Toxicity and kinetics of [3H]microcystin-LR in isolated perfused rat livers

Subacute and sublethal ingestion of microcystin-LR impairs lung mitochondrial function by an oligomycin-like effect

Microcystin-LR (MC-LR) is a potent cyanotoxin that can reach several organs. However subacute exposure to sublethal doses of MC-LR has not yet well been studied. Herein, we evaluated the outcomes of subacute and sublethal MC-LR exposure on lungs. Male BALB/c mice were exposed to MC-LR by gavage (30 µg/kg) for 20 consecutive days, whereas CTRL mice received filtered water. Respiratory mechanics was not altered in MC-LR group, but histopathology disclosed increased collagen deposition, immunological cell infiltration, and higher percentage of collapsed alveoli. Mitochondrial function was extensively affected in MC-LR animals. Additionally, a direct in vitro titration of MC-LR revealed impaired mitochondrial function. In conclusion, MC-LR presented an intense deleterious effect on lung mitochondrial function and histology. Furthermore, MC-LR seems to exert an oligomycin-like effect in lung mitochondria. This study opens new perspectives for the understanding of the putative pulmonary initial mechanisms of damage resulting from oral MC-LR intoxication.

Microcystin Toxicokinetics, Molecular Toxicology, and Pathophysiology in Preclinical Rodent Models and Humans

Microcystins are ubiquitous toxins produced by photoautotrophic cyanobacteria. Human exposures to microcystins occur through the consumption of contaminated drinking water, fish and shellfish, vegetables, and algal dietary supplements and through recreational activities. Microcystin-leucine-arginine (MCLR) is the prototypical microcystin because it is reported to be the most common and toxic variant and is the only microcystin with an established tolerable daily intake of 0.04 µg/kg. Microcystin toxicokinetics is characterized by low intestinal absorption, rapid and specific distribution to the liver, moderate metabolism to glutathione and cysteinyl conjugates, and low urinary and fecal excretion. Molecular toxicology involves covalent binding to and inhibition of protein phosphatases, oxidative stress, cell death (autophagy, apoptosis, necrosis), and cytoskeleton disruption. These molecular and cellular effects are interconnected and are commonly observed together. The main target organs for microcystin toxicity are the intestine, liver, and kidney. Preclinical data indicate microcystins may also have nervous, pulmonary, cardiac, and reproductive system toxicities. Recent evidence suggests that exposure to other hepatotoxic insults could potentiate microcystin toxicity and increase the risk for chronic diseases. This review summarizes the current knowledge for microcystin toxicokinetics, molecular toxicology, and pathophysiology in preclinical rodent models and humans. More research is needed to better understand human toxicokinetics and how multifactorial exposures contribute to disease pathogenesis and progression.

In Vivo and In Vitro Toxicity Testing of Cyanobacterial Toxins: A Mini-Review

Harmful cyanobacterial blooms are increasing and becoming a worldwide concern as many bloom-forming cyanobacterial species can produce toxic metabolites named cyanotoxins. These include microcystins, saxitoxins, anatoxins, nodularins, and cylindrospermopsins, which can adversely affect humans, animals, and the environment. Different methods to assess these classes of compounds in vitro and in vivo include biological, biochemical, molecular, and physicochemical techniques. Furthermore, toxic effects not attributable to known cyanotoxins can be observed when assessing bloom material. In order to determine exposures to cyanotoxins and to monitor compliance with drinking and bathing water guidelines, it is necessary to have reliable and effective methods for the analysis of these compounds. Many relatively simple low-cost methods can be employed to rapidly evaluate the potential hazard. The main objective of this mini-review is to describe the assessment of toxic cyanobacterial samples using in vitro and in vivo bioassays. Newly emerging cyanotoxins, the toxicity of analogs, or the interaction of cyanobacteria and cyanotoxins with other toxicants, among others, still requires bioassay assessment. This review focuses on some biological and biochemical assays (MTT assay, Immunohistochemistry, Micronucleus Assay, Artemia salina assay, Daphnia magna test, Radionuclide recovery, Neutral red cytotoxicity and Comet assay, Enzyme-Linked Immunosorbent Assay (ELISA), Annexin V-FITC assay and Protein Phosphatase Inhibition Assay (PPIA)) for the detection and measurement of cyanotoxins including microcystins, cylindrospermopsins, anatoxin-a, saxitoxins, and nodularins. Although most bioassay analyses often confirm the presence of cyanotoxins at low concentrations, such bioassays can be used to determine whether some strains or blooms of cyanobacteria may produce other, as yet unknown toxic metabolites. This review also aims to identify research needs and data gaps concerning the toxicity assessment of cyanobacteria.

Alteration of intracellular GSH levels and its role in microcystin-LR-induced DNA damage in human hepatoma HepG2 cells

Microcystin-LR (MCLR) is a liver-specific toxin known as a tumour promoter in experimental animals. Its mechanisms of hepatotoxicity have been well documented; however, the mechanisms of other effects, in particular those related to its genotoxicity, are not well understood. In our previous studies, we showed that MCLR-induced DNA strand breaks are transiently present and that the damage is mediated by reactive oxygen species (ROS). In this study, we show that exposure of HepG2 cells to non-cytotoxic doses of MCLR-induced time-dependent alterations in the level of intracellular reduced glutathione (GSH). These comprised a rapid initial decrease followed by a gradual increase, reaching a maximum after 6h of exposure, before returning to the control level after 8h. During the first 4h, expression of glutamate-cysteine ligase (GCL), the rate-limiting enzyme of GSH synthesis, increased, indicating an increased rate of de novo synthesis of GSH. The most important observation of this study, combined with the results of our previous studies is the correlation between the time course of alterations of intracellular GSH content and the formation and disappearance of MCLR-induced DNA damage. When the intracellular GSH level was reduced, MCLR-induced DNA damage was observed to increase. Later, when the level of intracellular GSH was normal or elevated, new DNA damage was not induced and existing damage was repaired. To confirm the role of GSH system in MCLR-induced genotoxicity, the intracellular GSH level was moderated by pre-treatment with buthionine-(S,R)-sulfoximine (BSO), a specific GSH synthesis inhibitor, and with N-acetylcysteine (NAC), a GSH precursor. Pre-treatment with BSO dramatically increased the susceptibility of HepG2 cells to MCLR-induced DNA damage, while pre-treatment with NAC almost completely prevented MCLR-induced DNA damage. Thus, intracellular GSH is shown to play a critical role in the cellular defence against MCLR-induced DNA damage in HepG2 cells.

Identification of human liver mitochondrial aldehyde dehydrogenase as a potential target for microcystin-LR

Microcystins (MCs) are hepatotoxins produced by a variety of freshwater cyanobacteria. The toxicity of these hepatotoxins is a severe health issue for both humans and livestock; MCs have been implicated in the development of liver cancer, necrosis, and even deadly intrahepatic bleeding. Microcystin-LR (MC-LR) is the MC variant most commonly encountered in a contaminated aquatic system. Thus far, MC-LR has only been shown to target the serine/threonine protein phosphatases 1 and 2A (PP1 and PP2A) and it is still unknown whether MC-LR can bind and inhibit any other protein targets inside the cell. To find potential MC-LR targets, we screened a phage display library for peptide ligands that specifically recognize MC-LR. Using these peptide sequences as guides, we performed a series of bioinformatics analyses revealing that MC-LR binds human liver aldehyde dehydrogenase 2 (ALDH2) at residues 447-451. We confirmed MC-LR binding of ALDH2 via automated docking computation, which yielded results matching our experimental and bioinformatics analyses. ALDH2 dysfunction may lead to aldehyde-induced reactive oxygen species (ROS) generation and, in turn, apoptosis. Therefore, ALDH2 could potentially be a target of MC-LR associated with the process of ROS-induced apoptosis. Our current study presents a new approach to the study of interactions of biological molecules by combining phage display technology with computational methods.

Cyanobacterial toxins - occurrence, biosynthesis and impact on human affairs

Mass developments of cyanobacteria ("blue-green algae") in lakes and brackish waters have repeatedly led to serious concerns due to their frequent association with toxins. Among these are the widespread hepatotoxins microcystin (MC) and nodularin (NOD). Here, we give an overview about the ecostrategies of the diverse toxin-producing species and about the genes and enzymes that are involved in the biosynthesis of the cyclic peptides. We further summarize current knowledge about toxicological mechanisms of MC and NOD, including protein phosphatase inhibition, oxidative stress and their tumor-promoting capabilities. One biotransformation pathway for MC is described. Mechanisms of cyanobacterial neurotoxins (anatoxin-a, homanatoxin-a, and anatoxin-a(s)) are briefly explained. We highlight selected cases of human fatalities related to the toxins. A special focus is given to evident cases of contamination of food supplements with cyanobacterial toxins, and to the necessary precautions.

Ecotoxicological effects of selected cyanobacterial secondary metabolites: a short review

Cyanobacteria are one of the most diverse groups of gram-negative photosynthetic prokaryotes. Many of them are able to produce a wide range of toxic secondary metabolites. These cyanobacterial toxins can be classified in five different groups: hepatotoxins, neurotoxins, cytotoxins, dermatotoxins, and irritant toxins (lipopolysaccharides). Cyanobacterial blooms are hazardous due to this production of secondary metabolites and endotoxins, which could be toxic to animals and plants. Many of the freshwater cyanobacterial blooms include species of the toxigenic genera Microcystis, Anabaena, or Plankthotrix. These compounds differ in mechanisms of uptake, affected organs, and molecular mode of action. In this review, the main focus is the aquatic environment and the effects of these toxins to the organisms living there. Some basic toxic mechanisms will be discussed in comparison to the mammalian system.

Expression modulation of multiple cytokines in vivo by cyanobacteria blooms extract from taihu lake, China

Cyanobacterial blooms that generate microcystins (MCs) are being increasingly recognized as a potent health hazard in aquatic ecosystems. However, immunomodulation induced by cyanotoxins has not been well documented. This paper reports the in vivo data on the immune disorder caused by crude microcystin (MC) extract of cyanobacteria blooms collected from Taihu Lake, China, with respect to cytokine mRNA levels. Using reverse-transcriptional polymerase chain reaction (RT-PCR), the expression of multiple cytokines, including proinflammatory (IL-1beta, TNF-alpha, and IL-6) and Th1/Th2-related cytokines (IL-2, IL-4 and IL-10), was evaluated following the cyanobacteria blooms extract containing MCs (CBE) exposure at four doses of 23, 38, 77, 115 mg lyophilized algae cells/kg body weight. The results showed that the mRNA levels of TNF-alpha, IL-1beta, IL-2 and IL-4 decreased significantly following injection of all doses as compared to the control (LPS or ConA only), while the IL-6 level was unaffected. Contrast to this decrease, the level of IL-10 mRNA was, however, transiently up regulated following injection of the lowest dose of CBE. The distinct patterns of expression of these cytokines suggested a modulation of cytokine network, the essential component of the host immune system. We further developed a mathematical model to simulate the interaction of T helper cell subsets and related cytokines, which proved to be a good approach to study the kinetics of the interaction of cells and cytokines in microcystin immunosuppression.

Microcystin-LR induces oxidative DNA damage in human hepatoma cell line HepG2

Microcystins are naturally occurring hepatotoxins produced by strains of Microcystis aeruginosa. They are involved in promoting primary liver tumours and a previous study showed that they might also be tumour initiators. In this study we demonstrate that microcystin-LR (MCLR) at doses that were not cytotoxic (0.01-1 microg/ml), induced dose and time dependent DNA strand breaks in human hepatoma cell line HepG2. These DNA strand breaks were transient, reaching a maximum level after 4h of exposure and declining with further exposure. In the presence of the DNA repair inhibitors cytosine arabinoside (AraC) and hydroxyurea (HU), together with MCLR, DNA strand breaks accumulated after prolonged exposure. These results suggest that DNA strand breaks are intermediates, produced during the cellular repair of MCLR induced DNA damage. Digestion of DNA with purified, oxidative DNA damage specific enyzmes, endonuclease III (Endo III) and formamidopyrimidine-DNA glycosylase (Fpg) markedly increased DNA strand breaks in MCLR treated cells, providing evidence that a substantial portion of the MCLR induced DNA strand breaks originate from excision of oxidative DNA adducts. A hydroxyl radical scavenger (DMSO) significantly reduced MCLR induced DNA damage. From these results we conclude that MCLR induces formation of reactive oxygen species that cause DNA damage, and that MCLR may act as an initiator of liver cancer.

Microcystic cyanobacteria extract induces cytoskeletal disruption and intracellular glutathione alteration in hepatocytes

Microcystins are a group of highly liver-specific toxins, although their exact mechanisms of action remain unclear. We examined the effects of microcystic cyanobacteria extract (MCE) collected from a contaminated water source on the organization of cellular microtubules (MTs) and microfilaments (MFs) in hepatocytes. We also investigated the effects on lactate dehydrogenase (LDH) leakage and intracellular glutathione (GSH). Primary cultured rat hepatocytes exposed to MCE (equivalent to 125 microg/mL lyophilized algae cells) showed a characteristic disruption of MTs and MFs in a time-dependent manner. Under these conditions, MCE caused aggregation of MTs and MFs and a severe loss of MTs in some cells. Moreover, MCE-induced cytoskeletal alterations preceded the LDH leakage. On the other hand, the treatment of cells with MCE led to a dose-dependent increase of intracellular GSH. However, time-course study showed a biphasic change of intracellular GSH levels with a significant increase in the initial stage followed by a decrease after prolonged treatment. Furthermore, pretreatment with N-acetylcystein (NAC), a GSH precursor, significantly enhanced the intracellular GSH level and decreased the MCE-induced cytotoxicity as well as cytoskeleton changes. In contrast, buthionine-(S, R)-sulfoximine, a specific GSH synthesis inhibitor, increased the cell susceptibility to MCE-induced cytotoxicity by depleting the intracellular GSH level. These findings suggest that intracellular GSH plays an important role in MCE-induced cytotoxicity and cytoskeleton changes in primary cultured rat hepatocytes. Increasing intracellular GSH levels protect cells from MCE-induced cytotoxicity and cytoskeleton changes.

Monitoring of microcystin-protein phosphatase adduct formation with immunochemical methods

Using anti-microcystin-LR monoclonal antibodies, an immunoblotting procedure was developed to monitor the formation of microcystin-protein phosphatase adducts in vitro and in vivo. The detection limits for the covalent binding of MCYST-LR with the recombinant protein phosphatase 1 (PP1) and rabbit liver cytosol proteins were found to be 0.1 ng and 0.3 ng per assay, respectively. MCYST-PP1 adducts were detected 30 s after the addition of MCYST-LR into the reaction mixture. Reduction of the methyldehydroalanine (Mdha) residue of MCYST-LR with ethanethiol totally abolished the covalent binding of the toxin to PP1, but retained its inhibitory toxicity on PP1. Immunoblotting analyses and enzyme-linked immunosorbant assay showed that between 5 min to 16 h after i.p. injection of single dose (35 microg/kg) of MCYST-LR into mice, approximately 0-27% of the injected toxin was found covalently bound while 0.2-9.2% existed as free form in liver cytosol.

Hepatic toxicity and persistence of ser/thr protein phosphatase inhibition by microcystin in the little skate Raja erinacea

Microcystin-induced ser/thr protein phosphatase (PP) inhibition and toxicity were examined in the little skate (Raja erinacea), an evolutionarily primitive marine vertebrate. As in mammals, PP inhibition and toxicity were exclusively hepatocellular, but were much more persistent in the skate. A dose of 63 microg/kg given iv to adult male skates resulted in the near complete inhibition of hepatic PP activity at 24 h. PP activity was still 95% inhibited 7 days after dosing in skates given 125 microg/kg microcystin. Mortality occurred at doses of 500 microg/kg or more. Hepatic lesions were only seen in animals with fully inhibited PP activity in liver. The histological changes seen at 125 microg/kg were mild periportal inflammatory changes increasing in severity together with hepatocyte necrosis at higher doses of microcystin. Microcystin persisted and could be detected in plasma up to 7 days after dosing. This finding shows that, in the skate, as in mammals, the liver is the only organ capable of uptake of microcystin, since there was no significant inhibition of PP activity in the rectal gland and small decreases in PP activity of the kidney that were not time or dose dependent. In vitro microcystin caused dose-dependent inhibition of PP activity in isolated skate hepatocytes, while it was without effect in cultured rectal glands. Uptake of microcystin and the accompanying inhibition of PP activity in skate hepatocytes was prevented by the addition of a series of organic dyes and bile acids. The spectrum of inhibitors of microcystin uptake in skate is similar to that seen in the rat, indicating common features of the carrier(s) in these diverse species.

Studies on oxidative damage induced by cyanobacteria extract in primary cultured rat hepatocytes

Contamination of water by cyanobacteria (blue-green algae) is a serious health problem around the world, largely due to the toxic effects of microcystins, a group of potent hepatotoxins. However, the mechanisms responsible for the cytotoxicity of microcystins have not been fully elucidated. In the present study, oxidative damage caused by lyophilized freshwater cyanobacteria extract was evaluated on primary cultured rat hepatocytes. A time- and dose-dependent increase of lactate dehydrogenase (LDH) leakage was observed in hepatocytes treated with cyanobacteria extract. Lipid peroxidation, a main manifestation of oxidative damage, was also studied and a time- and dose-dependent increase in malondiadehyde was observed. In addition, by using a fluorescent probe, 2',7'-dichlorofluorescein diacetate, it was found that cyanobacteria extract was able to enhance intracellular production of reactive oxygen species (ROS) in a dose- and time-dependent manner. Moreover, desferrioxamine, a specific iron chelator, could significantly decrease LDH leakage and ROS production caused by cyanobacteria extract treatment. These findings thus provide experimental evidence that oxidative damage is involved in cyanobacteria extract-induced hepatotoxicity. The understanding of this mechanism is believed to be beneficial to the prevention and control of the toxicity of microcystin and cyanobacteria contamination.

Distribution of tritiated dihydromicrocystin in swine

The distribution of tritiated dihydromicrocystin [3H]2H-MCLR was studied in anesthetized specific-pathogen-free pigs. Two doses were administered i.m. and one dose was given via an isolated ileal loop. At 4 hr after i.v. administration of the toxin at 25 micrograms/kg, 64.6% of the total dose (%TD) was located in the liver, with smaller amounts distributed to the kidneys (1.2% TD), lungs (1.75% TD), heart (0.22% TD), ileum (0.13% TD) and spleen (0.04% TD). A similar distribution was found at 4 hr postdosing in pigs given 75 micrograms/kg, although the liver contained a lower fraction of the total dose, at 46.99% TD, and the kidneys had somewhat more, at 2.19% TD, than the low dose. At the high dose, the fractions of the amount given accounted for by the lungs (0.55% TD), heart (0.23% TD), ileum (0.20% TD) and spleen (0.07% TD) were similar to those at the low dose. The livers of the pigs given 75 micrograms/kg via the ileal loop, at 5 hr postdosing, contained 49.5% TD and the ileum had 33.94% TD. Smaller amounts were distributed to kidneys (1.04% TD), lungs (0.65% TD), heart (0.81% TD) and spleen (0.16% TD). The livers of both groups dosed at 75 micrograms/kg contained higher concentrations of toxin, but lower percentages of the total dose, than the livers of pigs dosed at 25 micrograms/kg. Larger increases in serum arginase in the two 75 micrograms/kg groups were associated with histological evidence of more severe liver damage than at the 25 micrograms/kg dose. Analysis of radiolabeled compounds from hepatic tissue using fast atom bombardment mass spectrometry determined that the primary constituent was [3H]2H-MCLR, but two minor radioactive components were also isolated. These findings indicate that [3H]2H-MCLR is rapidly concentrated in the liver of swine, whether given i.v. or via an isolated ileal loop, that at extremely toxic doses uptake is slowed, and that it is as toxicologically active as the parent compound.

Biochemical characterization of microcystin toxicity in rainbow trout (Oncorhynchus mykiss)

The kinetics and biochemical effects of microcystins in rainbow trout were studied with freeze-dried toxic cells of Microcystis aeruginosa, strain PCC 7806. Following in vivo exposure the changes in liver histology were observed over a 72 hr period and the absorption of microcystins from the gastrointestinal tract into the blood and liver, as well as the inhibition of hepatic protein phosphatase 1 and 2A activities, were recorded using the protein phosphatase inhibition assay. The interaction between microcystins and trout liver phosphatases was further tested in vitro using the protein phosphatase inhibition assay. The in vivo experiments demonstrated a high organotropy of microcystins for the liver, where rapid and total inhibition of protein phosphatase 1 and 2A activity was observed. Maximal inhibition of phosphatases was observed 3 hr after gavage. At that time-point, approximately 63% of the toxin present in the liver was refractive to detection via the phosphatase inhibition assay and therefore most likely covalently bound to cellular proteins. The inhibition of hepatic protein phosphatases 1 and 2A proved to be transient only, as a progressive increase in phosphatase activity was observed beginning 12 hr after gavage of the fish, reaching approximately 50% of the control activity at 72 hr. In contrast, liver damage continued to progress despite this renewed protein phosphatase activity.

Hepatic necrosis in aged mice by oral administration of microcystin-LR

Aged mice (32 weeks) were orally administered microcystin-LR at 500 micrograms/kg, and injuries of the liver were estimated by microscopy 2 hr after treatment. Sixty-two per cent of aged mice proved to be sensitive to microcystin-LR, whereas such changes in the liver were not found in young mice (5 weeks). Uptake of the toxin into the liver was confirmed by high-performance liquid chromatography and frit-fast atom bombardment liquid chromatograph/mass spectrometry after clean-up with an immunoaffinity column. To verify the difference in sensitivity to microcystin-LR between aged and young mice, non-treated mice were examined, and among them aged mice were confirmed to have a rough surface of the stomach and small intestinal mucosa. These results suggested that the hepatotoxicity by oral administration of microcystin-LR is deeply related to aging, and particularly to conditions in the small intestine such as the permeability of capillaries in the villi.

Endothelins 1 and 3: potent cholestatic agents secreted and excreted by the liver that interact with cyclosporine

Autoradiographic studies have shown that the liver accumulates endothelin. High-affinity binding sites for endothelin have been identified on rat liver plasma membranes. We investigated the role of endothelin isopeptides as mediators of cholestasis with isolated rat liver perfused by a recirculating solution of buffer and blood. These studies demonstrated that endothelin-1, as measured by means of radioimmunoassay, was cleared from the perfusate by the liver and that the liver concentrated both endothelin-1 and endothelin-3 in bile. Addition of endothelin-1 to the liver perfusate solution increased the concentration of endothelin-3 measured in the perfusate, suggesting that endothelin-1 caused release or secretion of endothelin-3. Both endothelin-1 and endothelin-3 at 5 nmol/L caused almost complete cessation of bile flow, but this effect was more prolonged after endothelin-1 than after endothelin-3 administration. Because it has been reported that cyclosporine increases endothelin levels, we studied the interaction of these two compounds. Cyclosporine (100 mumol/L) also produced cholestasis. Endothelin-3 secretion in bile, however, was decreased in livers perfused with cyclosporine compared with secretion in controls. Simultaneous addition of endothelin-1 and cyclosporine that on their own were not significantly cholestatic produced cholestasis. In conclusion, endothelin is a potent cholestatic agent secreted and excreted by the liver. It may potentiate the cholestatic action of cyclosporine.

Association of microcystin-LR and its biotransformation product with a hepatic-cytosolic protein

Microcystin-LR (MCYST-LR), a cyclic peptide hepatotoxin, associates with high-molecular-weight, liver cytosolic components. Repetitive cycles of heat denaturation and pronase digestion released 80 +/- 6% of the bound radiolabel from these components, parent toxin (22%), and two biotransformation products, with high-performance liquid chromatography (HPLC) retention times of 6.7 (52%) and 5.6 (13%) min. Both parent and the biotransformed (6.7 min) toxin appeared to be covalently bound to a monomeric protein of molecular weight 40,000 (protein plus radiolabeled toxin). Binding and biotransformation reactions were time- and temperature-dependent and did not require endogenous molecules less than 6,000 daltons. The binding appeared to be saturable with a maximum of 20 pmol MCYST-LR bound per mg protein. The binding protein(s) and biotransformation activity were present in rat liver, brain, kidney, heart, lung, small intestine, large intestine, testes, skeletal muscle, and to a lesser extent, in fat. Okadaic acid, a specific protein phosphatase inhibitor, showed a concentration-dependent inhibition of [3H]MCYST-LR binding to hepatic cytosol. The molecular weight and organ distribution of the binding protein(s), and inhibition of binding by okadaic acid were consistent with one of the binding sites being the catalytic subunit of protein phosphatase type 2A.

Hepatotoxicity of microcystin-LR in fed and fasted rats

The LD50 (25 hr, i.p.) for microcystin-LR in fed rats (122 micrograms/kg) was significantly higher than that in fasted rats (72 micrograms/kg). At doses of 100, 150 and 200 micrograms of microcystin-LR per kg, the median times to death were 31.9, 18.2 and 11.2 hr for fed rats, and 1.8, 1.7 and 1.5 hr for fasted rats. A sublethal dose of microcystin (50 micrograms/kg) afforded protection to fasted, but not fed, rats against a subsequent lethal dose (200 micrograms/kg) challenge given 72 hr later. Biochemical and ultrastructural changes resulting from microcystin-LR (100 micrograms/kg, i.p.) were compared in fed and fasted rats 1 hr after injection. In both groups, liver weight and serum levels of sorbitol dehydrogenase and glucose significantly increased. Plasma membranes, isolated from livers of fed or fasted rats, exhibited similar toxin-induced changes in associated cytoskeletal elements. Liver mitochondria from toxin-treated, fasted rats exhibited complete inhibition of state 3 respiration, while those from toxin-treated, fed rats had ADP/O ratios and respiratory control indices comparable to control values. The primary event responsible for enhanced microcystin hepatotoxicity in the fasted state has not yet been identified. Depletion of glycogen stores and a decreased respiratory capacity may, however, play significant roles in this degenerative process.