Effects of glucagon on the redox states of cytochromes in mitochondria in situ in perfused rat liver

Theoretical studies on the regulation of oxidative phosphorylation in intact tissues

The theoretical studies on the regulation of oxidative phosphorylation that were performed with the aid of kinetic models of this process are overviewed. A definition of the regulation of the flux through a metabolic pathway is proposed and opposed to the control exerted by particular enzymes over this flux. Different kinetic models of oxidative phosphorylation proposed in the literature are presented, of which only the model proposed by myself and co-workers was extensively used in theoretical studies on the regulation and compensation in the oxidative phosphorylation system. These theoretical studies have led to the following conclusions: (1) in isolated mitochondria, an increase in the activity of an artificial ATP-using system stimulates mitochondria mainly via changes in [ADP], while changes in [ATP] and [P(i)] play only a minor role; (2) in non-excitable tissues (e.g. liver), hormones (acting via some cytosolic factor(s)) activate directly both ATP usage and at least some enzymes of the ATP-producing block; (3) in excitable tissues (e.g. skeletal muscle), neural signals stimulate (via some cytosolic factor(s)) in parallel all the steps of oxidative phosphorylation together with ATP usage and substrate dehydrogenation; (4) the decrease in the flux through cytochrome oxidase caused by a decrease in oxygen concentration is, at least partially, compensated by a decrease in Delta p and increase in the reduction level of cytochrome c. A theoretical prediction is formulated that there should exist and be observable a universal cytosolic factor/regulatory mechanism which directly activates (at least in excitable tissues) all complexes of oxidative phosphorylation during an increased energy demand.

Biochemistry and toxicology of the diterpenoid glycoside atractyloside

Atractyloside (Atr) is a diterpenoid glycoside that occurs naturally in plants (many of which are used in ethnomedicines) found in Europe, Africa, South America, Asia and the far East. It is also present in animal grazing forage. Atr (and its analogues) may be present at levels as high as 600 mg/kg dried plant material. Consumption of the plants containing Atr or carboxyatractyloside (carboxyAtr) has caused fatal renal proximal tubule necrosis and/or centrilobular hepatic necrosis in man and farm animals. Although pure Atr and crude plant extracts disrupt carbohydrate homeostasis and induce similar pathophysiological lesions in the kidney and liver, it is also possible that the toxicity of Atr may be confounded by the presence of other natural constituents in plants. Atr competitively inhibits the adenine nucleoside carrier in isolated mitochondria and thus blocks oxidative phosphorylation. This has been assumed to explain changes in carbohydrate metabolism and the toxic effects in liver and kidney. Although the acute toxicity of Atr is well described, many aspects of Atr toxicity (subchronic and chronic toxicity, reproductive toxicity, mutagenicity and carcinogenicity) have not been investigated and pharmacokinetic and metabolism data are limited. In vitro proximal tubular cells are selectively sensitive to Atr, whereas other renal cell types are quite resistant. There are also differences in the response of liver and renal tissue to Atr. Thus, not all of the clinical, biochemical and morphological changes caused by Atr can simply be explained on the basis of inhibition of mitochondrial phosphorylation. The relevance to a wider human risk is shown by the presence of Atr analogues in dried roasted Coffea arabica beans (17.5 32 mg/kg). There are no data to help identify the risk of low dose chronic exposure in human coffee consumers, nor is there information on the levels of Atr or its analogues in other commonly consumed human foodstuffs.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

Proportional activation coefficients during stimulation of oxidative phosphorylation by lactate and pyruvate or by vasopressin

A 'proportional activation' approach designed to deal with the influence of external effectors within biochemical systems is described. The proportional activation coefficient, which enables the quantitative determination of the relative stimulation of different parts of a system by a given effector, is defined. The proportional activation approach was used to calculate the relative activation of delta p-producing and delta p-consuming subsystems during stimulation of the respiration rate of cells by a variety of different effectors. Oxidative phosphorylation was stimulated by the addition of either lactate and pyruvate (10 mM and 1 mM) or vasopressin. The addition of lactate and pyruvate to suspensions of resting hepatocytes increased the respiration rate by about 50%. The delta p-consuming subsystem was stimulated about 60% as much as the delta p-producing subsystem. Quinolinic acid, commonly considered to be a specific inhibitor of gluconeogenesis, was found to block the delta p-producing oxidative subsystem as well as the delta p-consuming subsystem, indicating some nonspecific effects of this inhibitor. Addition of vasopressin to hepatocytes that were incubated in the presence of lactate and pyruvate resulted in an increase of the respiratory rate by up to 35%. The relative stimulation of the delta p-producing and delta p-consuming subsystems was essentially equal. Using the 'proportional activation approach' to analyse these and previously published data, it is shown that substrates (lactate/pyruvate and fatty acids), Ca(2+)-acting hormones (vasopressin and others) and calcium in muscles (heart muscle and skeletal muscle) activate both subsystems to a similar extent (it concerns especially Ca(2+)-dependent systems).

Theoretical studies on the control of the oxidative phosphorylation system

The dynamic model developed in our previous publications [1,2] was used to calculate the flux control coefficients of oxidation, phosphorylation and proton leak fluxes for isolated mitochondria and for three modes of work of intact cells (hepatocytes). The results obtained were compared with experimental data, especially those measured in the frame of the 'top-down approach' of the metabolic control theory. A good agreement for mitochondria and for intact cells was found. The control of the oxygen consumption flux is shared between the ATP utilization (main controlling factor), substrate dehydrogenation, proton leak and, in some conditions, the ATP/ADP carrier. The phosphorylation subsystem seemed to be controlled mainly by itself, while the proton leak was influenced by all three subsystems. It was also shown that the large relative change in the enzyme activity during inhibitor titration of mitochondria or cells could lead to the overestimation of some flux control coefficient values in experimental measurements. An influence of some hormones (glucagon, vasopressin, adrenaline and others) on the mitochondrial respiration was also simulated. Our results suggest that these hormones stimulate the substrate dehydrogenation as well as the phosphorylation system (ATP usage and, possibly, the ATP/ADP carrier).

Development and hormonal control of thioredoxin and the thioredoxin-reductase system in the rat liver during the perinatal period

The development and hormonal regulation of thioredoxin and of the thioredoxin-reductase system were investigated during the perinatal period in rat liver. An immunological procedure was developed in order to quantify thioredoxin in fetal and neonatal hepatocytes. Both immunoreactive thioredoxin and thioredoxin-reductase activity appeared on day 16.5 of pregnancy. The level of immunoreactive thioredoxin increased during the late fetal period, and its level was the same 24 h after birth. Moreover, its development was not subjected to hormonal regulation by corticosteroids and glucagon. In contrast, thioredoxin-reductase activity increased 3 times during the late fetal period and presented a marked increase 24 h after birth. In the absence of glucocorticoids there was no increase in the level of thioredoxin reductase, while administration of hydrocortisone acetate and glucagon to fetuses prematurely evoked its activity. This study suggests that if thioredoxin acts physiologically, this activity is related to the state of reduction of the molecule rather than to the total concentration in the liver.

Clofibrate induces carnitine acyltransferases in periportal and perivenous zones of rat liver and does not disturb the acinar zonation of gluconeogenesis

Clofibrate induces hypertrophy and hyperplasia and marked changes in the activities of various enzymes in rat liver. We examined the effects of treatment of rats with clofibrate on enzyme induction and on rates of metabolic flux in hepatocytes isolated from the periportal and perivenous zones of the liver. Clofibrate induced the activities of carnitine acetyltransferase (90-fold), carnitine palmitoyltransferase (3-fold) and NADP-linked malic enzyme (3-fold) to the same level in periportal as in perivenous hepatocytes, suggesting that these enzymes were induced uniformly throughout the liver acinus. Increased rates of palmitate metabolism and ketogenesis after clofibrate treatment were associated with: a more oxidised mitochondrial redox state; diminished responsiveness to glucagon and loss of periportal/perivenous zonation. Despite the marked liver enlargement and hyperplasia caused by clofibrate, the normal periportal/perivenous zonation of alanine aminotransferase and gluconeogenesis was preserved in livers of clofibrate-treated rats, indicating that clofibrate-induced hyperplasia does not disrupt the normal acinar zonation of these metabolic functions.

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.

Control of energy metabolism by glucagon and adrenaline in perfused rat liver

Changes in subcellular distribution of adenine nucleotides, mitochondrial/cytosolic proton gradients, rates of respiration, gluconeogenesis (fasted state) and glycogenolysis (fed state) were studied in isolated perfused rat livers following addition of glucagon (10(-8) M) or adrenaline (10(-7) M). Glucagon increased the gradient in all states. The cytosolic ATP/ADP ratio was increased in the fasted but decreased in the fed state which is consistent with a diminished futile cycling in gluconeogenesis (fasted state) or a decreased glycolytic rate (fed state). Adrenaline caused an increase in the proton gradient and the mitochondrial ATP/ADP ratio. The two effects are attributed to increased calcium entry into the matrix space.

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)

Glucagon-stimulated respiration and intracellular Ca2+

The effects of extra- and intracellular Ca2+ on glucagon-stimulated respiration were examined in perfused rat liver. Glucagon increased the uptake of O2 to a significantly greater extent in Ca2+-containing perfusate than in Ca2+-free perfusate. If, however, the livers were perfused first with Ca2+-containing perfusate for 60 min in order to load the hormone-sensitive Ca2+ pool(s) and subsequently with Ca2+-free perfusate, glucagon was able to stimulate O2 uptake to the same extent in Ca2+-free, as in Ca2+-containing perfusate. These experiments support previous observations of a connection between Ca2+ and the hormonal stimulation of respiration, but indicate a role for intracellular, rather than extracellular, Ca2+ in the process.

Thyroxine-induced changes in rat liver mitochondrial cytochromes

The effects of 10 days of thyroxine injection (15 micrograms/100 g body weight) on rat liver mitochondrial cytochrome concentration and on the percent reduction of the individual cytochromes during succinate-driven state III and IV respiration was spectrophotometrically determined at cytochrome-specific wave-length pairs. The concentrations of cytochromes b, c, total c (c + c1) and a a3 increased in hyperthyroid rats. The concentration of cytochrome c1 remained constant in euthyroid and hyperthyroid rats. Changes in the concentration of the membrane-bound cytochromes were also determined by difference spectra in cytochrome c-depleted mitochondrial membranes. Cytochromes b and a a3 showed increased concentrations in hyperthyroid rats while the concentration of cytochrome c1 remained unchanged. Hyperthyroid mitochondria showed increased reduction of cytochromes b, c1, c and total c during state III respiration and cytochromes c1, c, and total c during state IV respiration. The percent reduction of cytochrome b decreased during state IV respiration in the hyperthyroid mitochondria. These results suggest that the increase in respiration observed in the hyperthyroid state may be related to changes both in the mitochondrial cytochrome concentration and in the cytochrome reduction level.