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H M Mehendale

Publications and source records attributed to H M Mehendale.

At least 37 records · Page 2Linked to original sources
PubMed1996

Role of nutrition in the survival after hepatotoxic injury.

Nutritional status is an important factor in determining susceptibility to toxic chemicals. While macro and micronutrients may affect many aspects of Stage I and Stage II of toxicity, in this paper, the influence of macronutrients as sources of energy required for cell division and tissue repair mechanisms on the outcome of hepatic injury is discussed. Male Sprague-Dawley rats maintained on normal rodent chow and 15% glucose (as a source of energy for the centrilobular hepatocytes) in drinking water for 7 days experienced an increased lethality from structurally and mechanistically different centrilobular hepatotoxicants (acetaminophen, thioacetamide, chloroform and carbon tetrachloride), while male Sprague-Dawley (S-D) rats fed rat chow containing palmitic acid (PA, 8% w/w, as a source of energy for the periportal hepatocytes) and L-carnitine (LC, 2 mg/ml, as a mitochondrial carrier for the supplemented fatty acids) in drinking water for 7 days were protected from a LD100 dose (600 mg/kg, i.p.) of thioacetamide (TA). Indices of cell division revealed that cell cycle progression in the liver played a very critical role in determining the final outcome of hepatotoxic injury. These results confirmed our hypothesis that cell division and tissue repair play a critical role in survival after life-threatening hepatotoxic injury. Any manipulation directed towards altering a prompt and exacting compensatory cell division and tissue repair responses after hepatotoxic injury would also alter the final outcome of the toxicity. These studies indicate that the source of cellular energy can decisively influence the compensatory response of the target tissue to alter the outcome of hepatotoxic injury. Since nutritional status is known to vary widely among human populations, these could contribute enormously to susceptibility of human populations to toxic chemicals.

Administration, Oral↗
PubMed1996

Efficient tissue repair underlies the resiliency of postnatally developing rats to chlordecone + CCl4 hepatotoxicity.

It is often assumed that at a younger age populations are at higher risk of toxic effects from exposure to toxic chemicals. Recent studies have demonstrated that neonate and postnatally developing rats are resilient to a wide variety of structurally and mechanistically dissimilar hepatotoxicants such as galactosamine, acetaminophen, allyl alcohol, and CCl4. Most interestingly, young rats survive exposure to the lethal combination of chlordecone (CD) + CCl4 known to cause 100% mortality in adult male and female rats. In a study where postnatally developing (20- and 45-day), and adult (60-day) male Sprague Dawley rats were used, administration of CCl4 (100 microliters/kg, i.p.) alone resulted in transient liver injury regardless of age as indicated by plasma alanine transaminase (ALT), sorbitol dehydrogenase (SDH) levels and histopathological lesions. In CD-pretreated rats, CCl4-induced toxicity progressed with time culminating in 25 and 100% mortality by 72 h after CCl4 in 45- and 60-day rats, respectively, in contrast to regression of CCl4-induced toxicity and 0% mortality in 20-day rats. [3H]Thymidine (3H-T) incorporation and proliferating cell nuclear antigen (PCNA) studies revealed an association between delayed and diminished DNA synthesis, unrestrained progression of liver injury, and animal death. Time-course studies revealed that the loss of resiliency in the two higher age groups might be due to inability to repair the injured liver rather than due to infliction of higher injury. Intervention of cell division in 45-day CD rats by colchicine (CLC, 1 mg/kg, i.p.) 30 h after CCl4 challenge increased mortality from 25 to 85%, confirming the importance of stimulated tissue repair in animal survival. In contrast, efficient and substantial DNA synthesis observed in 20-day rats allows them to limit further progression of liver injury, thereby leading to full recovery of this age group with 0% mortality. Examination of growth factors and proto-oncogene expression revealed a 3- and 3.5-fold increase in transforming growth factor-alpha (TGF-alpha) and H-ras mRNA expressions, respectively, coinciding with maximal hepatocyte DNA synthesis in 20-day normal diet (ND) rats, as opposed to only 2- and 2.5-fold increases observed in 60-day ND rats, respectively. Increased expression of c-fos (10-fold) in 20-day rats occurred 1 h after CCl4 compared to less than a 2-fold increase in 60-day rats. These findings suggest that prompt stimulation of tissue repair permits efficient recovery from injury during early postnatal development of rats.

Animals↗
PubMed1996

Two-dimensional electrophoretic analysis of compartment-specific hepatic protein charge modification induced by thioacetamide exposure in rats.

Thioacetamide (TA) is a well-known hepatotoxicant. It has been reported that an obligate intermediate of TA binds to proteins with the formation of acetylimidolysine derivatives that are responsible for TA-induced hepatotoxic effects. TA has also been reported to cause chemically induced cell death via both apoptosis and necrosis. The objective of this study was 2-fold: first, to investigate the effect of TA exposure on protein charge modifications in the rat liver and second, to study the role of these molecular correlates in the regulation of cell death. Male Sprague-Dawley rats (200-225 g, 7-8 weeks old) were divided into four major groups and treated intraperitoneally with a 12-fold dose range of TA (50, 150, 300, and 600 mg TA/kg) dissolved in water. Using whole liver extracts, alterations in the hepatic protein pattern following treatment with the 12-fold dose range of TA were studied using high-resolution, two-dimensional polyacrylamide gel electrophoresis and computerized image analysis. The results indicate that charge modification was clearly evident as early as 2 hr with the lowest dose of 50 mg TA/kg. At this dose and time endoplasmic reticulum proteins, calreticulin, grp78, and ER6O exhibited acidic charge variants. The effect of TA became more prominent with dose and time. Generally the elevation of charge modification indices (CMI) by TA appeared to reach a peak between 4 and 6 hr and then while CMI either leveled off or declined in the lower two doses of 50 and 150 mg TA/kg, it continued to remain elevated with the higher doses of 300 and 600 mg TA/kg. This dichotomy in the elevation of CMI is in close correspondence to the pattern of cell death observed with a similar dose range of TA, where lower doses (50 and 150 mg TA/kg) predominantly cause cell death via apoptosis while higher doses cause cell death via necrosis. Delayed charge modification was observed with the cytosolic hsc70s with the 300 and 600 mg TA/kg treatments, indicating that the reactive metabolite(s) slowly leak out into the cytosol from the endoplasmic reticulum. There were no alterations in the mitochondrial proteins hsp60 and grp75, suggesting that TA has no effect on the mitochondrion, its effects primarily being confined to the endoplasmic reticulum. The concept of looking at these proteins as biomarkers of tissue injury has validity. These changes may be indicators of bioactivation and adduct formation and also may be signaling events in the regulation of the mode of cell death.

Animals↗
PubMed1996

Stimulated tissue repair prevents lethality in isopropanol-induced potentiation of carbon tetrachloride hepatotoxicity.

Published reports on the alcohol potentiation of CCl4 toxicity indicate that in spite of enhanced hepatotoxicity there is no increase in lethality. The objective of this study was to investigate the mechanism involved in animal survival despite significantly enhanced liver injury. Male Sprague-Dawley rats (175-225 g) were treated with isopropanol (ISOP, 2.5 ml/kg, 25% aqueous solution, po) 24 hr prior to CCl4 (1 ml/kg, ip) administration. Plasma enzymes (ALT and SDH), hepatic glycogen levels, and [3H]thymidine (3H-T) incorporation into hepatonuclear DNA were measured during a time course (0-96 hr) after CCl4 administration. Liver sections were examined for histopathology and cell cycle progression by proliferating cell nuclear antigen (PCNA) immunohistochemistry. Maximum injury was observed at 36 hr in both the groups as indicated by elevated plasma enzyme levels and by histopathology. The extent of injury in the ISOP + CCl4 group was higher than that in the H2O + CCl4 group. Plasma enzyme activity returned to control levels by 60 hr, indicating recovery from injury in both groups. Maximum 3H-T incorporation occurred at 48 hr in both groups (ISOP + CCl4; vehicle + CCl4), indicating maximum stimulation of S-phase synthesis. PCNA studies revealed a corresponding stimulation of cell cycle progression. The wave of S-phase synthesis and cell cycle progression returned to control levels in the H2O + CCl4 group by 60 hr but continued up to 72 hr in the ISOP + CCl4 group. These findings support the hypothesis that in response to increased infliction of CCl4 injury by isopropanol, augmented stimulation of cell division and tissue repair restrain the progression of injury and restore hepatic structure and function, thereby allowing the rats to survive. Further, antimitotic intervention with colchicine (1 mg/kg, ip) led to decreased S-phase synthesis, followed by 60% lethality in the isopropanol-pretreated group in contrast to 40% lethality in the group receiving CCl4 alone (H2O + CCl4). These findings suggest that greater stimulation of tissue repair restrains the progression of ISOP-enhanced infliction of CCl4 liver injury and accounts for recovery from enhanced liver injury and animal survival. The findings are consistent with a two-stage model of toxicity wherein liver injury is linked by progression or regression of injury, which is governed by the extent of tissue repair to the final outcome.

1-Propanol↗
PubMed1996

Physiologically based pharmacokinetic/pharmacodynamic modeling of the toxicologic interaction between carbon tetrachloride and Kepone.

Carbon tetrachloride (CCl4) lethality in Sprague-Dawley rats is greatly amplified by pretreatment of Kepone (decachlorooctahydro-1,3,2-metheno-2H-cyclobuta[cd] pentalen-2-one). The increase in lethality was attributed to the obstruction of liver regenerative processes. These processes are essential for restoring the liver to its full functional capacity following injury by CCl4. Based on the available mechanistic information on Kepone/CCl4 interaction, a physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model was constructed where the following effects of Kepone on CCl4 toxicity are incorporated: (1) inhibition of mitosis; (2) reduction of repair mechanism of hepatocellular injury; (3) suppression of phagocytosis. The PBPK/PD model provided computer simulation consistent with previously published time-course results of hepatotoxicity (i.e., pyknotic, injured and mitotic cells) of CCl4 with or without Kepone. As a further verification of this model, the computer simulations were also consistent with exhalation kinetic data for rats injected with different intraperitoneal (i.p.) doses of CCl4 in our laboratory. Subsequently, the PBPK/PD model, coupled with Monte Carlo simulation, was used to predict lethalities of rats treated with CCl4 alone and CCl4 in combination with Kepone. The experimental lethality studies performed in our laboratories were as follows: Sprague-Dawley rats were given either control diet or diet containing 10 ppm Kepone for 15 days. On day 16, rats in the Kepone treated group were given i.p. doses of 0, 10, 50, and 100 microliters/kg CCl4 (n = 9) while control rats were exposed to 0, 100, 1000, 3000, and 6000 microliters/kg CCl4 (n = 9). Lethality was observed at the 1000 (1/9), 3000 (4/9), and 6000 (8/9) microliters/kg doses for the control group and at the 50 (4/9) and 100 (8/9) microliters/kg for the treated group. Based on Monte Carlo simulation, which was used to run electronically 1000 lethality experiments for each dosing situation, the LD50 estimates for CCl4 toxicity with and without Kepone pretreatment were 47 and 2890 microliters/kg, respectively. Monte Carlo simulation coupled with the PBPK/PD model produced lethality rates which were not significantly different from the observed mortality, with the exception of CCl4 at very high doses (e.g., 6000 microliters/kg, p = 0.014). Deviation at very high doses of the predicted mortality from the observed may be attributed to extrahepatic systemic toxicities of CCl4, or solvent effects on tissues at high concentrations, which were not presently included in the model. Our modeling and experimental results verified the earlier findings of Mehendale (1990) for the 67-fold amplification of CCl4 lethality in the presence of Kepone. However, much of this amplification of CCl4 lethality with Kepone pretreatment was probably due to pharmacokinetic factors, because when target tissue dose (i.e., model estimated amount of CCl4 metabolites) was used to evaluate lethality, this amplification was reduced to 4-fold.

Animals↗
PubMed1996

Strain differences in tissue repair response to 1,2-dichlorobenzene.

Fischer 344 (F344) rats are reportedly 75-fold more sensitive than Sprague Dawley (S-D) rats to 1,2-dichlorobenzene (o-DCB) hepatotoxicity. Lethality studies were conducted since no information was available regarding the ultimate consequence of this sensitivity in terms of animal survival in the two strains. LD50S for o-DCB (1.66 ml/kg and 1.76 ml/kg in male F344 and S-D rats, respectively) did not differ. Several studies have shown the importance of tissue repair on animal survival following exposure to toxic chemicals. The objective of this study was to investigate if differential rates of cell division and tissue repair might explain the lack of difference in LD50 dose between the two strains despite higher hepatotoxic injury in F344 rats. Age-matched male S-D and F344 rats were administered o-DCB (0.2, 0.6, 1.2 ml/kg, i.p.); injury and tissue repair occurring as two dynamic but opposing events were measured over time. Liver injury was assessed by measuring plasma alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) activities and by liver histopathology. Higher plasma ALT elevations were observed in F344 rats following administration of 0.2 and 0.6 ml o-DCB/kg. Using SDH as a marker of liver injury, the strain difference was evident only at 0.2 ml o-DCB/kg. Liver regeneration was estimated by 3H-thymidine incorporation into hepatonuclear DNA and via proliferating cell nuclear antigen (PCNA) assay. Prompt and significantly higher hepatocellular regeneration beginning at 36 h was evident in F344 rats following administration of 0.2 and 0.6 ml o-DCB/kg. The significantly higher depletion of hepatic glycogen observed in F344 rats following administration of 0.2 and 0.6 ml o-DCB/kg occurred without significant changes in plasma glucose and is consistent with highly stimulated tissue repair seen in these rats at the corresponding doses. However, increasing the dose further to 1.2 ml o-DCB/kg results in a delayed (S-phase synthesis begins at 48 h) and diminished response to o-DCB. These findings suggest that a significantly higher rate of tissue repair in F344 rats helps them overcome higher liver injury inflicted by o-DCB. This differential in tissue repair in the two strains may play a vital role in equalizing the ultimate outcome of toxicity in the two strains.

Animals↗
PubMed1996

A review of the role of tissue repair as an adaptive strategy: why low doses are often non-toxic and why high doses can be fatal.

The role of tissue repair as an adaptive strategy by species is important to consider in both evolutionary and toxicological perspectives. This paper assesses the distinct and integrative roles of early phase regeneration (EPR) (i.e. arrested G2 hepatocytes chemically activated to proceed through mitosis) and secondary phase regeneration (SPR) (i.e. hepatocytes mobilized principally from G0/G1 to proceed through mitosis) in the repair of carbon tetrachloride (CCl4)-induced liver damage. The role of EPR as a triage system facilitating repair of minor toxic insults as well as providing an essential role in autoprotection as an initial step to augment and sustain SPR is proposed. The function of EPR is then compared with that of SPR in tissue recovery following more massive injury. The interrelationships of these two repair processes with EPR invoking and accelerated SPR following low-to-modest degrees of toxicant-induced hepatotoxicity as well as in auto- or hetero-protection supports the theory that the two responses are co-ordinated in time and functionality. The integration of these two repair processes as shown through experimental manipulation provides a new mechanistic framework to account for the previously reported profound (67-fold) potentiation of acute CCl4 hepatotoxicity by chlordecone (kepone) in adult male Sprague-Dawley rats as well as important interspecies variation in susceptibility to hepatotoxic agents in general and CCl4 in particular. On the basis of the distinct and integrative roles of EPR and SPR in liver responses to toxic injury, a generalized framework is presented that facilitates prediction of both toxic outcome, including shape of dose-response functions and interspecies variation to chemically induced liver damage.

Animals↗
PubMed1996

Hepatic cell division and tissue repair: a key to survival after liver injury.

The survival of patients suffering from severe liver damage depends heavily on the ability of the remaining hepatocytes to regenerate and replace the dead or dying cells; death usually occurs when the regenerating ability of the liver is compromised owing to heavy damage to the liver. The current approach to therapy aims only to block additional liver injury from hepatotoxicants or hepatic disease. If hepatocellular regeneration and tissue repair could be stimulated after hepatic damage by a therapeutically compatible mechanism, then it might be possible to prevent death arising from serious liver injury.

Age Factors↗
PubMed1996

Nutritional modulation of the final outcome of hepatotoxic injury by energy substrates: an hypothesis for the mechanism.

Survival after hepatocellular injury and necrosis may depend on the ability of the remaining hepatocytes to divide and restore an adequate population of functioning cells. Although adequate nutritional support is necessary for liver regeneration after severe liver damage, much is yet to be discovered concerning which nutritional factors are critical for liver regeneration. Clinically, nutritional substances are administered only from the energy aspect, without regard to whether or how these substrates may facilitate or impede liver tissue repair processes. Glucose is used as principal source of energy in liver damage because of accompanying marked hypoglycemia. But the contribution of glucose to compensatory liver regeneration and/or survival is unclear. This paper advances the hypotheses that: (1) glucose increases the toxicity of centrilobular hepatotoxicants by inhibiting hepatic cell division and tissue repair allowing unrestrained progression of injury; (2) fatty acids facilitate hepatic-cell division permitting hepatolobular restoration to occur thus preventing death from even a lethal dose. If hepatic tissue repair can be stimulated by some therapeutically compatible mechanism, then it might be possible to prevent death from even massive hepatocellular injury. In addition to nutritional manipulation, it should be possible to exploit molecular mechanisms that regulate organized cell division (tissue repair) to increase survival rates of patients suffering from fulminant hepatic failure. These findings have significant impact on tissue repair in a variety of other organs and tissues, particularly in diabetes-like conditions.

Animals↗
PubMed1996

Effect of an antimitotic agent colchicine on thioacetamide hepatotoxicity.

In an earlier study we established that timely and adequate tissue repair response following the administration of a six-fold dose-range of thioacetamide (TA; 50, 150, and 300 mg/kg) prevented progression of injury and led to recovery and animal survival. Delayed and attenuated repair response after the 600 mg/kg TA dose resulted in a marked progression of injury and 100% lethality. The objective of the present study was to further scrutinize this concept in an experimental protocol in which we hypothesized that a selective ablation of the tissue repair response should lead to lethality from the nonlethal, moderately toxic doses of 150 and 300 mg/kg TA. In this study we investigated the effect of the antimitotic agent colchicine (CLC, 1 mg/kg) on the outcome of TA hepatotoxicity. Male Sprague-Dawley rats (175-225 g) were injected intraperitoneally (ip) with 150 and 300 mg/kg TA. We assessed liver injury by serum enzyme elevations and histopathology. Tissue regeneration response was measured by 3H-thymidine incorporation into hepatonuclear DNA and by proliferating cell nuclear antigen (PCNA) assay. S-Phase stimulation, as indicated by 3H-thymidine incorporation, was noted at 36 and 48 hr following the administration of 150 mg/kg TA, whereas with the 300 mg/kg TA S-phase stimulation was elicited at 48 hr following treatment. Therefore, two doses of CLC (30 hr and 42 hr, 1 mg/kg, ip) were administered to the 150 mg/kg treated group while a single dose of CLC (42 hr, 1 mg/kg, ip) was administered to the 300 mg/kg group. CLC treatment resulted in 100% lethality in both groups. Thus, CLC administration converted nonlethal doses into lethal doses. The 150 mg/kg TA dose was then chosen to further investigate the underlying mechanism. Rats treated with TA alone recovered from injury by 36-48 hr while CLC treatment resulted in a progression of injury as indicated by serum enzyme elevation and histopathology. Tissue repair, as evidenced by 3H-thymidine incorporation and PCNA studies explained this dichotomy. Antimitotic intervention with CLC resulted in a significantly diminished repair response leading to unrestrained progression of injury and lethality even from nonlethal doses. This model demonstrates the critical role of tissue repair response in determining the final outcome of toxicity.

Animals↗
PubMed1995

Toxicodynamics of low level toxicant interactions of biological significance: inhibition of tissue repair.

Because of the complexity of studying the toxicological effects of mixtures of chemicals, much of the mechanistic information has become available through work with binary mixtures of toxic chemicals. Mechanisms derived from studies employing chemicals at individually nontoxic doses are more useful than the mechanisms of interactive toxicity at high doses from the perspective of environmental and public health. Several examples of chemical combinations and interactive toxicity at low doses are now available. Chlordecone-potentiated halomethane hepatotoxicity, where suppression of cell division and tissue repair response permits very high amplification of CCl4 injury culminating in animal mortality, is one such model. Phenobarbital-potentiated CCl4 injury does not lead to animal mortality in spite of much higher liver injury in comparison to the chlordecone+CCl4 model. Much higher stimulation of tissue repair allows the animals to survive despite higher liver injury. Similar interactions have been reported between alcohols and halomethane toxicants. These and other studies have revealed that infliction of toxicant-induced injury is accompanied by a parallel but opposing tissue repair stimulation response which allows the animals to overcome that injury up to a threshold dose. Beyond this threshold, tissue repair response is both diminished and delayed allowing unrestrained progression of injury. Large doses of chemicals can be predictably lethal owing to these two latter effects on tissue repair. Dose-response paradigms in which tissue repair response is measured as a parallel but opposing effect to toxic injury might be useful in more precise prediction of the ultimate outcome of toxic injury in risk assessment. Autoprotection experiments with CCl4, thioacetamide, 2-butoxyethanol and related chemicals as well as heteroprotection against acetaminophen-induced lethality with thioacetamide are examples where tissue repair stimulation has been shown to rescue the animals from massive and normally lethal liver injury. The concept of toxicodynamic interaction between inflicted injury and stimulated tissue repair offers mechanistic opportunity to fine-tune other aspects of human health risk assessment procedure. Tissue repair mechanisms may also offer a mechanistic basis to explain species and strain differences as well as to more accurately assess inter-individual differences in human sensitivity to toxic chemicals. Because tissue repair is affected by nutritional status, assessment of risk from exposure to chemicals without attention to nutritional status may be misleading. Finally, the concept of using maximum tolerated doses (MTDs) in long-term toxicity studies such as cancer bioassays may need to be re-examined. MTDs might be predictably expected to maximally stimulate cell division and it is known that increased cell division is likely to lead to increased number of errors in DNA replication thereby predisposing these animals to cancer. It is clear that detailed studies of toxicodynamic interaction between tissue injury and stimulated tissue repair are likely to yield significant dividends in fine-tuning risk assessment.

Animals↗
PubMed1995

Topical exposure to chlordane reduces the contact hypersensitivity response to oxazolone in BALB/c mice.

Previous studies have shown that prenatal exposure to the organochlorine pesticide chlordane significantly decreases the ear swelling response to the contact allergen oxazolone in BALB/c mice. Alterations of macrophage function in the efferent arm of the contact hypersensitivity response have also been reported. In the current study, chlordane was applied topically and the effects of oxazolone-induced contact hypersensitivity were determined. Initially, the reduction in oxazolone-induced ear swelling in topically-exposed female BALB/c mice was compared to 30-day-old BALB/c female mice exposed prenatally to chlordane. Prenatal chlordane exposure induced a 36% reduction in ear swelling compared to a 60% reduction following topical treatment at the challenge phase. Topically-applied chlordane also reduced the oxazolone-induced ear swelling by 40% when applied at sensitization. When applied at both sensitization and challenge, ear swelling was reduced by 71%. In a time-course study, it was determined that chlordane must be applied at the time of sensitization, challenge or both or within 1 h post-challenge to significantly reduce ear swelling. A dose-response study showed that the lowest concentration of chlordane resulting in a significantly reduced ear swelling response was 20 micrograms per ear.

Administration, Topical↗
PubMed1995

Pristane-induced effects on cytochrome P-4501A, ornithine decarboxylase and putrescine in rats.

The effects of pristane (2,6,10,14-tetramethylpentadecane) on cytochrome P-4501A (cP4501A) activity in microsomes, as well as on ornithine decarboxylase (ODC) activity and concomitant putrescine levels were examined in Copenhagen rats. In general, pristane treatment led to increased cP4501A levels when compared to basal levels, while co-treatment with 3-methylcholanthrene (3-MC) and pristane elicited augmented cP4501A responses when compared to responses induced by 3-MC alone. Increases in both ODC activity and putrescine levels were also observed in pristane treated rats. Collectively, these results indicate that pristane influences cP4501A activity and elicits promoter-like responses as reflected in elevated ODC activity and increased amount of putrescine.

Animals↗
PubMed1995

Age-related differences in TGF-alpha and proto-oncogenes expression in rat liver after a low dose of carbon tetrachloride.

The resiliency of rats during early post-natal development to CCl4 or to an interactive hepatotoxicity of chlordecone (CD) + CCl4 has been shown to be due to an efficient stimulation of tissue repair. The objective of the current study was to investigate if this is due to efficient expression of transforming growth factor-alpha (TGF-alpha) and proto-oncogenes. Postnatally developing (20 day old) and adult (60 day old) male Sprague-Dawley rats were challenged with a single low dose of CCl4 (100 microL/kg, ip) or corn oil. Liver samples were collected during a time course (0-96 h) after the administration of CCl4 and used to examine TGF-alpha and early (c-fos) and late (H-ras and K-ras) proto-oncogenes mRNA expressions. Significant increases in TGF-alpha, H-ras, and K-ras gene expressions were evident as early as 12 hours after CCl4 and peaked between 24 and 48 hours in an age-dependent manner as detected by slot-blot analysis. Results of the study revealed three- and twofold increases in TGF-alpha gene expression in 20 and 60 day old rats, respectively, after CCl4. There were 3.5- and 2.5-fold increases in H-ras and 4.4- and 3.4-fold increases in K-ras in 20 and 60 day old rats, respectively. In contrast, a 10-fold increase in c-fos mRNA expression was evident in 20 day old rats 1 hour after CCl4 treatment, returning to the baseline value by 3 hours, whereas in 60 day old rats, this increase was less than twofold. The overall findings of this study indicate that TGF-alpha and the early and late proto-oncogene mRNA expressions were enhanced in an age- and time-dependent manner in response to a low dose of CCl4. These results further strengthen the view that the remarkable resiliency of rats to hepatotoxicants during early postnatal development is due to substantial increases in stimulation of hepatocellular regeneration and tissue repair mechanisms, leading to regression of liver injury and recovery.

Aging↗
PubMed1995

2-Butoxyethanol autoprotection is due to resiliance of newly formed erythrocytes to hemolysis.

Pretreatment with a low dose of a toxic chemical protecting the animals from a subsequently administered lethal dose of the same chemical is called autoprotection. Autoprotection by model hepatotoxicants has been recently shown to be due to augmentation of cell division and tissue repair as well as an inherent resiliance of newly divided cells. The present studies were designed to investigate if an autoprotection model could be established in an extrahepatic tissue. The second objective was to test the hypothesis that inherent resiliance of newly divided cells is a major contributing mechanism for autoprotection. Female Sprague-Dawley rats (200-250 g) received a single administration of a moderately toxic but nonlethal dose (500 mg/kg, p.o.) 7 days prior to the administration of an LD90 dose (1500 mg/kg, p.o.) of the same compound. All rats receiving the initial protective dose are able to survive the lethal dose of butoxyethanol, in contrast to the death of those receiving the lethal dose alone. Following the administration of butoxyethanol, the hematocrit decreased from the normal 45% to 18% and by day 7, recovered to normal levels. Following the lethal challenge, hematocrit decreased to 13% in the naive rats, while decreasing only to 27% in rats receiving the protective dose, permitting animal survival. Administration of pyrazole to inhibit metabolism of butoxyethanol to butoxyacetic acid abolished autoprotection.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldehyde Dehydrogenase↗
PubMed1995

Injury and repair as opposing forces in risk assessment.

Recent advances in our understanding of the toxicodynamic events that follow infliction of injury have helped us to bridge the link between the tissue injury and the final outcome of that injury. In addition to infliction of tissue injury, toxic chemicals induce a biological compensatory response of tissue repair intended to overcome tissue injury through healing. Since stimulation of tissue repair is a simultaneous response accompanying injury, measuring this response in addition to quantifying injury might be helpful in tomorrow's risk assessment. Studies with model hepatotoxicants such as thioacetamide and CCl4, where tissue repair as well as injury were measured, reveal that endogenous mechanisms that drive the tissue repair response are responsible for more than just compensation for tissue injury. Up to a threshold dose, tissue repair is stimulated in a dose-dependent manner, and above this threshold it is both delayed and diminished. During this delay, tissue injury progresses unabated leading to tissue destruction and animal death. While dose-related stimulation of tissue repair leads to recovery, delayed and diminished tissue repair seen at the high doses leads to tissue destruction and animal death. These findings impact on the currently used maximum tolerated doses (MTDs) in cancer bioassays. MTDs represent maximal stimulation of cell proliferation thereby enhancing the likelihood of errors in DNA replication. Measuring tissue repair and injury as simultaneous biological responses to toxic agents might increase the usefulness of dose-response paradigms in risk assessment.

Animals↗
PubMed1995

The uptake and metabolism of cystamine and taurine by isolated perfused rat and rabbit lungs.

Cystamine has been reported to be taken up and metabolized to taurine by the rat lung slices. The objectives of the present study were to compare the uptake and metabolism of cystamine and taurine in isolated perfused lungs of rats and rabbits and examine the action of glutathione (GSH) on these processes. The uptake and metabolism of [14C]cystamine and [14C]taurine were studied at 20 microM concentrations each in isolated, ventilated, perfused rat and rabbit lungs. In some experiments, 1 microM GSH was included in the perfusate prior to the addition of cystamine. The perfusate and lung homogenate samples were analyzed for cystamine and its metabolites. [14C]cystamine uptake with and without GSH was 13 and 14% in rat lungs and 37 and 32% in rabbit lungs. [14C]taurine uptake was 10% in rat and 37% in rabbit lungs. The levels of radiolabeled cystamine and its metabolites were (nmol/g lung): 20.0 +/- 10.0 and 11.5 +/- 7.0 cystamine, 4.7 +/- 0.5 and 3.2 +/- 0.5 hypotaurine and 56.0 +/- 16.0 and 49.4 +/- 6.0 taurine, for rat and rabbit lungs, respectively, when perfused without GSH; and 18.0 +/- 1.0 and 2.5 +/- 0.5 cystamine, 6.6 +/- 0.5 and 18 +/- 10 hypotaurine and 60.0 +/- 12.0 and 33.6 +/- 9.0 taurine, when perfused with GSH, for rats and rabbit lungs, respectively. Taurine did not undergo any further metabolism in either of the lungs. These studies show that cystamine is taken up and metabolized to taurine via hypotaurine by both rat and rabbit lungs in a manner similar to that seen in rat lung slices. However, rat lungs have much greater capacity to metabolize cystamine to taurine than rabbit. Inclusion of GSH did not significantly alter the ability of lungs to sequester cystamine from the perfusate but the metabolism of hypotaurine to taurine was markedly decreased in rabbit lungs. Taurine was not metabolized any further. It is concluded that rat and rabbit lungs take up cystamine from the systemic circulation, metabolize it via hypotaurine to taurine, and effuse most of the latter in to the circulation.

Animals↗