High-energy phosphate transfer in human muscle: diffusion of phosphocreatine

Abstract

The creatine kinase (CK) reaction is central to muscle energetics, buffering ATP levels during periods of intense activity via consumption of phosphocreatine (PCr). PCr is believed to serve as a spatial shuttle of high-energy phosphate between sites of energy production in the mitochondria and sites of energy utilization in the myofibrils via diffusion. Knowledge of the diffusion coefficient of PCr (D PCr) is thus critical for modeling and understanding energy transport in the myocyte, but DPCr has not been measured in humans. Using localized phosphorus magnetic resonance spectroscopy, we measured D PCr in the calf muscle of 11 adults as a function of direction and diffusion time. The results show that the diffusion of PCr is anisotropic, with significantly higher diffusion along the muscle fibers, and that the diffusion of PCr is restricted to a ϳ28-m pathlength assuming a cylindrical model, with an unbounded diffusion coefficient of ϳ0.69 ϫ 10 Ϫ3 mm 2 /s. This distance is comparable in size to the myofiber radius. On the basis of prior measures of CK reaction kinetics in human muscle, the expected diffusion distance of PCr during its half-life in the CK reaction is ϳ66 m. This distance is much greater than the average distances between mitochondria and myofibrils. Thus these first measurements of PCr diffusion in human muscle in vivo support the view that PCr diffusion is not a factor limiting high-energy phosphate transport between the mitochondria and the myofibrils in healthy resting myocytes. myocyte; energy metabolism; creatine kinase shuttle; human studies PHOSPHOCREATINE (PCr) serves as the main short-term energy reserve in muscle, heart, and brain. PCr generates ATP to fuel cellular processes, including ion transport and muscular contraction, via the creatine kinase (CK) reaction. The CK reaction reversibly transfers a phosphoryl group between PCr and ATP, with unphosphorylated Cr and ADP as the other reactants and k as the forward pseudo-first-order rate constant of the reaction: PCr ϩ ADP ϩ H ϩ ↔ k ATP ϩ Cr . It was long ago hypothesized that the CK reaction serves as an intracellular energy shuttle, facilitating the transfer of high-energy phosphate from sites of de novo ATP creation in the mitochondria to sites of energy utilization, such as the myofibrils (4, 29, 45). The energy transfer mechanism in this PCr shuttle hypothesis involves 1) the creation of PCr at the mitochondria from the reverse CK reaction, 2) the diffusion of PCr to the point of use (including possible recycling through CK in the cytosol), and 3) the regeneration of ATP from ADP and PCr via the forward CK reaction to supply the needed energy (Fig. 1). Provided that the rate of ATP production via CK is manyfold higher than that generated by oxidative phosphorylation, the CK reaction could serve as a temporal-spatial energy buffer, supplying ATP when and where it is needed during periods of acute demand and/or stress.Since PCr diffusion is central to the PCr shuttle hypothesis, knowledge of its diffusion coefficient (D PCr ) is ...

In vivo diffusion MRS investigation of non‐water molecules in biological tissues

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“…where σ s is the standard deviation of the noise in S 0 and S . Assuming that f is the fraction of the time for acquiring S 0 , ψ the SNR per TR of the S 0 experiment, and N the total number of excitations in the combined experiment (the S 0 experiment + the S experiment), the relative error of ADC measurement can be given by …”

Cell signaling and mitochondrial dynamics: Implications for neuronal function and neurodegenerative disease

“…In situations where, without localized mitochondrial ATP production, diffusion limited high energy phosphate carriers do not adequately supply ATP needs, synaptic function may degrade. In striated muscle, the diffusion distance over the half-life of phosphocreatine has been estimated at ~66 μm (Gabr et al, 2011). If this finding can be extended to neurons with processes on the order of millimeters in length, it may explain why disruption of the mitochondrial dynamics machinery manifests in neurons with long processes as neuropathies.…”

Proton diffusion tensor spectroscopy of metabolites in human muscle in vivo

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