Metabolic consequences of functional complexes of mitochondria,myofibrils and sarcoplasmic reticulum in muscle cells

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Abstract
SUMMARY: Regulation of mitochondrial respiration both by endogenous and exogenous ADP in the cells in situ was studied in isolated and permeabilized cardiomyocytes, permeabilized cardiac fibers and `ghost' fibers (all with a diameter of 10–20 μm) at different (0–3 μmoll-1)free Ca2+ concentrations in the medium. In all these preparations,the apparent Km of mitochondrial respiration for exogenous ADP at free Ca2+ concentrations of 0–0.1μmoll-1 was very high, in the range of 250–350μmoll-1, in contrast to isolated mitochondria in vitro(apparent Km for ADP is approximately 20μmoll-1). An increase in the free Ca2+ concentration(up to 3 μmoll-1, which is within physiological range), resulted in a very significant decrease of the apparent Km value to 20–30 μmoll-1, a decrease of Vmax of respiration in permeabilized intact fibers and a strong contraction of sarcomeres. In ghost cardiac fibers, from which myosin was extracted but mitochondria were intact, neither the high apparent Km for ADP (300–350 μmoll-1) nor Vmax of respiration changed in the range of free Ca2+ concentration studied, and no sarcomere contraction was observed. The exogenous-ADP-trapping system (pyruvate kinase + phosphoenolpyruvate) inhibited endogenous-ADP-supported respiration in permeabilized cells by no more than 40%, and this inhibition was reversed by creatine due to activation of mitochondrial creatine kinase. These results are taken to show strong structural associations (functional complexes) among mitochondria, sarcomeres and sarcoplasmic reticulum. Inside these complexes, mitochondrial functional state is controlled by channeling of ADP, mostly via energy- and phosphoryl-transfer networks, and apparently depends on the state of sarcomere structures.