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biologyphysiology

    MECHANISMS RESPONSIBLE

    FOR THE ENHANCED CARDIAC PERFORMANCE

    OF WINTER FLOUNDER (Pleuronectes americanus)

     Paula da Costa Mendonça

    Ocean Sciences Centre, Memorial University of Newfoundland,

    St. John’s, NL, Canada

    Phone: +709 737 3112; Fax: +709 737 3220

    E-mail: mendonca@mun.ca

    Angela Gaylene Genge; Eric James Deitch; Anthony Kurt Gamperl

    EXTENDED ABSTRACT ONLY - DO NOT CITE

Introduction

    Despite their abundance and diversity (Fletcher, 1975; Moyle and Cech, 1996), flatfish cardiovascular biology/physiology has not been extensively studied, and much of the published data on cardiac function may be inaccurate due to the use of indirect measurements and/or techniques (e.g. the Fick principle). Recently, Joaquim et al. (in press) performed the first direct in vivo measurements of

    cardiac function in winter flounder and found that maximum stroke volume (S) V-1is extremely high in this species (1.5 ml g ventricle, 10ºC) compared with other

    teleosts. To examine the factors that contribute to the high S in this species, in V

    situ Starling curves and power curves, and in vitro pressure-volume curves were

    determined for the winter flounder (Pleuronectes americanus), Atlantic cod

    (Gadus morhua) and Atlantic salmon (Salmo salar).

Material and Methods

    Wild winter flounder were captured in Conception Bay (Newfoundland), while hatchery-reared cod and salmon were obtained from a cage-site operation (Bay D’Espoir) and the Ocean Sciences Centre (OSC), respectively. All fishes were acclimated at 8 to 10 ? 1ºC for at least 4 weeks prior to experimentation.

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    In situ heart preparations at 8 to 10ºC were performed as previously described by Farrell et al. (1982) for sea-raven, but adapted for the flounder, cod and salmon.

    To obtain pressure-volume curves (Forster and Farrell, 1994) the heart (without pericardium) was dissected free from the animal, and pressure-volume curves were generated for the atrium, ventricle and bulbus arteriosus of the 3 species. In addition, atrial:ventricular (A:V) and bulbus:ventricular (B:V) mass ratios were calculated to examine if the size of the heart chambers or their relative size

     influences the shape of the pressure-volume curves.

    After ln transforming the data for the Starling and pressure-volume curves, ANCOVA was used to test for homogeneity of slopes between species (p<0.05; rdSPSS Software). Maximum power values were obtained by fitting a 3 order

    power curve (SigmaPlot Software) to the data of each fish. Differences in maximum power output (P), cardiac output (Q), heart rate (f), stroke volume HH

    (S) and A:V and B:V mass ratios between species were assessed by ANOVAs V

    and pairise Tukey tests (SPSS Software, p<0.05).

Results and Discussion

    In situ maximum Q was not significantly between the three species, averaging -1-163 ml min kg. However, because of the small size of the flounder heart (RVM 0.05%), the maximum S achieved by the winter flounder was significantly V-1higher (2.2 ? 0.1 ml g ventricle) as compared with the Atlantic cod (1.7 ? 0.2) and Atlantic salmon (1.4 ? 0.1) (Fig.1A). The maximum P of the flounder heart H-1-1(7.6 ? 0.3 mW g) was significantly lower than the salmon (9.7 ? 0.5 mW g), -1but surprisingly similar to the cod (7.8 ? 0.6 mW g) (Fig.1B).

    Cod and salmon hearts could generate in vivo resting levels of Q at negative

    filling pressures (P), whereas the flounder heart required a positive Pof 0.4 inin cm HO to achieve resting Q. However, fewer increments in P were required 2in

    by the flounder heart to achieve elevated levels of S. For instance, to achieve a V-1S of 1.4 ml g ventricle, the flounder heart only needed a P increase of 1.9 cm Vin

    HO,whereas cod and salmon hearts required P increases of 2.9 and 6.9 cm 2 in-1HO, respectively. These data show that the high Svalues (in ml g ventricle) 2V

    measured for the flounder are partly related to an enhanced sensitivity to filling pressure. This conclusion is supported by the pressure-volume curves, which

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indicate that the flounder’s atrium, ventricle and bulbus are significantly more

    compliant when compared to the cod and salmon (Fig.2).

    3.0

     A 2.5 1 6 2.0 ventricle)-1

    1.5

    1.0

     Stroke Volume (ml g0.5

    0.0-4-202468 Input Pressure (cm HO)212 B 10 )-18

    6

    4 Power Output (mW g

    2

    0020406080100120

    Output Pressure (cm HO)2

    3

    Figure 1. Starling curves (A) and Power Curves (B) for the winter

    flounder (?), Atlantic cod (?) and Atlantic salmon (Δ) at 8 ºC. N=

    7-8, except when numbers appear next t data point.

     6 5A

     4O)2 3 2 Pressure (cm H 1 0

     0246810 120

     B 100

     80O)2 60

     40

     Pressure (cm H20 0 -200.00.51.01.52.02.5

     120

    100 C

    80O)2 60

    40

    Pressure (cm H20

    04

    0.00.51.01.52.02.53.03.5-1Volume (ml g chamber)

    Figure 2. Pressure-volume curves for the atrium (A), ventricle (B) and

    bulbus (C) of the winter flounder (?), Atlantic cod (?) and Atlantic

    salmon (Δ) at 8-10 ºC. N? 6, except salmon ventricle where N= 2.

Although, the flounder’s A:V ratio (0.22) was comparable to the cod (0.21) and

    the salmon (0.18), the flounder’s B:V ratio (0.59) was significantly higher (cod 0.37; salmon 0.22). In fact, the high B:V mass ratio may be partially responsible with the low arterial pressures reported for the flounder.

     In conclusion, the S measured in winter flounder (per gof ventricle) is V

    extremely high. This high S is related to 1) a pronounced Starling curve; 2) V

    more compliant heart chambers; and 3) a high B:V mass ratio. Our data support the in vivo data of Joaquim et al. (in press) and others, which show that the

    cardiovascular system of flatfish is a high volume, low pressure design. Further, these data suggest that the pericardium, and thus vis a fronte filling, may not be

    important for cardiac function in flatfishes.

Reference

    Farrell A, MacLeod K, Driedzic W. 1982. The effects of preload, after load, and

    epinephrine on cardiac performance in the sea raven, Hemitripterus

    americanus. Can. J. Zool. 60(3165-71).

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    Fletcher G. 1975. The effects of capture, "stress," and storage of whole blood on

    the red blood cells, plasma proteins, glucose, and electrolytes of the winter

    flounder (Pseudopleuronectes americanus). Can. J. Zool. 53(2):197-206.

    Forster M, Farrell A. 1994. The volumes of chambers of the trout heart. Comp.

    Biochem. Physiol. 109 A(1):127-32.

    Joaquim, N., Wagner, G.N. and Gamperl, A.K.. In Press. Cardiac function and

    critical swimming speed of the winter flounder (Pseudopleuronectes

    americanus) at two temperatures. Comp. Biochem. Physiol.

    rdMoyle PB, Cech J. 1996. Fishes: an introduction to ichthyology (3 edition).

    Prentice-Hall, Inc. p 339-45.

Acknowledgements

    PCM was supported by a Foundation for Science and Technology doctoral fellowship (Portugal), EJD was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) IPS scholarship, and AKG and AGG were supported by an NSERC discovery grant. We are also grateful to Dr. Trevor Avery for statistical advice, and AquaBounty (Canada) for supplying the Atlantic salmon used in this research.

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