Concentrative uptake of alanine in hepatocytes from fed and fasted rats

1983 ◽  
Vol 244 (5) ◽  
pp. G491-G500 ◽  
Author(s):  
L. O. Kristensen ◽  
L. Sestoft ◽  
M. Folke
Keyword(s):  
Steady State ◽  
Kinetic Model ◽  
Distribution Ratio ◽  
Passive Component ◽  
Electrochemical Gradient ◽  
Fed State ◽  
Fasted State ◽  
Threefold Increase ◽  
Maximal Distribution ◽  
Fasted Rats

The transport of alanine across the liver cell membrane was studied in hepatocytes isolated from fed and 48-h-fasted rats. Aminooxyacetate was used to render alanine nonmetabolizable. The steady-state intracellular-to-extracellular distribution ratio of alanine was maximal at extracellular concentrations below 0.5 mM and was increased from about 10 to about 20 by fasting. This increase was the net effect of a two- to threefold increase in the alanine influx and about a 50% increase in the rate constant of alanine efflux. The results suggest that alanine efflux occurred partly via the transport system mediating Na+-dependent alanine influx and partly by another pathway. The transmembrane Na+ electrochemical gradient remained unchanged by fasting and could apparently account for a maximal distribution ratio of alanine well above 20. In both nutritional states, the simplest kinetic model adequately describing the alanine influx implicated one saturable component besides a passive component. The apparent Vmax of the former was doubled by fasting while the apparent Km was insignificantly decreased. At low extracellular alanine concentrations, the rate of alanine consumption (aminooxyacetate absent) was only 40% of the alanine influx in the fed state but was increased at least fivefold by fasting and thereby balanced with the increased alanine influx. These results suggest a rate limitation at the transport level in the fasted state.

Glutamine carbon disposal and net glutamine uptake in fetuses of fed and fasted ewes

1993 ◽  
Vol 265 (5) ◽  
pp. E722-E727 ◽  
Author(s):  
L. L. Levitsky ◽  
B. S. Stonestreet ◽  
K. Mink ◽  
Q. Zheng
Keyword(s):  
Blood Flow ◽  
Steady State ◽  
Umbilical Vein ◽  
Carbon Uptake ◽  
Fed State ◽  
Glutamine Level ◽  
Fick Principle ◽  
Fasted State ◽  
Ovine Fetal ◽  
Glutamine Uptake

We have traced ovine fetal glutamine carbon uptake and disposal in 7 chronically catheterized fetuses of fed ewes and 10 fetuses of 48-h fasted ewes. Net fetal glutamine uptake (Fick principle, antipyrine blood flow) was 10.0 +/- 2.0 mumol.kg-1 x min-1 in fed fetuses and 6.4 +/- 1.4 mumol.kg-1 x min-1 in fasted fetuses [not significant (NS)]. However, net fetal glutamine uptake was linearly related to the umbilical vein glutamine level (P < 0.05) in fed and fasted fetuses. In contrast, fetal glutamate transfer to the placenta was 4.0 +/- 0.8 mumol.kg-1.min-1 in the fed state and 2.7 +/- 0.1 mumol.kg-1 x min-1 in the fasted state. Net fetal glutamine uptake and fetal glutamate transfer to the placenta were directly correlated (P < 0.05). Fetal glutamine carbon disposal was measured using a primed continuous infusion of [U-14C]-glutamine over a 3-h period and blood sampling during the last hour of infusion (steady state). Disposal was 20.9 +/- 2.6 mumol.kg-1 x min-1 in the fed state and 18.6 +/- 2.3 mumol.kg-1 x min-1 in the maternal fasted state (NS). Glutamine carbon disposal did not correlate with fetal arterial glutamine levels and was not influenced by maternal nutritional state.

Abstract 3560: Use of Hyperpolarized Magnetic Resonance for Non-Invasive Observation of Metabolic Regulation

Circulation ◽  
2008 ◽  
Vol 118 (suppl_18) ◽  
Author(s):  
Marie A Schroeder ◽  
Lisa C Heather ◽  
Helen J Atherton ◽  
Kieran Clarke ◽  
George K Radda ◽  
...  
Keyword(s):  
Magnetic Resonance ◽  
Metabolic Regulation ◽  
Krebs Cycle ◽  
Enzyme Regulation ◽  
Product Inhibition ◽  
Acetyl Coa ◽  
Fed State ◽  
Fasted Rats ◽  
Pyruvate Ratio

Hyperpolarized magnetic resonance (HP MR) has enabled real time visualization of in vivo metabolism. In this study, we postulated that HP MR could also non-invasively provide a measure of metabolic regulation. We focused on regulation of pyruvate dehydrogenase (PDH), a highly controlled enzyme that catalyzes the oxidation of pyruvate to acetyl CoA and CO2/HCO3-. We compared PDH flux in conditions of normal and attenuated enzyme activity, and in the presence of normal and augmented Krebs cycle flux, to determine the contributions of PDH activity and end product inhibition to enzyme regulation. Six rats were examined in the fed and fasted states (to modulate PDH activity), with 40 μmol HP 13C1-pyruvate alone and 40 μmol HP pyruvate co-infused with 40 μmol malate (to manipulate Krebs cycle flux/acetyl CoA uptake). HP tracer was infused into the rats in an MR scanner and cardiac spectra were acquired every second for 1 min. Conversion of pyruvate to 13HCO3-was monitored and the 13HCO3-/pyruvate ratio was used as a marker of PDH flux. Infusion of malate increased PDH flux by 31% compared with pyruvate alone, indicating that removal of acetyl CoA by incorporation into the Krebs cycle increased PDH flux. PDH flux was 57% lower in fasted rats injected with pyruvate alone compared with fed rats, and did not change with malate co-infusion. Here, low PDH activity prevented additional enzyme flux. These results suggest that end product inhibition limits fed state PDH flux, whereas PDH activity regulates pyruvate oxidation in the fasted state. In conclusion, this study has provided evidence that HP MR may be useful to obtain details of metabolic regulation, rather than just reflecting metabolic state. Figure 1 Bicarbonate/pyruvate ratio in fed and fasted rats, following an injection of pyruate or pyruate plus malate. In fed rats, co-infusion of malate increased PDH flux by 31% compared with injection of pyruvate alone (*p=0.02). Fasting reduced PDH flux by 57% (**p=0.002) following injection of pyruvate alone. Co-infusion with malate did not affect PDH flux in fasted rats.

Serial analysis of renal blood flow by positron tomography with rubidium-82

1986 ◽  
Vol 251 (5) ◽  
pp. H1024-H1030
Author(s):  
N. Tamaki ◽  
C. A. Rabito ◽  
N. M. Alpert ◽  
T. Yasuda ◽  
J. A. Correia ◽  
...  
Keyword(s):  
Blood Flow ◽  
Steady State ◽  
Kinetic Model ◽  
Renal Blood Flow ◽  
Constant Infusion ◽  
Flow Reduction ◽  
Arterial Blood ◽  
Steady State Kinetic ◽  
State Kinetic ◽  
Rubidium 82

To determine whether renal blood flow can be measured by positron-emission tomography (PET) during constant infusion of rubidium-82 (82Rb) using a steady-state kinetic model, studies were performed in 10 dogs at control (n = 10), during mild flow reduction (n = 7), during severe flow reduction (n = 10), and after reperfusion of the kidney (n = 3). PET data were quantified to determine mean concentration of 82Rb (Ct) in each transverse section of the kidney. The arterial concentration (Ca) of 82Rb was measured by well counting of arterial blood samples during the equilibrium scan. 82Rb renal uptake (Ct/Ca) correlated nonlinearly with microsphere renal blood flow according to a steady-state kinetic model (r = 0.90). 82Rb estimated flow was 3.16 +/- 1.36 ml X min-1 X g-1 at control and 1.56 +/- 0.57 and 0.37 +/- 0.59 during mild and severe flow reductions, respectively. Microsphere determined flow was 2.89 +/- 0.77 ml X min-1 X g-1 at control, 1.58 +/- 0.42 at mild reduction, and 0.27 +/- 0.49 at severe reduction. In the occlusion and reperfusion model, the 82Rb estimated flow during occlusion was 0.21 +/- 0.15 ml X min-1 X g-1 and on reperfusion went up to 2.13 +/- 1.08. The contralateral kidney demonstrated reductions in the 82Rb estimated flow of 3.02 +/- 1.62 ml X min-1 X g-1 (63%) and 2.92 +/- 0.89 (61%) during mild and severe flow reductions, respectively. We conclude that PET with 82Rb permits serial quantitative assessment of renal flood flow.

Endurance training attenuates stress hormone responses to exercise in fasted rats

1982 ◽  
Vol 243 (1) ◽  
pp. R179-R184 ◽  
Author(s):  
W. W. Winder ◽  
M. A. Beattie ◽  
R. T. Holman
Keyword(s):  
Blood Glucose ◽  
Plasma Concentrations ◽  
Liver Glycogen ◽  
Stress Hormone ◽  
Glucose Levels ◽  
Blood Glucose Levels ◽  
Fasted State ◽  
Hormone Responses ◽  
Exercise Muscle ◽  
Fasted Rats

Endurance exercise training produces major adaptations in hormonal and metabolic responses to exercise. This study was designed to determine whether the differences in hormone response persist in the fasted condition when liver glycogen is depleted. Rats were run on a motor-driven rodent treadmill 5 days/wk for periods up to 2 h/day for 10 wk. Trained and nontrained rats were then fasted 24 h and were run for periods ranging from 0- to 60 min. At the end of 60 min of exercise muscle glycogen was higher in trained rats (2.9 +/- 0.3 vs. 1.1 +/- 0.1 mg/g). Blood glucose was maintained at higher levels in trained rats throughout the course of the exercise (3.2 +/- 0.1 vs. 2.3 +/- 0.1 mM after 60 min). Plasma concentrations of glucagon and epinephrine increased in both groups during the exercise but were significantly lower in trained animals. Differences between trained and nontrained animals in stress hormone responses to exercise persist in the fasted state and appear to be a consequence of the capacity of trained animals to maintain higher blood glucose levels.

scholarly journals Calcium efflux and cycling across the synaptosomal plasma membrane

1985 ◽  
Vol 226 (1) ◽  
pp. 225-231 ◽  
Author(s):  
R Snelling ◽  
D Nicholls
Keyword(s):  
Plasma Membrane ◽  
Steady State ◽  
Potential Gradient ◽  
Electrochemical Potential ◽  
Electrochemical Gradient ◽  
Calcium Efflux ◽  
Normal Medium ◽  
Voltage Dependent ◽  
Synaptosomal Plasma Membrane ◽  
Ca2 Channels

Ca2+ efflux from intact synaptosomes is investigated. Net efflux can be induced by returning synaptosomes from media with elevated Ca2+ or high pH to a normal medium. Net Ca2+ efflux is accelerated when the Na+ electrochemical potential gradient is collapsed by veratridine plus ouabain. Under steady-state conditions at 30 degrees C, Ca2+ cycles across the plasma membrane at 0.38 nmol . min-1 . mg-1 of protein. Exchange is increased by 145% by veratridine plus ouabain, both influx and efflux being increased. Increased influx is probably due to activation of voltage-dependent Ca2+ channels, since it is abolished by verapamil. The results indicate that, at least under conditions of low Na+ electrochemical gradient, some pathway other than a Na+/Ca2+ exchange must operate in the plasma membrane to expel Ca2+.

scholarly journals Concentrative Accumulation of Choline by Human Erythrocytes

1968 ◽  
Vol 51 (4) ◽  
pp. 497-516 ◽  
Author(s):  
K. Martin
Keyword(s):  
Steady State ◽  
Human Erythrocytes ◽  
Electrochemical Gradient ◽  
Carrier System ◽  
Michaelis Constant ◽  
External Concentration ◽  
First Order ◽  
Maximum Flux ◽  
First Order Kinetics ◽  
Exchange Flux

Influx and efflux of choline in human erythrocytes were studied using 14C-choline. When incubated at 37°C with physiological concentrations of choline erythrocytes concentrate choline; the steady-state ratio is 2.08 ± 0.23 when the external choline is 2.5 µM and falls to 0.94 ± 0.13 as the external concentration is raised to 50 µM. During the steady state the influx of choline is consistent with a carrier system with an apparent Michaelis constant of 30 x 10-6 and a maximum flux of 1.1 µmoles per liter cells per min. For the influx into cells preequilibrated with a choline-free buffer the apparent Michaelis constant is about 6.5 x 10-6 M and the maximum flux is 0.22 µmole per liter cells per min. At intracellular concentrations below 50 µmole per liter cells the efflux in the steady state approximates first order kinetics; however, it is not flux through a leak because it is inhibited by hemicholinium. Influx and efflux show a pronounced exchange flux phenomenon. The ability to concentrate choline is lost when external sodium is replaced by lithium or potassium. However, the uphill movement of choline is probably not coupled directly to the Na+ electrochemical gradient.

Adipocyte lactate production remains elevated during refeeding after fasting

1990 ◽  
Vol 259 (6) ◽  
pp. E865-E871 ◽  
Author(s):  
F. D. Newby ◽  
L. K. Wilson ◽  
S. V. Thacker ◽  
M. DiGirolamo
Keyword(s):  
Adipose Tissue ◽  
Lactate Production ◽  
Hepatic Glycogen ◽  
Fed State ◽  
Fasted State ◽  
Rapid Restoration ◽  
Isolated Adipocytes ◽  
Elevated Rate ◽  
Total Glucose ◽  
Main Substrate

The metabolic state occurring with refeeding after fasting is characterized by the rapid restoration of hepatic glycogen. Recent evidence suggests that a main substrate for glycogenesis is lactate. Because adipose tissue is an active site of lactate production that increases with fasting, we examined the magnitude and duration of lactate production by isolated adipocytes from three adipose depots of rats fasted for 48 h and then refed for up to 96 h. The data show that 48 h of fasting results in a markedly elevated rate of adipocyte lactate production, which increased from 3-9% of total glucose metabolized in the fed state to 49-60% in the fasted state. During the refeeding period, lactate production remained elevated for 12-24 h and then declined. Mesenteric adipocytes had a higher rate and more prolonged elevation in lactate production than cells from the other two depots. We conclude that, with refeeding after a fast, adipocyte glucose conversion to lactate remains elevated during the time of hepatic glycogen restoration. This suggests that adipose tissue may actively produce lactate for glycogenesis during refeeding.

Peritubular uptake of lactate by Thamnophis proximal tubule

1980 ◽  
Vol 238 (4) ◽  
pp. F296-F304 ◽  
Author(s):  
P. H. Brand ◽  
R. Stansbury
Keyword(s):  
Steady State ◽  
Proximal Tubule ◽  
Water Content ◽  
Transport Mechanism ◽  
Lactate Concentration ◽  
Ringer Solution ◽  
Proximal Tubules ◽  
Electrochemical Gradient ◽  
Tubular Cells ◽  
Lactate Uptake

Lactate is absorbed in the proximal tubule and also enters tubular cells at the peritubular membrane. To characterize peritubular lactate entry, lactate uptake was measured in isolated nonperfused proximal tubules. Tubules were dissected and incubated in Ringer solution with L(+)-[U-14C]lactate and 3H2O. After incubation, the tubules were extracted, and the extracts were assayed for 14C and 3H or were chromatographed to determine the percentage of tubule 14C identifiable as lactate. Maximal steady-state tubular fluid-to-bath lactate concentration ratios (TF/B lactate) occurred by 30-60 min incubation at 25 degrees C. In 30 min, one-third of the tubules established a TF/B lactate ratio greater than 1.00, and 61.4 +/- 18.6% of tubule 14C was lactate. There was no difference in TF/B lactate ratio in proximal and distal proximal segments. Uptake was depressed at 5 degrees C. Mersalyl at 10(-4) M increased the TF/B lactate ratio and tubule water content. Probenecid at 7.5-30 x 10(-4) M also increased the TF/B lactate ratio. Distal proximal tubules incubated with [3H]PAH showed a control TF/B para-aminohippurate (PAH) ratio of approximately 30, but with 10(-4) M mersalyl the TF/B PAH ratio was approximately 1.00. Lactate uptake at the peritubular membrane occurs against an electrochemical gradient, independently from the PAH transport mechanism.

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