In fish as in mammals, the liver plays a fundamental role in a variety of physiological and metabolic pathways. Carbohydrate, lipid, and protein metabolism, as well as regulation of fluid osmolarity, blood functions, and digestive processes are some of the several functions ascribed to the hepatic parenchymal cells. The liver is, however, target of toxic chemicals of external origin, which are transported through the blood for detoxification or simply reach the organ as the first station after intestine absorption. Along with the kidney, the liver is indeed the major site for heavy metal accumulation in aquaticvertebrates (Olsvik et al., 2001).
Metals are ranked highly on the list of toxic substances, and among the top listed for occur- rence, toxicity, and potential exposure, are cad- mium and mercury (Vakharia et al., 2001). No role for Cd2+ and Hg2+ in metabolism of living organisms is yet known, however they are extre- mely hazardous to animal life. Cd2+ enters the food chain and concentrates within organisms because of its extremely long biological half-life, which renders the metal a cumulative toxin (Waalkes and Misra, 1996). Carp have been reported to accumulate and concentrate Cd2+ to several orders of magnitude above those found in their environment, and bioconcentration factors as high as 300 have been documented in the liver of individuals exposed to micromolar Cd2+ concen- trations (de Conto Cinier et al., 1999). Cd2+ induced apopotosis and DNA strand breaks in trout hepatocytes (Risso-de Faverney et al., 2001), and significantly decreased total protein concen- trations and glycolytic capacity in liver of tilapia exposed to natural water contamination (Almeida et al., 2001).
Toxicity of inorganic Hg2+ in water environ- ment is well documented, and while sediment is usually the primary source, the food web is considered the main pathway for metal accumula- tion. High trophic level species tend to accumulate the highest concentrations of Hg2+ , accordingly to the high biomagnification factor for inorganic Hg2+ reported in aquatic invertebrates (up to 10 000 in oysters; Birge et al., 1979). Fish liver and kidney tend to have higher percentages of inorganic Hg2+ than other tissues, although values may vary in different species (Riisga ̊rd and Han- sen, 1990). Hg2+ accumulation has been well documented in the liver of eels from contaminated areas (Batty et al., 1996), and effects are reported on cytosolic Ca2+ impairment in skate hepato- cytes (Nathanson et al., 1995).
The present study has been addressed to evalu- ate the possible effect of Cd2+ and Hg2+ on glucose release in the liver of the European eel, Anguilla anguilla, and an attempt has been made to clarify the mechanism by which these cations interfere with liver physiological functions. In particular, the influence of Cd2+ and Hg2+ on adenylyl cyclase (ACase)/cAMP transduction pathway has been studied, as this mechanism plays a crucial role in the control of glucose metabolism, and it is also involved in hormone signalling pathways regulating other physiological processes in fish liver (Fabbri et al., 1998b).