Muscle performance depends on the supply of metabolic fuels and disposal of end-products. Using circulating metabolite concentrations to infer changes in fluxes is highly unreliable because the relationship between these parameters varies greatly with physiological state. Quantifying fuel kinetics directly is therefore crucial to the understanding of muscle metabolism. This review focuses on how carbohydrates, lipids and amino acids are provided to fish muscles during hypoxia and swimming. Both stresses force white muscle to produce lactate at higher rates than it can be processed by aerobic tissues. However, lactate accumulation is minimized because disposal is also strongly stimulated. Exogenous supply shows that trout have a much higher capacity to metabolize lactate than observed during hypoxia or intense swimming. The low density of monocarboxylate transporters and their lack of upregulation with exercise explain the phenomenon of white muscle lactate retention. This tissue operates as a quasi-closed system, where glycogen stores act as an 'energy spring' that alternates between explosive power release during swimming and slow recoil from lactate in situ during recovery. To cope with exogenous glucose, trout can completely suppress hepatic production and boost glucose disposal. Without these responses, glycemia would increase four times faster and reach dangerous levels. The capacity of salmonids for glucoregulation is therefore much better than presently described in the literature. Instead of albumin-bound fatty acids, fish use lipoproteins to shuttle energy from adipose tissue to working muscles during prolonged exercise. Proteins may play an important role in fueling muscle work in fish, but their exact contribution is yet to be established. The membrane pacemaker theory of metabolism accurately predicts general properties of muscle membranes such as unsaturation, but it does not explain allometric patterns of specific fatty acids. Investigations of metabolic fuel kinetics carried out in fish to date have demonstrated that these ectotherms use several unique strategies to orchestrate energy supply to working muscles and to survive hypoxia.
Protein functions result from local and collective atomic motions that span a wide range of time scales. An integrated analysis of experimental and simulation data can shed light on the detailed mechanism of these motions. Applying a high electric field to protein crystals enables conformational changes that can be captured by time-resolved X-ray crystallography. Such an experiment (referred to as EF-X) carried out on a human PDZ domain obtained a series of atomistic ''snapshots'' of ensemble averages protein dynamics at 50 to 100 ns time intervals. Here, we present a molecular dynamics (MD) study of the same system and provide a detailed picture of the protein dynamics in between the experimental ''snapshots''. We replicated the experimental conditions and system geometry in the presence and absence of an electric field. By constructing a model of the protein crystal as a 3x3x3 supercell with a total of 108 individual proteins, we achieved extensive sampling of the protein conformational ensemble at the sub-millisecond time scale. A number of techniques, including principal component analysis and strain analysis, were utilized to quantify the effects of the electric field on the protein structure and crystal symmetry. This study demonstrates how MD simulations can complement information obtained in EF-X experiments by providing the higher spatial and temporal resolution of underlying dynamical processes.
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