Synthesis and degradation of hepatic metallothionein in mice differing in susceptibility to cadmium mortality

Proximal tubular transport of Metallothionein-Mercury complexes and protection against nephrotoxicity

Mercury (Hg) is an important environmental toxicant to which humans are exposed on a regular basis. Mercuric ions within biological systems do not exist as free ions. Rather, they are bound to free sulfhydryl groups (thiols) on biological molecules. Metallothionein (MT) is a cysteine-rich, metal-binding protein that has been shown to bind to heavy metals and reduce their toxic effects in target cells and organs. Little is known about the effect of MT on the handing and disposition of Hg. Therefore, the current study was designed to test the hypothesis that overexpression of MT alters the corporal disposition of Hg and reduces its nephrotoxicity. Furthermore, the current study examined the transport of Hg-MT complexes in isolated proximal tubules. Rats were treated with saline or Zn followed by injection with a non-nephrotoxic (0.5 µmol kg-1), moderately nephrotoxic (1.5 µmol kg-1), or significantly nephrotoxic (2.25 µmol kg-1) dose of HgCl2 (containing radioactive Hg). Pretreatment with Zn increased mRNA expression of MT and enhanced accumulation of Hg in the renal cortex of male and female rats. In addition, injection with Zn also protected animals from Hg-induced nephrotoxicity. Studies using isolated proximal tubules from rabbit kidney demonstrated that Hg-MT is taken up rapidly at the apical and basolateral membranes. The current findings suggest that at least part of this uptake occurs through an endocytic process. This study is the first to examine the uptake of Hg-MT complexes in isolated proximal tubules. Overall, the findings of this study suggest that supplementation with Zn may be a viable strategy for reducing the risk of Hg intoxication in at-risk populations.

Dermatotoxicity of oral cadmium is strain-dependent and related to differences in skin stress response and inflammatory/immune activity

Adverse effects of non-occupational exposure to cadmium (Cd) are increasingly acknowledged. Since our previous study has showed that orally acquired Cd affects skin, the contribution of genetic background to dermatotoxicity of oral cadmium was examined in two rat strains, Albino Oxford (AO) and Dark Agouti (DA), which differed in response to chemicals. While similar accumulation of Cd in the skin of both strains was noted, the skin response to the metal differed. DA rat individuals mounted antioxidant enzyme defense in the skin already at lower Cd dose, in contrast to AO rats which reacted to higher metal dose solely (and less pronounced), implying higher susceptibility of DA strain to Cd dermatotoxicity. Epidermal cells from both strains developed stress response, but higher intensity of antioxidant response in AO rats implied this strain`s better ability to defend against Cd insult. Cd induced epidermal cells' proinflammatory cytokine response only in DA rats. Increased IL-10 seems responsible for the lack of response in AO rats. Differences in the pattern of skin/epidermal cell responsiveness to cadmium give a new insight into repercussion of genetic variability to dermatotoxicity of orally acquired cadmium, bearing relevance for variations in the link between dietary cadmium and inflammation-based skin pathologies.

The vascular endothelium as a target of cadmium toxicity

Cadmium (Cd) is an important industrial and environmental pollutant that can produce a wide variety of adverse effects in humans and animals. A growing volume of evidence indicates that the vascular endothelium may be one of the primary targets of Cd toxicity in vivo. Studies over the past 20 years have shown that Cd, at relatively low, sublethal concentrations, can target vascular endothelial cells at a variety of molecular levels, including cell adhesion molecules, metal ion transporters and protein kinase signaling pathways. The purpose of this review is to summarize the results of these recent studies and to discuss the implications of these findings with regard to the mechanisms of Cd toxicity in specific organs including the lung, liver, kidney, testis and heart. In addition the possible roles of the vascular endothelium in mediating the tumor promoting and anticarcinogenic effects of Cd are discussed.

Further observations on the etiology of pre-eclampsia: mobilization of toxic cadmium-metallothionein into the serum during pregnancy

Cadmium-metallothionein, mobilized from the liver, might be the toxic serum factor associated with pre-eclampsia. We base this on four documented concepts. First, during pregnancy, maternal physiology adjusts to assure the fetus of the proper amounts of nutrients necessary for growth. Our focus is on zinc and progesterone. Second, because zinc and cadmium are similar, they compete for binding sites. Our focus is on the storage protein metallothionein. Third, the manifestations of cadmium toxicity closely mimic the manifestations of toxemia (i.e. hypertension, proteinuria, edema). Our focus is on cadmium-induced endovasculitis. Fourth is the concept that metallothionein-bound cadmium can be mobilized from the liver into the serum during pregnancy as it follows the mobilization of metallothionein-bound zinc. Our focus is on the extreme toxicity of extracellular cadmium-metallothionein. We correlate these four concepts into a rational theory on the etiology of toxemia, and we suggest a method of proof.

Interactions of metallothionein with murine lymphocytes: plasma membrane binding and proliferation

Metallothionein (MT) is a thiol rich protein that has been well characterized for its ability to bind and sequester heavy metal cations, free radicals and other reactive toxicants. In addition to induction by these stressors, MT gene expression is upregulated by several cytokines of the acute phase response. In previous work, we have shown that MT can alter aspects of lymphocyte function. MT alone induces modest proliferation of unfractionated splenocytes and acts synergistically with T cell- and B cell-specific mitogens. In contrast, MT inhibits humoral responsiveness in vivo and reduces in vitro T cell responses to processed antigen. In this report, we describe the effects of MT on specific lymphocyte subpopulations in order to further characterize the mechanism of MT-mediated alterations of immune activity. MT binds to the plasma membrane of both T and B lymphocytes, but, in the absence of a costimulatory agent, MT induces lymphoproliferation only in B cells. MT also enhances the capacity of naive B lymphocytes to differentiate into plasma cells. These results demonstrate differential immunomodulatory activities of MT and may explain some of the diverse immunoregulatory effects associated with exposure to environmental toxins.

Cadmium-induced hepatic endothelial cell injury in inbred strains of mice

Susceptibility to cadmium (Cd) hepatotoxicity differs among inbred strains of mice. For example, C3H/HeJ mice are sensitive to Cd-induced hepatotoxicity, whereas DBA/2J mice are resistant. The mechanism of genetic predisposition to Cd hepatotoxicity is unknown. A contemporary theory for acute target organ intoxication maintains that Cd initially damages vascular endothelium and parenchymal cell injury is a secondary event that results from localized ischemia. In the present study, the hypothesis that hepatic endothelial cells (EC) of C3H mice are more susceptible to Cd toxicity than those of DBA mice was tested. Hepatic parenchymal and endothelial cells were grown separately on monolayer cultures for 22 h and subsequently treated with various concentrations of Cd. Hepatocellular toxicity was assessed by lactate dehydrogenase leakage and intracellular K+ loss, whereas endothelial cell injury was assessed by trypan blue exclusion and the inhibition of protein synthesis. The susceptibility of hepatocytes to the cytotoxic effects of Cd was identical between strains. In contrast, the vulnerability of EC to Cd intoxication was strain-dependent. When exposed to 2.5-10.0 microM Cd, EC of Cd-sensitive mice were more susceptible to the cytotoxic effects of Cd than those of Cd-resistant mice. Basal metallothionein (MT) levels as well as Cd uptake into EC were similar in the two strains. Following Cd exposure, EC of Cd-sensitive mice accumulated similar amounts of MT as EC of Cd-resistant mice. These observations suggest that the microvasculature in livers of inbred mice is the target tissue responsible for strain-dependent susceptibility to Cd-induced liver injury. The mechanisms that account for this genetic variation in endothelial cell response to Cd are unknown, but do not appear to be related to the cellular disposition of Cd nor to a defect in the metabolism of MT.

Cd-metallothionein nephrotoxicity in inbred strains of mice

Genetic differences in the acute hepatic and testicular toxicity of Cd occur among different strains of mice. However, it is not known whether genetic variation to the renal damage caused by Cd-metallothionein (CdMT) exists. Therefore, male mice of the C3H/HeJ, C57/Bl10, CBA/CA, and DBA/2J strains, previously shown to differ in hepatic and testicular injury due to Cd, were treated with CdMT at dosages of 0.2, 0.4, 0.8, and 1.6 mg/kg (sc). For all strains of mice, tissue accumulation of Cd occurred predominantly in kidney, which had two to three times as much Cd as liver, while testes had no measurable amounts of Cd. Hepatic and renal metallothionein (MT) concentrations were increased with increasing dosage of CdMT, and no differences between strains were demonstrated. Urinary glucose was increased significantly at the three highest dosages of CdMT, with no differences between strains. At each dose level, light microscopic manifestations of CdMT nephropathy did not differ between strains. In summary, all CdMT-treated strains of mice responded similarly with respect to all measured renal parameters (accumulation of Cd and MT and nephrotoxicity). Unlike the strain differences in hepatic and testicular injury from Cd in these strains of mice, CdMT nephrotoxicity shows no such genetic variation.

Developmental variation in copper, zinc and metallothionein mRNA in brindled mutant and nutritionally copper deficient mice

The concentrations of copper, zinc and metallothionein-I (MT-I) mRNA were determined in the liver, kidney and brain of the brindled mutant mouse from birth until the time of death. Despite accumulation of copper in the kidney of the mutant, MT-I mRNA concentrations were normal. There was no difference between the MT-I mRNA in the brain of mutant and normal in the first 10 days of life, but after day 10 metallothionein mRNA levels were increased in the mutant. The concentration of copper was very low in the liver of the mutant, and on day 6 after birth the metallothionein mRNA was also reduced by about 50%. This reduction was not seen in copper-deficient 6-day-old pups, despite very low hepatic copper levels. This suggests that the lower hepatic MT-I mRNA in the day 6 brindled mouse was not simply due to the reduction in hepatic copper and also that hepatic copper is not regulating metallothionein gene expression the liver of neonatal mice. After day 12 hepatic MT-I mRNA levels were elevated in mutant and in copper deficient mice, both of which die at 14 to 16 days. These increases and the increase in brain MT-I mRNA in older mutant mice are likely to be caused by stress. Overall the results support the conclusions that the brindled mutation does not cause a constitutive activation of the metallothionein genes, and that the differences in metallothionein mRNA between mutant and normal are most probably secondary consequences of the mutation.

Cadmium-induced elevation of hepatic isometallothionein concentrations in inbred strains of mice

Susceptibility to Cd toxicity differs among inbred strains of mice. For example, C3H/He mice are sensitive to Cd-induced hepatotoxicity while DBA/2 mice are resistant. Metallothionein (MT), which in rodents exists predominantly as two isoproteins (MT-I and MT-II), is an important endogenous protein in the detoxication of Cd. The present investigation examines the possibility that strain-dependent susceptibility to Cd-induced liver injury is mediated by an inherited inability to accumulate a specific isoform of MT in response to Cd exposure. Hepatic concentrations of MT-I and MT-II were measured in C3H/He (Cd-sensitive) and DBA/2 (Cd-resistant) mice at various times after the administration of non-toxic (2.5 mumol Cd/kg) to hepatototoxic (80 mumol Cd/kg) dosages of Cd. The concentration of MT-I and MT-II in these strains was similar 24 h after injection of non-hepatotoxic dosages of Cd (10 mumol Cd/kg or less) as well as 6-12 h after a mildly hepatotoxic dose of Cd (20 mumol Cd/kg). The concentration of total MT in liver of Cd-sensitive mice was greater than that present in resistant mice 24-72 h after 20 mumol Cd/kg injection. The data indicates that susceptibility to Cd-induced hepatotoxicity observed in C3H/He mice is not due to a deficit in the induction of a particular isoform of MT.

Metallothionein induction in human peripheral blood lymphocytes by heavy metals

Human peripheral blood lymphocytes have the capacity to produce metallothioneins (MTs) as a protective response to cadmium exposure. To define the range of metal species inducing lymphocyte MTs, cellular proteins synthesized after exposure to each of 11 heavy metals were analyzed by gel electrophoresis. Toxic metals such as cadmium, mercury and silver were found to induce thioneins (apoproteins of MTs) at relatively low concentrations (maximum at approximately 10 microM), whereas less toxic metals such as zinc, copper and nickel were inductive at relatively high concentrations (maximum at approximately 200 microM). Tin, lead, iron, cobalt, and manganese did not induce thioneins. The heavy metal specificity of MT induction in the lymphocyte resembles that in the liver, and the regulatory mechanism of MT production seems to be similar in both of these tissues. In the cells exposed to highly toxic metals such as cadmium and mercury, expression of cytotoxicity (represented by decline of cysteine uptake) was remarkable at the metal concentrations higher than those saturating thionein induction, supporting the protective role of MTs against heavy metals.

Methylation status and organization of the metallothionein-I gene in livers and testes of strains of mice resistant and susceptible to cadmium

The methylation status, copy number and organization of the metallothionein-I (MT-I) gene was studied in hepatic and testicular DNAs of mouse strains resistant (BALB/c) and susceptible (NFS) to cadmium-induced testicular toxicity. Digestion of DNAs by the restriction enzymes BamHI, EcoRI and HindIII produced identical patterns for hepatic and testicular DNAs of both strains, indicating that there was no apparent difference in the gross genomic organization or in copy number of the MT-I gene in the 2 types of tissues from either strain. Digestion with MspI, HpaII, AvaII and HhaI indicated that the hepatic DNAs of both strains were under-methylated as compared to the testicular DNAs. However, the NFS DNAs lacked a fragment that was consistently observed in the MspI digests of BALB/c DNAs, suggesting the presence of a polymorphic CCGG site. This site was localized by double digestion of DNAs with BstEII or HindIII and MspI to the 3' end of the MT-I gene. Differences in methylation status may account for the differential susceptibility of the 2 tissues to cadmium toxicity. The higher degree of MT-I gene methylation may result in slower or inefficient induction of MT in the testes, resulting in greater sensitivity to metal toxicity in testes than in liver. However, differences in methylation status alone do not seem to account for the interstrain differences in cadmium toxicity, and other factors, such as differences in genetic organization, seem to be involved in the inducibility of MT-I gene in different strains.

Differences in immunological susceptibility to cadmium toxicity between two rat strains as demonstrated with cell biological methods. Effect of cadmium on DNA synthesis of thymus lymphocytes

When 2 inbred rat strains, the Brown-Norway rat and the Lewis rat were exposed to the same amount of CdCl2 for 15 days, a completely different immunological reaction pattern could be demonstrated. Despite the same amount of intrathymic cadmium in both strains, the Brown-Norway rat showed a significant decrease in thymocytes in the S-phase and a significant increase of thymocytes in the G2 phase and mitosis, in contrast with findings in the Lewis rats. A new method for estimating subtle forms of thymus atrophy showed a slight decrease in the number of the smallest thymocytes in the Brown-Norway rat after exposure to cadmium, in contrast with that in the Lewis rat. Evidence is presented that the approximately 1.7 times larger number of thymocytes/mg thymus in the Lewis rat, compared to the Brown-Norway rat, as well as the approximately 2.5 times lower proliferation rate of the thymocytes, and an approximately 1.5 times higher metallothionein content of the thymus medulla epithelial cells in the Lewis rat, might be responsible for the observed difference in toxicity. The zinc content of the thymus was not significantly decreased by exposure to CdCl2, and did not differ significantly between both strains.

Mechanisms of regulation of rat hepatic metallothionein-I and metallothionein-II levels following administration of zinc

The purpose of this study was to determine the role of transcription, translation, and protein degradation on the accumulation of metallothionein-I (MT-I) and metallothionein-II (MT-II) in rat liver following induction of these proteins by Zn. The time course of MT induction indicated that concentrations of MT-I and MT-II, quantitated by high-performance liquid chromatography, were similar at 6 hr after administration of 1000 mumol Zn/kg (sc), but thereafter the concentration of MT-II was always higher than that of MT-I. By 24 hr after Zn administration, the concentration of MT-II in liver was more than two times that of MT-I. This difference increased with time such that by 96 hr the concentration of MT-II was more than five times that of MT-I. MT-I and MT-II mRNA levels, measured by Northern blot hybridization with mouse cRNA probes, increased coordinately following administration of Zn. MT mRNAs increased to maximum levels 6-9 hr after Zn administration, at which times MT-II mRNA was about two times more abundant than MT-I mRNA. MT mRNA levels remained elevated above control for as long as 36 hr after Zn administration. The relative rates of synthesis for MT-I and MT-II were determined by quantitating incorporation of [35S]cysteine into MTs during a 2-hr period. For both proteins, the maximum relative rates of synthesis were observed 6-9 hr after administration of Zn, in parallel with the increase in mRNA levels. When MT synthesis was at maximal levels, there was approximately two times more [35S]cysteine incorporated into MT-II than MT-I, but at no other times were differences observed. In contrast to MT mRNA levels, MT synthesis returned to control levels by 24 hr after administration of Zn. Half-lives of the isometallothioneins, determined by pulse-labeling experiments, were calculated to be 12.2 +/- 0.8 and 21.9 +/- 3.0 hr for MT-I and MT-II, respectively. Thus, Zn treatment increases transcription of both MT-I and MT-II genes and the synthesis of MT-I and MT-II. However, Zn-induced MT-II is more stable than MT-I. These results suggest that differences in the rate of synthesis and degradation of MT-I and MT-II lead to a greater and more prolonged induction of MT-II following administration of Zn.

Cadmium-induced synthesis of metallothioneins in human lymphocytes and monocytes

Cd2+-binding proteins of peripheral blood lymphocytes and monocytes have not well been characterized so far, although they are expected to be a clue for understanding Cd2+ toxicity in those immune competent cells. We separated a family of Cd2+-binding proteins from Cd2+-exposed human peripheral blood lymphocytes by gel filtration chromatography, and characterized them by SDS-gel electrophoresis. The proteins showed electrophoretic behaviours closely similar to metallothioneins (MTs) of HeLa cells derived from human cervical carcinoma. The proteins were also found in Cd2+-exposed monocytes, and were inducible by Cd2+ in both lymphocytes and monocytes. Anti-MT serum specifically precipitated these proteins, which were thus identified as MTs. These results suggest that the two classes of the cells involved in the immune system possess a protective mechanism against Cd2+ through MTs. A variety of human lymphoid cell lines derived from both T and B cells were also found to have capacity to synthesize MTs in response to Cd2+.

Age-dependent variation for inducibility of metallothionein genes in mouse liver by cadmium

Cadmium is a toxic metal that induces the expression of metallothionein genes in many tissues and that binds avidly to metallothionein, a soluble transition metal binding protein. The present study examined the temporal pattern and magnitude of accumulation of metallothionein mRNA in liver of C57BL/6J mice of various ages treated with cadmium. In adult female mice, accumulation was dependent on the dosage level of cadmium and related to the concentration of this metal in liver. The accumulation of metallothionein mRNA in liver depended on age at exposure to cadmium. Intraperitoneal administration of 2 mg of cadmium per kg provoked small increases (two- to threefold) in levels of metallothionein mRNA in livers of 7- and 14-day-old mice. In contrast, cadmium treatment of 28- and 56-day-old mice resulted in 12- to 19-fold increases in levels of metallothionein mRNA in liver with maximum increases occurring 3 to 4 hr after treatment. Because similar patterns for the accumulation of cadmium of liver were found in 7-, 28-, and 56-day-old mice, observed age-dependent differences in induction of metallothionein mRNA in liver were probably not due to differences in the accumulation of cadmium in this organ. Taken together, these data suggest that tissue-specific factors controlling the expression of metallothionein genes may account for developmental variation in the inducibility of these genes by cadmium. Ontogenic variation in accumulation of metallothionein mRNA after cadmium treatment may be a factor in developmental variation in the acute lethality of cadmium in C57BL/6J mice.

Ontogenic variation in acute lethality of cadmium in C57BL/6J mice

Susceptibility of C57BL/6J mice to the lethal effects of parenterally administered Cd declined as a function of age at exposure. The 7-day LD50 increased from 1.65 mg Cd/kg body wt in 7-day-old mice to 4.08 mg/kg in adult mice. Survival time following treatment with Cd also increased as a function of age. High constitutive concentrations of metallothionein, a transition metal-binding protein, in livers of young mice did not protect against the lethality of Cd. These results suggest that, in the mouse, the interaction between Cd and this metal-binding protein may be affected by age at exposure to this toxic metal.

Increased absorption of and sensitivity to cadmium during late pregnancy: is there a relationship between markedly decreased maternal cadmium binding protein (metallothionein) and pregnancy-induced hypertension?

Laboratory animals have a unique sensitivity to cadmium toxicity in late pregnancy. This acute toxicity is not seen in non-pregnant, early pregnant, or lactating animals. Furthermore, during late pregnancy, laboratory animals absorb and retain substantially more cadmium from their diets than they do in the non-pregnant state. Both of these observations parallel the fact that a fivefold late gestational drop of maternal metallothionein (a metal-binding protein believed to detoxify cadmium) has been demonstrated in pregnant animals. Additional factors such as nutritional status and age affect cadmium absorption. As we have discussed previously, cadmium toxicity and toxemia of pregnancy have many common features including hypertension, proteinuria, edema, vasospasm and endovasculitis. Because of the above, we propose that cadmium plays a role in the etiology of toxemia.

Intestinal metallothionein in lethal-milk mice with systemic zinc deficiency

Lethal-milk (C57BL/6J-lm) mice over 12 months of age exhibit clinical signs of systemic Zn deficiency. Such lm mice have increased concentrations of metallothionein (MT) in the intestinal mucosa. Various concentrations of Cd or Zn were added to the drinking water. MT was assayed using the Cd-saturation/hemolysate method and for sulfhydryl concentration (MT has 33% cysteine residues) with Ellman's reagent. As assayed by both methods, mucosa from untreated lm mice contained approximately twice as much MT as did the C57BL/6J-(+lm/+lm) (B6) controls. Treatment with 150 and 500 ppm Zn removed the genotypic differences observed for the untreated and Cd-treated mice. These results are consistent with the lm mutation affecting Zn metabolism through impaired MT metabolism as measured for the intestinal mucosa. These studies do not eliminate the possibility that the liver may also contribute.

Strain differences in cadmium-mediated suppression of lymphocyte proliferation in mice

Strain differences were investigated on the proliferative responses of splenic lymphocytes obtained from C3H/He, BALB/c, and DBA/2 mice that were treated with cadmium (Cd) for 5 days (0.5 or 1.0 mg Cd/kg/day, sc), and the results were compared with those of in vitro treatment of spleen cells with Cd. Following in vivo treatment, splenocytes from the C3H strain were significantly more susceptible to suppressive effects of Cd exposure on all indices for proliferative responses to mitogens (concanavalin A, phytohemagglutinin, and lipopolysaccharide) and allogeneic lymphocytes, while those from DBA and BALB strains were fairly resistant. Among the three strains, the highest Cd concentrations in plasma and spleen were obtained in the C3H strain with the lowest hepatic concentration of Cd. On the other hand, the Cd exposure hardly affected the splenic concentration of zinc in the C3H strain in contrast to its decrease in the others. When spleen cells obtained from normal mice were treated in vitro with Cd, the C3H strain was more resistant to the suppressive effect of Cd than the other strains. These results indicate that the mouse strain variations in Cd-mediated suppression of lymphocyte proliferation are not based on intrinsic lymphocyte sensitivities, but likely are due to differences in the metabolism of Cd, which is under genetic control.

Zinc, cadmium, metallothionein, and progesterone: do they participate in the etiology of pregnancy induced hypertension?

Cadmium, a toxic heavy metal, has been incriminated in the etiology of essential hypertension. Zinc, an essential micronutrient necessary for growth, competes with cadmium for binding sites in biochemical processes; zinc deficiency states (i.e. pregnancy and low protein diet) might expose an individual to increased risk of cadmium toxicity. The increased sensitivity to cadmium during pregnancy could also be related to the effect of progesterone on zinc and cadmium metabolism through the actions of metallothionein (MT). MT is a low molecular weight protein believed to function in cadmium detoxification. Several studies in lab animals have documented a late gestation drop of maternal MT levels. This was thought to be due to rising progesterone levels. If there is also a late gestation drop in human maternal MT, then the propensity toward maternal cadmium toxicity would be enhanced. Therefore, we propose that when a zinc deficient woman becomes pregnant and is exposed to both the nutritional demands of the fetus and to the influence of progesterone, she will be likely to develop the manifestations of cadmium toxicity (i.e. hypertension, proteinuria, edema, etc.).

Cadmium hypersusceptibility in the C3H mouse liver: cell specificity and possible role of metallothionein

The possible involvement of metallothionein (MT) gene expression dysfunction was examined in a strain of mouse which is unusually sensitive to cadmium toxicity, the C3H. C3H mice, and the relatively cadmium-insensitive Swiss mice, were injected sc with 20 microM CdCl2/kg body wt. This dose caused liver damage, visible at the light microscopic level, in the C3H but not the Swiss mice. These studies showed that MT-I mRNA and MT protein accumulation, as well as binding of cadmium by MT, were very similar in the two strains. These data suggested that altered expression of MT in the hepatic parenchyma was not a factor in the C3H hypersusceptibility. An electron microscopic examination of the early effects of cadmium injection indicated that the primary targets for toxicity in the C3H liver may be the endothelial cells. It is hypothesized that the widespread damage seen at later times resulted, secondarily, from ischemia produced in response to endothelial cell damage.