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

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 mum) at different (0-3 mumol l(-1)) free Ca2+ concentrations in the medium. In all these preparations, the apparent K., of mitochondrial respiration for exogenous ADP at free Ca2+ concentrations of 0-0.1 mumol l(-1) was very high, in the range of 250-350 mumol l(-1), in contrast to isolated mitochondria in vitro (apparent K. for ADP is approximately 20 mumol l(-1)). An increase in the free Ca2+ concentration (up to 3 mumol l(-1), which is within physiological range), resulted in a very significant decrease of the apparent K-m value to 20-30 mumol l(-1), a decrease of V-max 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 K-m for ADP (300-350 mumol l(-1)) nor V-max 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.