Glucagon stimulation of citrulline formation in isolated hepatic mitochondria

Role of quercetin on hepatic urea production in acute renal failure

Acute renal failure (ARF) is a serious damage of renal function induced by various nephrotoxic drugs, ischemia, bilateral urethral obstruction, trauma and unilateral nephrectomy. Dramatic clinical syndrome, azotemia, develops as a result of hypovolemia, oliguria, reduced glomerular filtration and acidosis. In addition to classic medications recent studies give more attention to beneficial effect of natural plant products as bioflavonoids. We have studied the influence of bioflavonoid, quercetin, on hepatic urea production in glycerol induced ARF in the rats. Male Sprague Dawley rats were used in the experiment. The value of urea production in the liver was determined by measuring of liver arginase activity, the terminal enzyme of urea cycle. Arginase activity was increased (p < 0.01) as well as urea level (p < 0.001) 48 h after glycerol administration. Pretreatment by quercetin suppressed the arginase activity in the liver (p < 0.05) and plasma levels of urea (p < 0.01). So, we have concluded that quercetin may be beneficial in glycerol induced ARF.

Maintenance of energy-linked functions in rat liver mitochondria

EGTA and EDTA are compared with respect to their ability to preserve ATP-requiring reactions in rat liver mitochondria. The presence of EGTA sustains the high ATP requirement of citrulline synthesis. EDTA does not, even with excess Mg2+. Carboxylation of pyruvate, which has a lower ATP demand, is not influenced by the type of chelating agent. In mitochondria stored for 24 h at 4 degrees C, EGTA is more effective than EDTA in preventing loss of these energy-linked functions.

A smaller initial dose protects mice against several lethal doses of ammonium acetate

The synthesis of urea in the liver is the main mechanism for the elimination of excess ammonia. Rapid stimulation of the synthesis of urea (e.g. by administration of carbamyl glutamate, the analog of the physiological activator of carbamyl phosphate synthetase I) protects animals given lethal doses of ammonia. Since ammonia enhances the activity of the urea cycle, we tested and show here that administration of small doses of ammonium acetate supresses the mortality induced by a series of repeated LD100 of ammonium acetate separated by one hour, when the first LD100 is injected i.p. starting from 30 min to 5 hours after the initial smaller dose of ammonium acetate. Under these conditions, the levels of ammonia in blood are elevated more than ten times, but in spite of the greater amount of ammonia administered, the ammonemia is much lower than in mice dying after a single LD100. The enhanced synthesis of urea observed is correlated with an increase in the intramitochondrial content of N-acetyl glutamate. These findings are of interest as far as the short-term regulation of urea cycle, the mechanism of ammonia toxicity and have clinical implications.

The mechanism of Ca2+ stimulation of citrulline and N-acetylglutamate synthesis by mitochondria

ATP-driven citrulline synthesis by mitochondria treated with oligomycin, uncoupler and Ca2+ ionophore is stimulated more than 2-fold by an increase in extramitochondrial free [Ca2+] in the range 1-5 microM. Stimulation increases with the length of preincubation of mitochondria with Ca2+. EGTA prevents stimulation if added at the start of the preincubation period, but is without effect if added at the end, suggesting that Ca2+ acts indirectly on citrulline synthesis via the accumulation of an intermediate. Neither carbamoyl-phosphate synthase (ammonia) nor ornithine carbamoyltransferase are stimulated by up to 50 microM free Ca2+ in mitochondrial extracts, but N-acetylglutamate synthase is stimulated about 30% by 10 microM free Ca2+. We propose that an increase in the activity of N-acetylglutamate synthase in response to an increase in free [Ca2+] in the mitochondrial matrix may contribute to hormonal stimulation of the urea cycle.

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.

Sub-micromolar concentrations of extramitochondrial Ca2+ stimulate the rate of citrulline synthesis by rat liver mitochondria

1. In the presence of physiological concentrations of Na+ and Mg2+, the rate of citrulline synthesis by isolated rat liver mitochondria respiring on a range of substrates was stimulated by up to 60% when the extramitochondrial Ca2+ concentration was raised from 130 pM to 770 nM. 2. Our findings suggest that hormonal stimulation of the urea cycle may be mediated by Ca2+.

Aroclor 1254 treatment and fasting influences on rat liver mitochondrial carbamoyl phosphate synthesis with ADP and ATP

Previous studies have shown that the polychlorinated biphenyl mixture, Aroclor 1254 (ARO), -induced wasting in male rats is associated with increased permeability of hepatic mitochondria. This was correlated with hyperuremia and stimulated urea synthesis, hypoglycemia and suppressed glucogenesis after an ammonium acetate injection, and decreased retention of assimilated nitrogen and food intake. For ARO-toxic rats (100 mg/kg, ip, for 1, 2, and 4 days) versus Tween 80-treated, ad libitum-fed controls, mitochondrial carbamoyl phosphate (CP) formation (the initial step in urea synthesis from NH4+) was progressively stimulated for the duration of treatment from NH4+ and ATP but not from NH4+ and ADP. ARO maximal stimulation of CP formation also correlated with significant loss in body weight. Mitochondrial ornithine transcarbamoylase synthesis of citrulline from ornithine and carbamoyl phosphate was also stimulated. In comparison to fasted rats (24 hr), mitochondrial CP synthesis from NH4+ was enhanced with ADP but not with ATP. This ARO uncoupling of mitochondrial NH4+ metabolism and stimulation of CP formation with exogenous ATP and citrulline synthesis may have resulted from increased availability of substrates and cofactors in the matrix space, leakage of enzymes from the matrix, or a combination of these effects. These results are consistent with an increased inner membrane permeability and fragility during isolation and assays. In agreement with our previous studies, the data show that ARO exposure poises hepatic mitochondria toward the synthesis of urea intermediates.

Citrulline accumulation in mice induced by administration of L-hydrazinosuccinate

L-Hydrazinosuccinate has been shown to induce a marked inhibition of liver aspartate aminotransferase isoenzymes in mice. The effects of the drug on the amino acid content of liver were studied. Intraperitoneal administration of L-hydrazinosuccinate enormously increased the citrulline content of liver and plasma in 6 hr and, less markedly, increased the glutamate and ammonia content of liver with a simultaneous decrease in the aspartate content. Drug administration also induced a marked increase in the liver mitochondrial activity of citrulline formation from ornithine, ammonia and carbon dioxide, with a similar increase in N-acetylglutamate content; a prominent increase in liver tryptophan dioxygenase activity; and an elevated level of plasma corticosterone. The increase of citrulline was interpreted to be produced by decreased conversion of citrulline to argininosuccinate due to a lack of aspartate because of inhibition of aspartate aminotransferase by the drug and increased formation of citrulline due to increases of glutamate and ammonia, which further induced the increase of N-acetylglutamate, because of inhibition of aminotransferase as well as stimulation of amino acid degradation by glucocorticoids.

Biochemical analysis of decreased ornithine transport activity in the liver mitochondria from patients with hyperornithinemia, hyperammonemia and homocitrullinuria

Hyperornithinemia, hyperammonemia and homocitrullinuria (HHH disorder) is an inherited metabolic disorder which shows peculiar amino acid changes in the serum and urine. The primary defect is considered to be the transport of ornithine across the mitochondrial membrane, but there is no direct evidence for this so far. We have analyzed ornithine transport activities in the liver mitochondria from three patients with HHH disorder. In coupled liver mitochondria we demonstrated low activities of citrulline synthesis and low rates of ornithine uptake. However, there were no abnormalities in carbamoyl-phosphate synthetase activity, ornithine carbamoyltransferase activity, N-acetylglutamate levels or O2 uptake with succinate. We also performed a kinetic study of citrulline synthesis as a function of ornithine concentration. We found increased Km values for ornithine and varied Vmax values of citrulline synthesis, which suggested the presence of a mutant transport protein. From these results we conclude that the defect of hyperornithinemia, hyperammonemia and homocitrullinuria lies in the transport of ornithine across the mitochondrial membrane.

The potentiation of ammonia toxicity by sodium benzoate is prevented by L-carnitine

Sodium benzoate has been recommended and even been used for the treatment of hyperammonemia in humans. More recently, a note of caution was raised since it has been shown that in experimental animals, sodium benzoate potentiates ammonia toxicity and inhibits urea synthesis in vitro. This has been further confirmed in the work presented here and the mechanism by which benzoate increases mortality and the levels of blood ammonia in mice given ammonium acetate have also been studied. In hyperammonemia, urea production and N-acetylglutamate levels were decreased by sodium benzoate. Pretreatment of mice with L-carnitine suppressed mortality following ammonium acetate plus sodium benzoate administration. Under these conditions L-carnitine lowered blood ammonia and increased urea production and N-acetylglutamate levels.

Mitochondrial function after acute alteration of the endogenous insulin-to-glucagon ratio

Mannoheptulose (2g/kg i.p.) increases serum glucagon and decreases serum insulin via its effect on pancreatic islet cells. These changes in endogenous hormone status had effects on rat liver mitochondria that were comparable to the effects of injecting porcine glucagon (0.5 mg/kg i.p.). Mitochondrial adenine nucleotide content was increased 38 or 39% by mannoheptulose or glucagon respectively, citrulline synthesis by 165 or 193%, pyruvate carboxylation by 113 or 135%, coupled respiration by 34 or 42%, and uncoupled respiration by 40 or 54%. We conclude that the reciprocal changes in endogenous insulin and glucagon brought about by mannoheptulose offer a useful and interesting alternative to glucagon injection for studying the effects of these pancreatic hormones on liver mitochondria.

Stimulation of mitochondrial functions by glucagon treatment, starvation and by treatment of isolated mitochondria with glycogen-bound enzymes

Liver mitochondria isolated from rats starved overnight, or fed rats injected with glucagon, exhibited a similar increase of the respiration rate with succinate (by 30-40%) and glutamate plus malate (by 20-30%), as compared to mitochondria from control fed animals. The content of mitochondrial adenine nucleotides was elevated by 30-45% by glucagon treatment or starvation. Mitochondrial respiration and citrulline synthesis were stimulated by 30-40% when mitochondria isolated from fed rats were briefly preincubated with the extract from liver glycogen granules, ATP and MgCl2. This effect was abolished by heating the extract at 100 degrees C.

Inhibition of citrulline synthesis by octanoate and its modulation by adenine nucleotides

Liver mitochondria from octanoate-treated rabbits showed an impaired ability to synthesize citrulline. Two methods were used to evaluate citrulline synthesis in rat liver mitochondria. Under these conditions octanoate inhibited citrulline synthesis by over 50%. When ATP was included in the assay medium the inhibitory effect of octanoate was prevented. In the absence of ATP in the suspending medium, octanoate did not significantly lower total adenine nucleotides in rat liver mitochondria. However, under these conditions octanoate caused a change in the adenine nucleotide profile such that ATP content was decreased and AMP content was increased. When ATP was present in the assay medium, octanoate caused a similar increase in AMP content. However, ATP decreased only slightly. The alterations in mitochondrial adenine nucleotide profile by octanoate and the reversal of the effect by exogenous ATP suggests that octanoate inhibits citrulline synthesis via reduced intramitochondrial ATP levels. The ability of octanoate to lower mitochondrial ATP and elevate mitochondrial AMP may be related to its intramitochondrial activation by the medium chain fatty acid activating enzyme.

The stimulatory effect of alloxan diabetes on citrulline formation in rabbit liver mitochondria

The effect of alloxan diabetes on citrulline formation from NH4Cl and bicarbonate was studied in rabbit liver mitochondria incubated with glutamate or succinate as respiratory substrate, as well as with exogenous ATP in the presence of uncoupler and oligomycin. In contrast to ornithine transcarbamoylase, the activity of carbamoyl-phosphate synthetase (ammonia) was higher in mitochondria from diabetic animals than in those from normal ones. In diabetic rabbits the rates of citrulline synthesis were stimulated under all conditions studied. In contrast, levels of N-acetylglutamate, an activator of carbamoyl-phosphate synthetase (ammonia), were significantly increased only in the presence of glutamate, while the highest rates of citrulline formation occurred in uncoupled mitochondria incubated with exogenous ATP as energy source. Treatment of animals with alloxan resulted in an increase of both the intramitochondrial ATP level and the rate of adenine nucleotide translocation across the mitochondrial membrane. The results indicate that the stimulation of citrulline formation in liver mitochondria of diabetic rabbits is mainly due to an increase in carbamoyl-phosphate synthetase (ammonia) activity and an elevation of content of intramitochondrial ATP, a substrate of this enzyme.

The effect of glucagon on N-acetylglutamate-synthetase

The effect of glucagon and the protein content of the diet on the activity of N-acetylglutamate synthetase was studied. The activity of N-acetylglutamate synthetase depended on the protein content of the diet. Glucagon increased the activity of N-acetylglutamate synthetase and reduced the stimulatory effect of arginine. The enzyme of glucose-fed animals became arginine independent. It was concluded that glucagon induced some kind of covalent modification of the synthetase.

Stimulation of mitochondrial pyruvate metabolism and citrulline synthesis by dexamethasone. Effect of isolation and incubation media

Hepatic mitochondria isolated in 0.3 M-sucrose or 0.3 M-mannitol from rats treated for 3h with dexamethasone displayed stimulated rates of pyruvate carboxylation and decarboxylation and citrulline synthesis when compared with organelles from control animals. Mitochondria isolated in mannitol also displayed elevated rates of pyruvate carboxylation and decarboxylation when compared with those isolated in sucrose, and this stimulation was shown to be independent of the lengthy isolation procedure. Citrulline synthesis proceeded at similar rates in mitochondria isolated in either sugar. The concentration of exchangeable adenine nucleotides was identical in mitochondria isolated in sucrose or mannitol, suggesting that those prepared in the former sugar are not more permeable to metabolites than those prepared in the latter. The matrix volume of mitochondria isolated in mannitol was greater than that of mitochondria isolated in sucrose, and the effect of mannitol on pyruvate metabolism was mimicked by swelling the organelles in hypo-osmotic sucrose. Measurements of the extra-matrix volume by using [14C]sucrose or [14C]mannitol suggest that mannitol can permeate mitochondria to a greater extent than can sucrose. The possibility that mannitol elicits its effect by entering the mitochondrial matrix and so initiating swelling is discussed.

Glucagon and fasting do not activate peroxisomal fatty acid β-oxidation in rat liver

In a study of the endocrine control of peroxisomes, the effects of acute glucagon treatment and fasting on hepatic peroxisomal beta-oxidation in rats have been investigated. The activity of the rate-limiting peroxisomal beta-oxidation enzyme, fatty acyl-CoA oxidase, was measured to determine whether activation of peroxisomal beta-oxidation could account for the increase in total hepatic fatty acid oxidation following acute glucagon exposure. Catalase, a peroxisomal enzyme not directly involved in beta-oxidation, was also measured as a control for total peroxisomal activity. No changes with acute glucagon treatment of intact animals were observed with either activity as measured in liver homogenates or partially purified peroxisomal fractions. These observations indicate the lack of acute control by glucagon of peroxisomal function at the level of total enzyme activity. Previous work on the effects of fasting on hepatic fatty acid beta-oxidation [H. Ishii, S. Horie, and T. Suga (1980) J. Biochem. 87, 1855-1858] suggested an enhanced role for the peroxisomal beta-oxidation pathway during starvation. It was found that the peroxisomal beta-oxidation system, as measured by fatty acyl-CoA oxidase activity, does increase with duration of fast when expressed on a per gram wet weight liver basis. However, when this activity is expressed as total liver capacity, a decline in activity with increasing duration of fast is observed. Furthermore, this decline in peroxisomal capacity parallels the decline in total liver capacity for citrate synthase, a mitochondrial matrix enzyme, and total liver protein. These data indicate that peroxisomal beta-oxidation activity is neither stimulated nor even preferentially spared from proteolysis during fasting.

Possible role of Pi supply in mitochondrial actions of glucagon

Glucagon is able to diminish the net release of inorganic phosphate (Pi) occurring on incubation of isolated hepatocytes from 48-h-starved rats. Concomitantly the hormone increases the cellular Pi content. This is associated with a rise of Pi in the cytosolic fraction. Other hormonal effectors like phenylephrine, vasopressin and angiotensin II exert a smaller and transient effect as compared to glucagon. It is proposed that this increase in Pi availability to the mitochondria, by favouring substrate level phosphorylation at the succinyl-CoA synthetase step plays a role in the development of the metabolite pattern found in the mitochondrial matrix space after exposure of hepatocytes to glucagon or the above agents. With regard to the glutamate level this view is evidenced by the finding that its hormone-dependent decrease was inversely correlated to the respective increase in the cytosolic Pi concentration. Further evidence is provided by experiments with isolated mitochondria incubated under state-3 conditions at medium Pi concentrations corresponding to those metabolically active in the cytosolic compartment of control and glucagon-stimulated hepatocytes, being 2 mM and 3 mM, respectively. Increasing medium phosphate concentration from 2 mM to 3 mM caused a marked decrease in the level of succinyl-CoA and increased the rates of 2-oxoglutarate utilization and of malate and phosphoenolpyruvate production. Citrulline synthesis also was found to be stimulated at 3 mM Pi. Taken together our results suggest a role of Pi supply in mitochondrial actions of glucagon in intact hepatocytes. Moreover, they could contribute to a better interpretation of glucagon effects on isolated mitochondria from hormone-pretreated liver cells.

Effects of L-carnitine on urea synthesis following acute ammonia intoxication in mice

L-Carnitine protects mice against acute ammonia intoxication. The effect of L-carnitine on in vivo incorporation of [14C] bicarbonate into urea has been investigated in mice given large doses of ammonium acetate. The hepatic content of N-acetylglutamate has been measured. Following ammonia administration the animals given L-carnitine have much higher production of urea than the unprotected mice. The marked protective effect of L-carnitine on ammonium acetate-induced hyperammonemia and on the increase in urea synthesis is not due primarily to activation of N-acetylglutamate synthetase.

Effects of glucagon in vivo on the N-acetylglutamate, glutamate and glutamine contents of rat liver

Glucagon injected into rats caused an increase in liver N-acetylglutamate content, and coincidentally the glutamate content decreased. The liver glutamine content decreased only after 10 min, consistent with an activation of glutaminase after a lag. These observations indicate that the increased N-acetylglutamate content was not due to an increase in glutamate content caused by an activation of glutaminase (EC 3.5.1.2).

The stimulation of hepatic oxidative phosphorylation following dexamethasone treatment of rats

The effect of short-term treatment of rats with the synthetic glucocorticoid, dexamethasone, on mitochondrial oxidative phosphorylation has been examined. Treatment of rats for 3 h increased the oxidative capacity of the subsequently isolated mitochondria such that they displayed increased uncoupled and State 3 rates of respiration with NAD-linked substrates, succinate or durohydroquinone. The oxidation of ascorbate plus N,N,N',N'-tetramethyl-p-phenylenediamine was unaffected. No change was apparent in the activity of a variety of dehydrogenase enzymes nor was there any increase in the mitochondrial content of cytochromes a, b, c1 or c. The uncoupler-dependent ATPase activity of the mitochondria was slightly enhanced following hormone treatment, but not the basal or the total ATPase activity measured in the presence of Triton X-100 plus Mg2+. The mitochondria prepared from dexamethasone-treated rats also displayed increased intramitochondrial concentrations of Mg2+, K+ and exchangeable adenine nucleotides but not Ca2+. It is suggested that the effect of glucocorticoids on mitochondrial respiration may be both the result of a direct activation of the respiratory chain within Complex III and an elevated intramitochondrial adenine nucleotide concentration. The evidence for the de novo synthesis of mitochondrial proteins which mediate the response remains inconclusive.

Elevated intramitochondrial adenine nucleotides and mitochondrial function

Several groups of investigators have shown that treatment of rats with glucagon produces an increase in the adenine nucleotide content of hepatic mitochondria. It has been suggested that this enlarged pool of exchangeable nucleotides may be responsible for several of glucagon's stimulatory effects on mitochondrial functions by accelerating the transport of adenine nucleotides across the inner mitochondrial membrane. This hypothesis was tested by loading rat liver mitochondria in vitro with adenine nucleotides to supranormal levels. This procedure did result in stimulation of several metabolic and bioenergetic functions including pyruvate carboxylation, uncoupler-dependent ATPase, and succinic dehydrogenase activity but not formation of citrulline. However, a sham loading that did not increase the nucleotide content of the mitochondria was essentially as effective as the loading procedure in stimulating those functions assayed. Mitochondria, loaded in vitro with supranormal levels of adenine nucleotides, were shown to have an enlarged pool of exchangeable nucleotides. This exchange was atractyloside sensitive, but the rate of exchange was only slightly increased as a consequence of enlargement of the pool. Similarly, mitochondria isolated from glucagon-treated rats showed no increase in the rate of exchange, although the exchangeable pool was increased. There was no correlation between the rate of nucleotide exchange and the rate of the uncoupler-dependent ATPase.

Stimulation of mitochondrial functions by glucagon treatment. Evidence that effects are not artifacts of mitochondrial isolation

(1) Activation of rat liver mitochondrial functions following glucagon treatment was demonstrated in mitochondria that had not been isolated by the conventional technique of differential centrifugation and washing in sucrose solutions. Crude liver homogenates in 0.3 M-sucrose or 0.15 M-KCl prepared from rats treated with glucagon showed stimulation of State-3 and uncoupled respiration, carboxylation of pyruvate, and citrulline synthesis comparable with those previously reported in isolated mitochondria. (2) During the isolation procedure of mitochondria the hormonal stimulations of pyruvate carboxylation and citrulline formation were shown not to be enhanced by sequential washing. (3) Mitochondria isolated from glucagon-treated rats by differential centrifugation and washing in 0.3 M-mannitol/1 mM-EGTA, pH 7.0, exhibited a mean rate of citrulline synthesis that was greater than twice that of the control. Liver homogenates prepared in 0.3 M-sucrose or 0.3 M-mannitol showed identical rates of State-3 respiration and percentage stimulations of respiration by glucagon treatment. (4) Addition of glucagon led to a rapid accumulation of malate and aspartate and decreased the amounts of glutamate and citrate in isolated hepatocytes incubated with L-lactate. When gluconeogenesis was inhibited at the phosphoenolpyruvate carboxykinase (EC 4.1.1.32) reaction these phenomena were accentuated, lending support to the interpretation that they are the direct result of stimulation of carboxylation and oxidation reactions in the mitochondria. These results do not support the proposal [Siess, Fahimi & Wieland (1981) Hoppe-Seyler's Z. Physiol. Chem. 362. 1643-1651] that the mitochondrial effects of glucagon treatment result from a stabilization of mitochondria to detrimental effects of sucrose during their isolation. (5) The mean hormonal stimulation of pyruvate carboxylation in mitochondria isolated in 0.3 M-sucrose was shown to be approx. 2.5-fold when assayed either at 37 degrees C or 25 degrees C. In contrast, on the basis of similar experiments, Siess et al. (1981) concluded that the effects of glucagon on hepatic mitochondria are not characteristic of a true hormonal stimulation. Our data indicate this conclusion to be unjustified.

Influence of isolation media on the preservation of mitochondrial functions

1) In the present study the influence of sucrose and mannitol-based isolation media on the degree of functional preservation of rat liver mitochondria has been investigated. Apparently intact mitochondria conventionally prepared with a 0.3M sucrose medium displayed significantly lower rates of state-3 respiration, pyruvate carboxylation, ATP hydrolysis and thiol group production than mitochondria prepared from the same livers with mannitol. 2) Extracts from the latter, furthermore, showed a significantly higher activity of succinate dehydrogenase activity, whereas no difference in glutamate dehydrogenase activity was demonstrable. 3) The low activities apparent with the sucrose medium could be brought to the level of the mannitol medium by the addition of potassium phosphate (4mM). A similar effect was exerted by K2SO4, whereas KCl and the respective sodium salts were significantly less effective. 4) Sucrose-prepared mitochondria display decreased contents of metabolites such as ATP, glutamate, citrate and malate. 5) Comparative studies with a variety of carbohydrates indicated that isolation media based on disaccharides are inferior to those based on monosaccharides in the preparation of functionally intact mitochondria from rat liver. 6) The results reported herein appear to be of general interest as sucrose-prepared mitochondria have been employed in the past in a great number of studies and are still widely used at present.

Acute effects of glucagon on citrulline biosynthesis

Mitochondria isolated from livers of rats fed on different diets showed altered capacity to synthesize citrulline. Glucagon, 15 min after injection, increases citrulline biosynthesis, except after the high-protein diet. A significant correlation between citrulline biosynthesis and N-acetylglutamate content with and without glucagon treatment was shown when rats were fed on a standard or a carbohydrate diet. Different diets modified carbamoyl phosphate synthetase I (EC 6.3.4.16) and N-acetylglutamate synthase (acetyl-CoA:L-glutamate N-acetyltransferase, EC 2.3.1.1) activities. Glucagon did not modify these activities.

The nature of the changes in liver mitochondrial function induced by glucagon treatment of rats. The effects of intramitochondrial volume, aging and benzyl alcohol

(1) The effects of changes in the intramitochondrial volume, benzyl alcohol treatment and calcium-induced mitochondrial aging on the behaviour of liver mitochondria from control and glucagon-treated rats are reported. (2) The stimulatory effects of glucagon on mitochondrial respiration, pyruvate metabolism and citrulline synthesis could be mimicked by hypo-osmotic treatment of control mitochondria and reversed by calcium-induced aging of mitochondria or by treatment with 20 mM benzyl alcohol. Hypo-osmotic treatment increased the matrix volume whilst aging but not benzyl alcohol decreased this parameter. (3) Liver mitochondria from glucagon and adrenaline-treated rats were shown to be less susceptible to damage by exposure to calcium than control mitochondria and frequently showed slightly (15%) elevated intramitochondrial volumes. (4) Aging, benzyl alcohol and hypo-osmotic media increased the susceptibility of mitochondria to damage caused by exposure to calcium. (5) Glucagon-treated mitochondria were less leaky to adenine nucleotides than control mitochondria. (6) These results suggest that glucagon may exert its action on a wide variety of mitochondrial parameters through a change in the disposition of the inner mitochondrial membrane, possibly by stabilisation against endogenous phospholipase A2 activity. This effect may be mimicked by an increase in the matrix volume or reversed by calcium-dependent mitochondrial aging.

Evidence that glucagon stabilizes rather than activates mitochondrial functions in rat liver

The present study is concerned with the question as to whether the acute treatment of intact rats or hepatocytes with glucagon and dibutyryl cAMP, respectively, leads to a stabilization or an activation of mitochondrial functions, such as state-3 respiration, succinate dehydrogenase activity and pyruvate carboxylase activity. For this purpose, the influence of various parameters of mitochondria preparation (isolation medium, washing steps, storage) as well as of phospholipase A inhibitors (cinchocain, chloroquine) on the expression of the hormone effect was examined. With regard to the above mentioned functions, the values displayed by control mitochondria were found to be considerably higher if mannitol instead of sucrose had been used for isolation. Accordingly, only small effects of hormone treatment became apparent. The addition of cinchocain or chloroquine to the sucrose medium yielded results similar to those obtained with mannitol. Furthermore, the hormone effect on state-3 respiration and succinate dehydrogenase activity was only small if the mitochondria had been prepared faster than usual and had been used without washing. Regarding pyruvate carboxylase, a considerably smaller glucagon effect was observed when it was assayed at 25 degrees C and not (as usual) at 37 degrees C. Our results indicate that glucagon application stabilizes rather than activates mitochondrial functions.

Increased activity of phosphate-dependent glutaminase in liver mitochondria as a result of glucagon treatment of rats

1. Injection of rats with glucagon leads to an increased effective activity of glutaminase in subsequently isolated liver mitochondria. 2. This effect of glucagon is manifested as a decreased requirement of glutaminase for phosphate in the presence of HCO3-. The HCO3--concentration-dependence is unchanged. 3. The effect of glucagon is lost on disruption of the mitochondria. 4. In accordance with previous reports, incubation of mitochondria in hypo-osmotic media also increases the effective activity of glutaminase. Glucagon increases glutamine hydrolysis at intermediate osmolarities of the suspending medium, but does not affect glutaminase activity when it is already maximally activated by hypo-osmotic conditions. 5. From this and previous work, it seems that hypo-osmotic incubation conditions, EDTA and glucagon may all activate glutaminase by a common mechanism. It is postulated that this mechanism involves modification of the interaction of glutaminase with the mitochondrial inner membrane.

Relationship between the rate of citrulline synthesis and bulk changes in the intramitochondrial ATP/ADP ratio in rat-liver mitochondria

1. The relationship between intramitochondrial and extramitochondrial ATP-utilizing systems and the intramitochondrial ATP/ADP ratio was studied in isolated rat-liver mitochondria. Citrulline synthesis was used as an intramitochondrial ATP-utilizing system, and glucose-6-phosphate synthesis as an extramitochondrial ATP-utilizing system. The intramitochondrial ATP/ADP ratio was manipulated in three ways: with succinate and different concentrations of malonate and/or hexokinase; with 2-oxoglutarate (plus oligomycin) and different concentrations of hexokinase; and with added ATP in uncoupled mitochondria (oligomycin present). 2. Under all conditions used, citrulline synthesis was strictly correlated with the bulk intramitochondrial ATP/ADP ratio. 3. The curve relating citrulline synthesis and intramitochondrial ATP/ADP was shifted towards lower ATP/ADP ratios when the activity of carbamoyl-phosphate synthetase was enhanced by increasing the mitochondrial content of N-acetylglutamate. 4. It is concluded that under the experimental conditions used the intramitochondrial adenine nucleotides behave as a homogeneous pool.

Early kinetics of glucagon action in isolated hepatocytes at the mitochondrial level

The temporal relationship between the effect of glucagon on respiratory functions and the changes in metabolites related to gluconeogenesis has been studied. Mitochondria prepared from hepatocytes after incubation with glucagon for 1 min already displayed a maximal stimulation of state-3 respiration. The increase in succinate dehydrogenase activity was almost fully expressed 3 min after glucagon. With respect to the utilization of pyruvate, 2-oxoglutarate and glutamate, glucagon produced a significant effect within 1 min. The rate of this decrease was linear for about 3 min slowing down thereafter. The stimulation of glucose production from lactate became significant within 1 min and remained constant up to 15 min. The influence of glucagon on the mitochondrial redox state also was an early event. It was maximally shifted to the more reduced state within 2 min and declined within 15 min. Under the conditions employed no effect of glucagon on urea synthesis or branched-chain amino acid release up to 15 min incubation time was discernible. Glucagon influenced the respiratory parameters virtually independent of Ca2+, in contrast to its action on intermediary metabolism. As to the hormone specificity, no enhancement of state-3 respiration and succinate dehydrogenase activity was caused by phenylephrine or isoproterenol. From the time course studies presented, it appears that the mitochondrial effects of glucagon might be causally interrelated with the regulation of gluconeogenesis. Moreover, our results indicate that the stimulation of state-3 respiration represents the earliest, specific action of glucagon at the mitochondrial level.

Rapid action of glucagon on hepatic mitochondrial calcium metabolism and respiratory rates

The time course for the effects of acute, in vivo glucagon treatment on energy-linked functions of isolated hepatic mitochondria has been studied. After 1 min of glucagon treatment, two changes are observed in mitochondrial function. State 4 (nonphosphorylating) respiratory rates with L-glutamate as substrate are decreased. No significant change is observed in the State 4 respiratory rates with succinate as substrate at 1 min of treatment. Concurrent with the change in nonphosphorylating respiratory rates is a decrease in the half time of spontaneous calcium release from mitochondria preloaded with calcium in a phosphate-containing medium. After 2-4 min of treatment, the previously reported stimulations in rates of State 3 (phosphorylating) respiration and calcium influx into mitochondria are observed. After approximately 6 min of treatment, these changes have reached their maxima. The combined effects of increased calcium uptake rate and decreased calcium efflux rate leads to a decrease in the calcium cycling rate of mitochondria. This decrease in the cycling rate should lead to the increased efficiency of mitochondrial energy transduction and may be responsible, in part, for the increased functional capability of mitochondria isolated from glucagon-treated animals. A correlate of the reduced cycling rate is a decrease in the steady-state concentration at which the mitochondria can buffer the calcium concentration of the incubation medium. The changes observed in calcium efflux rates and respiratory rates exhibit a time course consistent with possible intermediates in the glucagon-induced stimulation of hepatic gluconeogenesis.

The relationship between intramitochondrial N-acetylglutamate and activity of carbamoyl-phosphate synthetase (ammonia). The effect of glucagon

1. The relationship between urea synthesis, intracellular N-acetylglutamate and the capacity of rat-liver mitochondria to synthesize citrulline was investigated. 2. Treatment of rats with glucagon prior to killing results not only in an increased intramitochondrial ATP concentration and an increased capacity of the mitochondria to synthesize citrulline, but also in an increased concentration of intramitochondrial N-acetylglutamate. 3. Comparison of the rate of citrulline synthesis in mitochondria from glucagon-treated and from control rats, incubated under different conditions, shows that the increased N-acetylglutamate concentration after glucagon treatment is at least in part responsible for the observed increased capacity of the mitochondria to synthesize citrulline. 4. Ureogenic flux in isolated hepatocytes under different incubation conditions correlated with the intracellular concentration of N-acetylglutamate and with the capacity of the mitochondria to synthesize citrulline. 5. When isolated hepatocytes were incubated with NH3, ornithine, lactate and oleate, intracellular N-acetylglutamate increased about eightfold in the first 10 min; during this period the rate of urea synthesis increased considerably. 6. It is concluded that the concentration of intramitochondrial N-acetylglutamate plays an important role in the short-term control of flux through the urea cycle under different nutritional and hormonal conditions.

The effects of ammonium chloride and bicarbonate on the activity of glutaminase in isolated liver mitochondria

1. Glutamine hydrolysis in liver mitochondria was studied by measuring the production of glutamate under conditions where this compound could not be further metabolized. 2. Glutaminase activity in intact mitochondria was very low in the absence of activators. 3. Glutamine hydrolysis was markedly stimulated by NH4Cl and also by HCO3- ions. 4. The stimulation by each of these compounds was much decreased if the mitochondria were uncoupled. 5. Maximum rates of glutamine hydrolysis required the addition of phosphate. A correlation was observed between the activity of glutaminase in the presence of NH4Cl plus HCO3- and the intramitochondrial content of ATP. 6. In disrupted mitochondria, NH4Cl stimulated glutaminase to a much smaller extent than in intact mitochondria. The NH4Cl stimulation in disrupted mitochondria was much increased by the addition of ATP. KHCO3 also stimulated glutaminase activity in disrupted mitochondria, and ATP increased the magnitude of this stimulation. 7. It was concluded that maximum rates of glutaminase activity in liver mitochondria require the presence of phosphate, ATP and either HCO3- or NH4+. A comparison of the results obtained on intact and broken mitochondria indicates that these effectors have a direct effect on the glutaminase enzyme system rather than an indirect effect mediated by changes in transmembrane ion gradients or in the concentrations of intramitochondrial metabolites.