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ssJEB Classics is an occasionalcolumn, featuring historicpublications from The Journal ofExperimental Biology. Thesearticles, written by modern expertsin the field, discuss each classicpaperʼs impact on the field ofbiology and their own work. APDF of the original paper isavailable from the JEB Archive(http://jeb.biologists.org/).

A JEB CLASSIC ON FISHEXERCISE

Tobias Wang and Hans Malte discuss the1977 paper by Joe W. Kiceniuk and DavidR. Jones entitled: ʻThe oxygen transportsystem in trout (Salmo gairdneri) duringsustained exercise.ʼA copy of the paper can be obtained fromhttp://jeb.biologists.org/content/69/1/247.abstract

All animals raise metabolism duringexercise, and the increased call for oxygenin muscles must be attended by increaseddelivery of oxygen from the surroundingsto the mitochondria within the workingmuscles. Because this transport of oxygen,and the opposing transfer of metabolicallyproduced CO2, depends on a coordinatedinterplay between all parts of the oxygencascade, the cardiovascular and respiratoryresponses to exercise represent animportant research area withincomparative physiology. In addition toallowing for an understanding of thestructural and physiological limitations foroxygen transport, it is also likely thatevolution acts particularly on thephysiological ability to cope with stressfulsituations such as exercise; anunderstanding of how the cardio-respiratory systems respond to exercise istherefore pivotal to understanding whythese structures and their underlyingcontrol have evolved.

The appreciation of exercise physiology isprobably as old as physiologicalinvestigations, but because comprehensivephysiological studies on live animalsperforming voluntary exercise aretechnically extremely challenging, it wasnot really until the 1970s that such studieswere performed on non-mammalian

vertebrates. In terms of exercise studies onfishes, the 1977 JEB paper by JoeKiceniuk and David Jones really raised thebar and provided the first truly completequantitative description of oxygentransport in the steady state at variousswimming speeds. In this light, it isperhaps not surprising that Kiceniuk andJones (Kiceniuk and Jones, 1977) isamongst the most cited JEB papers fromthe 1970s, and their data continue to beused today – simply because no one hasbeen able to surpass the completeness ofthis tour de force in cardiorespiratoryphysiology!

To describe the changes in gill ventilationand the cardiovascular system, Kiceniukand Jones placed rainbow trout in aswimming respirometer where the rate ofoxygen consumption (VO2) could bemeasured at various swimming speeds.The trout were equipped with wires torecord the electrocardiogram to measurefor heart rate (see Fig.1A), and cannulaeinserted in both the dorsal and ventralaorta, as well as in a central veinimmediately before the heart (commoncardinal vein), allowed for repeated bloodsampling and measurements of bloodpressures. The blood samples from thedorsal aorta allowed Kiceniuk and Jones toassess whether branchial gas exchangewould continue to oxygenate the blood atthe various speeds, while the difference inoxygen concentrations between thecardinal vein and the dorsal aorta allowedfor calculation of cardiac output, i.e. therate of blood flow (Q) generated by theheart and stroke volume (Vs) of the heart.Finally, the duo measured gill ventilationby placing a skirt – ingeniouslyconstructed from a disposable surgicalglove – above the opercula such thatsampling of expired water, withoutcontamination from ambient water, couldbe performed and gill ventilationcalculated from the oxygen concentrationdifferences between inspired and expiredwater (see water sampling cannula inFig.1A).

Together, they showed that arterial oxygenlevels [i.e. both the partial pressure (PO2)and concentration of oxygen] remain higheven at maximal aerobic exercise, and thatthe increased call for oxygen was met byincreased cardiac output (Q) throughincreased heart rate and stroke volume incombination with increased extraction ofoxygen in the blood as revealed by alowering of the venous oxygenconcentration (Kiceniuk and Jones, 1977)(Fig. 1B, Eqn 3). This, we know now, seemsto be a uniform pattern during exercise inall vertebrates even across a wide range oftemperatures (e.g. Åstrand et al., 1964;

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JEB Class ics

Kiceniuk and Jones, 1977; Butler et al.,1977; Andersen and Wang, 2003; Clark etal., 2005).

Two additional observations emerge fromthe observation that arterial PO2 remainshigh during exercise in fish and othervertebrates. Firstly, the high arterial PO2shows that ventilation of the gas exchangesurfaces (i.e. gills in fish and lungs in air-breathers) increases proportionally to therise in aerobic metabolism during exercise.Secondly, the high arterial PO2 demonstratesthat the gill has sufficient capacity to allowfor the tenfold rise in oxygen diffusionacross the branchial epithelium, eventhough the oxygen partial pressuredifference between the blood and the wateronly increases to a rather limited extentbecause PO2 of the venous blood onlydecreases by a few kPa. Thus, the increasedflux of oxygen implies that the diffusionconductance or ‘transfer factor’, i.e. theflux of oxygen for a unit partial pressuredifference, increases considerably duringexercise. The mechanisms for the increasedconductance probably stems from acombination of increased perfusionrecruiting more area for diffusion and a

shortening of the diffusion distance asblood pressures increases (Jones andRandall, 1979; Farrell et al., 1980). Similarresponses are characteristic of vertebratelungs (Turino et al., 1963; Hsia et al.,2008), but in the fish gill the actualmechanism and structural changes remaindifficult to quantify.

What makes Kiceniuk and Jones’ 1977 JEBpaper a classic? Earlier studies on fish hadshown that the increase in oxygen uptake isalmost exponential as swimming speedincreases and reaches maximal values thatare up to tenfold higher than resting values(e.g. Brett, 1964; Brett, 1965; Brett, 1972).Several other studies reported on parts ofthe oxygen transport cascade (Stevens andRandall, 1967a; Stevens and Randall,1967b). However, the relevant changes inventilation, the cardiovascular system andthe blood gases had not been measuredsimultaneously until Kiceniuk and Jonespublished this classic paper. Thus, Kiceniukand Jones’ tenacity and perseveranceallowed the first calculations of transferfactors in the gills and the systemiccirculation of exercising fish. In fact, theoriginal data from Kicenuik and Jones’

1977 publication provided the mostcomplete set of input parameters (includinggill ventilation, cardiac output and gilldiffusion conductance) available when werecently devised an integrativemathematical model to describe maximaloxygen uptake and transport from water totissues (Wang and Malte, 2011). Using thismodel we were able to clarify to whatextent the oxygen affinity of the blood isimportant for maximal oxygen uptake andtransport in the steady state.10.1242/jeb.073288

Tobias WangAarhus University

tobias.wang@biology.au.dk

Hans MalteAarhus University

hans.malte@biology.au.dk

ReferencesAndersen, J. B. and Wang, T. (2003).Cardiorespiratory effects of forced activity anddigestion in toads. Physiol. Biochem. Zool. 76, 459-470.Åstrand, P.-O., Cuddy, T. E., Saltin, B. andStenberg, J. (1964). Cardiac output duringsubmaximal and maximal work. J. Appl. Physiol., 19,268-274.

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Fig.1. (A) An illustration from Joe Kiceniukʼs PhD dissertation demonstrating the placement of the cannulae for withdrawal of arterial and venous bloodsamples from the dorsal aorta and the common cardinal vein, respectively. These cannulae were used to determine blood gases and, in combinationwith the simultaneous measurements of oxygen consumption (VO2), allowed for a calculation of cardiac output (Q) using Eqn1 (where fH is heart rate).The skirt placed above the opercula allowed for measurements of expired water, such that the rate of gill ventilation (Vgill) could be calculated usingEqn2 (where PIO2 and PEO2 are the partial pressures of inspired and expired O2, respectively). (B) As shown in the lower panel, Kicenuik and Jones(Kicenuik and Jones, 1977) demonstrated that arterial oxygen levels are maintained during exercise and that the increase in oxygen consumption isgoverned by a rise in cardiac output – primarily by increased stroke volume (Vs) – in combination with a widening of the O2 concentration differencebetween arterial and venous blood ([O2]a–[O2]v), i.e. increased extraction (Eqn 3). This response is shown as a graphical solution to the Fick principle onthe right-hand side of the figure.

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JEB ClassicsBrett, J. R. (1964). The respiratory metabolism andswimming performance of young sockeye salmon. J.Fish. Res. Board Can. 31, 1183-1226.Brett, J. R. (1965). The relation of size to rate ofoxygen consumption and sustained swimming speedof sockeye salmon (Oncorhynchus nerka). J. Fish.Res. Board Can. 33, 1491-1501.Brett, J. R. (1972). The metabolic demand for oxygen in fish, particularly salmonids, and acomparison with other vertebrates. Respir. Physiol. 14,151-170.Butler, P. J., West, N. H. and Jones, D. R. (1977).Respiratory and cardiovascular responses of thepigeon to sustained, level flight in a wind-tunnel. J.Exp. Biol. 71, 7-26.Clark, T. D., Wang, T., Butler, P. J. and Frappell, P.B. (2005). Factorial scopes of cardio-metabolic

variables remain constant with changes in bodytemperature in the varanid lizard, Varanus rosenbergi.Am. J. Physiol. 288, R992-R997.Farrell A. P., Sobin, S. S., Randall, D. J. andCrosby, S. (1980). Intralamellar blood flow patterns infish gills. Am. J. Physiol. 239, R428-R436.Hsia, C. C. W., Wagner, P. D., Merrill Dane, D.,Wagner, H. E., and Johnson, R. L., Jr (2008).Predicting diffusive alveolar oxygen transfer fromcarbon monoxide-diffusing capacity in exercisingfoxhounds. J. Appl. Physiol. 105, 1441-1447.Jones, D. R. and Randall, D. J. (1979). Therespiratory and circulatory systems during exercise.Fish Physiol. 7, 425-501.Kiceniuk, J. W. and Jones, D. R. (1977). The oxygentransport system in trout (Salmo gairdneri) duringsustained exercise. J. Exp. Biol. 69, 247-260.

Stevens, E. D. and Randall, D. J. (1967a). Changesin blood pressure, heart rate and breathing rate duringmoderate swimming activity in rainbow trout. J. Exp.Biol. 46, 307-315.Stevens, E. D. and Randall, D. J. (1967b). Changesof gas concentrations in blood and water duringmoderate swimming activity in rainbow trout. J. Exp.Biol. 46, 329-337.Turino, G. M., Bergofsky, E. H., Goldring, R. M. andFishman, A. P. (1963). Effect of exercise onpulmonary diffusing capacity. J. Appl. Physiol. 18, 447-456.Wang, T. and Malte, H. (2011). O2 uptake andtransport: the optimal P50. In Encyclopedia of FishPhysiology: From Genome to Environment, Vol. 2 (ed.A. P. Farrell), pp. 893-898. San Diego, CA: AcademicPress.

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