Page 383 - The Toxicology of Fishes

Liver Toxicity 363

et al., 2002). A multispecific organic solute and steroid transporter that requires the coexpression of two
gene products, Ostα and Ostβ, has also been isolated and functionally characterized in skate. The cDNA
and predicted amino acid sequences of Ostα and Ostβ appear to be novel and are likely to be members
of an additional family of transporters. The overall substrate specificity of Ostα and Ostβ is most similar
to that of the OATP family, in particular to that of human OATP-C (also known as LST1 and OATP-2)
(Wang et al., 2001). Jacquemin et al. (1995) also identified an organic anion transport protein in skate
liver, and these studies suggest that the primitive skate OATP is most closely related to humans.

Multidrug Resistance-Associated Proteins (MRPs)
Genes for MRPs have been identified in red mullet (Mullus barbatus) (Sauerborn et al., 2004), and an
MRP2 ortholog has been characterized in the liver of the small skate (Raja erinacea) (Cai et al., 2003).
Antibodies directed against mammalian MRP2-specific epitopes labeled a 180-kDa protein band in skate
liver plasma membranes and stained canaliculi by immunofluorescence, indicating that skate livers
expressed a homologous protein (Rebbeor et al., 2000). Investigations of ATP-dependent transport in
skate liver plasma membrane vesicles suggest that the transport of anions is mediated by a functional
analog of mammalian MRP.

Functional and Structural Evidence for a BSEP/SPGP
A BSEP-like protein in the canalicular membrane of the skate liver has been identified and shown to
transport both bile salts and bile alcohols (Ballatori et al., 2000). A BSEP-like protein has also been
identified in medaka (Oryzias latipes) (Hinton et al., unpublished studies).

Bile Flow

Bile flow rates, perhaps expectedly, vary between piscine and mammalian species. Although few aquatic
species have been studied, bile flow information for some cartilaginous fish and teleosts exists. For
comparative purposes, in humans bile acid secretion by the hepatocyte approximates 1 g/hr, 95% of
which is recovered following absorption from the distal ileum into the portal venous system (Wolkoff
and Cohen, 2003). The human bile salt pool size is approximately 50 to 60 µM/kg body weight and
averages 3 to 4 g (Trauner and Boyer 2003). In humans, bile salts are rapidly extracted from enterohepatic
+
circulation by the liver with single-pass extraction rates as high as 80%. Na -dependent uptake of a
variety of bile salts is considered to be the rate-limiting step in bile-acid-dependent bile flow, a mechanism
+
that finds a Na /bile acid cotransport with stoichiometry of >1:1. After common bile duct ligation and
cannulation, both the spiny dogfish shark (Squalus acanthias) and the skate (Raja erinacea) were
observed to secrete bile for periods of 4 to 7 days with maximum rates of 1.77 ± .89 mL/kg per 24
hours in Squalus acanthias (approximately 100 times slower than rat) and 2.66 ± .89 mL/kg per 24
hours in Raja erinacea (Boyer et al., 1976a,b). Other reported bile flow rates are 33 to 74 µL/kg/hr for
Squalus acanthias and 75 to 111 µL/kg/hr for Raja erinacea (Klaassen and Watkins, 1984). Bile flow
in the common bile duct of cannulated rainbow trout (Oncorhynchus mykiss) was observed to be constant
at 75 µL/kg/hr over 108 hours of experimentation (Grosell et al., 2000). The bile acid concentration in
the hepatic bile of rainbow trout (15 to 50 mM) is within the range reported for mammals (Klaassen
and Watkins, 1984; Strange 1984). A comparison of bile composition and bile flow rates can be found
in Table 7.6.
Reported bile flow rates are known to vary among vertebrate species, with aquatic species showing
slower rates of bile flow. Where flow rates vary markedly, the fundamental composition of bile, or bile
fluid, can be similar, as can be seen in freshwater trout, rats, and humans. Saltwater fish show approx-
imately twice the concentration of electrolytes (Na, K, Cl). Although sparse information is available for
aquatic species, current data generally show that bile synthesis and transport are fairly well-conserved
physiological functions; for example, skate BSEP clones were identified that share approximately 65%
identity with human BSEP. Other homologous mammalian transporters identified in fish are Pgp, OATP,
MRP, and NTCP. Similarly, the human nuclear receptors FXR, LXR, CAR, PXR, VDR, and PPARα,
-β, and -γ have also been identified in piscine species. Given our increasing understanding of conserved