Mechanism of manganese-induced tolerance to cadmium lethality and hepatotoxicity

Abstract

Pretreatment with Mn 2+ is known to produce tolerance to Cd 2+-induced lethality. This study was designed to determine the mechanism of tolerance to Cd 2+-induced lethality and hepatotoxicity following Mn 2+ pretreatment. Rats given 36 μmoles Cd 2+/kg, i.v., died within 10–20 hr while only one of nine rats pretreated with Mn 2+ (250 μmoles/kg, s.c., 48 and 24 hr prior to Cd 2+ challenge) died. Ten hours after Cd 2+, plasma aspartate aminotransferase and sorbitol dehydrogenase activities were elevated markedly, and extensive histopathologic lesions of the liver were evident in control rats but not in Mn 2+- pretreated rats. To examine the mechanism of this tolerance, distribution of Cd 2+ to fourteen organs and the subcellular distribution in six organs were determined in control and Mn 2+-pretreated rats. Two hours after challenge (31 μmoles Cd 2+/kg, i.v., 0.75 μCi 109Cd 2+/μmole Cd 2+), the distribution of Cd 2+ to liver markedly increased after Mn 2+ pretreatment with concomitant decreases in other tissues. Mn 2+ pretreatment also resulted in a marked difference in the hepatic subcellular distribution of Cd 2+ with more present in cytosol and less associated with organelles. Gel-filtration chromatography indicated that most cytosolic Cd 2+ was bound to a low molecular weight protein. Isolation and partial characterization of this protein suggest that it is identical to metallothionein (MT); it had a similar relative elution following gel-filtration chromatography, had low absorbance at 280 nm and, after separation into two isoproteins by DEAE A-25 anion exchange chromatography, had the same mobility after electrophoresis on non-denaturing polyacrylamide gels as Cd 2+-induced metallothioneins. These data suggest that Mn 2+ pretreatment reduces Cd 2+-induced hepatotoxicity by altering the hepatic subcellular distribution of Cd 2+ with more Cd 2+ binding to MT in the cytosol. This decreased hepatotoxicity is probably responsible for tolerance to Cd 2+-induced lethality.