Intramitochondrial adenine nucleotides and energy-linked functions of heart mitochondria

Isolated rabbit heart mitochondria were incubated with varying amounts of inorganic pyrophosphate in 250 mM sucrose to specifically decrease the pool size of endogenous adenine nucleotides. The endogenous adenine nucleotide content decreased by as much as 80% as a result of this treatment. Phosphorylating respiration (state 3) declined from about 340 to 180 nAtoms O.min^-1.protein^-1 over the full range of intramitochondrial adenine nucleotides measured (approx 7.5-1.5 nmol/mg protein). Uncoupled and nonphosphorylating (state 4) rates of respiration were not greatly affected by adenine nucleotide depletion. Respiratory activity of the adenine nucleotide-depleted mitochondria was enhanced by addition of exogenous adenosine 5'-triphosphate (ATP). Partial depletion (approx 40%) of the intramitochondrial adenine nucleotides resulted in an impaired ability of heart mitochondria to retain Ca²⁺. Premature Ca²⁺ efflux was associated with organelle swelling and altered energy coupling. Exogenous ATP or adenosine 5'-diphosphate (ADP) added prior to Ca²⁺ efflux restored Ca²⁺ retention in these mitochondria. Atractyloside inhibited the restoration of Ca²⁺ retention. This study indicates a significant role for endogenous adenine nucleotides in maintaining oxidative phosphorylation and Ca²⁺ transport in heart mitochondria. The results are discussed with regard to significance in ischemic heart damage.