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R Rognstad

Publications and source records attributed to R Rognstad.

At least 37 records · Page 2Linked to original sources
PubMed1982

14CO2 fixation by phosphoenolpyruvate carboxykinase during gluconeogenesis in the intact rat liver cell.

During gluconeogenesis from L-glutamine, 14CO2 is fixed into glucose. Inhibitors of pyruvate transport or pyruvate carboxylase only slightly decrease the 14CO2 incorporation, indicating that a pathway of formation of pyruvate, followed by pyruvate carboxylation, is not primarily involved. These results suggest that 14CO2 fixation is effected by a reverse (exchange) reaction of P-enolpyruvate carboxykinase. MnCl2 (0.5 mM) stimulates the 14CO2 fixation in glucose from L-glutamine by nearly 50%. This result is in accord with a recent study (Colombo, G., Carlson, G. M., and Lardy, H. A. (1981) Biochemistry 20, 2749-2757) showing that Mn2+ greatly stimulates the reverse reaction (P-enolpyruvate leads to oxalacetate) of purified rat liver P-enolpyruvate carboxykinase. Preliminary calculations suggest that 14CO2 is also fixed by reversible P-enolpyruvate carboxykinase activity during gluconeogenesis from L-lactate, in addition to the fixation of H14CO3(-) in the pyruvate carboxylase forward reaction.

Animals↗
PubMed1982

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

1. We have examined effects, on gluconeogenesis from lactate, of altering energy metabolism in two ways: (a) by primarily lowering cytosolic ATP levels with the use of atractyloside or 2,5 anhydromannose; and (b) by decreasing mitochondrial energy generation with the use of the classical uncoupler, dinitrophenol. 2. Agents which lower cytosolic ATP inhibit gluconeogenesis and increase pyruvate kinase flux (PK) correspondingly, while pyruvate carboxylase and P-enolpyruvate carboxykinase fluxes are unchanged, at least until gluconeogenesis is inhibited by more than 50%. 3. Dinitrophenol, on the other hand, although it also induces a (smaller) increase in PK, primarily decreases gluconeogenesis by an effect on a mitochondrial step in the gluconeogenic pathway. 4. Low concentrations of dinitrophenol increase Krebs cycle oxidation by at least 50% before significant inhibition of gluconeogenesis from lactate occurs.

Adenosine Triphosphate↗
PubMed1981

Effects of glucose and of lactate on phosphofructokinase flux during gluconeogenesis.

We have examined the effects of glucose and lactate, the products of the gluconeogenic-glycolytic pathways, on phosphofructokinase flux during gluconeogenesis in hepatocytes from fasted rats. With dihydroxyacetone as substrate, phosphofructokinase flux is rather active. Addition of lactate, at concentrations of 5-10 mM, causes a lowering of this flux to the levels found when lactate alone is the substrate. Inhibitor studies suggest that a mitochondrially formed metabolite of lactate is the likely effector involved. Addition of glucose (10mM or greater) to dihydroxyacetone causes an increase in phosphofructokinase flux. Only small effects are seen unless the cells are preincubated with glucose, in which case an estimated 2-3-fold increase in phosphofructokinase flux occurs.

Animals↗
PubMed1981

Control of ethanol utilization by rat hepatocytes.

Ethanol oxidation by hepatocytes from fasted rats was determined in the presence and absence of 0.2 mM ethyl hydrazinoacetate, a transaminase inhibitor which blocks the malate-aspartate cycle. 20 muM phenazine methosulfate caused the largest increase (nearly 150%) in ethanol utilization. 5 muM norepinephrine caused a 50% increase in ethanol oxidation, and most of this increase was caused by stimulation of the alpha-glycerophosphate shuttle, since it remained in the presence of ethyl hydrazinoacetate. 1 muM glucagon caused a 25% increase in ethanol uptake, and most of this increase was abolished by ethyl hydrazinoacetate, indicating that the malate-aspartate cycle was involved. 25 muM dinitrophenol increased ethanol use by 20% and this increase was nearly unaffected by ethyl hydrazinoacetate. The results indicate that ethanol utilization, under the conditions used, is primarily controlled by the capacity of the shuttle systems, and not by the capacity of the respiratory chain.

Animals↗
PubMed1981

A comparison of 13C nuclear magnetic resonance and 14C tracer studies of hepatic metabolism.

The gluconeogenic pathway from 13C-labeled substrates, each of which contained the 14C-labeled counterpart at a tracer level, has been followed in isolated rat liver cells and in isolated perfused mouse liver. The gluconeogenic flux from glycerol, the synthesis of glycogen, the stimulation of glycogenolysis by glucagon, the recycling of triacylglycerol, and an increase in pentose cycle activity under the influence of phenazine methosulfate were all observed directly in the 13C NMR spectra of perfused liver or isolated hepatocytes. The relative concentrations of 13C label at specific carbons measured by the NMR spectra under these conditions agreed closely with 14C isotopic distributions measured in extracts of the same doubly labeled samples for specific activities of greater than or equal to 3%. The label distributions measured by both methods were the same to within the experimental errors, which ranged from +/- 2% to +/- 7% in these experiments.

Animals↗
PubMed1981

Manganese effects on gluconeogenesis.

Manganese (Mn2+) does not significantly increase gluconeogenesis from lactate (10 mM) plus pyruvate (1 mM) in hepatocytes from fasted rats. In hepatocytes not treated with Mn2+, glucagon (1 microM) and epinephrine (10 microM) at these optimal concentrations both stimulate gluconeogenesis from lactate/pyruvate (10:1), but the hormonal effects are not additive. In the presence of Mn2+ the hormonal effects are slightly larger, and the effects of glucagon (1 microM) and epinephrine (10 microM) become nearly completely additive. Mn2+ increases the specific activity of glucose formed from lactate plus NaH14CO3 by nearly 20%. The increase may be attributed to an increased exchange reaction of either pyruvate carboxylase or phosphoenolypyruvate carboxykinase, suggesting that one of these may be markedly stimulated by Mn2+, the increased exchange reaction possibly signifying an approach toward "near equilibrium" status.

Animals↗
PubMed1981

Metabolism of 3H- and 14C-labelled lactate in starved rats.

1. [2-(3)H,U-(14)C]- or [3-(3)H,U-(14)C]-Lactate was administered by infusion or bolus injection to overnight-starved rats. Tracer lactate was injected or infused through indwelling cannulas into the aorta and blood was sampled from the vena cava (A-VC mode), or it was administered into the vena cava and sampled from the aorta (V-A mode). Sampling was continued after infusion was terminated to obtain the wash-out curves for the tracer. The activities of lactate, glucose, amino acids and water were followed. 2. The kinetics of labelled lactate in the two modes differed markedly, but the kinetics of labelled glucose were much the same irrespective of mode. 3. The kinetics of (3)H-labelled lactate differed markedly from those for [U-(14)C]lactate. Isotopic steady state was attained in less than 1h of infusion of [(3)H]lactate but required over 6h for [U-(14)C]lactate. 4. (3)H from [2-(3)H]lactate labels glucose more extensive than does that from [3-(3)H]lactate. [3-(3)H]Lactate also labels plasma amino acids. The distribution of (3)H in glucose was determined. 5. Maximal radioactivity in (3)HOH in plasma is attained in less than 1min after injection. Near-maximal radioactivity in [(14)C]glucose and [(3)H]glucose is attained within 2-3min after injection. 6. The apparent replacement rates for lactate were calculated from the areas under the specific-radioactivity curves or plateau specific radioactivities after primed infusion. Results calculated from bolus injection and infusion agreed closely. The apparent replacement rate for [(3)H]lactate from the A-VC mode averaged about 16mg/min per kg body wt. and that in the V-A mode about 8.5mg/min per kg body wt. The apparent rates for [(14)C]lactate (;rate of irreversible disposal') were 8mg/min per kg body wt. for the A-VC mode and 5.5mg/min per kg body wt. for the V-A mode. Apparent recycling of lactate carbon was 55-60% according to the A-VC mode and 35% according to the V-A mode. 7. The specific radioactivities of [U-(14)C]glucose at isotopic steady state were 55% and 45% that of [U-(14)C]lactate in the A-VC and V-A modes respectively. We calculated, correcting for the dilution of (14)C in gluconeogenesis via oxaloacetate, that over 70% of newly synthesized glucose was derived from circulating lactate. 8. Recycling of (3)H between lactate and glucose was evaluated. It has no significant effect on the calculation of the replacement rate, but affects considerably the areas under the wash-out curves for both [2-(3)H]- and [3-(3)H]-lactate, and calculation of mean transit time and total lactate mass in the body. Corrected for recycling, in the A-VC mode the mean transit time is about 3min, the lactate mass about 50mg/kg body wt. and the lactate space about 65% of body space. The V-A mode yields a mass and lactate space about half those with the A-VC mode. 9. The area under the wash-out curve for [(14)C]lactate is some 20-30 times that for [(3)H]lactate, and apparent carbon mass is 400-500mg/kg body wt. and presumably includes the carbon of glucose, pyruvate and amino acids, which are exchanging rapidly with that of lactate.

Animals↗
PubMed1980

Effects of ethyl hydrazinoacetate on gluconeogenesis and on ethanol oxidation in rat hepatocytes.

Both ethyl hydrazinoacetate and aminooxyacetate strongly inhibit gluconeogenesis from L-lactate, but not from pyruvate or fructose, in rat hepatocytes. Ethyl hydrazinoacetate partially inhibits gluconeogenesis from polyols, and also partially inhibits ethanol oxidation. In contrast to results obtained with aminooxyacetate, increasing the ethyl hydrazinoacetate concentration from 0.2 to 2 mM does not tend to diminish the inhibitory effect of this transaminase inhibitor on ethanol or polyol utilization.

Aminooxyacetic Acid↗
PubMed1979

Effects of 2,4-dihydroxybutyrate on lipogenesis in rat hepatocytes.

Hepatocytes were prepared from rats fasted 2 days and refed a high carbohydrate diet for 1 day. These cells contained very high levels of glycogen (about half the defatted dry weight) and carried out high rates of lipogenesis (up to 800 micron at tritium incorporation from 3HOH/g (defatted dry weight)/h), even in the absence of added substrates. Pentose cycle flux was estimated by a method involving the use of [1-14C]galactose (Rognstad, R. (1976) Int. J. Biochem. 7, 221-228). In hepatocytes from normal fasted refed rats, the amount of NADPH produced by the pentose cycle was sufficient for about one-half to three-fourths of that required for fatty acid synthesis. 2,4-Dihydroxybutyrate, a malic enzyme inhibitor (Schimerlik, M.I. & Cleland, W.W. (1977) Biochemistry 16, 565-570) markedly depressed the randomization of 14C in lactate from [6-14C]hexoses, indicating an inhibition of the pyruvate cycle. 2,4-Dihydroxybutyrate (10 mM) had only a slight inhibitory effect on overall lipogenesis, but increased the rate of the pentose cycle by 40 to 90%.

Animals↗
PubMed1979

Pyruvate cycling involving possible oxaloacetate decarboxylase activity.

With high concentrations of pyruvate as substrate for hepatocytes from fasted rats, high rates of cycling between pyruvate and the dicarboxylic acids occur, as shown isotopically. This rate of cycling is adequate to account for the hydrogen translocation from the mitochondria to the cytosol to furnish NADH for lactate formation. Addition of sufficiently high concentrations of mercaptopicolinate to block almost completely glucose formation from pyruvate, depresses isotopic cycling and lactate formation by only about 50-75%. Under some conditions, when the normal phosphoenolpyruvate carboxykinase activity is inhibited, cytosolic oxaloacetate may be decarboxylated directly to pyruvate, possibly via the decarboxylase activity of phosphoenolpyruvate carboxykinase.

Animals↗
PubMed1979

Rate-limiting steps in metabolic pathways.

A method is proposed to detect whether a given enzyme catalyzes a rate-limiting step in a metabolic pathway. With the use of a range of concentrations of specific inhibitors of an enzyme, the finding of a biphasic response with an initial null effect indicates the non-rate-limiting nature of the enzyme. With this method, phosphoenolpyruvate carboxykinase is indicated to catalyze a rate-limiting step in lactate gluconeogenesis in hepatocytes from fasted rats.

Aminooxyacetic Acid↗
PubMed1979

Starvation diabetes in the rat: onset, recovery, and specificity of reduced responsiveness of pancreatic beta-cells.

A 24-h starvation markedly diminished the stimulant action of 8 mM glucose on insulin secretion from isolated perifused rat islets of Langerhans. The response to a supramaximal glucose stimulus (27.5 mM) remained normal, but prolonged fasting (48 or more) also reduced its efficacy. Refeeding of 24-h fasted animals resulted in complete restoration of glucose sensitivity within 24 h. The responses to glyceraldehyde (2 mM) and alpha-ketoisocaproate (8 mM) at concentrations which elicit approximately half-maximal stimulation were unaltered by a 24-h fast, while that to a half-maximally effective dose of mannose (15 mM) was decreased. Theophylline (5 mM) could not normalize the reduced secretory response to glucose seen in this state. The islets' ability to metabolize glucose, using various in vitro pretreatment protocols and different incubation times, was not affected by a 24-h fast. Mannose and glyceraldehyde metabolism were also unaltered. Prolonged fasting (48 h) reduced glucose metabolism by 25% at both 8 and 27.5 mM. The acute adaptive changes in islet sensitivity to moderate glucose and mannose concentrations during short term fasting (24 h) cannot be explained by an altered usage of the added hexoses.

Animals↗