties of a mercurial-dependent adenosine triphosphatase activity have been examined in isolated beef heart mitochondria. The reaction differs from that induced by uncouplers in that it is associated with extensive ion uptake and osmotic swelling, is highly specific for K⁺ over Na⁺, and is enhanced by respiration. Evidence is presented which suggests that the following events can account for the observations: (1) The mercurial blocks the phosphate transporter so that phosphate hydrolyzed from ATP is trapped in the matrix. (2) This interior negative potential causes cations to move inward and swelling results. (3) Permeability to K⁺ but not to Na⁺ is enhanced greatly by the reaction of the mercurial with the membrane. The inward movement of K⁺ closely resembles that produced by valinomycin, in that it is accompanied by proton ejection into the medium and it rapidly establishes a condition in which ion gradients cannot be maintained. This marked increase in permeability may be related to the pH gradient and is manifest as additional passive swelling in the absence of sucrose and passive contraction when sucrose is present. A comparison of the kinetics of swelling and of ATP hydrolysis shows that the elevated rates of ATPase are correlated with this condition of high permeability. When a corresponding condition of high permeability to Na⁺ is established by treatment with gramicidin or EDTA, the mercurial-dependent ATPase is nearly as rapid in Na⁺ as in the K⁺ medium. It appears, therefore, that the K⁺ specificity resides at the level of membrane permeability and is not a feature of the ATPase reaction per se. (4) Respiration appears to affect the ATPase reaction by virtue of its ability to extrude ions from the matrix in the presence of the mercurial. p-Chloromercuriphenyl sulfonate causes a switch from respiration-dependent ion accumulation to respiration-dependent ion extrusion to occur. A model to explain these reactions is presented.     

Ion transport by heart mitochondria. XXII. Spontaneous, energy-linked accumulation of acetate and phosphate salts of monovalent cations

The accumulation of monovalent cations by isolated beef heart mitochondria has been studied by evaluating the efficiency of energy-dependent osmotic swelling. Extensive osmotic swelling occurs spontaneously when isolated heart mitochondria are suspended in 0.1 m acetate or phosphate salts. The swelling and ion uptake depend on either respiration or the presence of exogenous ATP, and the initial rate of swelling is proportional to the initial rate of respiration or ATP hydrolysis, respectively. The efficiency of the reaction varies somewhat from preparation to preparation but approaches a limit of about 2 cations accumulated per pair of electrons traversing a phosphorylation site. All monovalent cations tested support the reaction, but the most efficient energy-dependent swelling occurs with K⁺. Weak acid anions are required for the ion accumulation and swelling and the reaction appears to depend on the amount of free acid available in the suspension. Permeant strong acid anions, such as NO₃⁻, fail to support the swelling reaction in the presence of energy. Valinomycin increases both the amount and the efficiency of ion uptake under these conditions. Mg²⁺ decreases both of these values whereas p-chloromercuriphenyl sulfonate increases both. These responses are discussed in terms of current models of mitochondrial ion transport.