Effects of ethanol on the structure and function of rat liver mitochondrial and microsomal membranes

Betaine treatment attenuates chronic ethanol-induced hepatic steatosis and alterations to the mitochondrial respiratory chain proteome

Introduction. Mitochondrial damage and disruption in oxidative phosphorylation contributes to the pathogenesis of alcoholic liver injury. Herein, we tested the hypothesis that the hepatoprotective actions of betaine against alcoholic liver injury occur at the level of the mitochondrial proteome. Methods. Male Wister rats were pair-fed control or ethanol-containing liquid diets supplemented with or without betaine (10 mg/mL) for 4-5 wks. Liver was examined for triglyceride accumulation, levels of methionine cycle metabolites, and alterations in mitochondrial proteins. Results. Chronic ethanol ingestion resulted in triglyceride accumulation which was attenuated in the ethanol plus betaine group. Blue native gel electrophoresis (BN-PAGE) revealed significant decreases in the content of the intact oxidative phosphorylation complexes in mitochondria from ethanol-fed animals. The alcohol-dependent loss in many of the low molecular weight oxidative phosphorylation proteins was prevented by betaine supplementation. This protection by betaine was associated with normalization of SAM : S-adenosylhomocysteine (SAH) ratios and the attenuation of the ethanol-induced increase in inducible nitric oxide synthase and nitric oxide generation in the liver. Discussion/Conclusion. In summary, betaine attenuates alcoholic steatosis and alterations to the oxidative phosphorylation system. Therefore, preservation of mitochondrial function may be another key molecular mechanism responsible for betaine hepatoprotection.

Mechanisms of signal transduction in the stress response of hepatocytes

Adaptation of animals to stress is a unique property of life which allows the survival of the species. The stress response of hepatocytes is a very complex phenomenon, sometimes involving a cascade of events. The general stress signals are elucidated by mobilization of carbohydrate stores and akin to the insulin mediators. Oxidative signals are generated by pesticides, heavy metals, drugs, and alcohol which may or may not be under the purview of peroxisomes. Peroxisomal responses are well-defined involving specific receptors, whereas nonperoxisomal responses may be signaled by calcium, the Ah receptor, or built-in antioxidant systems. The intoxication signals are generally thought to be membrane defects induced by xenobiotics which then lead to highly nonspecific responses of hepatocytes. Detoxication signals, on the other hand, are specific responses of hepatocytes triggering de novo syntheses of detoxifier proteins or enzymes. Evidence reveals the existence of two distinct mechanisms of signal transduction in stressed hepatocytes--one involving the peroxisome and the other the plasma membrane.

Effects of acute alcohol intoxication on gluconeogenesis and its hormonal responsiveness in isolated, perfused rat liver

Rats were acutely administered ethanol as a primed constant infusion in order to produce sustained blood ethanol levels of 8-12 or 55-65 mM. At the end of ethanol infusion the livers were either freeze-clamped in vivo or isolated and perfused for metabolic studies. The rate of gluconeogenesis and its responsiveness to phenylephrine (10 microM), prostaglandin F2 alpha (5 microM) and glucagon (10 nM), as well as the redox state of the cytosolic NAD(+)-NADH system were assessed in livers isolated from acutely ethanol-treated rats, and subsequently perfused without ethanol. For liver clamped in vivo, high- but not low-ethanol treatment decreased the ATP content by 31% and slightly increased ADP and AMP content, resulting in a decreased energy charge (11%). Glutamate and aspartate content was also increased in high-dose ethanol-infused rats with no changes in malate and 2-oxoglutarate content. Gluconeogenesis with saturating concentrations of lactate (4 mM)+pyruvate (0.4 mM) was delayed in reaching a plateau in the livers of high-dose ethanol-treated rats and its response to all three stimulators was impaired. Low-dose ethanol treatment only decreased the liver response to phenylephrine. While the perfused livers of low-dose ethanol-treated rats displayed no changes in adenine nucleotide content, the livers of high-dose ethanol-treated rats had a decreased ATP (35%) and an increased AMP (77%) content, paralleled by a fall in the total adenine nucleotides (14%) and energy charge (14%). No differences were observed between the saline- and ethanol-treated rats with respect to malate-aspartate shuttle intermediate concentration in perfused livers. Also, the livers of high-, but not low-dose ethanol-treated rats had a more negative value of NAD(+)-NADH redox state as compared to the livers of control rats. The data suggest that acute ethanol intoxication produces changes in liver metabolism and its responsiveness to hormones/agonists that are demonstrable for at least 2 hr after isolation and perfusion of the liver.

Maintenance of structural and functional characteristics of skeletal-muscle mitochondria and sarcoplasmic-reticular membranes after chronic ethanol treatment

The effect of long-term ethanol intake on the structural and functional characteristics of rat skeletal-muscle mitochondria and sarcoplasmic reticulum was investigated. Functionally, skeletal-muscle mitochondria were characterized by a high respiratory control index and ADP/O ratio and a high State-3 respiration rate with different substrates. These parameters were not significantly different in preparations from control and ethanol-fed rats, except for a small increase in the rate of oxidation of alpha-oxoglutarate/malate in the latter. In submitochondrial particles from the two groups of animals there was no significant difference in cytochrome content, ATPase activity or the activity of respiratory-chain complexes. Mitochondrial membranes from untreated and ethanol-fed rats showed no difference in the baseline e.s.r. order parameter, and both preparations were equally sensitive to disordering by ethanol in vitro. Similarly, sarcoplasmic-reticulum preparations were not significantly affected by long-term ethanol feeding with respect to Ca2(+)-ATPase activity or in baseline order parameter and susceptibility to membrane disordering by ethanol in vitro. These membranes were also equally sensitive to degradation by exogenous phospholipase A2. Ethanol feeding did not alter the class composition of mitochondrial or sarcoplasmic-reticulum membrane phospholipids, nor the acyl composition of individual phospholipid classes. Specifically, the changes in acyl composition that characteristically occur in liver microsomal phosphatidylinositol and liver mitochondrial cardiolipin were not observed in the corresponding phospholipids from skeletal-muscle membranes. In experiments where membrane preparations from liver and skeletal muscle from the same ethanol-fed animals were compared, the liver membranes developed membrane tolerance, with the muscle membranes retaining normal sensitivity to disordering effects by ethanol. It is concluded that: (a) different tissues from the same animals differ in their susceptibility to ethanol; (b) the tissue-specific lack of development of membrane tolerance correlates with a lack of chemical changes in the phospholipids and with a retention of normal function of mitochondria and sarcoplasmic reticulum; (c) effects of chronic ethanol intake on muscle function are not due to a defect in the mitochondrial energy supply.

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.