Mitochondrial creatine kinase activity and phosphate shuttling are acutely regulated by exercise in human skeletal muscle
Authors: Perry Christopher G. R. , Kane Daniel A. , Herbst Eric A. F. , Mukai Kazutaka , Lark Daniel S. , Wright David C. , Heigenhauser George J. F. , Neufer P. Darrell , Spriet Lawrence L. , Holloway Graham P. 
Published: August 2012

Key point
ATP transfer from mitochondria to the cytoplasm occurs mainly through phosphate transfer to creatine by mitochondrial creatine kinase (miCK) but also by transport and/or diffusion of ADP and ATP through specific mitochondrial transport protein complexes.
Determining the effect of exercise on phosphate shuttling may require contractile signals in situ and varying creatine concentrations to alter miCK activity.
Mitochondrial respiratory sensitivity to ADP was assessed in permeabilized muscle fibre bundles (PmFBs) before and after 2 h cycling exercise in human skeletal muscle.
In relaxed PmFBs, ADP sensitivity decreased post‐exercise when miCK phosphate shuttling was low (no creatine) with no change in net ADP sensitivity in the presence of creatine, whereas in contracting fibres post‐exercise ADP sensitivity was higher with creatine.
This shows miCK activity is increased post‐exercise, especially during contraction in PmFBs, and suggests exercise regulates phosphate shuttling, which would improve maintenance of energy homeostasis during contraction.
Abstract  Energy transfer between mitochondrial and cytosolic compartments is predominantly achieved by creatine‐dependent phosphate shuttling (PCr/Cr) involving mitochondrial creatine kinase (miCK). However, ADP/ATP diffusion through adenine nucleotide translocase (ANT) and voltage‐dependent anion carriers (VDACs) is also involved in this process. To determine if exercise alters the regulation of this system, ADP‐stimulated mitochondrial respiratory kinetics were assessed in permeabilized muscle fibre bundles (PmFBs) taken from biopsies before and after 2 h of cycling exercise (60%) in nine lean males. Concentrations of creatine (Cr) and phosphocreatine (PCr) as well as the contractile state of PmFBs were manipulated in situ. In the absence of contractile signals (relaxed PmFBs) and miCK activity (no Cr), post‐exercise respiratory sensitivity to ADP was reduced in situ (up to 126% higher apparent Km to ADP) suggesting inhibition of ADP/ATP diffusion between matrix and cytosolic compartments (possibly ANT and VDACs). However this effect was masked in the presence of saturating Cr (no effect of exercise on ADP sensitivity). Given that the role of ANT is thought to be independent of Cr, these findings suggest ADP/ATP, but not PCr/Cr, cycling through the outer mitochondrial membrane (VDACs) may be attenuated in resting muscle after exercise. In contrast, in contracted PmFBs, post‐exercise respiratory sensitivity to ADP increased with miCK activation (saturating Cr; 33% lower apparent Km to ADP), suggesting prior exercise increases miCK sensitivity in situ. These observations demonstrate that exercise increases miCK‐dependent respiratory sensitivity to ADP, promoting mitochondrial–cytosolic energy exchange via PCr/Cr cycling, possibly through VDACs. This effect may mask an underlying inhibition of Cr‐independent ADP/ATP diffusion. This enhanced regulation of miCK‐dependent phosphate shuttling may improve energy homeostasis through more efficient coupling of oxidative phosphorylation to perturbations in cellular energy charge during subsequent bouts of contraction.