General Energy Metabolism

Provided for non-commercial research and educational use. Not for reproduction, distribution or commercial use. This article was originally published in Encyclopedia of Fish Physiology: From Genome to Environment, published by Elsevier, and the attached copy is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial Nelson J.A. and Chabot D. (2011) General Energy Metabolism. In: Farrell A.P., (ed.), Encyclopedia of Fish Physiology: From Genome to Environment, volume 3, pp. 1566–1572. San Diego: Academic Press. ª 2011 Elsevier Inc. All rights reserved. Author's personal copy General Energy Metabolism JA Nelson, Towson University, Towson, MD, USA D Chabot, Institut Maurice-Lamontagne, Mont-Joli, QC, Canada ª 2011 Elsevier Inc. All rights reserved. Metabolic Rates of Fish Further Reading Conclusions Glossary M˙ Mass of oxygen consumed by an organism. The SI O2 Adenosine triphosphate (ATP) An almost universal unit is micromoles or millimoles per unit time, but often carrier of chemical bond potential energy; fish use ATP expressed as milligrams of oxygen per unit time. It can –1 made from catabolism of foodstuff or body reserve also be divided by the mass of the fish (e.g., mg-O2 h –1 molecules to fuel energy-dependent processes. kg ) in which case, it is called ‘mass-specific oxygen Direct calorimetry The measurement of waste heat consumption’. produced by metabolic processes to assess the rate of Quantile A value that divides a data set into parts. these processes. Where the division is made depends upon the Entropy A thermodynamic property measuring the parameter q.Ifq = 0.5, half the data are below the amount of disorder in the system. Greater disorder is quantile and half above, which gives the median. If q is energetically favorable; thus, entropy favors unfolding of given as a percent instead of a proportion, the quantile proteins. can be called a percentile. Indirect calorimetry The measurement of O2 or Standard metabolic rate (SMR) The minimum foodstuff molecules consumed or CO2 produced to metabolic rate of survival. Typically, SMR is assess the rate of metabolic processes. measured on resting, unstressed adult animals in the Mass exponent, � (also called allometric post-absorptive state under normothermic exponent) The power function exponent for the conditions. For fish, normothermic is defined as a relationship of metabolic rate as a function of body size. temperature well within the species tolerance limits Metabolic scaling How metabolism changes as body for which the animals have had ample time to size changes; typically summarized by the mass exponent. acclimate. Metabolic Rates of Fish of fish result in a low ‘signal:noise’ ratio for direct calori metry in aquatic studies. Therefore, we find limited Measuring Metabolic Rate measurements of metabolic rate by direct calorimetry Organismal energy use is thermodynamically inefficient, available for the fishes; most studies instead rely upon a that is, about two-thirds of the potential energy of the process called ‘indirect calorimetry’. reactants is lost as a less-useful form of energy known as Indirect calorimetry takes advantage of the fact that heat during the execution of essential life processes. In substances are consumed or produced during the addition, the catabolic reactions of the body that convert catabolic conversion of foodstuffs to useful ATP energy. the energy in food to the adenosine triphosphate (ATP) Eqn (2) depicts the complete aerobic respiration of an required to fuel these process are themselves inefficient example foodstuff molecule, glucose: and generate heat. Thus, although the most useful indi þ cator of total metabolic activity would be to measure the C6H12O6 þ 36ADP þ 36Pi þ 36H þ 6O2 ) 6CO þ 36ATP þ 42H O þ Heat ð1Þ rate of total ATP turnover in an organism, in the absence 2 2 of a convenient way to measure ATP turnover, account Both oxygen (O2) and carbon dioxide (CO2) are gases at ing for the heat produced by these aggregate reactions is the range of temperatures and pressures encountered by the best way to assess total metabolic activity, a process fish; their stoichiometric consumption (O2) or release called ‘direct calorimetry’. Unfortunately, the high heat (CO2) can be monitored to gauge the rate of this reaction. capacity of water and the relatively low metabolic activity Some studies also follow the decline in storage 1566 Encyclopedia of Fish Physiology: From Genome to Environment, 2011, Vol. 3, 1566-1572, DOI: 10.1016/B978-0-1237-4553-8.00148-9 Author's personal copy Energetics | General Energy Metabolism 1567 metabolites in starved fish as a way to monitor rates of ectotherms. R allows for no activity, digestion, or reproduc respiration reactions. However, because starvation is a tion, and care needs to be taken to exclude other sources of nonstandard physiological state, these types of experi energy expenditure when measuring it. Measurement meth ments will not be discussed further here (see also Gut odologies for BMR in endotherms are well established. In Anatomy and Morphology: Gut Anatomy). Because humans, for example, BMR is measured in subjects who are each foodstuff (protein, fat, or carbohydrate) produces awake, supine, fasted for 12 h, motionless, after a 20–30-min different amounts of energy per amount of O2 consumed period of rest, and isolated from external stimuli in a ther or CO2 emanated, accurate use of indirect calorimetry for moneutral dark environment. Even so, BMR is not without bioenergetics requires a strict assessment of the substrate criticism because metabolic rate can decrease further during being respired. This is not a great challenge in laboratory sleep. Nevertheless, the stringent measurement conditions situations, but may be impossible for most field studies facilitate comparisons between studies and can also be because the exact composition of the diet is often applied to many nonhuman mammals. unknown. Because measuring [CO2] in water is more By contrast, the conditions in which SMR is measured in challenging than measuring [O2], studies of fish metabolic aquatic ectotherms are more loosely defined. SMR is typi rate over the past 30 years have relied upon measuring O2 cally observed in postabsorptive (but not starving), resting consumption almost exclusively, often without any organisms after acclimation to the experimental tempera attempt to relate the measurement back to energy usage. ture and apparatus, isolated from outside stimuli (including As such, oxygen consumption ( M_ ) has actually become O2 sensory stimuli from potential predators and possibly con a measurement in its own right because without account specifics), during the part of the circadian activity cycle ing for the substrate being oxidized or accounting for any _ when MO2 is lowest. Strictly speaking, SMR should be anaerobic metabolism that occurs, it may be quite differ measured only in animals that are in a steady state (no ent from the actual metabolic rate. Recent careful growth or reproduction). However, the minimum metabolic comparisons of metabolic rate in endotherms using both rate of juvenile fish is often called SMR and the growth of direct and indirect calorimetry on the same animals sug many fish species is indeterminate, so the no-growth criter gest that the use of indirect calorimetry incurs routine ion is often not met. SMR of adult fish should be measured errors of the order of 20%, with isolated cases as high as outside of the reproductive season, although in practice, the 35%. reproductive status of fish used in SMR determinations is not always known. Because there is usually some activity during measurements of metabolic rate, or because activity Standard Metabolic Rate and its Measurement has not been monitored, many authors refrain from using the term SMR and instead use resting, fasting, or most commonly resting routine metabolic rate. However, this C ¼ðF þ U ÞþðR þ W þ SDAÞþðB þ GÞ ð2Þ may be unduly restrictive and the term SMR should be In eqn (2), a standard bioenergetics equation, (see also allowed when activity level is measured and known to be at Energetics: Energetics: An Introduction) ‘R’ represents the minimum possible for the species. the minimum rate of energy expenditure needed to keep Handling stress has undoubtedly inflated many pub a fish alive. It involves three broad classes of processes, the lished estimates of SMR in fishes. Human contact and air first of which is biosynthesis of macromolecules. Even exposure need to be minimized before measuring SMR. when a fish is not growing, there is a constant, energy- Visual contact with humans or laboratory noises can ele requiring renewal of the macromolecules that make up vate metabolic rate considerably even in the absence of movement. An acclimation period is required before M_ the fish with molecules from the diet. The second class of O2 processes concerns the chemical work of moving ions and measurements can be used to estimate SMR. Handling molecules against concentration gradients or moving stress and the novelty of the experimental setup may _ polar compounds across nonpolar membranes. These elevate MO2 in fish for hours or even days. Sufficient _ energy-requiring processes are essential for osmoregula acclimation can be verified by measuring MO2 during the tion, the maintenance of internal cellular integrity, acclimation period to confirm that the fish has reached a cellular communication, intra-organismal communication state where repeatable measurements of minimum meta (e.g., action potentials or hormone release), and the trans bolic rate can be made.

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