Sources of intramitochondrial malate.

Liver mitochondria from rats treated with gluconeogenic hormones or subjected to vigorous exercise consume oxygen more rapidly than do mitochondria from control rats. These treatments result in elevated mitochondrial malate concentrations, which facilitate the entry of added substrate into the mitochondria. In this paper we describe experiments conducted to determine the source of the extra malate. Injections of glutamate plus alanine, two amino acids that are increased in blood after exercise and hormone treatment, caused liver mitochondrial malate to be increased. Injections of glucagon, cortisol, or both hormones elevated liver mitochondrial malate concentrations in both adrenalectomized and sham-operated rats.

Ca2+-mediated action of long-chain acyl-CoA on liver mitochondria energy-linked processes.

The decrease of steady-state transmembrane potential (delta psi) and loss of accumulated Ca2+ are magnified if palmitoyl-CoA is added to rat liver mitochondria exposed to Ca2+ and phosphate. The extent of this damage increases with increasing concentration of long-chain acyl-CoA. Addition of L-carnitine with or without the addition of palmitoyl-CoA considerably delays the deenergization. In the latter case, there is a substantial decrease in the assayed endogenous long-chain acyl-CoA content. This protective action of L-carnitine is abolished by L-aminocarnitine, a powerful inhibitor of carnitine palmitoyl transferase (palmitoyl-CoA: L-carnitine O-palmitoyltransferase, EC 2.3.1.21.). The removal of Ca2+ by EGTA, or the inhibition of its uptake by Ruthenium red or Mg2+ further enhances the degree of protection.

Changes in mitochondrial activity caused by ammonium salts and the protective effect of carnitine.

Ammonium salts added to isolated rat liver mitochondria deviate alpha-ketoglutarate to glutamate synthesis, thus decreasing its availability as respiratory substrate. As a consequence a decrease of respiratory rate is observed which is paralleled by progressive mitochondrial swelling. It was demonstrated that L-carnitine may abolish this swelling thus improving structural and metabolic state of mitochondria.

Effect of amrinone on myocardial mitochondria function.

The effect of amrinone on cardiac mitochondria of guinea pig was studied. It was found that amrinone does not change the respiratory function of cardiac mitochondria in the presence of alpha-ketoglutarate, whereas it inhibits glutamate oxidation. It was also found that amrinone strongly inhibits the activity of glutamic dehydrogenase of both crude extract from sonicated heart mitochondria and of purified preparation from bovine liver. This inhibition may explain the effect of amrinone on the oxidation of glutamate in mitochondria. These results are discussed in view of the contradictory effects of amrinone on cardiac and other tissues.

The role of malate in exercise-induced enhancement of mitochondrial respiration.

Liver mitochondria isolated from rats immediately after exercise oxidize substrates more rapidly than do mitochondria from resting animals. In both fed and fasted rats, a 1-h period of exercise resulted in increased concentrations of malate in their livers and in the mitochondria isolated therefrom. This increase occurred in both untrained and exercise-trained rats. Because mitochondrial malate is known to facilitate mitochondrial uptake of other carboxylic substrates, it seems likely that the increased mitochondrial malate is responsible for the increased rate of oxidation. Rats injected with small amounts of malate (4.6 mumol/100 g body wt) yielded liver mitochondria with increased malate concentration and increased rates of oxidation of citrate, alpha-ketoglutarate, and succinate. The beta adrenergic antagonist propranolol (0.25 mg/100 g body wt) and the alpha 1 antagonist prazosin (same dose) did not abolish the effect of exercise on mitochondrial malate concentration or substrate oxidation.

The role of malate in hormone-induced enhancement of mitochondrial respiration.

Shortly after the injection of glucagon, epinephrine, norepinephrine, vasopressin, or angiotensin II into fasted rats, mitochondria isolated from their livers contained elevated concentrations of malate and oxidized citrate, alpha-ketoglutarate, and, in some cases, succinate more rapidly than mitochondria from fasted, control rats. The administration of tryptophan, lactate, or ethanol and refeeding of rats fasted 24 h result in similar elevations of mitochondrial malate concentration and oxidation of added substrates. Treatments that resulted in elevated mitochondrial malate resulted also in increased uptake of added citrate, alpha-ketoglutarate, pyruvate, and, in some cases, succinate. It is postulated that the well-documented effect of gluconeogenic hormones on mitochondrial oxidation of carboxylic substrates may be mediated by malate which not only yields oxalacetate to support the tricarboxylic acid cycle but also facilitates the transport of added substrates, and which is regenerated in the tricarboxylic acid cycle.

The effect of different types of physical exercise on glucose and citrulline synthesis in isolated rat liver parenchymal cells.

Rates of gluconeogenesis from lactate or pyruvate in hepatocytes from untrained rats were not increased by an acute (1 h) bout of exercise (running at 20 m/min). Hepatocytes from rats that had been exercise-trained for 1 month had lower rates of gluconeogenesis from lactate than cells from unexercised controls; the rates with pyruvate were identical. Hepatocytes from livers of trained animals immediately after 1 h of exercise synthesized glucose more rapidly and accumulated more citrulline than cells from resting rats.

Stabilising action of carnitine on energy linked processes in rat liver mitochondria.

Rat liver mitochondria exposed to stressing conditions - ageing at room temperature, incubation in the presence of t-butyl hydroperoxide or damaging concentrations of Ca2+ and phosphate- undergo a rapid fall in their membrane potential (delta psi) with a concomitant release of endogenous Mg2+ and accumulated Ca2+. Addition of L-carnitine to the incubation medium considerably delays mitochondrial deenergization. A similar, though lower, protection has also been observed in L-carnitine pretreated and subsequently washed rat liver mitochondria. Furthermore mitochondria isolated from livers of starved rats, treated with L-carnitine 30 minutes before death and exposed to the same stressing conditions show similar delay in the decrease of delta psi and concurrent energy linked processes as compared with untreated animals. Both the in vitro and in vivo results strongly indicate that the stabilising action of L-carnitine on liver mitochondria is due to the removal of membrane bound long chain acyl CoA.

The effect of nutritional state and hormones on the number of circulating red cells in Gallus domesticus.

When chickens are fasted, the liver becomes dark, whereas the spleen is bleached. Microscopic and spectrophotometric analysis indicated the colour resulted from red blood cells accumulating in enlarged sinusoids of the liver. In birds fasted for 48 hr the hematocrit increases 30%. Blood volume and red cell volume are not altered; the extra cells are not newly synthesised. The extra cells arise from the spleen, possibly under alpha-adrenergic control if given epinephrine by injection and under beta-adrenergic control during fasting as indicated by experiments with alpha and beta-receptor blocking agents.

The influence of fasting on chicken liver metabolites, enzymes and mitochondrial respiration.

Chicken liver mitochondria were isolated in relatively pure form as indicated by electron microscopy and marker enzyme assay. The rate of respiration, respiratory control index and ADP/O ratios with several different substrates indicated that chicken liver mitochondria are more uncoupled than rat liver mitochondria. Chickens have ten-fold higher malate concentrations in liver than do rats, 2-oxoglutarate was also more abundant in chicken livers. Fasted birds had a five-fold increase in beta-hydroxybutyrate as compared with fed birds; whereas malate and lactate concentrations decreased. Fasted birds had increased levels of isocitrate dehydrogenase (NADP dependent) and lactate dehydrogenase in the cytosol, and increased malate dehydrogenase (NAD dependent), isocitrate dehydrogenase (NADP dependent) and malic enzyme activities in the mitochondria.