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Metabolism at the Max: How Vertebrate Organisms Respond to Physical Activity

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ABSTRACT

Activity metabolism is supported by phosphorylated reserves (adenosine triphosphate, creatine phosphate), glycolytic, and aerobic metabolism. Because there is no apparent variation between vertebrate groups in phosphorylated reserves or glycolytic potential of skeletal muscle, variation in maximal metabolic rate between major vertebrate groups represents selection operating on aerobic mechanisms. Maximal rates of oxygen consumption in vertebrates are supported by increased conductive and diffusive fluxes of oxygen from the environment to the mitochondria. Maximal CO2 efflux from the mitochondria to the environment must be matched to oxygen flux, or imbalances in pH will occur. Among vertebrates, there are a variety of modes of locomotion and vastly different rates of metabolism supported by a variety of cardiorespiratory architectures. However, interclass comparisons strongly implicate systemic oxygen transport as the rate‐limiting step to maximal oxygen consumption for all vertebrate groups. The key evolutionary step that accounts for the approximately 10‐fold increase in maximal oxygen flux in endotherms versus ectotherms appears to be maximal heart rate. Other variables such as ventilation, pulmonary/gill, and tissue diffusing capacity, have excess capacity and thus are not limiting to maximal oxygen consumption. During maximal activity, the ratio of ventilation to respiratory system blood flow is remarkably similar among vertebrates, and CO2 extraction efficiency increases while oxygen extraction efficiency decreases, suggesting that the respiratory system provides the largest resistance to maximal CO2 flux. Despite the large variation in modes of activity and rates of metabolism, maximal rates of oxygen and CO2 flux appear to be limited by the cardiovascular and respiratory systems, respectively. © 2015 American Physiological Society. Compr Physiol 5:1677‐1703, 2015.

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Figure 1. Figure 1. A summary of resting (A) Creatine Phosphate (CP) and (B) ATP concentrations within red and white skeletal muscle fibers from vertebrates (see Refs. ). For mammals, red vs. white [CP], P = 0.0006 (two‐tailed t test); for all vertebrates combined, red versus white [CP], P = 0.0085 (two‐tailed t test). For mammals, red vs. white muscle [ATP], P = 0.0005 (two‐tailed t test); for fish, red versus white muscle [ATP], P = 0.012 (two‐tailed t test); for all vertebrates combined, red versus white muscle [ATP], P = 0.0064 (two‐tailed t test).
Figure 2. Figure 2. A summary of (A) skeletal muscle, (B) blood (P = 0.73; ANOVA) and (C) whole body lactate (P = 0.51; ANOVA) concentrations following maximal activity in various vertebrate groups (see Ref. ).
Figure 3. Figure 3. Anaerobic potential quantified as the number of minutes that would be possible to supply ATP at a rate equal to VO2max based on class‐wide averages of maximal muscle lactate, muscle CP, and muscle ATP. Values are calculated based on typical class muscle composition and assumption of complete dephosphorylation of CP, complete conversion of ATP to ADP, and that lactate accumulation occurs solely due to glycogen catabolism. Five representative species and respective VO2max values were taken, with permission, from Hillman et al. ().
Figure 4. Figure 4. A summary of VO2max data and corresponding body temperature data that were collected and reported for vertebrates; (A) ectotherms only and (B) all vertebrate groups together: fish (); amphibians () summary, (); reptiles: (); birds: (); mammals: () summary, () summary, (). For ectotherms only: 15°C (P = 0.063, ANOVA), 20°C (P = 0.0002; amphibians > fish = reptiles; Tukey's post‐hoc test, P < 0.001); 25°C (P < 0.0001; amphibians > fish = reptiles; Tukey's post‐hoc test, P < 0.0001); 30°C (P = 0.0074; amphibians > fish = reptiles; Tukey's post‐hoc test, P = 0.015).
Figure 5. Figure 5. The Q10 relationship for VO2max and body temperature from representatives of three ectothermic groups. The fish (Carassius auratus) data are, with permission, from Fry and Hart (), the toad (Bufo boreas) data are, with permission, from Carey (), and the lizard (Sceloporus occidentalis) data are, with permission, from Bennett and Gleeson ().
Figure 6. Figure 6. A schematic diagram of the four steps in the transport of O2 from the environment into the mitochondria and CO2 out of the cell to the environment, and the variables involved at each step used in this analysis. Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 7. Figure 7. (A) Mean VO2 values for rat (Rattus norvegicus; ), pigeon (Columba livia; ), lizard (Varanus mertensi; ), toad (Rhinella marina; ), and trout (Oncorhynchus mykiss; ) at rest (filled bars) and at VO2max (unfilled bars), and (B) The ratio of VO2max to VO2rest (aerobic scope) for the five representative species. Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 8. Figure 8. (A) The ratio of inspiratory minute volume (VI) to VO2 and (B) respiratory O2 extraction efficiencies (%) for the five representative vertebrate species at VO2rest (filled bars) and VO2max (unfilled bars). The references are the same as for Figure . Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 9. Figure 9. Mean (+SEM) values for (A) Arterial PO2 (PaO2) and (B) Arterial O2 content at rest and during VO2max for the five representative vertebrate species. The references are the same as for Figure .
Figure 10. Figure 10. Cardiovascular data at rest (filled bars) and at VO2max (unfilled bars) for the five representative vertebrate species. (A) heart rate (Fh), (B) stroke volume (Vs), (C) systemic blood flow, and (D) arterial‐venous O2 content difference. Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 11. Figure 11. (A) The relationship of systemic blood flow rate (Qsys) to VO2 from individual species from the five representative vertebrate species at rest (filled bars) and at maximal (unfilled bars), and (B) resting and maximal tissue O2 extraction efficiencies for the same species. The references are the same as for Figure . Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 12. Figure 12. The relationship between systemic O2 transport and VO2max for a variety of vertebrates. The inset figure is the ectotherm data expanded for clarity. Data are from two species of Chondrichthyes [Triakis semifasciata (); Scyliorhinus stellaris ()], three species from Osteichthyes [Gadus morhua as in (); Oncorhynchus mykiss (); Oncorhynchus tshawytscha ()], one species of Amphibian [Rhinella marina ()], three species from Reptilia [Iguana iguana (); Varanus exanthematicus (); Varanus mertensi ()]; two species from Aves [Columbia livia (); Dromiceius novaehollandiae ()], and seven species from Mammalia [Rattus norvegicus (); Trichosurus vulpecula (); Canis familiaris (); Capra hircus (); Bos taurus; Equus caballus (); Homo sapiens ()]. Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 13. Figure 13. Tissue diffusion capacities calculated by two methods for the five representative vertebrate species: (A) the negative exponential of PaO2 and (B) via a Fick calculation using mean capillary PO2. The methods are tightly correlated (r2 = 0.98) with the Fick calculation being 1.6 times greater on average. Ectothermic data are normalized to the lizard temperature of 35°C assuming a Q10 of 2.
Figure 14. Figure 14. Representative dimensions of (A) red blood cells (RBC) and (B) muscle fiber diameters from different vertebrate groups. Data taken, with permission, from Snyder and Sheafor ().
Figure 15. Figure 15. Blood‐alveolar gradients, systemic venous blood partial pressure, and alveolar partial pressure for O2 and CO2 at rest (filled bars) and at VO2max (unfilled bars) from representative species of five vertebrates classes. (A) Alveolar‐venous O2 gradient, (B) venous‐alveolar CO2 gradient, (C) venous PO2 (PvO2), (D) venous PCO2 (PvCO2), (E) alveolar PO2 (PAO2), and (F) alveolar PCO2 (PACO2). The references are the same as for Figure . Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 16. Figure 16. (A) The values for the ventilation perfusion ratios from representative species of five vertebrates classes for the ventilation: perfusion ratio (VI/Qpul) at VO2rest (filled bars) and during VO2max (unfilled bars), and (B) the ratio of VI: Qpul at VO2max to VO2rest. The references are the same as for Figure .
Figure 17. Figure 17. Arterial (A) PCO2, and (B) pH at VO2rest (filled bars) and VO2max (unfilled bars) from representative species of five vertebrate classes. The references are the same as for Figure . Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 18. Figure 18. Respiratory extraction efficiencies of CO2 from the blood at VO2rest (filled bars) and VO2max (unfilled bars) from representative species of five vertebrate classes. The references are the same as for Figure . Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.
Figure 19. Figure 19. (A) The net change in respiratory extraction efficiencies from VO2rest to VO2max for CO2 and O2, and (B) the changes in extraction from rest to activity expressed relative to the resting minus active alveolar PCO2 and PO2. Note that despite similar alveolar pressure changes affecting gas movement, CO2 extraction increases while O2 extraction decreases indicating a respiratory limitation for CO2 efflux. Derived, with permission, from the data presented in Figures , and .


Figure 1. A summary of resting (A) Creatine Phosphate (CP) and (B) ATP concentrations within red and white skeletal muscle fibers from vertebrates (see Refs. ). For mammals, red vs. white [CP], P = 0.0006 (two‐tailed t test); for all vertebrates combined, red versus white [CP], P = 0.0085 (two‐tailed t test). For mammals, red vs. white muscle [ATP], P = 0.0005 (two‐tailed t test); for fish, red versus white muscle [ATP], P = 0.012 (two‐tailed t test); for all vertebrates combined, red versus white muscle [ATP], P = 0.0064 (two‐tailed t test).


Figure 2. A summary of (A) skeletal muscle, (B) blood (P = 0.73; ANOVA) and (C) whole body lactate (P = 0.51; ANOVA) concentrations following maximal activity in various vertebrate groups (see Ref. ).


Figure 3. Anaerobic potential quantified as the number of minutes that would be possible to supply ATP at a rate equal to VO2max based on class‐wide averages of maximal muscle lactate, muscle CP, and muscle ATP. Values are calculated based on typical class muscle composition and assumption of complete dephosphorylation of CP, complete conversion of ATP to ADP, and that lactate accumulation occurs solely due to glycogen catabolism. Five representative species and respective VO2max values were taken, with permission, from Hillman et al. ().


Figure 4. A summary of VO2max data and corresponding body temperature data that were collected and reported for vertebrates; (A) ectotherms only and (B) all vertebrate groups together: fish (); amphibians () summary, (); reptiles: (); birds: (); mammals: () summary, () summary, (). For ectotherms only: 15°C (P = 0.063, ANOVA), 20°C (P = 0.0002; amphibians > fish = reptiles; Tukey's post‐hoc test, P < 0.001); 25°C (P < 0.0001; amphibians > fish = reptiles; Tukey's post‐hoc test, P < 0.0001); 30°C (P = 0.0074; amphibians > fish = reptiles; Tukey's post‐hoc test, P = 0.015).


Figure 5. The Q10 relationship for VO2max and body temperature from representatives of three ectothermic groups. The fish (Carassius auratus) data are, with permission, from Fry and Hart (), the toad (Bufo boreas) data are, with permission, from Carey (), and the lizard (Sceloporus occidentalis) data are, with permission, from Bennett and Gleeson ().


Figure 6. A schematic diagram of the four steps in the transport of O2 from the environment into the mitochondria and CO2 out of the cell to the environment, and the variables involved at each step used in this analysis. Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 7. (A) Mean VO2 values for rat (Rattus norvegicus; ), pigeon (Columba livia; ), lizard (Varanus mertensi; ), toad (Rhinella marina; ), and trout (Oncorhynchus mykiss; ) at rest (filled bars) and at VO2max (unfilled bars), and (B) The ratio of VO2max to VO2rest (aerobic scope) for the five representative species. Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 8. (A) The ratio of inspiratory minute volume (VI) to VO2 and (B) respiratory O2 extraction efficiencies (%) for the five representative vertebrate species at VO2rest (filled bars) and VO2max (unfilled bars). The references are the same as for Figure . Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 9. Mean (+SEM) values for (A) Arterial PO2 (PaO2) and (B) Arterial O2 content at rest and during VO2max for the five representative vertebrate species. The references are the same as for Figure .


Figure 10. Cardiovascular data at rest (filled bars) and at VO2max (unfilled bars) for the five representative vertebrate species. (A) heart rate (Fh), (B) stroke volume (Vs), (C) systemic blood flow, and (D) arterial‐venous O2 content difference. Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 11. (A) The relationship of systemic blood flow rate (Qsys) to VO2 from individual species from the five representative vertebrate species at rest (filled bars) and at maximal (unfilled bars), and (B) resting and maximal tissue O2 extraction efficiencies for the same species. The references are the same as for Figure . Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 12. The relationship between systemic O2 transport and VO2max for a variety of vertebrates. The inset figure is the ectotherm data expanded for clarity. Data are from two species of Chondrichthyes [Triakis semifasciata (); Scyliorhinus stellaris ()], three species from Osteichthyes [Gadus morhua as in (); Oncorhynchus mykiss (); Oncorhynchus tshawytscha ()], one species of Amphibian [Rhinella marina ()], three species from Reptilia [Iguana iguana (); Varanus exanthematicus (); Varanus mertensi ()]; two species from Aves [Columbia livia (); Dromiceius novaehollandiae ()], and seven species from Mammalia [Rattus norvegicus (); Trichosurus vulpecula (); Canis familiaris (); Capra hircus (); Bos taurus; Equus caballus (); Homo sapiens ()]. Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 13. Tissue diffusion capacities calculated by two methods for the five representative vertebrate species: (A) the negative exponential of PaO2 and (B) via a Fick calculation using mean capillary PO2. The methods are tightly correlated (r2 = 0.98) with the Fick calculation being 1.6 times greater on average. Ectothermic data are normalized to the lizard temperature of 35°C assuming a Q10 of 2.


Figure 14. Representative dimensions of (A) red blood cells (RBC) and (B) muscle fiber diameters from different vertebrate groups. Data taken, with permission, from Snyder and Sheafor ().


Figure 15. Blood‐alveolar gradients, systemic venous blood partial pressure, and alveolar partial pressure for O2 and CO2 at rest (filled bars) and at VO2max (unfilled bars) from representative species of five vertebrates classes. (A) Alveolar‐venous O2 gradient, (B) venous‐alveolar CO2 gradient, (C) venous PO2 (PvO2), (D) venous PCO2 (PvCO2), (E) alveolar PO2 (PAO2), and (F) alveolar PCO2 (PACO2). The references are the same as for Figure . Modified from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 16. (A) The values for the ventilation perfusion ratios from representative species of five vertebrates classes for the ventilation: perfusion ratio (VI/Qpul) at VO2rest (filled bars) and during VO2max (unfilled bars), and (B) the ratio of VI: Qpul at VO2max to VO2rest. The references are the same as for Figure .


Figure 17. Arterial (A) PCO2, and (B) pH at VO2rest (filled bars) and VO2max (unfilled bars) from representative species of five vertebrate classes. The references are the same as for Figure . Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 18. Respiratory extraction efficiencies of CO2 from the blood at VO2rest (filled bars) and VO2max (unfilled bars) from representative species of five vertebrate classes. The references are the same as for Figure . Reproduced from Hillman et al. () with permission from the Journal of Comparative Physiology B.


Figure 19. (A) The net change in respiratory extraction efficiencies from VO2rest to VO2max for CO2 and O2, and (B) the changes in extraction from rest to activity expressed relative to the resting minus active alveolar PCO2 and PO2. Note that despite similar alveolar pressure changes affecting gas movement, CO2 extraction increases while O2 extraction decreases indicating a respiratory limitation for CO2 efflux. Derived, with permission, from the data presented in Figures , and .
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Michael S. Hedrick, Thomas V. Hancock, Stanley S. Hillman. Metabolism at the Max: How Vertebrate Organisms Respond to Physical Activity. Compr Physiol 2015, 5: 1677-1703. doi: 10.1002/cphy.c130032