Linear relation between time constant of O2 uptake kinetics and total creatine in vitro

In 1988, Meyer (Am J Physiol 254:C548, 1988) proposed an electrical analog model of oxidative phosphorylation in which the cellular total creatine pool (TCr = PCr + Cr) acts as a metabolic capacitor. In this model the time constant (τ) for O2 uptake (Jo) is given by: τ = RC, where “R” is the mitochondrial resistance to energy transfer and “C” is the metabolic capacitance (C). The purpose of this study was to evaluate O2 uptake kinetics polarographically using a novel in vitro system. Isolated rat skeletal muscle mitochondria (n=7 separate preparations), 0.15 mg protein, were suspended in 2.0 ml KCl-based medium containing 5.0 mM ATP, varying TCr pools (0.1 to 1.5 mM), 150 U creatine kinase, and an ATP-splitting system of glucose + hexokinase (HK). Pyruvate (1 mM) and malate (1 mM) were added as oxidative substrates. Jo at steady state (Jo(ss))was determined from HK additions in the absence of a creatine pool. In the presence of various TCr levels, Jo(t) was measured across time after HK addition at one of two levels (40% and 60% of state 3 Jo). τwas evaluated according to Jo(t) = Jo(ss)(1-e−/τ). At TCr levels(mM) of 0.1, 0.2, 0.3, 0.75, and 1.5 the corresponding τ values (sec, means ± SE) were, respectively, 32.6 ± 3.0, 44.2 ± 2.2, 69.9 ± 4.3, 227.3 ± 22.2, and 347.6 ± 25.9. Thus, τ increased linearly with TCr (R2 = 0.945 for all, not mean, data). Moreover, τ was not influenced by ATP turnover rate; at any given TCr? values were not different at 40% andτ60% state 3 Jo. These in vitro results are consistent with the concept that TCr acts as a metabolic capacitance, as proposed in the Meyer electrical analog model. Supported by NSF IBN-0116997

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