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Biomedical subjects

R Rognstad

Publications and source records attributed to R Rognstad.

At least 19 recordsLinked to original sources
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↗
PubMed1978

On the biochemical nature of triose- and hexose-stimulated insulin secretion.

The differential effects of several specific inhibitors of intermediary metabolism, mannoheptulose, 2-deoxylucose, and iodoacetate, were studied with isolated perifused pancreatic islets stimulated with glucose, mannose, glyceraldehyde, dihydroxyacetone, or alpha-ketoisocaproate. Insulin release rates and/or capacities to metabolize these caloric stimuli served as indicators of the inhibitors' actions. Mannoheptulose and 2-deoxyglucose blocked hexose-stimulated hormone release and hexose metabolism concomitantly, but left the functional and metabolic actions of trioses unaltered. Iodoacetate blocked hexose- and triose-stimulated hormone release as well as their metabolism in a parallel fashion. The action of alpha-ketoisocaproate was not affected by any of these three inhibitory agents. The data are most easily explained by a theory that incorporates metabolic signals, arising during the degradation of insulin-releasing fuel molecules, as an integral component in the process of beta-cell stimulation.

Animals↗
PubMed1977

Role of pyruvate kinase in the regulation of gluconeogenesis from L-lactate.

Glucagon and L-epinephrine stimulate gluconeogenesis from 20 mM L-lactate, the effect being about 3 times greater in liver cells from fed rats than in those from fasted rats. The rate of pyruvate kinase flux was estimated to be less than 10% of the rate of gluconeogenesis from lactate in hepatocytes from fasted rats, and neither glucagon nor epinephrine lowered the absolute rate significantly. In hepatocytes from fed rats, however, the rate of pyruvate kinase was nearly one-half that of gluconeogenesis. Glucagon caused a marked depression of pyruvate kinase flux, with 1 muM glucagon lowering the rate to nearly the level found in cells from fasted rats Epinephrine at concentrations from 10(-8) to 10(-6) M actually increased pyruvate kinase flux during gluconeogenesis from lactate in cells from fed rats. These results are in accord with the view that the effects of glucagon and epinephrine on gluconeogenesis are not identical.

Animals↗
PubMed1977

Sources of ammonia for urea synthesis in isolated rat liver cells.

L-Leucine inhibits urea synthesis in rat hepatocytes from a number of nitrogen sources, including ammonia. The inhibition by L-leucine is largely overcome by addition of 1 mM L-ornithine, suggesting that the main site of L-leucine action is at ornithine transcarbamylase, rather than at glutamate dyhydrogenase. L-Norvaline is a more potent inhibitor of urea synthesis than is L-leucine, but again the inhibition is largely counteracted by L-ornithine. Addition of aminooxyacetate and L-norvaline strongly suppresses the formation of glucose and lactate from L-asparagine, suggesting that an alternate pathway of aspartate metabolism, the purine nucleotide cycle, in not a major pathway. Hadacidin, an inhibitor of adenylosuccinate synthetase, an enzyme of the purine nucleotide cycle, has no effect on urea synthesis in rat liver cells.

Alanine↗
PubMed1976

The metabolism of l-[3-3h]lactate by isolated hamster liver cells.

The rate of tritium removal from L-[3-3H]lactate by hamster liver cells is faster than the analytical rate of lactate utilization, or the rate of 14C disappearance from 6-[U-14C, 3-3H]Llactate decrease. However, addition of low concentrations (0.1 to 1.0 mM) of L-cycloserine, a glutamate pyruvate transaminase inhibitor, nearly equalizes the rates of isotope utilization from L-[3-3H]lactate and L-[U-14C]lactate. The results suggest a very limited rate of recycling of phosphoenolpyruvate back to pyruvate during gluconeogenesis from lactate in fasted hamster liver cells.

Alanine Transaminase↗
PubMed1975

Recycling of glucose by rat hepatocytes.

1. The metabolism of glucose labeled uniformly with 14C, and in positions 2, 3 and 5 with tritium by hepatocytes from fed and fasted rats were studied. Cells were incubated with glucose as sole substrate, or with glucose and a variety of glucose precursors, and uptake or production of glucose, and the utilization of the isotopes was determined. 2. There was no uptake of glucose at concentration of up to 15 mM, and net glucose synthesis in the presence of precursors. 14C was however recovered in CO2, lactate and amino acids, and tritium in water. Considerable incorporation into glycogen from 14C and 3H-labeled glucose occurred at high (above 20 mM) glucose concentrations. 3. The yield in water always exceeded that in 14C-labeled products. The yield in 3HOH from [2-3H] glucose exceeded that from [5-3H] glucose, and the latter was greater than from [3-3H] glucose. 4. Utilization of labeled glucose does not follow Michaelis-Menten kinetics. The fractional rate of uptake of 14C and tritium-labeled glucose increases with glucose concentration with a maximum at about 15 mM and then declines. 5. The effect of numerous gluconeogenic substrates on the isotope utilization and the 3H/14C ratio in glycogen was studied. The uptake of 14C was always depressed. Addition of lactate and dihydroxyacetone has little effect on the detritiation of [2-3H] glucose, but it is depressed by other substrates. The detritiation of [3-3H]-and[5-3H]glucose is depressed in gluconeogenesis, that from [3-3H]glucose usually more than from [5-3H]glucose. In the presence of lactate detritiation of [3-3H]glucose is about half that from [5-3H]glucose. 6. Equations to calculate the phosphorylation of glucose and fructose 6-phosphate in the presence of futile cycling between glucose and glucose 6-phosphate and fructose 6-phosphate and fructose 1,6-bisphosphate were derived. 7. The estimate of glucose phosphorylation requires determination of the specific activity of glucose 6-phosphate from [2-3H]glucose. It appears that futile cycling between glucose and glucose 6-phosphate is extensive in cells with a high glycogen content, but is low in cells from starved rats and nearly absent in those from diabetic animals. 8. The estimation of the phosphorylation of fructose 6-phosphate in the presence of cycling requires knowledge of the specific activities of fructose 6-phosphate and fructose 1,6-bisphosphate from [3-3H]glucose. At present there are no adqquate data to calculate phosphorylation and recycling of fructose 6-phosphate, but under some conditions the rate may be quite high.

Amino Acids↗
PubMed1975

Metabolism of [5-T]fructose by isolated liver cells.

Glucose formed from [5-T]fructose in rat hepatocytes contains about 10 to 30% tritium. This does not appear to be due to fructose metabolism via hexokinase since neither the initial presence of glucose, nor wide variations in the original fructose concentration, have much effect on the relative labeling of glucose versus water from [5-T]fructose. Comparison of the T:14C ratios in glucose produced from [U-14C, 5-T]fructose and D-[U-14C, 2-T]glyceraldehyde indicate that there is tritium retention in the metabolism of fructose via the fructokinase-initiated pathway. The tritium retention can cause significant errors in the estimation of the futile cycle between fructose-1,6-P2 and fructose 6-P, by methods involving the use of [5-T]glucose.

Animals↗