Lack of biliary excretion of Cd linked to an inherent defect of the canalicular isoform of multidrug resistance protein (cMrp) does not abnormally stimulate accumulation of Cd in the Eisai hyperbilirubinemic (EHB) rat liver

Decreased hepatobiliary secretion of inorganic mercury, its deposition and toxicity in the Eisai hyperbilirubinemic rat with no hepatic canalicular organic anion transporter

Eisai hyperbilirubinemic (EHB) rats, a new mutant strain inbred from Sprague-Dawley (SD) rats, show no inherent expression of the canalicular multidrug resistance protein (cMrp) and lack canalicular multispecific organic anion transporter (cMOAT) activity. A sample of 203Hg (40 microCi with 40 microg Hg/kg) was injected intravenously (i.v.) into four male SD and EHB rats. Biliary excretion of reduced-glutathione (GSH) was 426 and 2 microg/bile for 15 min in the SD and EHB rats, respectively. Biliary excretion of 203Hg for 45 min in EHB rats significantly decreased to 1/4 of that of the SD rats. However, there was no difference in the hepatic uptake of 203Hg between the two strains. Other rats were injected subcutaneously (s.c.) with HgCl2 solution (at 0.2 and 1.6 mg/kg) containing 203Hg. Some 4 days after the injection of 0.2 mg/kg, about 3 and 13% of the total dose was found in the liver in SD and EHB rats, respectively. The hepatic supernatant Hg was recovered mainly in the void volume of a Sephadex column. Some 2 days after the injection of 1.6 mg/kg, these values were 3 and 23% in SD and EHB rats, respectively. The increased retention stimulated hepatic metallothionein (MT) induction and increased the proportion of Hg in the MT region on the Sephadex column. On the other hand, biliary excretion of 203Hg for 15 min in EHB rats was about 1/6-1/4 of that in SD rats. With the injection of 1.6 mg/kg, hepatic and renal functions worsened in EHB rats. In particular, severe necrosis was found in the renal tubules. Our results suggest that biliary secretion of inorganic Hg may be partly regulated by the ATP-dependent transport system, the glutathione S-conjugate export pump (GS-X pump) composed of Mrp and MOAT. Significantly decreased excretion stimulates hepatic retention of inorganic Hg. However, the hepatic lesions are less predictive. The MT induction may reduce the toxicity of metal to the liver cells.