The effects of alpha-adrenergic agonists on the regulation of the branched chain alpha-ketoacid oxidation in the perfused rat liver

Modulation of the hepatic alpha 1-adrenoceptor responsiveness by colchicine: dissociation of free cytosolic Ca(2+)-dependent and independent responses

1. The cytoskeletal depolymerizing agent, colchicine, prevents the hepatic alpha 1-adrenoceptor-mediated stimulation of respiration, H+ and Ca2+ release to the effluent perfusate, intracellular alkalosis, and glycogenolysis. Unlike the other parameters, colchicine does not perturb the alpha 1-agonist-induced stimulation of gluconeogenesis or phosphorylase 'a' activation, and enhances the increase in portal pressure response. The lack of effect of colchicine on the hepatic alpha 2-adrenoceptor-mediated effects indicates that its actions are alpha 1-specific. 2. Colchicine enhances the acute alpha 1-adrenoceptor-mediated intracellular Ca2+ mobilization and prevents the activation of protein kinase C. This differential effect on the two branches of the alpha 1-adrenoceptor signalling pathway is a distinctive feature of the colchicine action. 3. The lack of effect of colchicine in altering the alpha 1-adrenoceptor ligand binding affinity suggests that it might interact with some receptor-coupled regulatory element(s). 4. The acuteness of the colchicine effect and the ability of its isomer beta-lumicolchicine to prevent all the alpha 1-adrenoceptor-mediated responses but the increase in vascular resistance, indicate that its action cannot be merely ascribed to its effects in depolymerizing tubulin. 5. Colchicine perturbs the hepatic responses to vasoactive peptides. It enhances the vasopressin-induced rise of cytosolic free Ca2+ in isolated hepatocytes and prevents the sustained decrease of Ca2+ in the effluent perfusate. It also inhibits the stimulation of glycogenolysis, without altering the stimulation of gluconeogenesis. 6. It is concluded that there are at least two major alpha 1-adrenoceptor signalling pathways. One is colchicine-sensitive, independent of variations in free cytosolic Ca2+, and protein kinase C-dependent; the other one is colchicine-insensitive, dependent on variations in free cytosolic Ca2+, and protein kinase C-independent.

Metabolism of alpha-ketoisocaproic acid in isolated perfused liver of cirrhotic rats

To determine the hepatic fate of alpha-ketoisocaproate (KIC) in cirrhosis, six groups of isolated rat livers were perfused with 0, 0.5, 1 (with or without alpha-[1-14C]KIC), 2, and 5 mM KIC; control livers from healthy rats were studied in parallel under similar conditions. KIC was rapidly removed by the normal livers, whereas uptake was lower in the cirrhotic livers at all concentrations tested (at 2 mM, 4.04 +/- 0.33 vs. 6.32 +/- 0.58 mumol/min; P < or = 0.05). The transamination pathway, evaluated by leucine exchanges, was more important in the cirrhotic livers (25.4 vs. 6.8% in controls at 2 mM). The incorporation of alpha-[1-14C]KIC in proteins of cirrhotic liver was increased compared with controls (0.25 +/- 0.04% of alpha-[1-14C]KIC was incorporated in proteins excreted in perfusate vs. 0.20 +/- 0.04 in controls; P < or = 0.05). In addition, a line of evidence suggests that glutamine rather than glutamate is the N donor for leucine synthesis from KIC. The decarboxylation pathway evaluated by beta-hydroxybutyrate production and by 14CO2 release from alpha-[1-14C]KIC was reduced, respectively, by 40-85% (according to KIC dose) and by 24% at 90 min in cirrhotic livers compared with healthy livers. These results indicate a dramatic modification of KIC metabolism in the cirrhotic liver; its uptake by the liver is decreased and its incorporation into proteins is increased via an enhancement of transamination to leucine, probably as a consequence of an inhibition of branched-chain keto acid dehydrogenase.

Comparison of the effect of a mitochondrial uncoupler, 2,4-dinitrophenol and adrenaline on oxygen radical production in the isolated perfused rat liver

Using the isolated perfused rat liver, we examined the effect of stimulation of mitochondrial respiration by 2,4-dinitrophenol (2,4-DNP) and adrenaline on reactive oxygen species (ROS) production, liver damage and lipid peroxidation. ROS production was monitored by luminol- and lucigenin-enhanced chemiluminescence and oxygen uptake was measured simultaneously. Liver damage and lipid peroxidation were evaluated by measuring hepatic lactate dehydrogenase (LDH) and thiobarbituric acid reacting substances (TBARS) release. Tissue ROS level decreased and oxygen uptake increased soon after 2,4-DNP infusion. On termination of 2,4-DNP infusion, there was a sharp increase in lucigenin-enhanced chemiluminescence, which declined slowly, but luminol-enhanced chemiluminescence did not change prominently. Hepatic LDH and TBARS release increased gradually during 2,4-DNP infusion and were manifested by termination of the infusion. Allopurinol did not affect ROS production and TBARS release, but delayed increases in LDH release after termination of 2,4-DNP infusion. Adrenaline, which stimulates mitochondrial respiration without uncoupling caused similar but smaller ROS changes observed in 2,4-DNP. LDH and TBARS release were not affected significantly by adrenaline infusion. These results indicate that uncoupling of oxidative phosphorylation decreases ROS production and restoration of oxidative phosphorylation enhances ROS production and liver damage. Xanthine oxidase is unlikely to contribute to enhanced ROS production after termination of 2,4-DNP but has some protective effect during uncoupling.

Oxidation of leucine by Leishmania donovani

The metabolism of leucine by Leishmania donovani was investigated. Washed promastigotes were incubated with [1-14C]- or [U-14C]leucine or [1-14C]alpha-ketoisocaproate (KIC) and 14CO2 release was measured. The amount of KIC-derived acetyl-CoA oxidized in the citric acid cycle was computed. Promastigotes from mid-stationary phase cultures oxidized each of these labeled substrates less rapidly than cells from late log phase cultures, and significantly less acetyl-CoA derived from KIC oxidation was oxidized in the citric acid cycle. Glucose was a stronger inhibitor than was acetate of CO2 formation in the citric acid cycle in log phase promastigotes, but the reverse was observed in cells from mid-stationary phase. Alanine also inhibited leucine catabolism, but glutamate had little effect. Acute hypo-osmotic stress did not affect leucine catabolism, but hyper-osmotic stress caused appreciable inhibition of leucine oxidation. Cells grown under hypo- or hyper-osmotic conditions showed no changes in the effects of hypo- or hyper-osmotic stress on leucine catabolism, i.e. L. donovani is not an osmoconformer with respect to leucine metabolism. Leucine utilization in L. donovani was insensitive to a number of drugs that affect leucine metabolism in mammalian cells, indicating that the leucine pathway in L. donovani is not regulated in the same manner as in mammalian cells.

Compartmental analysis of 45Ca2+ efflux in perfused rat liver: effects of hormonal stimulation

The kinetics of calcium movements in the isolated perfused rat liver were examined using compartmental analysis of the efflux profiles of 45Ca2+ from 45Ca(2+)-equilibrated livers under a variety of calcium concentrations and hormonal treatments. From the 45Ca2+ efflux profiles, we determined that a three compartment model was appropriate to describe the movements of calcium in the liver on the time scale of the experiments. Hormonal treatment with the alpha-adrenergic agonist, phenylephrine, or the vasoactive peptide, vasopressin, during the efflux period lowered significantly the rate of transfer of Ca2+ between the internal compartments at all of the calcium concentrations employed. Also, phenylephrine treatment leads to increased transfer of Ca2+ into the liver from the perfusate. The temporal characteristics of the phenylephrine and vasopressin sensitive Ca2+ pools were examined by pulsing livers, loaded for variable periods of time with 45Ca2+, with the two hormones during the efflux of 45Ca2+ to measure the kinetics of Ca2+ exchange in the hormone-sensitive pools. Results from these experiments indicate that the rate of unstimulated Ca2+ efflux, k2, for the phenylephrine and vasopressin sensitive Ca2+ pools, modeled as a one compartment system, are the same, 0.074 and 0.078 min-1 for phenylephrine and vasopressin respectively, corresponding to half times for turnover of the pool(s) of 9.3 and 8.9 min, respectively.

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.

Measurement of branched-chain alpha-keto acid dehydrogenase flux rates in perfused heart and liver

The methods described here represent a flexible set of procedures for investigating the metabolism of the branched-chain alpha-keto acids and other substances in perfused organs, notably the rat heart and liver. These procedures have been used to investigate many aspects of the metabolism of the branched-chain alpha-keto acids not discussed here, such as the effects on branched-chain alpha-keto acid metabolism by exposure to alpha-adrenergic agents, by inhibition of the monocarboxylate translocator, and by the coinfusion of other metabolites.

Utilization of alpha-ketoisocaproate for protein synthesis in uremic rats

We have recently shown that the nutritional efficiency, R, of alpha-ketoisocaproate (KIC) as a substitute for leucine, defined as the ratio of the dose of leucine to the dose of KIC (on a leucine-free diet) for equal growth, can be evaluated isotopically: 14C-KIC and 3H-leucine are administered p.o.; six hours later, 14C/3H in the leucine of whole body protein, divided by 14C/3H in the injectate, gives a value distinguishable from R assessed in the same animals by growth experiments. To see how chronic uremia affects R, 11/12 nephrectomized rats and sham-operated controls were fed a regular diet for 15 days and then given these isotopes p.o. Six hours later, R, measured in whole body protein, and in the protein of brain, heart, muscle, salivary gland, liver, and the kidney remnant was significantly greater than in sham-operated controls. The greatest difference (39%) was seen in liver protein and the smallest difference (19%) in muscle. Thus chronic uremia increases the efficiency, relative to leucine, with which KIC is utilized for protein synthesis in all of these organs and in the body as a whole. Possible explanations are discussed.

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.

Transport of 2-oxoisocaproate in isolated hepatocytes and liver mitochondria

The transport of 2-oxoisocaproate into isolated hepatocytes and liver mitochondria of rat was studied using [U-14C]2-oxoisocaproate and the silicone oil filtration procedure. 2-Oxoisocaproate uptake by hepatocytes was composed of: rapid adsorption, unmediated diffusion and carrier-mediated transport. The carrier-mediated transport was strongly inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulphonic acid and p-chloromercuribenzoate, was less sensitive to alpha-cyano-4-hydroxycinnamate and insensitive to p-chloromercuriphenylsulphonate. Other 2-oxo acids: pyruvate, 2-oxoisovalerate and 2-oxo-3-methylvalerate, were also inhibitory. The kinetic parameters of the carrier-mediated transport were Km 30.6 mM and Vmax 23.4 nmol/min per mg wet wt, at 37 degrees C. It is concluded that at its low, physiological, concentration, 2-oxoisocaproate penetrates the hepatocyte membrane mainly by unmediated diffusion. The uptake of 2-oxoisocaproate by isolated liver mitochondria was partly inhibited by alpha-cyano-4-hydroxycinnamate, the inhibitor of mitochondrial monocarboxylate carrier. The remaining uptake was linearly dependent on 2-oxoisocaproate concentration and represented unmediated diffusion. The carrier-mediated transport exhibited the following kinetic parameters: Km 0.47 mM, Vmax 1.0 nmol/min per mg protein at 6 degrees C; and Km 0.075 mM and Vmax about 8 nmol/min per mg protein at 37 degrees C.