Studies on alpha-adrenergic-induced respiration and glycogenolysis in perfused rat liver

p-Synephrine suppresses glucose production but not lipid accumulation in H4IIE liver cells

p-Synephrine, the primary protoalkaloid in the extract of bitter orange and other citrus species, has gained interest due to its lipolytic activity in adipose tissues. We previously found that p-synephrine stimulates glucose consumption via AMP-activated protein kinase (AMPK) in L6 skeletal muscle cells. This study investigated the effect of p-synephrine on glucose production and lipid accumulation in H4IIE rat liver cells. Glucose production was increased in H4llE cells that were incubated in glucose-free medium but decreased dose dependently (1-100 μM) with p-synephrine treatment. Protein levels of glucose-6-phosphatase (G6Pase) and phosphoenol pyruvate carboxykinase (PEPCK) were also decreased by treatment (4 h) with p-synephrine. Antagonists against α- and β-adrenergic receptors (phentolamine and propranolol) and other inhibitors against signaling molecules did not interrupt p-synephrine-induced suppression in glucose production. However, H7 (an inhibitor of serine/threonine kinases PKA, PKC, and PKG) significantly blocked p-synephrine-induced suppression of glucose production and further increased basal glucose production. Unlike the suppressive effect on glucose production, p-synephrine failed to affect palmitic acid-induced cytoplasmic lipid accumulation. Protein levels of fatty acid synthase (FAS) and phosphorylation levels of AMPK and ACC were not changed by p-synephrine. Altogether, p-synephrine can suppress glucose production but does not affect lipid accumulation in H4IIE liver cells.

The action of p-synephrine on hepatic carbohydrate metabolism and respiration occurs via both Ca(2+)-mobilization and cAMP production

Citrus aurantium extracts, which contain large amounts of p-synephrine, are widely used for weight loss purposes and as appetite suppressants. In the liver, C. aurantium (bitter orange) extracts affect hemodynamics, carbohydrate metabolism, and oxygen uptake. The purpose of the present work was to quantify the action of p-synephrine and also to obtain indications about its mechanism of action, a task that would be difficult to accomplish with C. aurantium extracts due to their rather complex composition. The experimental system was the isolated perfused rat liver. p-Synephrine significantly stimulated glycogenolysis, glycolysis, gluconeogenesis, and oxygen uptake. The compound also increased the portal perfusion pressure and the redox state of the cytosolic NAD(+)/NADH couple. A Ca(2+)-dependency for both the hemodynamic and the metabolic effects of p-synephrine was found. p-Synephrine stimulated both cAMP overflow and the initial Ca(2+) release from the cellular stores previously labeled with (45)Ca(2+). The metabolic and hemodynamic actions of p-synephrine were strongly inhibited by α-adrenergic antagonists and moderately affected by β-adrenergic antagonists. The results allow to conclude that p-synephrine presents important metabolic and hemodynamic effects in the liver. These effects can be considered as both catabolic (glycogenolysis) and anabolic (gluconeogenesis), they are mediated by both α- and β-adrenergic signaling, require the simultaneous participation of both Ca(2+) and cAMP, and could be contributing to the overall stimulation of metabolism that usually occurs during weight loss periods.

Adrenergic metabolic and hemodynamic effects of octopamine in the liver

The fruit extracts of Citrus aurantium (bitter orange) are traditionally used as weight-loss products and as appetite suppressants. A component of these extracts is octopamine, which is an adrenergic agent. Weight-loss and adrenergic actions are always related to metabolic changes and this work was designed to investigate a possible action of octopamine on liver metabolism. The isolated perfused rat liver was used to measure catabolic and anabolic pathways and hemodynamics. Octopamine increased glycogenolysis, glycolysis, oxygen uptake, gluconeogenesis and the portal perfusion pressure. Octopamine also accelerated the oxidation of exogenous fatty acids (octanoate and oleate), as revealed by the increase in ¹⁴CO₂ production derived from ¹⁴C labeled precursors. The changes in glycogenolysis, oxygen uptake and perfusion pressure were almost completely abolished by α₁-adrenergic antagonists. The same changes were partly sensitive to the β-adrenergic antagonist propranolol. It can be concluded that octopamine accelerates both catabolic and anabolic processes in the liver via adrenergic stimulation. Acceleration of oxygen uptake under substrate-free perfusion conditions also means acceleration of the oxidation of endogenous fatty acids, which are derived from lipolysis. All these effects are compatible with an overall stimulating effect of octopamine on metabolism, which is compatible with its reported weight-loss effects in experimental animals.

Metabolic effects of antihypertensive agents: role of sympathoadrenal and renin-angiotensin systems

Reports of beneficial, neutral and adverse impacts of antihypertensive drug classes on glucose and lipid metabolism can be found in human data. Furthermore, mechanisms for these diverse effects are often speculative and controversial. Clinical trial data on the metabolic effects of antihypertensive agents are highly contradictory. Comparisons of clinical trials involving different agents are complicated by differences in the spectrum of metabolic disturbances that accompany hypertension in different groups of patients. Two physiological systems are predominant at the interface between metabolic and cardiovascular regulation: the sympathetic nervous system (SNS) and the renin-angiotensin system (RAS). These two systems are major targets of antihypertensive drug actions, and also mediate many of the beneficial and adverse effects of antihypertensive agents on glucose and lipid metabolism. Thiazides and beta-adrenergic antagonists can adversely affect glucose and lipid metabolism, which are frequently compromised in human essential hypertension, and increase the incidence of new cases of diabetes. Laboratory studies confirm these effects, and suggest that compensatory activation of the SNS and RAS may be one mechanism. Other antihypertensives directly targeting the SNS and RAS may have beneficial effects on glucose and lipid metabolism, and may prevent diabetes. Resolution of the controversies surrounding the metabolic effects of antihypertensive agents can only be resolved by further laboratory studies, in addition to controlled clinical trials.

Zonation of the metabolic action of vasopressin in the bivascularly perfused rat liver

Predominance of the vasopressin binding capacity in the hepatic perivenous area leads to the hypothesis that the metabolic effects of the hormone should also be more pronounced in this area. Until now this question has been approached solely by experiments with isolated hepatocytes where an apparent absence of metabolic zonation was found. We have reexamined this question using the bivascularly perfused liver. In this system periportal cells can be reached in a selective manner with substrates and effectors via the hepatic artery when retrograde perfusion (hepatic vein --> portal vein) is done. The action of vasopressin (1-10 nM) on glycogenolysis, initial calcium efflux, glycolysis and oxygen uptake were measured. The results revealed that the action of vasopressin in the liver is heterogeneously distributed. Glycogenolysis stimulation and initial calcium efflux were predominant in the perivenous area, irrespective of the vasopressin concentration. Oxygen uptake was stimulated in the perivenous area; in the periportal area it ranged from inhibition at low vasopressin concentrations to stimulation at high ones. Lactate production was generally greater in the perivenous zone, whereas the opposite occurred with pyruvate production. Analysis of these and other results suggests that at least three factors are contributing to the heterogenic response of the liver parenchyma to vasopressin: a) receptor density, which tends to favour the perivenous zone; b) cell-to-cell interactions, which tend to favour situations where the perivenous zone is amply supplied with vasopressin; and c) the different response capacities of perivenous and periportal cells.

Sources for superoxide release: lessons from blockade of electron transport, NADPH oxidase, and anion channels in diaphragm

Isolated diaphragm releases low levels of superoxide (O2*-) at rest and much higher levels during heat stress. The molecular source is unknown. The hypothesis was tested that heat stress stimulates mitochondrial complex activity or NADPH oxidases, resulting in increased O2*- release. The mitochondria within intact rat diaphragm were inhibited at complex I (amobarbital or rotenone) or complex I and II (rotenone plus thenoyltrifluoroacetone). NADPH oxidases were blocked by diphenyliodonium. None of these treatments inhibited O2*- release. Conversely, most blockers stimulated O2*- release. As intracellular O2*- generators require a mechanism for O2*- transport across the membrane, anion channel blockers, probenecid and 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid, were also tested. Neither blocker had any inhibitory effect on O2*- release. These results suggest that O2*- released from diaphragm is not directly dependent on mitochondrial complex activity and that it is not a reflection of passive diffusion of O2*- through anion channels. Although the molecular source for extracellular O2*- remains elusive, it is clearly sensitive to temperature and conditions of "chemical hypoxia" induced by partial or complete mitochondrial inhibition.

Antagonism between the metabolic responses induced by epinephrine and piroxicam on isolated rat hepatocytes

Nonsteroidal anti-inflammatory drugs (NSAIDs) are one of the most employed therapeutic agents. They have a wide spectrum of biological effects, some of which are independent of cyclooxygenase inhibition, such as the alterations on the components of signal transduction systems. In particular, previous data from our laboratory suggested an antagonism between epinephrine and piroxicam, one of the most prescribed NSAIDs. Thus, this study deals with the epinephrine-piroxicam antagonism recorded for metabolic responses in isolated rat hepatocytes. The obtained results show that epinephrine stimulates lactate and ethanol consumption, stimulates glucose release from lactate only, and has no effect on cellular triacylglycerides content. Otherwise, in a dose-dependent basis, piroxicam stimulates lactate and ethanol consumption accompanied by an increase in triacylglycerides content, without changes in glucose release by hepatocytes. Piroxicam blocks the epinephrine-induced stimulation of glucose release from lactate, and epinephrine blocks the piroxicam-mediated increase in triacylglycerides content from lactate or ethanol. In contrast, the effects of epinephrine and piroxicam, promoting the consumption of lactate and ethanol, are not antagonized or added after the simultaneous administration of both compounds. This last result is probably related to the ability of both compounds to stimulate oxygen consumption. On isolated rat liver mitochondria, micromolar doses of piroxicam partially uncouple oxidative phosphorylation, and paradoxically stimulates an ATP-dependent mitochondrial function as citrullinogenesis. These results show for first time, on isolated rat hepatocytes, an antagonism between the metabolic responses of epinephrine and piroxicam, at the concentration found in plasma after its therapeutical administration.

Acute effect of leptin on hepatic glycogenolysis and gluconeogenesis in perfused rat liver

Leptin circulates in blood and is involved in body weight control primarily via hypothalamic receptors. To examine its direct metabolic action, effects of short-term portal leptin infusion: 1) on postprandial basal and epinephrine-stimulated glycogenolysis; and 2) on postabsorptive lactate-stimulated gluconeogenesis were studied in isolated perfused rat livers. Incremental epinephrine (150 pmol x min-1 x g-1 liver)-stimulated glucose release (in micromol/g liver within 30 minutes; control: 28.3 +/- 2.8) was suppressed (P <.05) by 44% (15.8 +/- 1.6), by 48% (14.6 +/- 4.1), and by 53% (13.3 +/- 2.1) during insulin (3 pmol x min-1 x g-1 liver), leptin (30 pmol x min-1 x g-1 liver), and simultaneous leptin + insulin infusion. Perfusate cyclic adenosine monophosphate increased approximately twofold during epinephrine stimulation in all groups. Neither leptin nor insulin affected hepatic lactate production, bile flow, or portal pressure in the fed state. In the postabsorptive state (20-hour fasting), rates of lactate (10 mmol/L)-dependent hepatic glucose release (in micromol. min-1 x g-1 liver; control: 0.12 +/- 0.01) were increased (P <.01) to 0.35 +/- 0.02 and to 0.24 +/- 0.01 by glucagon (3 pmol x min-1 x g-1 liver) and by leptin (15 pmol x min-1 x g-1 liver), respectively. In parallel, lactate uptake rates (in micromol x min-1 x g-1 liver) were higher in the presence of both glucagon (0.90 +/- 0. 03) and leptin (0.84 +/- 0.02) compared with control (0.68 +/- 0.04). In conclusion, leptin modulates hepatic glucose fluxes and may contribute to direct humoral regulation of liver glycogen stores in the fasted as well as in the fed state.

cAMP and Ca2+ involvement in the mitochondrial response of cultured fetal rat hepatocytes to adrenaline

The effect of adrenaline on the control of respiratory activity of mitochondria from fetal hepatocytes in primary culture was studied. In the absence of adrenaline, the respiratory control ratio (RCR) of mitochondria increased during the first 3 days of culture due to a decrease in the rate of state 4 respiration. The presence of adrenaline in the incubation medium further increased the mitochondrial RCR through a decrease in the rate of respiration in state 4 and to an increase in the respiration rate in state 3. The effect of adrenaline was mimicked by dibutyryl-cAMP, forskolin, and isobutyl methyl xanthine. All these compounds increased cAMP concentrations, suggesting that cAMP may be involved in the effect of adrenaline. The increase in intracellular free Ca2+ concentrations caused by phenylephrine, vasopressin, or thapsigargin was also accompanied by an increase in the RCR, suggesting that both phenomena are associated. Dibutyryl-cAMP also increased free Ca2+ concentrations, suggesting that the effects of cAMP may be mediated by free Ca2+ concentrations. Adrenaline, dibutyryl-cAMP, phenylephrine, vasopressin, and thapsigargin promoted adenine nucleotide accumulation in mitochondria; this may be an intermediate step in the activation of mitochondrial respiratory function. These results suggest that the stimulatory effect of adrenaline on mitochondrial maturation in cultured fetal rat hepatocytes may be exerted through a mechanism in which both cAMP and Ca2+ act as second messengers. It is concluded that the effect of adrenaline on mitochondrial maturation is exerted by both alpha- and beta-adrenergic mechanisms and is mediated by the increase in adenine nucleotide contents of mitochondria.

Activation of glycogenolysis by methotrexate. Influence of calcium and inhibitors of hormone action

The influence of Ca2+ and the possible action of hormone blockers on the activation of glycogenolysis by methotrexate were investigated. Methotrexate was inactive on glycogenolysis and oxygen uptake when the liver, depleted of intracellular Ca2+, was perfused with Ca(2+)-free medium. The action of methotrexate in calcium-depleted hepatocytes could be restored by the addition of extracellular Ca2+. When Ca2+ was absent in the extracellular medium, but the intracellular stores were not depleted, methotrexate produced transient and progressively attenuated increases in glycogenolysis and oxygen uptake. Like many agonists, methotrexate produced transient increases in Ca2+ efflux. The action of methotrexate was not blocked by the antagonists of norepinephrine, phenylephrine, isoproterenol, vasopressin and angiotensin II. It was concluded that methotrexate acts through a Ca(2+)-dependent mechanism, which is similar to that of the Ca(2+)-dependent agonists. This action, however, seems not to be receptor mediated.

Inhibition by noradrenaline and adrenaline of the increase in glucose and lactate output and decrease in flow after sympathetic nerve stimulation in perfused rat liver: possible involvement of protein kinase C

In perfused rat liver stimulation of the hepatic nerve plexuses increased via alpha 1-receptors glucose and lactate output decreased flow and caused an overflow of noradrenaline into the hepatic vein. Infusion of noradrenaline and adrenaline also elicited similar metabolic and hemodynamic alterations via alpha 1-receptors, whereas infusion of isoproterenol via beta 2-receptors enhanced glucose output and slightly reduced lactate release without affecting flow. The influence of circulating catecholamines on the nerve stimulation-dependent changes was investigated. Noradrenaline (100 nmol/L) or adrenaline (40 nmol/L) but not isoproterenol (1 mumol/L), which themselves caused about half-maximal alterations, strongly inhibited the nerve stimulation-induced increase in glucose and lactate output and decrease in flow but had no effect on noradrenaline overflow. The protein kinase C activator (4 beta)phorbol 12-myristate, 13-acetate (100 nmol/L) but not its analog (4 alpha)phorbol 12,13-didecanoate (100 nmol/L) strongly inhibited the metabolic and hemodynamic changes caused by nerve stimulation or noradrenaline infusion. The protein kinase C inhibitor H7 (20 mumol/L) partially prevented the inhibition of the nerve actions by noradrenaline. The results lead us to conclude that noradrenaline and adrenaline inhibited the metabolic and hemodynamic nerve actions by means of a mechanism involving protein kinase C rather than presynaptic alpha-receptors or beta-receptors. The catecholamines apparently increased via alpha 1-receptors inositol 1,4,5-trisphosphate, which in turn enhanced cytosolic Ca2+ and thus altered metabolism and in part hemodynamics, and diacylglycerol, which in turn activated protein kinase C and thus feedback inhibited the signal chain from alpha 1-receptors via G proteins to phospholipase C.

Dehydrogenase activation by Ca2+ in cells and tissues

The activation of intramitochondrial dehydrogenases by Ca2+ provides a link between the intensity of work performance by a tissue and the activity of pyruvate dehydrogenase and the tricarboxylate cycle, and hence the rate of ATP production by the mitochondria. Several aspects of this model of the control of oxidative phosphorylation are examined in this article, with particular emphasis on mitochondrial functioning in situ in cardiac myocytes and in the intact heart. Recent use of the fluorescent Ca2+ chelating agents indo-1 and fura-2 has allowed a more quantitative description of the dependence of dehydrogenase activity upon concentration of free intramitochondrial Ca2+, in experiments with isolated mitochondria. Further, a novel technique developed by Miyata et al. has allowed description of free intramitochondrial Ca2+ within a single cardiac myocyte, and the conclusion that this parameter changes in response to electrical excitation of the cell over a range which would be expected to give substantial modulation of dehydrogenase activity.

Control of glycogenolysis and blood flow by arterial and portal norepinephrine in perfused liver

In isolated rat liver single pass perfused via both the hepatic artery (80 mmHg, 30% flow) and the portal vein (10 mmHg, 70% flow), norepinephrine (NE) was infused either singly or jointly via the hepatic artery or the portal vein in the absence or presence of the alpha 1-blocker prazosin or the beta 2-blocker butoxamine. Arterial NE caused an increase in glucose output and a shift from lactate uptake to release that was slower in onset and smaller in peak height but longer in duration than the alterations affected by portal NE. The sum of the metabolic changes by arterial and portal NE was not equal to the changes by jointly applied arterial plus portal NE. The metabolic alterations by arterial NE were mediated via alpha 1-receptors, with beta 2-receptors probably having a permissive function, but those by portal NE were transmitted only via alpha 1-receptors. Arterial NE caused a strong decrease in arterial flow and contralaterally also a smaller reduction of portal flow. Portal NE decreased portal flow but did not significantly influence arterial flow. The sum of the alterations in flow by arterial and portal NE was not equal to the changes by jointly applied NE. The hemodynamic alterations in the artery by arterial NE were the results of actions via alpha 1-receptors and counteractions via beta 2-receptors, whereas the changes in the portal vein by arterial NE and portal NE were mediated via alpha 1-receptors. About 65% of arterial and only 30% of portal NE was extracted during a single path. The results indicate that the hepatic artery and the portal vein can function as independent sites of hormonal signal input, which interact by complex but still undefined mechanisms in the regulation of metabolism and hemodynamics.

Effect of epinephrine on ethanol metabolism by isolated rat hepatocytes

The effect of epinephrine on ethanol metabolism was determined in isolated rat hepatocytes. Epinephrine (10 microM) enhanced an initial rapid rate of ethanol elimination observed in the first 5 min. Thereafter, between 5 and 90 min, the rate of ethanol elimination was slower and not affected by epinephrine. Epinephrine resulted in higher acetaldehyde concentrations at 2 min, but not thereafter. Acetaldehyde production in the presence and absence of epinephrine was inhibited by 4-methylpyrazole, by a low free extracellular calcium concentration, and by the alpha 1-adrenergic blocker prazosin. Ethanol alone and epinephrine alone increased oxygen consumption, but the effects were not additive. The ethanol-induced decreases in the cytosolic NAD-/NADH and NADP++NADPH ratios and in the mitochondrial NAD+/NADH ratio were delayed by the presence of epinephrine. An accelerated initial alcohol dehydrogenase activity sufficient to account for the rapid initial rate of ethanol elimination shown with epinephrine was demonstrated by coupling ethanol oxidation with lactaldehyde reduction, a system which increases the rate of dissociation of NADH from the enzyme and its oxidation back to NAD+. The findings in this study indicate that an increased reoxidation of NADH during ethanol oxidation by alcohol dehydrogenase is the basis for the rapid transient increase in ethanol elimination produced by epinephrine.

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.

The effects of radical stress and N-ethylmaleimide on rat hepatic alpha 1-adrenergic receptors

Liver plasma membranes of non-treated rats or rats treated with carbon tetrachloride were prepared. Liver plasma membranes obtained from non-treated rats were preincubated with cumene hydroperoxide, N-ethylmaleimide or hydrogen peroxide in the absence or presence of Fe2+. The effects of in-vivo or in-vitro treatment on hepatic alpha 1-adrenergic receptors were assessed in a [3H]prazosin binding assay. All treatments except preincubation with hydrogen peroxide alone caused a decrease in the number of alpha 1-adrenergic receptors without a change in the affinity of [3H]prazosin. The decrease in receptor number was accompanied by an increase in the level of lipid peroxidation in the plasma membranes. The results indicate that the hepatic alpha 1-adrenergic receptor is vulnerable to free radical stress.

Control of gluconeogenesis: role of fatty acids in the alpha-adrenergic response

Phenylephrine increases hepatic gluconeogenesis for as long as it is present in the extracellular medium. This effect is accompanied by a parallel increase in oxygen consumption. No apparent stoichiometric relationship exists between the phenylephrine-stimulated respiration and the energy required to meet the demands of gluconeogenesis. In the absence of extracellular calcium, no sustained stimulation of respiration was observed and phenylephrine failed to enhance gluconeogenesis; however, acute and transient effects of the alpha-adrenergic agonist were still observable. The following observations indicate that fatty acids are not involved in the alpha-adrenergic response: (1) the effects of phenylephrine and octanoate on respiration and gluconeogenesis were found to be additive; (2) unlike phenylephrine, octanoate is capable of stimulating gluconeogenesis in calcium-depleted liver; (3) in the absence of calcium, phenylephrine was incapable of further stimulating respiration or gluconeogenesis in the presence of octanoate. It is concluded that the conditions of increased lipid mobilization and/or oxidation are not sufficient to explain the metabolic response to alpha-adrenergic agonists. Fatty acids and alpha-adrenergic stimulation share a common role of stimulating gluconeogenesis in a manner dependent on their ability to stimulate respiration; however, the additive nature of their effects and distinct calcium requirements indicate that they act to trigger different mechanisms.

Characterization of the alpha-adrenergic stimulation of hepatic respiration

The alpha-adrenergic agonist phenylephrine induces a biphasic stimulation of respiration in perfused isolated rat liver. The first phase, of rapid onset and short duration, is paralleled by increased glycogenolysis, glycolysis, and NAD redox potential. The second phase lasts for as long as the alpha-agonist is present and is accompanied by increased gluconeogenic flux. Only the second phase of sustained increased respiration is clearly dependent on extracellular Ca2+. In contrast, normal respiratory responses were obtained under Ca2+-loading conditions or in the presence of the Ca2+ ionophore A23187, indicating that the alpha-adrenergic action on respiration is not simply mediated by its ability to increase the cytosolic Ca2+ concentration. No stimulation of gluconeogenesis is observed in the absence of a sustained increase of respiration. However, it is not energy support that leads to the stimulation of glucose production. The adrenergic response is influenced by the nutritional status of the animal and the availability of oxidizable fuels. In livers from starved animals, the alpha-adrenergic respiratory response is abolished when long chain fatty acid oxidation is prevented by the addition of tetradecylglycidate. In the presence of pyruvate the respiratory response is partially restored. It is concluded that increased beta-oxidation is not mandatory for the alpha-adrenergic stimulation of respiration; however, maximal respiratory responses are obtained only when fatty acid oxidation is allowed to proceed. The latter finding appears to be the result of a limited flux through the tricarboxylic acid cycle when long chain fatty acid oxidation is impeded, secondary to a limiting acetyl CoA supply.

Influence of epinephrine on alcohol dehydrogenase activity in rat hepatocyte culture

The effects of epinephrine on alcohol dehydrogenase activity and on rates of ethanol elimination were determined in rat hepatocyte culture. Continuous exposure of the hepatocytes to epinephrine (10 microM) in combination with dexamethasone (0.1 microM) enhanced alcohol dehydrogenase activity on days 4-7 of culture, whereas neither hormone alone had an effect. The increased alcohol dehydrogenase activity was associated with an increased rate of ethanol elimination. Acute addition of 10 microM epinephrine to hepatocytes maintained in culture with 0.1 microM dexamethasone did not change alcohol dehydrogenase activity, but resulted in an immediate marked, but transitory, increase in ethanol elimination within the first 5 min after the addition of the hormone. Prazosin, an alpha 1-adrenergic blocker, and antimycin, an inhibitor of mitochondrial respiration, were powerful inhibitors of the transient increase in ethanol elimination, whereas 4-methylpyrazole was only partially inhibitory. These observations indicate that epinephrine has a chronic effect in increasing alcohol dehydrogenase activity and ethanol elimination and, also, an acute transient effect of increasing ethanol elimination which is not limited by alcohol dehydrogenase activity.

A water-soluble derivative of prazosin prazosinamine hydrochloride [1-(4'-amino-6',7'-dimethoxyquinazolin-2'-yl)-4-(6''-aminohexanoyl) piperazine hydrochloride], reversibly inhibits the calcium-mobilizing action of alpha 1-adrenergic agonists in the perfused rat liver

A newly-synthesized derivative of prazosin, prazosinamine hydrochloride, was examined for its ability to antagonize the interaction of the alpha 1-adrenergic agonist phenylephrine with liver cells. Using a Ca2--selective electrode to measure changes in perfusate Ca2+ concentration, prazosinamine was found to be as effective as prazosin in inhibiting the phenylephrine-induced efflux of Ca2+ from the perfused liver. Maximal and half-maximal inhibition occurred at 150 nM and 25 nM prazosinamine, respectively. Prazosinamine appears to share the alpha 1-specificity of prazosin, but has other unique and desirable properties. Its solubility in aqueous media is about three orders of magnitude higher than that of prazosin. Also, its antagonistic effects are rapid in onset, and are reversed within seconds of terminating its infusion into the liver. These attributes seem to make this agent more useful than prazosin for adrenergic receptor studies in perfused tissues. The molecule can also be readily coupled to other ligands.

Stimulation of anaerobic metabolism in rats at high altitude hypoxia--adrenergic effects dependent on dietary states

1. Plasma lactate and pyruvate were increased more markedly in fed rats than in fasted rats exposed to an 8000 m altitude. 2. The increase in plasma lactate and pyruvate was enhanced and inhibited by the alpha 1-adrenergic antagonist prazosin and the beta-blocker propranolol, respectively, in fasted rats exposed to an 8000 m altitude. Blood glucose was not changed by adrenergic blockades under the same conditions. 3. Prazosin and propranolol showed no effect on glycolytic metabolites in plasma in fed rats submitted to an 8000 m altitude. Blood glucose of fed rats was increased by alpha 1-blockade during severe hypoxia. 4. In fasted rats whose energy metabolism depends on oxidation mainly, alpha 1- and beta-adrenergic receptors can participate in the stimulation of respiration and the glycogen degradation, respectively, during an exposure to severe hypoxia. In fed rats energy metabolism depends on glycolysis, which utilizes blood glucose as the substrate preferentially during hypoxia.

Hormonal regulation of the tricarboxylic acid cycle in the isolated perfused rat liver

The effect of Ca2+-mobilizing hormones, vasopressin, angiotensin II and the alpha-adrenergic agonist phenylephrine, on the metabolic flux through the tricarboxylic acid cycle was investigated in isolated perfused rat livers. All three Ca2+-mobilizing agonists stimulated 14CO2 production and gluconeogenesis in livers of 24-h-fasted rats perfused with [2-14C]pyruvate. Prazosin blocked the phenylephrine-elicited stimulation of 14CO2 and glucose production from [2-14C]pyruvate whereas the alpha 2-adrenergic agonist, BHT-933, did not affect the rates of 14CO2 and glucose production from [2-14C]pyruvate indicating that the phenylephrine-mediated response involved alpha 1-adrenergic receptors. Phenylephrine, vasopressin and angiotensin II stimulated 14CO2 production from [2-14C]acetate in livers derived from fed rats but not in livers of 24-h-fasted rats. In livers of 24-h-fasted rats, perfused with [2-14C]acetate, exogenously added pyruvate was required for an increase in the rate of 14CO2 production during phenylephrine infusion. This last observation suggests increased pyruvate carboxylation as one of the mechanisms involved in stimulation of tricarboxylic acid cycle activity by the Ca2+-mobilizing agonists, vasopressin, angiotensin II and phenylephrine.

On the stimulation of respiration by alpha-adrenergic agonists in perfused rat liver

Interactions between phenylephrine-induced oxygen consumption, lactate and pyruvate output, and urea and glucose production were examined in perfused livers from fed or 48-h-fasted rats. Within 2 min of phenylephrine infusion, oxygen consumption in perfused livers was increased by approximately 40%. Increases in oxygen consumption induced by phenylephrine were essentially abolished in the presence of carboxyatractyloside, whereas those induced by dinitrophenol were still evident. Phenylephrine-induced increases in oxygen consumption were accompanied by enhanced rates of gluconeogenesis and ureogenesis in livers from fed or 48-h-fasted animals. These data indicate that phenylephrine-induced increases in respiration in perfused rat liver may result from an enhanced rate of mitochondrial oxidative phosphorylation in response to an increased cellular energy requirement.

Control of oxygen uptake, microcirculation and glucose release by circulating noradrenaline in perfused rat liver

The effect of noradrenaline on oxygen uptake, on periportal and perivenous oxygen tension at surface acini, on microcirculation and on glucose output were studied in isolated rat livers perfused at constant flow with Krebs-Henseleit-hydrogen carbonate buffer containing 5mM glucose and 2mM lactate. Noradrenaline at 1 microM concentration caused a decrease in oxygen uptake, while at 0.1 microM it led to an increase. Both high and low doses of noradrenaline decreased the tissue surface oxygen tension in periportal and - after a transient rise - in perivenous areas. Noradrenaline at an overall constant flow caused an increase of portal pressure and an alteration of the intrahepatic distribution of the perfusate: at the surface of the liver and in cross sections infused trypan blue led to only a slightly heterogeneous staining after a low dose of noradrenaline but to a clearly heterogeneous staining after a high dose. Both high and low doses of noradrenaline stimulated glucose release. All effects could be inhibited by the alpha-blocking agent phentolamine. In conclusion, control of hepatic oxygen consumption by circulating noradrenaline is a complex result of opposing hemodynamic and metabolic components: the microcirculatory changes inhibit oxygen uptake; they dominate after high catecholamine doses. The metabolic effects include a stimulation of oxygen utilization; they prevail at low catecholamine levels. The noradrenergic control of glucose release is also very complex, involving direct, metabolic and indirect, hemodynamic components.

Accumulation of Ca2+ induced by cytotoxic levels of menadione in the isolated, perfused rat liver

Previous studies have indicated that the presence of cytotoxic levels of menadione (2-methyl-1,4-naphthoquinone) causes rapid changes in intracellular thiol and Ca2+ homeostasis in isolated rat hepatocytes. The present investigation was undertaken to examine these effects in the intact liver. Rat livers were therefore perfused with Krebs-Henseleit buffer containing 1.3 mM Ca2+ using a single-pass mode, and the perfusate Ca2+ level was monitored with an on-line Ca2+-selective electrode. Infusion of menadione elicited an increased O2 uptake by the liver, followed by a dose-dependent decrease in the perfusate level of Ca2+. Hepatic accumulation of Ca2+ was accompanied by stimulation of cytosolic phosphorylase a activity. Cessation of menadione infusion resulted in gradual recovery of perfusate Ca2+ to base levels. Ca2+ uptake was not accompanied by decreases in reduced pyridine nucleotide or ATP levels in the liver as evidenced by measurements either during maximal Ca2+ uptake or after recovery. However, Ca2+ uptake was correlated with decreased glutathione and increased glutathione disulfide levels in the liver, both of which reversed during recovery from Ca2+ uptake. Moreover, depletion of hepatic glutathione by pretreatment with diethylmaleate resulted in increased Ca2+ uptake during menadione infusion. The amount of protein-bound mixed disulfides showed a particularly striking relationship to Ca2+ uptake, reaching a maximal level during Ca2+ uptake and reversing toward normal value during recovery from Ca2+ accumulation. The present findings suggest that menadione-induced Ca2+ uptake is due to plasma membrane dysfunction as a result of loss of protein thiol groups critical for maintaining the plasma membrane Ca2+ extrusion mechanism. Our model offers a particularly useful opportunity to study mechanisms underlying toxic disturbances in Ca2+ homeostasis in the intact liver, since Ca2+ fluxes can be monitored under conditions in which cellular control mechanisms are not obliterated by excessive toxicity.

Regulation of oxygen consumption in perfused rat liver: decrease by alpha-sympathetic nerve stimulation and increase by the alpha-agonist phenylephrine

In livers perfused with Krebs-Henseleit bicarbonate buffer containing bovine red cells, 5 mM glucose and 2 mM lactate, electrical stimulation round the hepatic artery and the portal vein caused via alpha-receptors a decrease in oxygen consumption and portal flow, an increase in glucose output and a switch from lactate uptake to output. In livers perfused with erythrocyte- and substrate-free buffer both in a volume- or pressure-constant system stimulation of the liver nerves resulted in similar changes. Infusion of the alpha-agonist phenylephrine mimicked the metabolic and hemodynamic nerve effects, but led to an increase in oxygen uptake. The converse effects of alpha-sympathetic nerve stimulation and alpha-agonist infusion on oxygen consumption indicate either a different mode of action or a complex mechanism with opposing metabolic and hemodynamic components.

Adrenergic regulation of glycogenolysis in liver of Xenopus laevis in vitro

Glycogenolysis was stimulated by catecholamines in in vitro cultures of hepatic tissue of Xenopus laevis. Dose response curves showed that adrenaline and isoprenaline were equally effective while noradrenaline and phenylephrine were progressively less effective in eliciting glycogen breakdown. Neither oxymetazoline nor methoxamine had any effect on glycogenolysis. Administration of adrenaline to cultures was followed within 1 min by a rise in tissue cyclic AMP concentration and within 2 min by an increase in phosphorylase a activity. Both these responses were blocked by propranolol but little affected by phenoxybenzamine. These findings suggest that catecholamines activate glycogenolysis via a beta-adrenergic receptor in X. laevis and that alpha-adrenergic receptors play no role in regulating hepatic glycogenolysis in this species.

Alpha 1-adrenergic stimulation of hamster brown adipocyte respiration

Respiration was increased approximately 5-fold with 0.05 microM norepinephrine and to a maximum of 10-fold by 0.30 microM norepinephrine. Prazosin, an alpha-adrenergic blocking agent highly selective for alpha 1-type receptors, partially inhibited the response to norepinephrine (0.05 microM) by 20-25% at a concentration of 0.10-1 microM. In contrast, when the stimulus for respiration was provided by isoproterenol or 3-isobutyl-1-methylxanthine, prazosin was without effect up to a concentration of 10 microM. Yohimbine, an alpha-adrenergic blocking drug preferential for alpha 2-receptors, did not influence norepinephrine-stimulated oxygen uptake. Respiration was increased two- to fourfold by phenylephrine or methoxamine, agents preferential for alpha 1-adrenergic receptors but not at all by clonidine, an agent preferential for alpha 2-adrenergic receptors. The stimulatory effect of phenylephrine on oxygen uptake was fully blocked by prazosin but not propranolol. Removal of extracellular calcium with ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid prevented phenylephrine stimulation of respiration but was without effect when isoproterenol was the stimulus. These results support the participation of alpha 1-adrenergic receptors in control of respiration and are consistent with the possibility that changes in cell calcium are intimately involved in this response.

alpha-Adrenergic stimulation of glycolysis and Na+, K+-transport in perfused rat liver

Interactions between alpha-adrenergic-induced lactate production, respiration and K+ uptake of perfused rat livers were investigated. Inhibition of mitochondrial electron transport by cyanide had no effect on K+ uptake, decreased phenylephrine-induced lactate production by 60% and suppressed extra O2 consumption by more than 90%. Ouabain did not affect alpha-adrenergic-induced respiration but inhibited K+ uptake and extra lactate production. Effects of ouabain and K+-free medium on lactate production were additive and independent of cyanide, indicating that they were mediated by extramitochondrial mechanisms. Extra ATP utilized for K+ uptake in response to phenylephrine was apparently of glycolytic rather than mitochondrial origin. We propose that alpha-adrenergic-induced lactate production is secondary to an enhanced operation of the plasma membrane (Na+,K+)-ATPase.