323
Chapter fourteen: Toxicology and poisons
14.5.1 Cetaceans
MeHg + biomagnifies through trophic levels and is present in high concentrations in tissues such as the liver, the kidney, and the pelage of marine mammals (for review of concentrations in Arctic marine mammals see Dietz et al. 2013). Mercury has both natural
and anthropogenic sources, although temporal trends of Hg in ice cores indicate that concentrations increased markedly following industrialization of the early twentieth century.
Temporal trends of Hg concentrations in biota are scant and difficult to compare, and
analyses are confounded by well described spatial trends and difficulties in comparing
different age, sex, and tissues (Braune et al. 2005). Some species/locations show increasing
concentrations of Hg across multiple decades, and for some species where preindustrial
archived tissues exist (generally hard tissues, i.e., Dietz et al. 2009).
The relationship between Hg and Se has been known for several decades (Koeman
et al. 1975) although the mechanisms of demethylation and subsequent detoxification are
only recently beginning to be described (Khan and Wang 2009). Tiemmanite crystals (mercury selenide; HgSe) have been reported in the livers of Guiana dolphins (Lailson-Brito
et al. 2012), pilot whales (Caurant et al. 1996), and in the lung tissue of Atlantic bottlenose
dolphins and short finned pilot whales (Globicephala macrorhynchus) (Rawson et al. 1995).
Similarly, using micro-x-ray fluorescence, Nakazawa et al. (2011) found evidence for tiemmanite molecules in the liver, the kidney, the lung, the spleen, the pancreas, the muscle,
and the brain of Striped dolphin (Stenella coeruleoalba). It is unclear whether the tiemmanite
crystals found in the lung and other tissues of the latter study were present due to localized demethylation and conjugation, sequestration from other tissues, or, in the case of the
lungs, inhalation of the particles directly.
Cultured cells selected for Cd resistance have been shown to have elevated levels of
MT protein compared to non-resistant cells (Durnam and Palmiter 1987). In fact, the induction of MT by various compounds has been shown to protect against Cd-induced toxicity
(for review see Klaassen et al. 1999). Interestingly, it has been shown in rats that immature
individuals have higher circulating MT concentrations compared to adults (Goering and
Klaassen 1984), and similar trends have been described in Franciscana dolphins (Pontoporia
blainvillei) where concentrations of MT were higher in juveniles than adults (Polizzi et al.
2014). Although the differences in the aforementioned study were not statistically significant, the relationship of higher MT in immature > mature animals has been described in
gray seals (Halichoerus grypus) as well as in mice (Andrews et al. 1984; Teigen and Andersen
1999). Maternal milk has been suggested to be a significant source of dietary Cd exposure
during nursing, and studies investigating mineral contents of harp seal milk have found
concentrations of 0.059–0.101 µg/g (Webb et al. 1984) and 0.021–0.089 µg/g (Wagemann
and Stewart 1988). Whether the elevated levels of MT in immature mammals are due to
induction via maternal contaminant transfer or differences in developmental physiology
is unknown (perhaps a higher demand for Cu or Zn), however, high concentrations of MT
in fetal tissues must certainly confer protections against Cd toxicity.
Similar to Hg, reports of concentrations of Cd bound to MT are varied among marine
mammals. Amiard-Triquet and Caurant (1997) found relatively high concentrations of
Cd in the livers of pilot whales, and found that concentrations of MT-like proteins were
correlated with Cd concentrations in adults. However, they found that in the summer
(July) only 51% of Cd was bound to MTs, and in November the percent bound was only
6%. This is in contrast to Wagemann et al. (1984), who found 77% of cytosolic Cd bound to
MTs in the livers of narwhals. Das et al. (2000) suggested that this might be evidence of an
adaptation of Arctic narwhals to high concentrations of Cd in prey, since environmental
Chapter fourteen: Toxicology and poisons
14.5.1 Cetaceans
MeHg + biomagnifies through trophic levels and is present in high concentrations in tissues such as the liver, the kidney, and the pelage of marine mammals (for review of concentrations in Arctic marine mammals see Dietz et al. 2013). Mercury has both natural
and anthropogenic sources, although temporal trends of Hg in ice cores indicate that concentrations increased markedly following industrialization of the early twentieth century.
Temporal trends of Hg concentrations in biota are scant and difficult to compare, and
analyses are confounded by well described spatial trends and difficulties in comparing
different age, sex, and tissues (Braune et al. 2005). Some species/locations show increasing
concentrations of Hg across multiple decades, and for some species where preindustrial
archived tissues exist (generally hard tissues, i.e., Dietz et al. 2009).
The relationship between Hg and Se has been known for several decades (Koeman
et al. 1975) although the mechanisms of demethylation and subsequent detoxification are
only recently beginning to be described (Khan and Wang 2009). Tiemmanite crystals (mercury selenide; HgSe) have been reported in the livers of Guiana dolphins (Lailson-Brito
et al. 2012), pilot whales (Caurant et al. 1996), and in the lung tissue of Atlantic bottlenose
dolphins and short finned pilot whales (Globicephala macrorhynchus) (Rawson et al. 1995).
Similarly, using micro-x-ray fluorescence, Nakazawa et al. (2011) found evidence for tiemmanite molecules in the liver, the kidney, the lung, the spleen, the pancreas, the muscle,
and the brain of Striped dolphin (Stenella coeruleoalba). It is unclear whether the tiemmanite
crystals found in the lung and other tissues of the latter study were present due to localized demethylation and conjugation, sequestration from other tissues, or, in the case of the
lungs, inhalation of the particles directly.
Cultured cells selected for Cd resistance have been shown to have elevated levels of
MT protein compared to non-resistant cells (Durnam and Palmiter 1987). In fact, the induction of MT by various compounds has been shown to protect against Cd-induced toxicity
(for review see Klaassen et al. 1999). Interestingly, it has been shown in rats that immature
individuals have higher circulating MT concentrations compared to adults (Goering and
Klaassen 1984), and similar trends have been described in Franciscana dolphins (Pontoporia
blainvillei) where concentrations of MT were higher in juveniles than adults (Polizzi et al.
2014). Although the differences in the aforementioned study were not statistically significant, the relationship of higher MT in immature > mature animals has been described in
gray seals (Halichoerus grypus) as well as in mice (Andrews et al. 1984; Teigen and Andersen
1999). Maternal milk has been suggested to be a significant source of dietary Cd exposure
during nursing, and studies investigating mineral contents of harp seal milk have found
concentrations of 0.059–0.101 µg/g (Webb et al. 1984) and 0.021–0.089 µg/g (Wagemann
and Stewart 1988). Whether the elevated levels of MT in immature mammals are due to
induction via maternal contaminant transfer or differences in developmental physiology
is unknown (perhaps a higher demand for Cu or Zn), however, high concentrations of MT
in fetal tissues must certainly confer protections against Cd toxicity.
Similar to Hg, reports of concentrations of Cd bound to MT are varied among marine
mammals. Amiard-Triquet and Caurant (1997) found relatively high concentrations of
Cd in the livers of pilot whales, and found that concentrations of MT-like proteins were
correlated with Cd concentrations in adults. However, they found that in the summer
(July) only 51% of Cd was bound to MTs, and in November the percent bound was only
6%. This is in contrast to Wagemann et al. (1984), who found 77% of cytosolic Cd bound to
MTs in the livers of narwhals. Das et al. (2000) suggested that this might be evidence of an
adaptation of Arctic narwhals to high concentrations of Cd in prey, since environmental