Page 630 - The Toxicology of Fishes

610 The Toxicology of Fishes

mechanisms of tolerance (Klerks and Weis, 1987). During evaluation of the toxicity of zinc sulfate to
rainbow trout (Oncorhynchus mykiss), Lloyd (1960) noticed that preexposure to zinc significantly
enhanced median percent survival in subsequent exposures to elevated zinc concentrations. Similarly,
Edwards and Brown (1967) found that rainbow trout maintained in a zinc solution were less susceptible
to subsequent zinc toxicity as measured by LC values. By the 1970s, interest in the development of
50
toxicity resistance to metals had become widespread, and numerous efforts were made to better charac-
terize this response. Pretreatment of rainbow trout eggs with zinc (Sinley et al., 1974) or with cadmium
(Beattie and Pascoe, 1978) increased the survivability of hatching and larval fish to subsequent metal
exposures. Similar results were reported by Spehar et al. (1978) in studies with flagfish (Jordanella
floridae). In another study, preexposure of larval rainbow trout to sublethal doses of cadmium produced
fish with resistance to higher doses of cadmium (Pascoe and Beattie, 1979). Evidence for both physio-
logical acclimation and genetic adaptation began to appear in the literature shortly thereafter; for example,
Dixon and Sprague (1981a,b) investigated toxicity resistance in rainbow trout to copper, arsenic, or
cyanide. Preexposure to these toxicants resulted in enhanced survivability during subsequent, elevated
exposures. Acclimation to copper was lost within 3 weeks of maintenance in clean water, indicating
physiological acclimation. Numerous other accounts followed, describing acclimation responses following
preexposure to copper, cadmium, mercury, zinc, aluminum, and selenium (Buckley et al., 1982; Duncan
and Klaverkamp, 1983; Orr et al., 1986), as well as mixtures of metals (Roch and McCarter, 1984a).
Dixon and Sprague (1981c) provided some of the first mechanistic evidence for the involvement of
metal-binding proteins in physiological acclimation to metals. In that study, the authors reported increased
synthesis of a low-molecular-weight soluble protein in the liver of copper-tolerant fish. Other investigators
subsequently addressed the specific involvement of metallothioneins and concluded that these proteins
play important or dominant roles in reducing metal toxicity (Roch and McCarter, 1984a,b). In one study,
McCarter and Roch (1983) demonstrated a strong relationship between LC values for copper in Coho
50
salmon and hepatic metallothionein levels (Figure 13.5).

200

Metallothionein Concentration 150

100

50
200 250 300 350 400 450 500 550
Copper 168-hr LC50
FIGURE 13.5 Plot of 168-hour LC 50 values vs. metallothionein levels in juvenile Coho salmon exposed to copper. (Adapted
from McCarter, J.A. and Roch, M., Comp. Biochem. Physiol., 74C, 133–137, 1983.)