Effects of alterations in energy supply on gluconeogenesis from L-lactate

The action of extracellular NAD+ on gluconeogenesis in the perfused rat liver

In the rat liver NAD+ infusion produces increases in portal perfusion pressure and glycogenolysis and transient inhibition of oxygen consumption. The aim of the present work was to investigate the possible action of this agent on gluconeogenesis using lactate as a gluconeogenic precursor. Hemoglobin-free rat liver perfusion in antegrade and retrograde modes was used with enzymatic determination of glucose production and polarographic assay of oxygen uptake. NAD+ infusion into the portal vein (antegrade perfusion) produced a concentration-dependent (25-100 microM) transient inhibition of oxygen uptake and gluconeogenesis. For both parameters inhibition was followed by stimulation. NAD+ infusion into the hepatic vein (retrograde perfusion) produced only transient stimulations. During Ca2+-free perfusion the action of NAD+ was restricted to small transient stimulations. Inhibitors of eicosanoid synthesis with different specificities (indo-methacin, nordihydroguaiaretic acid, bromophenacyl bromide) either inhibited or changed the action of NAD+. The action of NAD+ on gluconeogenesis is probably mediated by eicosanoids synthesized in non-parenchymal cells. As in the fed state, in the fasted condition extracellular NAD+ is also able to exert two opposite effects, inhibition and stimulation. Since inhibition did not manifest significantly in retrograde perfusion it is likely that the generating signal is located in pre-sinusoidal regions.

A method for the determination of glucose synthesis in isolated bovine hepatocytes

A simple method for determining glucose synthesis from radiolabeled precursors in isolated bovine hepatocytes using ion exchange resins is presented. This method allows processing of multiple small volume samples using suspensions of anion and cation exchange resins rather than traditional stacked column separation methods. Hepatocytes were isolated from calf liver by collagenase perfusion of the caudate lobe and were incubated with (14)C-labeled lactate or propionate as gluconeogenic substrates. Glucose synthesis was determined in an aliquot of cell suspension that was vortexed with a slurry of anion exchange (acetate form) resin, followed by a slurry of cation exchange resin. Newly synthesized, labeled glucose was recovered in the supernatant after centrifugation and quantitated by scintillation counting. Using this procedure, more than 98% of the unused labeled precursor was bound to the ion exchange resin and essentially 100% of a labeled glucose tracer was recovered in the supernatant. Pretreatment of hepatocyte suspensions with glucose oxidase was shown to eliminate the accumulation of radioactivity in the supernatant, thus confirming the specificity of this technique for measurement of newly synthesized glucose. This method was sensitive to changes in the rate of hepatic gluconeogenesis that resulted from changes in substrate concentration or the addition of glucagon or fatty acids to the hepatocyte incubations.

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.

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.

Inhibition of gluconeogenesis by 2,5-anhydro-D-mannitol in isolated rat hepatocytes

2,5-Anhydro-D-mannitol inhibited glucose synthesis, increased the pyruvate/phosphoenolpyruvate ratio and altered adenine nucleotide concentrations in hepatocytes isolated from fasted rats. The accumulations of 2,5-anhydro-D-mannitol 1,6-diphosphate, an allosteric activator of pyruvate kinase, and of ADP in treated hepatocytes can account for the increase in pyruvate/phosphoenolpyruvate ratio and the inhibition of glucose synthesis from lactate.

Loss of responsiveness to glucagon following underperfusion of the isolated rat liver

Increases in glucose and urea output in response to increasing glucagon concentration have been studied in isolated livers perfused with physiological concentrations of amino acids. Glucose output was more sensitive to glucagon than urea output. A period of non-perfusion caused a subsequent loss of responsiveness to glucagon concentrations, at the high end of the physiological range, but the response to glucagon at the low end of the physiological range was unaffected. This probably represents a post-receptor effect rather than alterations at the receptor level.

Enzymatic analysis of 2,5-anhydro-D-mannitol and related compounds

Two enzymatic assay procedures for the measurement of 2,5-anhydrohexitol fructose analogs have been devised. Both procedures are based on the measurement of ADP formed during enzymatic phosphorylation of the analogs either by hexokinase or by fructokinase. The actual measurement makes use of the coupled assay system using pyruvate kinase, PEP, lactate dehydrogenase, and NADH. Both systems can be used to measure fructose and appropriate analogs at cuvette concentrations up to 0.10 mM. The hexokinase procedures allows the measurement of fructose, 2,5-anhydromannitol, and 2,5-anhydromannose. Glucose, which also reacts, can be removed by pretreatment of the samples with glucose oxidase. The fructokinase procedure allows the measurement of fructose, 2,5-anhydromannitol, 2,5-anhydroglucitol, and 2,5-anhydrotalitol.