Page 727 - The Toxicology of Fishes

Biomarkers 707

Respiratory and Cardiovascular Responses as Biomarkers
The main functions of the gills are in gas exchange (Randall and Daxboeck, 1984) and osmoregulation
(Payan et al., 1984; Laurent and Hebibi, 1988). Investigations on respiration and circulation during
exercise and hypoxia have revealed aspects of the control of gas exchange. Under normal physiological
conditions, gas exchange is controlled mainly by adjustments in ventilation, while blood flow through
the gills (perfusion) is kept reasonably consistent (Randall and Daxboeck, 1984). As the partial pressure
of oxygen (pO ) in the blood falls (e.g., during exercise or as a result of low environmental pO ), then
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the ventilation rate increases and a marked fall in heart rate (bradycardia) is observed. Blood flow to
the gills is maintained by an increase in cardiac stroke volume which offsets the bradycardia to maintain
cardiac output for all but the most extreme cases of hypoxia in the blood of trout (Randall, 1982). Thus,
the primary control of respiration appears to be efferent arterial pO in fish. Changes in the partial
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pressure of carbon dioxide (pCO ) which alter blood pH may influence pO via acid–base effects on
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hemoglobin oxygen affinity and saturation (Eddy et al., 1977), so carbon dioxide can indirectly alter
respiration, as well (Desforges et al., 2001, 2002; Perry and Gilmour, 2002). Adequate blood flow through
the gills is also essential for normal physiological function; in fact, the ratio of water ventilation to blood
perfusion is an important factor in maintaining optimum diffusion gradients across the gills. The
ventilation/perfusion ratio is normally about 10 to 20 in fishes (Taylor, 1985). Blood flow is influenced
by many factors (e.g., catecholamines, exercise), and fish tend to maintain blood flow to the gills by
compensatory changes in heart function rather than direct changes in vascular resistance in the gills
(Randall and Daxboeck, 1984). In addition to water and blood flow (diffusion gradients), gas diffusion
and ion movements across the gills may depend on gill permeability, the functional surface area, and
the diffusion distance between the water and blood. It is therefore not surprising that changes in the
composition or status of the gill epithelial cells have profound effects on both respiration and osmoreg-
ulation (Bindon et al., 1994; Laurent and Hebibi, 1988). Thus, from a functional perspective any pollutant
that ultimately alters blood pO , pCO , pH, vascular resistance, or blood flow (heart function) or induces
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changes in gill cell proliferation will probably cause a change in respiratory function.
Laboratory studies have shown that acute exposure to aquatic pollutants can cause edema and epithelial
lifting in the gills (Alazemi et al., 1996; Mallat, 1985; Skidmore and Tovell, 1972), which may be
accompanied by the hypersecretion of mucus (Handy and Eddy, 1989). These structural effects will at
least increase the diffusion distance for the respiratory gases to cause hypoxia or hypercapnia (Piiper,
1998; Sellers et al., 1975), which ultimately leads to changes in ventilation frequency and volume (Sellers
et al., 1975). Pollutants that deoxygenate or acidify the water may also stimulate ventilation (Wright et
al., 1986) in the absence of gross gill pathology. Chronic sublethal exposure to pollutants may have
more subtle effects on respiratory and cardiovascular responses. Fish may show anatomical adjustment
of gill surface area in the form of changes in the length or thickness of the gill filaments (Handy et al.,
1999; Wilson et al., 1994) or altered proportions of the cell types in the epithelia (Wendelaar Bonga et
al., 1990), which may result in the need for only minor changes in ventilation in the long term to retain
physiological function. Alternatively, fish may reduce energy expenditure on activities such as locomotion
in order to minimize extra demands on respiration during pollutant exposure (e.g., dietary copper)
(Campbell et al., 2002; Handy et al., 1999).
The question arises as to which of these responses can be used as a biomarker and which parameters
(blood gases, oxygen uptake, ventilation frequency and volume, blood flow, blood pressure, cardiac
output, or its components) can be measured on a routine basis so the biomarker can be used in
environmental monitoring programs as well as fundamental research (Handy and Depledge, 1999; Handy
et al., 2002a). In the laboratory, evaluating blood gases and blood pressure and conducting electrocar-
diograms usually require invasive techniques (Houston, 1990), although non-invasive approaches have
also been tried for ventilation (Kramer and Botterweg, 1991). Optical techniques such as online cardio-
vascular monitoring have been developed for crustaceans, but these have not been tried on fish species
(Handy and Depledge, 1999). Oxygen consumption rates measured in a fish respirometer are particularly
useful, as this measurement indicates the aerobic cost of pollutant exposure (Campbell et al., 2002). For
aqueous exposures, at least, correlations exist between oxygen consumption rates and xenobiotic transfer
rates across the gills (Randall et al., 1996). Oxygen consumption may therefore be a useful tool for
predicting branchial exposure, and several models have been suggested (Yang et al., 2000a,b). Whatever