Linear relation between time constant of oxygen uptake kinetics, total creatine, and mitochondrial content in vitro

Following the onset of moderate aerobic exercise, the rate of oxygen consumption (J_o) rises monoexponentially toward the new steady state with a time constant (τ) in the vicinity of 30 s. The mechanisms underlying this delay have been studied over several decades. Meyer's electrical analog model proposed the concept that the τ is given by τ = R _m·C, where R_m is mitochondrial resistance to energy transfer, and C is metabolic capacitance, determined primarily by the cellular total creatine pool (TCr = phosphocreatine + creatine). The purpose of this study was to evaluate in vitro the J_o kinetics of isolated rat skeletal muscle mitochondria at various levels of TCr and mitochondrial protein. Mitochondria were incubated in a medium containing 5.0 mM ATP, TCr pools of 0-1.5 mM, excess creatine kinase, and an ATP-splitting system of glucose + hexokinase (HK). Pyruvate and malate (1 mM each) were present as oxidative substrates. J_o was measured across time after HK was added to elicit one of two levels of J_o (40 and 60% of state 3). At TCr levels (in mM) of 0.1, 0.2, 0.3, 0.75, and 1.5, the corresponding τ values (s, means ± SE) were 22.2 ± 3.0, 36.3 ± 2.2, 65.7 ± 4.3, 168.1 ± 22.2, and 287.3 ± 25.9. Thus τ increased linearly with TCr (R₂ = 0.916). Furthermore, the experimentally observed τ varied linearly and inversely with the mitochondrial protein added. These in vitro results consistently conform to the predictions of Meyer's electrical analog model.