The stimulation of the mitochondrial uncoupler-dependent ATPase in isolated hepatocytes by catecholamines and glucagon and its relationship to gluconeogenesis

The role of hormones in sepsis: an integrated overview with a focus on mitochondrial and immune cell dysfunction

Sepsis is a dysregulated host response to infection that results in life-threatening organ dysfunction. Virtually every body system can be affected by this syndrome to greater or lesser extents. Gene transcription and downstream pathways are either up- or downregulated, albeit with considerable fluctuation over the course of the patient's illness. This multi-system complexity contributes to a pathophysiology that remains to be fully elucidated. Consequentially, little progress has been made to date in developing new outcome-improving therapeutics. Endocrine alterations are well characterised in sepsis with variations in circulating blood levels and/or receptor resistance. However, little attention has been paid to an integrated view of how these hormonal changes impact upon the development of organ dysfunction and recovery. Here, we present a narrative review describing the impact of the altered endocrine system on mitochondrial dysfunction and immune suppression, two interlinked and key aspects of sepsis pathophysiology.

The responses of rat hepatocytes to glucagon and adrenaline. Application of quantified elasticity analysis

The internal control of hepatocyte metabolism has been previously analysed using metabolic control analysis. The aim of this paper is to extend this analysis to include the responses of the cells to hormonal stimulus. Hepatocyte metabolism was divided into nine reaction blocks: glycogen breakdown, glucose release, glycolysis, lactate production, NADH oxidation, pyruvate oxidation, proton leak, mitochondrial phosphorylation and ATP consumption, linked by five intermediates: mitochondrial membrane potential, cytoplasmic NADH/NAD and total cellular ATP, glucose 6-phosphate and pyruvate. The kinetic responses of the reaction blocks to the intermediates were determined previously in the absence of added hormones. In this study, the changes in flux and intermediate levels that occurred upon addition of either glucagon or adrenaline were measured. From comparison of the fractional changes in fluxes and intermediate levels with the known kinetics of the system, it was possible to determine the primary sites of action of the hormones. The results show that the majority of processes in the cell are responsive to the hormones. The notable exception to this is the failure of adrenaline to have a direct effect on glycolysis. The activity change of each metabolic block observed in the presence of either hormone was quantified and compared to the indirect effects on each block caused by changes in metabolite levels. The second stage of the analysis was to use the calculated activity changes and the known control pattern of the system to give a semiquantitative analysis of the regulatory pathways employed by the hormones to achieve the changes in fluxes and metabolite levels. This was instructive in analysing, for example, how glucagon caused a decrease in flux through glycolysis and an increase in oxidative phosphorylation without large changes in metabolite levels (homeostasis). Conversely, it could be seen that the failure of adrenaline to maintain a constant glucose 6-phosphate concentration was due to the stimulation of glycogen breakdown and inhibition of glucose release.

The mechanisms by which mild respiratory chain inhibitors inhibit hepatic gluconeogenesis

(1) Liver cells from starved rats were incubated with 10 mM L-lactate, 1 mM pyruvate and 0.3 microM glucagon in the presence and absence of the mild respiratory inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) at 0.5 mM. (2) The whole cell concentrations of phosphoenolpyruvate, 2-phosphoglycerate and 3-phosphoglycerate increased about 2-fold, whilst the triose and hexose phosphate concentrations all decreased significantly. Similar results were obtained with 0.15 microM oligomycin and 10 microM atractyloside. (3) These data can be explained by a substantial decrease in the cytosolic free concentration ratio of ATP/ADP acting on the equilibrium of glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase. (4) The increase in cytosolic phosphoenolpyruvate concentration can account for the observed increase in pyruvate kinase flux that occurs under these conditions (Pryor et al. (1987) Biochem. J. 247, 449-457). (5) An inhibition of pyruvate carboxylase was also implied by a decrease in calculated tissue oxaloacetate concentrations, confirming a role for both enzymes in the inhibition of gluconeogenesis. (6) Whole cell concentrations of effectors of pyruvate carboxylase activity were measured; only the ATP/ADP ratio decreased significantly. (7) Subcellular fractionation studies showed a good correlation between the measured mitochondrial ATP/ADP ratio and rates of gluconeogenesis both in the presence and absence of oleate. (8) A similar correlation could be observed between rates of pyruvate carboxylation and the measured matrix ATP/ADP ratio in isolated liver mitochondria from starved rats. (9) Data are also presented suggesting an additional effect of DCMU on the rate pyruvate carboxylation in situ under some circumstances, mediated by decreases in mitochondrial acetyl-CoA and cytosolic pyruvate concentrations. (10) It is noted that the effects of phenylethylbiguanide (phenformin) on the rate of gluconeogenesis and metabolite profiles in the perfused liver (Cooke et al. (1973) J. Biol. Chem. 248, 5272-5277) are similar to those caused by DCMU, supporting a mitochondrial locus of action for this hypoglycaemic agent.

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.

Inorganic pyrophosphate is located primarily in the mitochondria of the hepatocyte and increases in parallel with the decrease in light-scattering induced by gluconeogenic hormones, butyrate and ionophore A23187

1. The effects of a variety of hormones on the PPi content and light-scattering of isolated rat liver cells was studied. 2. The basal PPi content was about 130 pmol/mg of cell protein, and increased after hormone addition, in parallel with a decrease in light-scattering which we have observed previously [Quinlan, Thomas, Armston & Halestrap (1983) Biochem. J. 214, 395-404]. 3. The mean increases in PPi content with the agonists shown (as pmol/mg of protein) were: 0.1 microM-glucagon, 25; 20 microM-phenylephrine, 30; 25 nM-vasopressin, 127; glucagon + phenylephrine, 115; glucagon + vasopressin, 382; 100 microM-ADP, 50; 15 microM-A23187, 72; 1 mM-butyrate, 80. 4. In the absence of extracellular Ca2+, vasopressin had little effect on either the PPi content or the light-scattering of hepatocytes. 5. The magnitude of the increase in PPi content correlated with that of the decrease in light-scattering irrespective of the stimulating agent, provided that the PPi did not exceed 300 pmol/mg of protein. Above this value little additional change in light-scattering was observed. 6. Subcellular fractionation showed that over 90% of the cellular PPi was intramitochondrial in both control and stimulated cells. 7. The data support the conclusions of previous experiments using isolated liver mitochondria [Davidson & Halestrap (1987) Biochem. J. 246, 715-723] that hormones increase the mitochondrial matrix volume through a Ca2+-induced rise in matrix [PPi]. 8. It is further proposed that this increase in mitochondrial [PPi] allows entry of ADP into the mitochondria in exchange for PPi and is therefore responsible for the increase in total mitochondrial adenine nucleotides observed after hormone treatment.

The involvement of calcium in the stimulation of respiration in isolated rat hepatocytes by adrenergic agonists and glucagon

The effects of adrenergic agonists and glucagon on respiration and cytosolic free Ca2+, measured with quin2, in female rat hepatocytes have been compared. In the presence of lactate, all three agonists caused a permanent stimulation of respiration. However, unlike phenylephrine, glucagon induced only a transient increase in cytosolic free Ca2+, and isoprenaline caused no detectable change. Mechanisms other than Ca2+ for respiratory stimulation by cAMP-linked agonists in liver are indicated.

The mechanism of the hormonal activation of respiration in isolated hepatocytes and its importance in the regulation of gluconeogenesis

The effects of hormones on the cytochrome spectra of isolated hepatocytes were recorded under conditions of active gluconeogenesis from L-lactate. Glucagon, phenylephrine, vasopressin and valinomycin, at concentrations that caused stimulation of gluconeogenesis, increased the reduction of the components of the cytochrome bc1 complex, just as has been observed in liver mitochondria isolated from glucagon-treated rats [Halestrap (1982) Biochem. J. 204, 37-47]. The effects of glucagon and phenylephrine were additive. The time courses of the increased reduction of cytochrome c/c1 and NAD(P)H/NAD(P)+ caused by hormones, valinomycin, A23187 and ethanol were measured by dual-beam spectrophotometry and fluorescence respectively. Ethanol (14 mM) produced a substantial rise in NAD(P)H fluorescence, beta-hydroxybutyrate/acetoacetate and lactate/pyruvate ratios, no change in cytochrome c/c1 reduction, a 10% decrease in O2 consumption and a 60% decrease in gluconeogenesis. Glucagon, phenylephrine and vasopressin caused a substantial and transient rise in NAD(P)H fluorescence, but a sustained increase in cytochrome c/c1 reduction and the rates of O2 consumption and gluconeogenesis. The transience of the fluorescence response was greater in the absence of Ca2+, when the cytochrome c/c1 response also became transient. The fluorescence response was smaller and less transient, but the cytochrome c/c1 response was greater, in the presence of fatty acids. Both responses were greatly decreased by the presence of 1 mM-pent-4-enoate. Valinomycin (2.5 nM) caused a decrease in NAD(P)H fluorescence coincident with an increase in cytochrome c/c1 reduction and the rate of gluconeogenesis and O2 consumption. A23187 (7.5 mM) caused increases in both NAD(P)H fluorescence and cytochrome c/c1 reduction. The effects of hormones and valinomycin on the time courses of NAD(P)H fluorescence, cytochrome c/c1 reduction and light-scattering by hepatocytes were compared with those of 0.5 microM-Ca2+ or 1 nM-valinomycin on the same parameters of isolated liver mitochondria. It is concluded that hormones increase respiration by hepatocytes in a biphasic manner. An initial Ca2+-dependent activation of mitochondrial dehydrogenases rapidly increases the mitochondrial [NADH], which is followed by a volume-mediated stimulation of fatty acid oxidation and electron flow between NADH and cytochrome c. 10. Amytal (0.5 mM) was able to reverse the effects of hormones on the reduction of cytochromes c/c1 and the rates of gluconeogenesis and O2 consumption without significantly lowering tissue [ATP].(ABSTRACT TRUNCATED AT 400 WORDS)

The stimulation of tricarboxylic acid-cycle flux by alpha-adrenergic agonists in perfused rat liver

Output of 14CO2 from 1-14C-labelled glutamate, 2-oxoglutarate or octanoate and from 4-methyl-2-oxo[2-14C]pentanoate was increased by more than 100% after infusion of phenylephrine into perfused livers of fed rats. Infusion of ethanol or sorbitol raised 3-hydroxybutyrate/acetoacetate ratios and decreased the output of 14CO2. Increases in 14CO2 output induced by phenylephrine were observed in the presence or absence of ethanol or sorbitol and were accompanied by elevated 3-hydroxybutyrate/acetoacetate ratios under all conditions examined. Phenylephrine had no effect on total tissue ATP/ADP ratios in livers from fed or starved rats. The data suggest that phenylephrine-induced increases in tricarboxylic acid-cycle flux do not arise from lowered matrix NADH/NAD+ or ATP/ADP ratios.

A re-evaluation of the role of mitochondrial pyruvate transport in the hormonal control of rat liver mitochondrial pyruvate metabolism

The inhibitor of mitochondrial pyruvate transport alpha-cyano-beta-(1-phenylindol-3-yl)-acrylate was used to inhibit progressively pyruvate carboxylation by liver mitochondria from control and glucagon-treated rats. The data showed that, contrary to our previous conclusions [Halestrap (1978) Biochem. J. 172, 389-398], pyruvate transport could not regulate metabolism under these conditions. This was confirmed by measuring the intramitochondrial pyruvate concentration, which almost equilibrated with the extramitochondrial pyruvate concentration in control mitochondria, but was significantly decreased in mitochondria from glucagon-treated rats, where rates of pyruvate metabolism were elevated. Computer-simulation studies explain how this is compatible with linear Dixon plots of the inhibition of pyruvate metabolism by alpha-cyano-4-hydroxycinnamate. Parallel measurements of the mitochondrial membrane potential by using [3H]triphenylmethylphosphonium ions showed that it was elevated by about 3 mV after pretreatment of rats with both glucagon and phenylephrine. There was no significant change in the transmembrane pH gradient. It is shown that the increase in pyruvate metabolism can be explained by a stimulation of the respiratory chain, producing an elevation in the protonmotive force and a consequent rise in the intramitochondrial ATP/ADP ratio, which in turn increases pyruvate carboxylase activity. Mild inhibition of the respiratory chain with Amytal reversed the effects of hormone treatment on mitochondrial pyruvate metabolism and ATP concentrations, but not on citrulline synthesis. The significance of these observations for the hormonal regulation of gluconeogenesis from L-lactate in vivo is discussed.

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.

Effect of treatment of rats with dexamethasone in vivo on gluconeogenesis and metabolite compartmentation in subsequently isolated hepatocytes

Hepatocytes prepared from rats treated with dexamethasone for 2 or 3h and maintained in the presence of 10 microM-dexamethasone in the preparation and incubation buffers showed significantly elevated rates of gluconeogenesis compared with those prepared from control animals. Dexamethasone treatment also increased the sensitivity of the cells to glucagon and the catecholamines. Analysis of the concentrations of metabolites in the gluconeogenic pathway indicated that dexamethasone decreased the intracellular concentration of pyruvate and increased those of phosphoenolpyruvate, acetyl-CoA and citrate, suggesting a stimulation of the reaction(s) converting pyruvate into phosphoenolpyruvate. This was substantiated by analysis of the pattern of metabolites found in the mitochondrial compartment after digitonin fractionation of the cells. Inclusion of 3-mercaptopicolinate in the incubation enhanced the effect of the hormone on the distribution of metabolites. Thus, in the absence of an effect of the steroid at the level of phosphoenolpyruvate carboxykinase or pyruvate kinase, dexamethasone treatment still increased the formation of malate, aspartate and citrate from pyruvate, indicating a stimulation in the intact cell of pyruvate carboxylase. It is suggested that the stimulation of pyruvate carboxylase is a result of a general activation of mitochondrial function, with an increase in the intramitochondrial concentrations of acetyl-CoA and ATP, a decrease in glutamate and an enhanced intramitochondrial [ATP]/[ADP] ratio.

The stimulation of mitochondrial pyruvate carboxylation after dexamethasone treatment of rats

Treatment of rats for 3 h with dexamethasone was shown to stimulate both pyruvate carboxylation and decarboxylation in the subsequently isolated mitochondria. The effect of hormone treatment on pyruvate carboxylation was also apparent in liver homogenates assayed within minutes of killing the animal and was independent of the temperature at which the assay was performed, suggesting that it was not an artifact of the mitochondrial preparation procedure. The stimulation of both aspects of pyruvate metabolism in the intact organelle was independent of the induction of either pyruvate carboxylase or pyruvate dehydrogenase. Similarly, there was no change in the percentage of pyruvate dehydrogenase in the active form, indicating that the effect of steroid treatment on pyruvate oxidation was not via changes in the degree of phosphorylation of the enzyme. Adrenalectomizing the animals for a period of 14 days before the experiment had no effect on either parameter. Glucocorticoid treatment of the animals increased the rate of pyruvate uptake into the mitochondria, as measured by the titration of pyruvate metabolism with alpha-cyano-4-hydroxycinnamate, a specific inhibitor of the pyruvate translocator. It also increased the intramitochondrial concentrations of acetyl-CoA and ATP and led to an elevated [ATP]/[ADP] ratio within the mitochondria. It is suggested that both enzymes of pyruvate metabolism exist in the mitochondria under considerable restraint and that glucocorticoids act to relieve this restraint by alterations in substrate supply and the intramitochondrial concentrations of effector molecules.

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.

Measurement of the intramitochondrial volume in hepatocytes without cell disruption and its elevation by hormones and valinomycin

Methods have been developed to measure the lysophospholipid content and matrix volume of liver cell mitochondria in situ in order to test the hypothesis that these parameters may be important in the hormonal control of mitochondrial function [Armston, Halestrap & Scott (1982) Biochim. Biophys. Acta 681, 429-439]. No change in the labelling of mitochondrial lysophospholipids with [32P]Pi was detected after treatment of liver cells with glucagon, phenylephrine or vasopressin. Incorporation of [32P]Pi into mitochondrial phosphatidylinositol was enhanced by phenylephrine and vasopressin. Mitochondrial volumes were measured using rapid disruption of cells by sonication into 3H2O and [14C]sucrose or without cell disruption using 3H2O and [14C]mannitol. In control cells the two methods gave values of 1.09 and 0.40 microliters/mg of mitochondrial protein respectively, which represent 19 and 7% respectively of the total cell volume measured with 3H2O and inulin [14C]carboxylic acid. Both methods showed that glucagon, phenylephrine and 1 nm-valinomycin produced significant increases (13% and 26% using sucrose and mannitol respectively) in mitochondrial volume. The increase was coincident with the stimulation of gluconeogenesis from L-lactate and pyruvate and of mitochondrial respiratory chain activity. The effects of glucagon and phenylephrine were additive on both mitochondrial volume and respiratory chain activity, but not on gluconeogenesis. Liver cells exposed to gluconeogenic hormones or low concentrations of valinomycin showed a decrease in light scattering at 520 nM correlating with the change in mitochondrial volume but without a change in whole-cell volume. The time course and hormone sensitivity of this response were similar to those for the hormonal stimulation of gluconeogenesis. The light-scattering response to glucagon, phenylephrine and vasopressin, but not to valinomycin, were greatly reduced or abolished in Ca2+-free media.

Regulation of mitochondrial pyruvate carboxylation in isolated hepatocytes by acute insulin treatment

The effect of acute insulin treatment of hepatocytes on pyruvate carboxylation in both isolated mitochondria and cells rendered permeable by filipin was examined. Challenging the cells with insulin alone had no effect on either the basal rate of pyruvate carboxylation or gluconeogenesis, although it did suppress the responses to both glucagon and catecholamines. Insulin treatment was unable to antagonize the enhanced rate of pyruvate carboxylation caused by stimulation of the cells with either angiotensin or vasopressin. Neither insulin nor the gluconeogenic hormones altered the total extractable pyruvate carboxylase activity in the isolated mitochondria, suggesting that the effect of hormones at the level of the isolated intact organelle was mediated via alterations in the intramitochondrial concentrations of effector molecules, notably ATP and the [ATP]/[ADP] ratio and substrate availability. The alterations in pyruvate carboxylation correlate well with glucose synthesis in terms of sensitivity to effector molecules, putative second messengers and time of onset of the response, indicating that alterations in the flux through this enzyme are compatible with it being an important site in the control of gluconeogenesis from C3 precursors.

Hormonal stimulation of mitochondrial pyruvate carboxylation in filipin-treated hepatocytes

A method is described for measuring rates of mitochondrial pyruvate carboxylation in hepatocytes treated with the polyene antibiotic, filipin, to render the plasma membrane permeable to substrates. With this approach it was possible to demonstrate that treatment of cells with glucagon or catecholamines results in a stimulation of mitochondrial CO2 fixation measured in situ comparable with that observed in the isolated mitochondria, in terms of time of onset of the response, hormone selectivity and sensitivity. In addition, angiotensin II and vasopressin were shown to enhance the activity of pyruvate carboxylase in both the intact mitochondria and filipin-treated cells, thus strengthening the postulate that this site is a major locus of hormone action in the control of gluconeogenesis. Addition of 3-mercaptopicolinic acid, to inhibit gluconeogenesis at the level of phosphoenolpyruvate carboxykinase, had no significant effect on the stimulation of pyruvate carboxylation by adrenaline, suggesting that the effect of the hormone at this site is independent of changes in activity of other enzymes further on in the pathway. The data presented preclude the possibility that acute effects of hormones on mitochondrial metabolism are solely artifacts of the preparation procedure.

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 [1-14C]oleate oxidation to 14CO2 in isolated rat hepatocytes by the catecholamines, vasopressin and angiotensin. A possible mechanism of action

Adrenaline, noradrenaline, vasopressin and angiotensin increased 14CO2 production from [1-14C]oleate by hepatocytes from fed rats but not by hepatocytes from starved rats. The hormones did not increase 14CO2 production when hepatocytes from fed rats were depleted of glycogen in vitro. Increased 14CO2 production from ]1-14C]oleate in response to the hormones was observed when hepatocytes from starved rats were incubated with 3-mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase. 3-Mercaptopicolinate inhibited uptake and esterification of [1-14C]oleate, slightly increased 14CO2 production from [1-14C]oleate and greatly increased the [3-hydroxybutyrate]/[acetoacetate] ratio. In the presence of 3-mercaptopicolinate 14CO2 production in response to the catecholamines was blocked by the alpha-antagonist phentolamine and required extracellular Ca2+. The effects of vasopressin and angiotensin were also Ca2+-dependent. The actions of the hormones of 14CO2 production from [I-14C]oleate by hepatocytes from starved rats in the presence of 3-mercaptopicolinate thus have the characteristics of the response to the hormones found with hepatocytes from fed rats incubated without 3-mercaptopicolinate. The stimulatory effects of the hormones on 14CO2 production from [1-14C]oleate were not the result of decreased esterification (as the hormones increased esterification) or increased beta-oxidation. It is suggested that the effect of the hormones to increase 14CO2 production from [1-14C]oleate are mediated by CA2+-activation of NAD+-linked isocitrate dehydrogenase, the 2-oxoglutarate dehydrogenase complex, and/or electron transport. The results also demonstrate that when the supply of oxaloacetate is limited it is utilized for gluconeogenesis rather than to maintain tricarboxylic acid-cycle flux.

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.

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.

Effects of adrenaline on ketogenesis from long- and medium-chain fatty acids in starved rats

1. Injection of adrenaline into 24 h-starved rats caused a 69% decrease in blood [ketone-body] (3-hydroxybutyrate plus acetoacetate), accompanied by a decreased [3-hydroxybutyrate]/[acetoacetate] ratio. Blood [glucose] and [lactate] increased, but [alanine] was unchanged. 2. Adrenaline also decreased [ketone-body] after intragastric feeding of both long- and medium-chain triacylglycerol. The latter decrease was observed after suppression of lipolysis with 5-methylpyrazole-3-carboxylic acid, indicating that the antiketogenic action of adrenaline was not dependent on the chain length of the precursor fatty acid. 3. The actions of adrenaline to decrease blood [ketone-body] and to increase blood [glucose] were not observed after administration of 3-mercaptopicolinate, an inhibitor of gluconeogenesis. This suggests that these effects of the hormone are related. 4. The possible clinical significance of the results is discussed with reference to the restricted ketosis often observed after surgical or orthopaedic injury.

Hormone-induced changes in NADH fluorescence and O2 consumption of rat hepatocytes

Hepatocytes isolated from fasted male rats were incubated with a mixture of glucose, ribose, mannose, glycerol, and acetate and then treated with vasopressin (ADH), glucagon, or vasoactive intestinal polypeptide (VIP). Each of these hormones causes a rapid transient increase in the fluorescence signal arising from NADH and a sustained increase in the rate of metabolic oxygen consumption (QO2). The NADH transient was largest in response to glucagon, followed by ADH and VIP, respectively. For each hormone the responses were prevented by addition of a calcium-chelating agent. These results show that a transient, Ca2+-dependent redox shift in NADH and stimulation of QO2, perhaps resulting from an increase in the rate of delivery of reducing equivalents to the mitochondrion, occur early in the sequence of events by which several hormones that increase gluconeogenesis act.

The rôle of mitochondrial pyruvate transport in the stimulation by glucagon and phenylephrine of gluconeogenesis from L-lactate in isolated rat hepatocytes

The sensitivity of glucose production from L-lactate by isolated liver cells from starved rats to inhibition by alpha-cyano-4-hydroxycinnamate was studied. A small percentage of the maximal rate of gluconeogenesis was insensitive to inhibition by alpha-cyano-4-hydroxycinnamate, and evidence is presented to show that this is due to pyruvate entry into the mitochondria as alanine. After subtraction of this rate, Dixon plots of the reciprocal of the rate of gluconeogenesis against inhibitor concentration were linear both in the absence and presence of glucagon, phenylephrine or valinomycin, each of which stimulated gluconeogenesis by 30-50%. Pyruvate kinase activity was decreased by glucagon, but not by phenylephrine or valinomycin. Inhibition of gluconeogenesis by quinolinate (inhibitor of phosphoenolpyruvate carboxykinase) or monochloroacetate (probably inhibiting pyruvate carboxylation) caused a significant deviation from linearity of the Dixon plot obtained with alpha-cyano-4-hydroxycinnamate. Amytal, however, inhibited gluconeogenesis without affecting the linearity of this plot. These data, coupled with a computer simulation study, suggest that pyruvate transport may control gluconeogenesis from L-lactate and that hormones may stimulate this process through an effect on the respiratory chain. An additional role for pyruvate kinase and pyruvate carboxylase is quite compatible with the data presented.

Effect of glucagon on ethanol oxidation in isolated rat liver cells

Addition of glucagon 5 min after ethanol was found to stimulate the rate of ethanol oxidation in hepatocytes isolated from starved rats. This stimulation is of the same order of magnitude as that mediated by asparagine. The glucagon effect is suppressed by antiproteolytic agents such as insulin or NH4Cl. The stimulating effect of glucagon on ethanol oxidation is probably linked to enhanced proteolysis and an elevated glutamate level in the hepatocytes.

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.