Role of the creatine/phosphocreatine system in the regulation of mitochondrial respiration

Abstract

The mechanism of metabolic regulation of mitochondrial respiration in cardiac muscle cells was studied experimentally in the permeabilized heart fibres of mice and by computer modelling in silico. The experiments showed that the rate of mitochondrial respiration could be controlled by local production of ADP by mitochondrial creatine kinase in the intermembrane space of mitochondria. The spatially inhomogenous reaction-diffusion model of compartmentalized energy transfer was used to analyse which metabolite level in cytoplasm may be important for regulation of respiration. At low and moderate workloads, up to VO2 equal to 70 micromol min-1 g-1 dry weight, the only factor to which respiration responded was inorganic phosphate. At the values of VO2 higher than 70 micromol min-1 g-1 dry weight, the respiration rate responded mostly to changes in creatine, phosphocreatine and then time-averaged (over the contractile cycle) ADP concentrations in the cytoplasm. These results are taken to show that under conditions of moderate workloads, creatine kinase activity at given physiological creatine and phosphocreatine concentrations (apparent maximal activity achievable under these conditions) is in excess to oxidative phosphorylation rate, which is controlled by Pi concentration changes starting from very low values of the latter. At higher workloads mi-CK should be upregulated by increasing creatine and decreasing phosphocreatine concentrations, and only at very high workloads the ADP diffusion flux should be increased to upregulate oxidative phosphorylation. Thus, on the basis of the study in silico of compartmentalized energy transfer by phophocreatine/creatine system, the authors conclude that there exist multiple parallel regulatory factors controlling the rate of oxygen consumption in dependence of the workload. If creatine kinase is inhibited (and there is no myokinase activity), respiration requires high diffusive flux of ADP back into mitochondria, which is the sole regulator of respiration. This needs, however, increased ADP concentrations in the cytoplasm, which in turn result in inhibition of contraction.

A Biophysical Model of the Mitochondrial Respiratory System and Oxidative Phosphorylation

“…For example, the widely used model of Korzeniewski and Zoladz [4][5][6] invokes an empirical linear relationship between the difference in pH across the mitochondrial inner membrane (matrix pH minus cytosol pH) and the magnitude of inner membrane potential. While the Korzeniewski model has been validated and verified based on a number of studies and is widely applied [7][8][9], the central empirical relationship between electrostatic potential and pH difference is not expected to apply under all conditions. For example, in the extensive study of isolated cardiac mitochondria published by Bose et al [10], this relationship is not obeyed.…”

Oral creatine monohydrate supplementation improves brain performance: a double–blind, placebo–controlled, cross–over trial

“…This latter study also showed reduced activation-stimulated oxyhaemoglobin delivery to the activated area following creatine supplementation. This suggests that creatine supplementation is acting to smooth fluctuations in the blood oxygen level dependent response curve which results from brain activation (Gjedde et al 1999;Madsen et al 1999), possibly by altering rates of ATP synthesis in the mitochondrion through the mitochondrial creatine kinase-adenine nucleotide translocase-porin complex (Wallimann et al 1992;Saks et al 2000). Most recently, deletion of cytosolic brain-type creatine kinase in mice was shown to result in slower learning of a spatial task and diminished open-field habituation as well as increased intra-and infra-pyramidal hippocampal mossy fibre area suggesting that the creatine-creatine kinase network is also involved in brain plasticity in addition to metabolism ( Jost et al 2002).…”

Oxidative ATP synthesis in skeletal muscle is controlled by substrate feedback

“…The mechanism of control of oxidative phosphorylation is through feedback of substrates for ATP synthesis. No additional regulatory mechanisms, such as feed-forward control of certain enzymes via cytosolic calcium levels (9) or functional coupling between mitochondrial creatine kinase and ANT (23,26,27), are necessary to explain the majority of the observed data. In addition, the impact of specific protein deficiencies on the relationship between oxidative phosphorylation and cytoplasmic ADP and P i is successfully explained by making the appropriate modifications to the mitochondrial enzymes altered in the diseases.…”

“…Although the central component of the model-mitochondrial oxidative phosphorylation-is based on a mitochondrial model previously developed to match data on isolated mitochondria from rat heart, the integrated model matches a rich set of data on in vivo phosphate compounds from human skeletal muscle in healthy and complex I deficient individuals. The model also produces reasonable predictions for the ANT deficient subject, although the data available for comparison are sparse.The analysis predicts that the rate of oxidative phosphorylation is primarily regulated through concentrations of the substrates for ATP synthesis (ADP and P i ), since no additional control mechanisms, such as feed-forward control of certain enzymes via cytosolic calcium levels (9) and functional coupling between mitochondrial creatine kinase and ANT (23,26,27) that have been proposed to operate in the heart, were incorporated into the model. The current analysis does not rule out the possibility that ancillary control mechanisms are active in skeletal muscle (16,28); however, it shows that major contributions of such mechanisms to the overall regulation of the mitochondrial ATP synthetic pathway are not necessary to explain the thrust of the observed data.…”