Compartmentation of Adenine Nucleotides in the Isolated Working Guinea Pig Heart Stimulated by Noradrenaline

Abstract

Relationships between subcellular adenine nucleotides (ATP, ADP), heart function and oxidative myocardial metabolism were studied in the isolated working guinea pig heart. The heart preparations were stimulated by noradrenaline [norepinephrine] and utilized pyruvate alone or in combination with glucose as energy-providing substrates. Using density gradient centrifugation of lyophilized myocardial homogenates in nonaqueous media the following subcellular distribution of ATP and ADP, respectively, was obtained: The concentration of ATP in the cytosol was higher than in the mitochondria, while the content of ADP was not different. The overall ATP/ADP ratio in the cytosol was more than 10 times lower than the concentration ratio of free ATP and ADP in the cytosol as derived from the cytosolic creatine kinase equilibrium. The mitochondrial ATP/ADP ratio was much lower than the free cytosolic ATP/ADP ratio. The concentration term of the phosphorylation potential of ATP was higher in the cytosol than in the mitochondria. Myocardial function and substrate oxidation exhibited typical augmentations during infusion of 0.08 .mu.M noradrenaline. Increased heart performance and oxidative myocardial metabolism were not associated with major changes in the cytosolic ATP or ADP contents. The free ATP/ADP ratio and particularly the phosphorylation state of ATP, i.e., the ratio [ATP]/[ADP] .times. [Pi], were decreased in the cytosol. In the mitochondria, adenine-nucleotide concentration ratios were not substantially changed under the same conditions. The results are compatible with an asymmetrical translocation of adenine nucleotides across the mitochondrial membrane in working hearts. The reciprocal relationship between rates of oxidative metabolism and free cytosolic ATP/ADP ratio indicates that mitochondrial respiration in the intact heart could be controlled by the phosphorylation state of the extramitochondrial ATP.