Induction of Na+ / K+ exchange in swollen heart mitochondria

Heart mitochondria swollen passively in nitrate salts contract in a respiration-dependent reaction which can be attributed to an endogenous cation/H⁺ exchange component (or components). The rate of contraction increases with increased extent of passive swelling in both Na⁺ and K⁺ salts. Since nearly constant internal cation concentrations are maintained during osmotic swelling, this result suggests that both Na⁺/H⁺ and K⁺/H⁺ exchange is enhanced by increased matrix volume. Endogenous Mg²⁺ is also lost with increased matrix volume, and this observation, in conjunction with other evidence available in the literature, suggests that monovalent cation/H⁺ exchanges may be regulated by divalent cations. Passive exchange of Na⁺/K⁺,⁴²K⁺/K⁺, and²⁴Na⁺/Na⁺ can be readily demonstrated in mitochondria swollen in nitrate. All these exchanges are low or not detectable in unswollen control mitochondria, and it appears that they are manifestations of the activated cation/H⁺ component (or components) functioning in the absence of pH.