Stimulated CO2 Production Research Articles

Priming effect of stable C pool in soil and its temperature sensitivity

It is important to understand the effects of exogenous carbon (C) additions on the mineralization of native soil C in agricultural soils under changing global temperatures. The slowly mineralizable C pools represent the major component of soil carbon. Once the labile fraction of soil C is depleted through mineralization, additions of exogenous labile C may enhance decomposition of slowly mineralizable soil C through the so-called priming effect (PE). The effect of exogenous C on mineralization of slowly mineralizable C in soils that have undergone extended substrate depletion is not well understood. The aim of this study was to assess the effects of exogenous C addition on the PE and its temperature sensitivity (Q10) in two substrate depleted soils. The Mollisols with two levels of C 6.8% and 2.9% were pre-incubated at 15 and 25 ℃ for 864 days, followed by 42-day incubations with and without 13C-enriched corn (Zea mays) leaf addition at 15 and 25 ℃. Soil organic carbon (SOC) decreased by 7%−9% in low and 1%−3% in high C soil after the long-term pre-incubation, respectively. In the 42-day incubation following the long-term pre-incubation, the incubation at the higher temperature (25 ℃) significantly increased cumulative CO2 production compared to that at the lower temperature (15 °C) for both high and low C soils with or without exogenous C addition. Furthermore, corn leaves addition stimulated CO2 production derived from resistant OC in both soils, resulting in a positive PE. Higher incubation temperature resulted in a greater PE in both soils independent of pre-incubation temperature. The temperature sensitivity (Q10) of the PE in the low C soil pre-incubated at 25 °C was greater than that pre-incubated at 15 °C, but in the high C soil, the Q10 of PE was lower after pre-incubation at 25 °C. The results confirmed that exogenous C addition and a higher temperature accelerate native SOC mineralization and suggested that the temperature sensitivity of the PE depended on the soil C content and the long-term temperature regime a soil is subjected to.

Microplastic fibers affect dynamics and intensity of CO2 and N2O fluxes from soil differently

Microplastics may affect soil ecosystem functioning in critical ways, with previously documented effects including changes in soil structure and water dynamics; this suggests that microbial populations and the processes they mediate could also be affected. Given the importance for global carbon and nitrogen cycle and greenhouse warming potential, we here experimentally examined potential effects of plastic microfiber additions on CO2 and N2O greenhouse gas fluxes. We carried out a fully factorial laboratory experiment with the factors presence of microplastic fibers (0.4% w/w) and addition of urea fertilizer (100 mg N kg− 1) using one target soil. The conditions in an intensively N-fertilized arable soil were simulated by adding biogas digestate at the beginning of the incubation to all samples. We continuously monitored CO2 and N2O emissions from soil before and after urea application using a custom-built flow-through steady-state system, and we assessed soil properties, including soil structure. Microplastics affected soil properties, notably increasing soil aggregate water-stability and pneumatic conductivity, and caused changes in the dynamics and overall level of emission of both gases, but in opposite directions: overall fluxes of CO2 were increased by microplastic presence, whereas N2O emission were decreased, a pattern that was intensified following urea addition. This divergent response is explained by effects of microplastic on soil structure, with the increased air permeability likely improving O2 supply: this will have stimulated CO2 production, since mineralization benefits from better aeration. Increased O2 would at the same time have inhibited denitrification, a process contributing to N2O emissions, thus likely explaining the decrease in the latter. Our results clearly suggest that microplastic consequences for greenhouse gas emissions should become an integral part of future impact assessments, and that to understand such responses, soil structure should be assessed.

Marine microbial community responses related to wetland carbon mobilization in the coastal zone

AbstractHere, we examine how marine microbial communities respond when dissolved organic matter (DOM) is mobilized from coastal wetlands. Biological transformations of this DOM may increase in the presence of reactive substrates, such as algal‐derived DOM (ADOM) in the coastal zone—a process known as priming. We performed laboratory experiments examining transformations of DOM derived from coastal wetland peat (PDOM) with and without the presence ADOM. Associated shifts in microbial community composition and functional gene abundance were measured to evaluate mechanisms of priming effects. ADOM presence stimulated CO2 production when compared to the seawater control, which was further enhanced in the copresence of PDOM. DOM characterization showed a substantial difference in features present at the end of the incubation when PDOM was present with and without ADOM, while metagenomic sequencing indicated shifts in microbial community composition and identified 23 unique functional genes associated with pathways for the breakdown of aromatic compounds.

Carbon mineralization and microbial activity in agricultural topsoil and subsoil as regulated by root nitrogen and recalcitrant carbon concentrations

Aims Mechanisms of subsoil carbon sequestration from deep-rooted plants are elusive, but may contribute to climate change mitigation. This study addressed the role of root chemistry on carbon mineralization and microbiology in a temperate agricultural subsoil (60 and 300 cm depth)compared to topsoil (20 cm depth). Methods Roots from different plant species were chemically characterized and root-induced CO2 production was measured in controlled soil incubations (20 weeks). Total carbon losses, β-glucosidase activity, carbon substrate utilization, and bacterial gene copy numbers were determined. After 20 weeks, resultant carbon mineralization responses to mineral nitrogen (N) were tested. Results Root-induced carbon losses were significantly lower in subsoils (32–41%) than in topsoil (58%). Carbon losses varied according to root chemistry and were mainly linked to root N concentration for subsoils and to lignin and hemicellulose concentration for topsoil. Increases in β-glucosidase activity and bacterial numbers in subsoils were also linked to rootN concentration. Added mineral N preferentially stimulated CO2 production from roots with low concentrations of N, lignin and hemicellulose. Conclusions The results were compatible with a concept of N availability and chemically recalcitrant root compounds interacting to control subsoil carbon decomposition. Implications for carbon sequestration from deep-rooted plants are discussed.

Biofilm and temperature controls on greenhouse gas (CO2 and CH4) emissions from a Rhizophora mangrove soil (New Caledonia)

Seasonal variations of CO2 and CH4 fluxes were investigated in a Rhizophora mangrove forest that develops under a semi-arid climate, in New Caledonia. Fluxes were measured using closed incubation chambers connected to a CRDS analyzer. They were performed during low tide at light, in the dark, and in the dark after having removed the top 1–2 mm of soil, which may contain biofilm. CO2 and CH4 fluxes ranged from 31.34 to 187.48 mmol m−2 day−1 and from 39.36 to 428.09 μmol m−2 day−1, respectively. Both CO2 and CH4 emissions showed a strong seasonal variability with higher fluxes measured during the warm season, due to an enhanced production of these two gases within the soil. Furthermore, CO2 fluxes were higher in the dark than at light, evidencing photosynthetic processes at the soil surface and thus the role of biofilm in the regulation of greenhouse gas emissions from mangrove soils. The mean δ13C-CO2 value of the CO2 fluxes measured was −19.76 ± 1.19‰, which was depleted compared to the one emitted by root respiration (−22.32 ± 1.06‰), leaf litter decomposition (−21.43 ± 1.89‰) and organic matter degradation (−22.33 ± 1.82‰). This result confirmed the use of the CO2 produced within the soil by the biofilm developing at its surface. After removing the top 1–2 mm of soil, both CO2 and CH4 fluxes increased. Enhancement of CH4 fluxes suggests that biofilm may act as a physical barrier to the transfer of GHG from the soil to the atmosphere. However, the δ13C-CO2 became more enriched, evidencing that the biofilm was not integrally removed, and that its partial removal resulted in physical disturbance that stimulated CO2 production. Therefore, this study provides useful information to understand the global implication of mangroves in climate change mitigation.

Can Fire Residues (Ash and Char) Affect Microbial Decomposition in Wetland Soils?

With increasing fire frequency expected with climate change, understanding the direct and indirect effect of fire on the global carbon budget is critical. While many studies focus on carbon loss through biomass removal and shifts in greenhouse gas (GHG) emission, minimal studies have taken a mechanistic approach to determine the indirect effect of fire residues on GHG production post-fire. The study objective was to isolate the effect of vegetation-derived fire residues (char and ash) on microbial decomposition in wetland soils of varying phosphorus concentrations. Carbon GHG production was influenced by residue type and oxygen availability and results varied by soil phosphorus concentrations. Aerobic decomposition decreased following ash addition in low-phosphorus soil initially; however, there were no additional effects of residues on aerobic decomposition. Under anaerobic conditions, residues had no effect on decomposition in high-phosphorus soil. In contrast, char stimulated CO2 production; and regardless of residue, CH4 production was stimulated in low-phosphorus soil. While char contained higher available phosphorus and carbon relative to ash, ash still provided phosphorus suggesting variable gas production in situ may be dependent on fire residue proportion and associated nutrient input. Taken together, increased fire frequency associated with climate change may stimulate CH4 production in low-phosphorus wetlands.

Effect of thaw depth on fluxes of CO2 and CH4 in manipulated Arctic coastal tundra of Barrow, Alaska

Changes in CO2 and CH4 emissions represent one of the most significant consequences of drastic climate change in the Arctic, by way of thawing permafrost, a deepened active layer, and decline of thermokarst lakes in the Arctic. This study conducted flux-measurements of CO2 and CH4, as well as environmental factors such as temperature, moisture, and thaw depth, as part of a water table manipulation experiment in the Arctic coastal plain tundra of Barrow, Alaska during autumn. The manipulation treatment consisted of draining, controlling, and flooding treated sections by adjusting standing water. Inundation increased CH4 emission by a factor of 4.3 compared to non-flooded sections. This may be due to the decomposition of organic matter under a limited oxygen environment by saturated standing water. On the other hand, CO2 emission in the dry section was 3.9-fold higher than in others. CH4 emission tends to increase with deeper thaw depth, which strongly depends on the water table; however, CO2 emission is not related to thaw depth. Quotients of global warming potential (GWPCO2) (dry/control) and GWPCH4 (wet/control) increased by 464 and 148%, respectively, and GWPCH4 (dry/control) declined by 66%. This suggests that CO2 emission in a drained section is enhanced by soil and ecosystem respiration, and CH4 emission in a flooded area is likely stimulated under an anoxic environment by inundated standing water. The findings of this manipulation experiment during the autumn period demonstrate the different production processes of CO2 and CH4, as well as different global warming potentials, coupled with change in thaw depth. Thus the outcomes imply that the expansion of tundra lakes leads the enhancement of CH4 release, and the disappearance of the lakes causes the stimulated CO2 production in response to the Arctic climate change.

How much oxygen is needed for acetylene to be consumed in soil?

Acetylene (C2H2) is employed for the quantification of important biological processes such as nitrogen fixation, nitrous oxide reduction, ammonium and methane oxidation, and methanogenesis. Although acetylene is not a natural product, the ability of bacteria to grow on C2H2 is a phenomenon common to soils and sediments. Our experiment was designed to study the modification of CO2 production, O2 uptake and microbial biomass (Cmic) in soil in response to the consumption of added acetylene. Two soils (peat-muck and Eutric Cambisol) were incubated under well aerated (60% water holding capacity [WHC]) or flooded conditions, and enriched with C2H2 in the range 0.1–10 kPa (initially, 0.434–63.4 mmol C2H2 kg−1) at a constant temperature of 20°C. Gases were measured chromatographically, while Cmic was measured by the physiological SIR (substrate-induced respiration) method based on the initial response of microorganisms to glucose amendment. We used a simple calculation of net CO2, net O2 and net Cmic (as differences between amended and not amended soils) to estimate the contribution of C2H2 to the total respiration and microbial growth during the incubation. Peat-muck soil consumed more C2H2 than Cambisol (maximum 54.03 vs. 19.25 mmol kg−1, p < 0.001). Acetylene utilization was faster and larger in flooded than in wet soils (16.2 and 7.81 mmol kg−1, respectively, p < 0.05), and followed the tendency observed for Cmic. Depending on C2H2 enrichment, air–water conditions, and soil tested, both reduction and stimulation of measured activities were observed in response to acetylene consumption. Low C2H2 uptake, especially in Cambisol incubated at 60% WHC, resulted in the reduction of soil respiration and biomass (by 1–29%). Large C2H2 consumption in flooded soils stimulated CO2 production, O2 uptake and Cmic, even by 78%, 72% and 43%, respectively. Net CO2, net O2, and net Cmic were linearly positively related to the quantity of consumed C2H2 (p < 0.001). Acetylene utilization was a combined effect of initial C2H2, soil properties, and air–water status. The amount of consumed C2H2 was the highest in flooded peat-muck soil enriched with 10 kPa, and lowest in Cambisol at 60% WHC with 0.1 kPa C2H2. Consumption of 1 mol acetylene induced production of 0.5 mol CO2, and uptake of 0.4 mol O2. Low acetylene consumption (<6 mmol kg−1) resulted in reduction of both soil respiration and microbial biomass.

Co‐operation between cytosolic and plastidic oxidative pentose phosphate pathways revealed by 6‐phosphogluconate dehydrogenase‐deficient genotypes of maize

SummaryThe aim of this work was to examine the extent to which the oxidative steps of the pentose phosphate pathway in the cytosol contribute to the provision of reductant for biosynthetic reactions. Maize (Zea mays L.) contains at least two loci (pgd1 and pgd2) that encode 6‐phosphogluconate dehydrogenase. Ten genotypic combinations of wild‐type (Pgd1+3.8;Pgd2+5) and null alleles of pgd1 and pgd2 were constructed in the B73 background. The maximum catalytic activity of 6‐phosphogluconate dehydrogenase in the roots of seedlings of these lines correlated with the number of functional pgd1 and pgd2 alleles. Enzyme activity in the double‐null homozygote (pgd1‐null;pgd2‐null) was 32% of that in B73 wild‐type suggesting the presence of at least one other isozyme of 6‐phosphogluconate dehydrogenase in maize. Subcellular fractionation studies and latency measurements confirmed that the products of pgd1 and pgd2 are responsible for the vast majority, if not all, of the cytosolic 6‐phosphogluconate dehydrogenase activity in maize roots. Essentially, all of the residual activity in the double‐null homozygote was confined to the plastids. Low concentrations (0.1–0.5 mM) of sodium nitrite stimulated 14CO2 production by detached root tips of both wild‐type and 6‐phosphogluconate dehydrogenase‐deficient maize seedlings fed [U‐14C]glucose. Analysis of the ratio of 14CO2 released from [1–14C]glucose relative to [6–14C]glucose (C1/C6 ratio) showed that stimulation of the oxidative pentose phosphate pathway by nitrite correlated with the dosage of wild‐type alleles of pgd1 and pgd2. The failure of 6‐phosphogluconate dehydrogenase‐deficient lines to respond to nitrite indicates that perturbation of the cytosolic oxidative pentose phosphate pathway can influence the provision of reductant in the plastid. We conclude that the plastidic and cytosolic oxidative pentose phosphate pathways are able to co‐operate in the provision of NADPH for biosynthesis.

Regulation of pancreatic beta-cell mitochondrial metabolism: influence of Ca2+, substrate and ADP.

To gain insight into the regulation of pancreatic beta-cell mitochondrial metabolism, the direct effects on respiration of different mitochondrial substrates, variations in the ATP/ADP ratio and free Ca2+ were examined using isolated mitochondria and permeabilized clonal pancreatic beta-cells (HIT). Respiration from pyruvate was high and not influenced by Ca2+ in State 3 or under various redox states and fixed values of the ATP/ADP ratio; nevertheless, high Ca2+ elevated pyridine nucleotide fluorescence, indicating activation of pyruvate dehydrogenase by Ca2+. Furthermore, in the presence of pyruvate, elevated Ca2+ stimulated CO2 production from pyruvate, increased citrate production and efflux from the mitochondria and inhibited CO2 production from palmitate. The latter observation suggests that beta-cell fatty acid oxidation is not regulated exclusively by malonyl-CoA but also by the mitochondrial redox state. alpha-Glycerophosphate (alpha-GP) oxidation was Ca(2+)-dependent with a half-maximal rate observed at around 300 nM Ca2+. We have recently demonstrated that increases in respiration precede increases in Ca2+ in glucose-stimulated clonal pancreatic beta-cells (HIT), indicating that Ca2+ is not responsible for the initial stimulation of respiration [Civelek, Deeney, Kubik, Schultz, Tornheim and Corkey (1996) Biochem. J. 315, 1015-1019]. It is suggested that respiration is stimulated by increased substrate (alpha-GP and pyruvate) supply together with oscillatory increases in ADP [Nilsson, Schultz, Berggren, Corkey and Tornheim (1996) Biochem. J. 314, 91-94]. The rise in Ca2+, which in itself may not significantly increase net respiration, could have the important functions of (1) activating the alpha-GP shuttle, to maintain an oxidized cytosol and high glycolytic flux; (2) activating pyruvate dehydrogenase, and indirectly pyruvate carboxylase, to sustain production of citrate and hence the putative signal coupling factors, malonyl-CoA and acyl-CoA; and (3) increasing mitochondrial redox state to implement the switch from fatty acid to pyruvate oxidation.

Regulation of murine acid secretion by CO2

To determine whether endogenous metabolic sources alone provide sufficient CO2 for acid secretion in mammals, basal and stimulated acid secretion and metabolic CO2 production were measured concurrently in mouse stomachs, in vitro, without exogenous CO2, and after addition of 5% CO2 serosally. Basal acid secretion was varied by changing luminal pH from 3.2 to 4.0. In the absence of an exogenous supply of CO2 acid secretion was stable under basal conditions and increased during cholinergic stimulation with carbachol. Serosal CO2 supply increased basal and stimulated acid secretion. The increase in basal acid secretion depended on the initial level of acid secretion. At pH 4.0, exogenous CO2 increased acid output (mean +/- SD) by 13% from 112 +/- 11 nmol/min to 126 +/- 8 nmol/min (P < 0.03), whereas at pH 3.6 the increase was 40% (63 +/- 14 to 88 +/- 20 nmol/min, P < 0.04) and 157% at pH 3.2 (21 +/- 13 to 54 +/- 14 nmol/min, P < 0.002). Following cholinergic stimulation a maximal acid output of 321 +/- 38 nmol/min was attained without serosal CO2, whilst addition of 5% CO2 to the serosal solution increased maximal acid secretion by 49% to 479 +/- 96 nmol/min (P < 0.005). Metabolic activity, measured as total gastric CO2 production, was greater as acid secretion rates increased [239 +/- 20 nmol/min at 21 +/- 13 nmol/min (luminal pH 3.2) versus 406 +/- 28 nmol/min at 321 +/- 17 nmol/min (after cholinergic stimulation)]. The data support the concept that basal and sub-maximal acid secretion can be maintained by CO2 available from metabolic sources, but full expression of the acid secretory apparatus requires exogenous CO2.

Noradrenaline‐induced stimulation of glutamine metabolism in primary cultures of astrocytes

Effects of noradrenaline and of adrenergic subtype specific agonists on the uptake and metabolism of [14C]glutamine and [14C]glutamate in primary cultures of mouse astrocytes have been investigated. The total uptake of radioactivity from extracellular [14C]glutamine into the cells was enhanced during exposure to 100 microM noradrenaline, isoproterenol, or clonidine. This is partly due to an increased radioactivity in the glutamine pool and partly due to an increased formation of labeled glutamate from glutamine, which had become very marked (66%) after 240 min of incubation. The CO2 formation from labeled glutamine during 4 hr of incubation was enhanced about twofold in the presence of noradrenaline. Ten millimolar amino oxyacetic acid (AOAA), a transamination inhibitor, had no effect on CO2 formation from glutamine, indicating that the formation of alpha-ketoglutarate from glutamate occurs as an oxidative deamination. The stimulation of 14CO2 production from labeled glutamine was at least as large when glucose was deleted from medium, suggesting that the increased 14CO2 formation represents a stimulation of glutamine metabolism as such and is not only a reflection of an increase in oxidative metabolism of glucose and a bidirectional exchange between alpha-ketoglutarate and glutamate. The opposite process, incorporation of radioactivity from labeled glutamate into glutamine, was not enhanced in the presence of noradrenaline. The findings suggest that noradrenaline stimulates the rates of glutamine uptake, glutamate synthesis, and CO2 production from glutamine and thus increases energy supply to astrocytes but has no effect on the opposite reaction, i.e., glutamine formation from glutamate, a reaction of importance for neuronal-astrocyte interations.

Submicromolar Ca2+ regulates phosphorylating respiration by normal rat liver and AS-30D hepatoma mitochondria by different mechanisms.

The stimulation of 2-oxoglutarate and NAD(+)-isocitrate dehydrogenase by Ca2+ in mitochondria from normal tissues has been proposed to mediate partially the activation of oxidative energy metabolism elicited by physiological elevations in cytosolic Ca2+. This mode of regulation may also occur in tumor cells in which several aspects of mitochondrial metabolism are known to be altered. This study provides a comparison of the stimulation by submicromolar concentrations of Ca2+ on the rates of ATP-generating (state 3) respiration under physiologically realistic conditions by mitochondria isolated from normal rat liver and from highly malignant rat AS-30D ascites hepatoma cells. The K0.5 for activation of glutamate-dependent state 3 respiration by Ca2+ in the presence of ATP at 37 degrees C was determined to be 0.70 +/- 0.05 (S.E.) microM for hepatoma mitochondria and 0.90 +/- 0.03 microM for rat liver mitochondria. This activation was also reflected by a Ca2(+)-induced shift in the oxidation-reduction state of hepatoma mitochondrial pyridine nucleotides to a more reduced level and Ca2+ stimulation of 14CO2 production from [1-14C]glutamate. Whereas the Ca2+ sensitivity of state 3 respiration by hepatoma mitochondria can be explained by the activation of 2-oxoglutarate and possibly NAD(+)-isocitrate dehydrogenases, the Ca2+ sensitivity of liver mitochondrial respiration appears to be predominantly mediated by activation of electron flow through ubiquinone and Complex III of the electron transport chain, as indicated by the specificity of the effects of Ca2+ on respiration with different oxidizable substrates. Although rat liver and hepatoma mitochondria employ different modes of Ca2(+)-activated ATP generation, these results support the hypothesis that changes in cytosolic Ca2+ play a significant role in the potentiation of energy production in tumor, as well as normal tissue.

Pharmacological responsiveness of dissociated native and cultured eccrine secretory coil cells

Methacholine (MCh)- and isoproterenol (Iso)-stimulated 14CO2 production was compared between freshly dissociated rhesus sweat secretory coil cells (mainly clear cells) and cultured cells (grown on a collagen-coated plastic plate) derived from native cells. 14CO2 production was enhanced by MCh and by Iso in native coil cells (but not in ductal cells) in a pharmacologically specific and dose-dependent manner. 14CO2 production in subcultured coil cells (19-45 days in culture) was only one-third to one-fifth that of native cells. MCh-stimulated 14CO2 production was inhibited by ouabain and furosemide in both native and cultured coil cells. A decrease in 14CO2 production, of about one-half, was already evident in primary cells cultured for less than 1 wk. The decreased pharmacological responsiveness of the cultured coil cells was seen, although the cultured cells showed the typical epithelioid appearance, abundant mitochondria, the occasional presence of intercellular lacunae resembling intercellular canaliculi, and the persistence of immunoreactive keratin. We conclude that 1) a primary culture of sweat gland cells can be initiated from dissociated cells; 2) cultured sweat secretory coil cells qualitatively, but not quantitatively, retain the pharmacological responsiveness and transport activity of the native cells as determined by 14CO2 production; 3) collagenase-dissociated cells represent an excellent in vitro system for the study of glandular function at the cellular level; and 4) the decrease in pharmacological responsiveness is not simply due to trypsin treatment during harvesting of cultured cells, because that of organ-cultured, intact, secretory coils also declines with time of culture.

Metabolism of round spermatids: gossypol induces uncoupling of respiratory chain and oxidative phosphorylation.

The effect of gossypol on energy metabolism of round spermatids of rats was examined. When spermatids were treated with various concentrations of gossypol for 30 min at 32 degrees C, a biphasic response (stimulation at low concentrations and inhibition at high concentrations) was seen in pyruvate and CO2 production from lactate. At the early period of incubation, gossypol at even high concentrations stimulated CO2 production to an extent similar to that stimulated by 2,4-dinitrophenol (DNP). At longer periods of incubation, however, the rates of CO2 production from lactate dropped to those seen in the rotenone-treated cells. The rates of oxygen consumption were not increased further by DNP when cells were pretreated with gossypol. The adenosine triphosphate (ATP) content in spermatids was reduced markedly, although lactate oxidation to CO2 and mitochondrial respiration were stimulated by gossypol. These results suggest that gossypol probably exerts its effect on spermatids by uncoupling respiratory chain and oxidative phosphorylation.

Hepatocyte heterogeneity in response to icosanoids

1. The metabolic and hemodynamic effects of prostaglandin F2 alpha, leukotriene C4 and the thromboxane A2 analogue U-46619 were studied during physiologically antegrade (portal to hepatic vein) and retrograde (hepatic to portal vein) perfusion and in a system of two rat livers perfused in sequence. 2. The stimulatory effects of prostaglandin F2 alpha (3 microM) on hepatic glucose release, perfusion pressure and net Ca2+ release were diminished by 77%, 95% and 64%, respectively, during retrograde perfusion when compared to the antegrade direction, whereas the stimulation of 14CO2 production from [1-14C]glutamate by prostaglandin F2 alpha (which largely reflects the metabolism of perivenous hepatocytes) was lowered by only 20%. Ca2+ mobilization and glucose release from the liver comparable to that seen during antegrade perfusion could also be observed in retrograde perfusions; however, higher concentrations of the prostaglandin were required. 3. The glucose, Ca2+ and pressure response to leukotriene C4 (20 nM) or the thromboxane A2 analogue U-46619 (200 nM) of livers perfused in the antegrade direction were diminished by about 90% during retrograde perfusion. Sodium nitroprusside (20 microM) decreased the pressure response to leukotriene C4 (20 nM) and U-46619 (200 nM) by about 40% and 20% in antegrade perfusions, respectively, but did not affect the maximal increase of glucose output. 4. When two livers were perfused antegradely in series, such that the perfusate leaving the first liver (liver I) entered a second liver (liver II), infusion of U-46619 at concentrations below 200 nM to the influent perfusate of liver I increased the portal pressure of liver I, but not of liver II. At higher concentrations of U-46619 there was also an increase of the portal pressure of liver II and with concentrations above 800 nM the pressure responses of both livers were near-maximal [19.6 +/- 0.8 (n = 7) cm H2O and 16.5 +/- 1.1 (n = 8) cm H2O for livers I and II, respectively]. There was a similar behaviour of glucose release from livers I and II in response to U-46619 infusion. When liver I was perfused in the retrograde direction, a significant pressure or glucose response of liver II (antegrade perfusion) could not be observed even with U-46619 concentrations up to 1000 nM. 5. Similarly, the perfusion pressure increase and glucose release induced by leukotriene C4 (10 nM) observed with liver II was only about 20% of that seen with liver I.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Na+ on intestinal succinate transport and metabolism in vitro.

The effect of Na+ on 14CO2 production from [14C]succinate was studied in isolated rat enterocytes, and Na+-dependent succinate transport was characterized in pig intestinal brush-border membrane vesicles. The production of 14CO2 from [14C]succinate by enterocytes was decreased 12-fold when Na+ was replaced by N-methyl-D-glucamine in the absence of glutamine and 20-fold in the presence of 0.2 or 0.5 mM glutamine. The ratio of 14CO2 produced from [1,4-14C]succinate to that produced by [2,3-14C]succinate was not affected by Na+ replacement, indicating that the pattern of tricarboxylic acid cycle metabolism was not altered. The uptake of [14C]succinate by brush-border membrane vesicles was stimulated 10-fold in the presence of 100 mM NaCl compared with 100 mM KCl. When succinate uptake was corrected to transport into an osmotically sensitive space, the magnitude of the Na+ stimulation was 20-fold. Succinate transport into brush-border membrane vesicles was Na+ dependent, electroneutral, nonconcentrative, with an apparent Na+-succinate coupling ratio of 2:1. Results of this study indicate that Na+-stimulated CO2 production by enterocytes can be explained by the effect of Na+ on succinate transport across the brush-border membrane.

Carnitine stimulation of pyruvate dehydrogenase complex (PDHC) in isolated human skeletal muscle mitochondria.

L-carnitine stimulated CO2 production from 1-14C pyruvate in mitochondria from human skeletal muscle nearly twofold. A comparable increase in the pyruvate dehydrogenase complex (PDHC) activity was seen. Moreover, in the presence of L-carnitine and at pyruvate concentration greater than 0.25 mM, this effect was associated with a marked increase of acetylcarnitine synthesis. Deoxycarnitine, an inhibitor of carnitine acetyltransferase (CAT), partially reversed the effect of carnitine on PDHC activity. The stimulatory effect of carnitine on PDHC activity in human mitochondria is mediated by the modulation of the intramitochondrial acetyl-CoA/CoASH ratio.

Insulin-like effects of vanadate in isolated rat adipocytes.

Vanadate has been shown to have a number of insulin-like effects in various cells, including isolated rat adipocytes. In the present study we compared the activities of vanadate and insulin in isolated fat cells using a number of different assays of insulin-like activity. Both insulin and vanadate stimulated [2-3H]glucose incorporation into fat cell lipid in a dose-dependent manner, but the maximal effect of vanadate was markedly greater than that of insulin. At 10(-2) M vanadate the effect was 3-4 times as great as the maximal effect of insulin. This effect was dependent on specific glucose transport. Combinations of insulin and vanadate were not more effective than vanadate alone. Vanadate also produced antilipolysis with an effect somewhat greater than that of insulin. Using [U-14C]glucose both vanadate and insulin stimulated 14CO2 production and [14C]glucose incorporation into lipid, and again the effect of vanadate was greater than that of insulin. Vanadate had a greater effect on 14CO2 production than on [14C]glucose incorporation into lipid. When [1-14C]glucose was used vanadate again had a significantly greater effect on 14CO2 production than did insulin, but when [6-14C]glucose was used the effects of vanadate and insulin were equal. These results demonstrate that vanadate has insulin-like effects in isolated fat cells, but it selectively stimulates certain pathways to a greater extent than does insulin. The greater effect of vanadate than insulin appears to be primarily on the pentose phosphate shunt, suggesting that this agent may be useful for examination of this intracellular pathway in fat cells.

The effects of recombinant tumour necrosis factor (cachectin) on metabolism in isolated rat adipocyte, hepatocyte and muscle preparations.

Tumour necrosis factor (TNF) did not stimulate lipolysis in isolated rat adipocytes, though preincubation with TNF increased adrenaline-stimulated fatty acid release. Glycogenolysis, gluconeogenesis and ketogenesis in isolated rat hepatocytes were not influenced by TNF in short-term (30-60 min) incubations. TNF stimulated 14CO2 production from [U-14C]glucose in rat hemidiaphragm preparations, but lactate production and alanine release were not significantly altered. It is concluded that TNF does not regulate short-term metabolism in adipocytes, hepatocytes and muscle preparations in the manner of a catabolic hormone.