Measurement of urea kinetics in vivo by means of a constant tracer infusion of di-15N-urea

1981 ◽  
Vol 240 (4) ◽  
pp. E428-E434 ◽  
Author(s):  
R. R. Wolfe
Keyword(s):  
Steady State ◽  
Constant Infusion ◽  
Selected Ion Monitoring ◽  
Plasma Urea ◽  
Urea Production ◽  
Nonsteady State ◽  
Urea Metabolism ◽  
The Difference ◽  
15N Urea

We have assessed the feasibility of studying urea metabolism in vivo in both steady state and nonsteady state situations by means of the primed constant infusion of di-15N-urea and the analysis of the resulting enrichment in plasma urea. Both hepatectomized dogs with known rates of urea infusion and intact dogs were studied. The enrichment of the bistrimethylsilyl derivative of urea was determined on a gas chromatograph-mass spectrometer. Selected ion monitoring was set for m/e 189 (M - 15), m/e 190 (A + 1), and m/e 191 (A + 2), thus enabling the calculation of the rate of urea production from nonrecycled NH3 (from A + 2 data) (Ra N), the rate of recycling of NH3 into urea (Ra R) (from A + 1 data), and thus the total rate of urea production (Ra N + Ra R). When urine collections were made, the incorporation of urea-N into protein was estimated from the difference between Ra N and urea excretion. We found that, in the steady state in a hepatectomized dog, the rate of appearance of urea can be determined accurately. In the nonsteady state in both hepatectomized and intact dogs, urea appearance could be estimated within +/- 20% in most situations. The only situation in which this was not the case was when we attempted to measure rapid changes in Ra R. Thus, within limits, this can be a useful technique enabling the quantitation of various aspects of urea metabolism.

Isotopic analysis of leucine and urea metabolism in exercising humans

1982 ◽  
Vol 52 (2) ◽  
pp. 458-466 ◽  
Author(s):  
R. R. Wolfe ◽  
R. D. Goodenough ◽  
M. H. Wolfe ◽  
G. T. Royle ◽  
E. R. Nadel
Keyword(s):  
Acid Metabolism ◽  
Human Subjects ◽  
Isotopic Analysis ◽  
Constant Infusion ◽  
Leucine Oxidation ◽  
Plasma Pool ◽  
Urea Production ◽  
Urea Metabolism ◽  
15N Urea ◽  
Plasma Leucine

We have used the primed constant infusion of di-[15N]urea and [1–13C]leucine to determine the effects of mild exercise (approx 30% Vo2max for 105 min) on urea production and leucine metabolism in human subjects. The oxidation of plasma leucine was distinguished from the oxidation of leucine that never entered the plasma pool (“intracellular” leucine) by means of determining the enrichment of alpha-ketoisocaproic acid (alpha-KICA). Total leucine oxidation increased from 0.38 +/0 0.05 to 1.41 +/- 0.14 micromol . kg-1 . min-1 during exercise due to increases in the oxidation of plasma leucine (150%) and intracellular leucine (600%). Plasma leucine flux decreased slightly, but not significantly (0.1 greater than P greater than 0.05), and the percent of alpha-KICA derived from plasma leucine dropped significantly (P less than 0.05) from 79.5 +/- 4.3 at rest to 62.0 +/- 5.3% over the last 30 min of exercise. Despite the increase in leucine oxidation during exercise, urea concentration and production did not change. Thus in exercise urea production does not accurately reflect all aspects of amino acid metabolism.

Reassessment of primed constant-infusion tracer method to measure urea kinetics

1987 ◽  
Vol 252 (4) ◽  
pp. E557-E564 ◽  
Author(s):  
F. Jahoor ◽  
R. R. Wolfe
Keyword(s):  
Steady State ◽  
Constant Infusion ◽  
Dependent Manner ◽  
Tracer Technique ◽  
Tracer Method ◽  
Priming Dose ◽  
Short Term ◽  
Urea Production ◽  
Urea Kinetics ◽  
Dose Dependent

The validity of the primed constant-infusion tracer technique to make short-term measurements of urea production rates (Ra) in humans in a physiological steady state and during disruption of steady state was evaluated. Four subjects received a primed constant infusion (P/I = 560 min) of [13C]urea for 8 h. A plateau in urea enrichment was reached after 2 h and maintained throughout. When [13C]- and [18O]urea were simultaneously infused into four subjects at P/I ratios of 560:1 and 360:1, respectively, both tracers reached plateau enrichment at the same time (2-4 h). The enrichment at plateau was a function of the infusion rate rather than the priming dose, and calculated urea Ra was the same with either prime. In five additional experiments the technique responded acutely to a physiological perturbation (alanine infusion) in a dose-dependent manner. The results confirm that this technique is appropriate for short-term measurements of urea Ra, and the requirement for accuracy in estimating the priming dose is not impractically stringent.

Constant specific activity input allows reconstruction of endogenous glucose concentration in non-steady state

1990 ◽  
Vol 258 (6) ◽  
pp. E1037-E1040 ◽  
Author(s):  
C. Cobelli ◽  
G. Toffolo
Keyword(s):  
Steady State ◽  
Glucose Concentration ◽  
Specific Activity ◽  
Radioactive Tracer ◽  
In Vivo Studies ◽  
Constant Infusion ◽  
Glucose Turnover ◽  
Glucose System ◽  
Constant Specific Activity

In vivo studies on the glucose system often require its perturbation by an exogenous input of glucose, whereas glucose turnover is assessed by infusing a glucose tracer. The constant infusion represents the usual format of tracer administration, but it has no clear advantage other than simplicity. Here we propose a different tracer infusion format. It consists of infusing the tracer in parallel with unlabeled glucose so as to maintain a constant specific activity in the infusate. This protocol does not increase experimental complexity and provides new information on the glucose system in non-steady state by allowing reconstruction of the endogenous component of glucose concentration. This reconstruction only requires very general assumptions, such as tracer-tracee indistinguishability and mass conservation; in particular it is independent of the glucose model structure, i.e., number of compartments and their interconnections. A proof of the result is given for a general nonlinear model of the glucose system. The constant specific activity input is also advantageous for non-steady-state calculations, because it reduces the variation in the measured plasma glucose specific activity. The glucose system has served as the prototype, but the protocol is applicable to other blood-borne substances. The radioactive tracer case has been considered, but the same results apply to stable isotope tracers as well; in this case they also become relevant in a somewhat different context, i.e., kinetic studies in steady state.

Drawbacks of the constant-infusion technique for measurement of renal function

1995 ◽  
Vol 268 (4) ◽  
pp. F543-F552 ◽  
Author(s):  
B. A. Van Acker ◽  
G. C. Koomen ◽  
L. Arisz
Keyword(s):  
Renal Function ◽  
Steady State ◽  
Plasma Concentrations ◽  
Constant Infusion ◽  
Urinary Recovery ◽  
Infusion Method ◽  
Pah Clearance ◽  
The Difference ◽  
Infusion Technique ◽  
Extrarenal Clearance

We investigated the validity of the steady-state constant infusion method (CIM), in which quantitative urinary recovery and constant plasma concentrations of the solute infused are required. Successive 3-h clearances of inulin and p-aminohippuric acid (PAH) were determined for 27 h in 25 patients with renal disease. Results were compared with the standard method of bladder clearance (StM) and with a modified CIM (ModCIM). The 24-h urinary recovery was incomplete for both inulin and PAH. Mean 24-h ModCIM inulin clearance overestimated StM by 4.5 ml.min-1 x 1.73 m-2 (range 0–9, P < 0.001) independent of the extent of renal impairment and pointed to slow distribution and/or extrarenal clearance of inulin. For PAH, the difference between ModCIM and StM clearance was related to the average PAH clearance by ModCIM and StM (r = 0.78). Furthermore, neither plasma inulin nor PAH became completely constant, because of the circadian rhythm in renal function. In conclusion, the conditions of the steady-state CIM technique are not fulfilled, and the method is not suitable for accurate measurement of inulin and PAH clearance, especially when the clearance is low.

Mivacurium Arteriovenous Gradient during Steady State Infusion in Anesthetized Patients

2002 ◽  
Vol 97 (3) ◽  
pp. 622-629 ◽  
Author(s):  
Samia Ezzine ◽  
François Donati ◽  
France Varin
Keyword(s):  
Steady State ◽  
High Performance ◽  
Sampling Site ◽  
Plasma Concentrations ◽  
Plasma Cholinesterase ◽  
Constant Infusion ◽  
Pharmacokinetic Parameters ◽  
Brachial Vein ◽  
The Difference ◽  
Total Body

Background Mivacurium and isomers undergo rapid hydrolysis by plasma cholinesterase. As this enzyme is largely distributed, it cannot be excluded that these isomers might undergo peripheral elimination. This hypothesis was investigated in patients by measuring the difference between arterial and venous concentrations under a constant-rate continuous infusion of mivacurium. Methods During propofol-remifentanil anesthesia, eight adult consenting patients received an intravenous bolus dose of 0.2 mg/kg mivacurium, followed by a constant infusion (3, 5, or 7 microg. kg. min ) into the brachial vein. One hour after starting the infusion, arterial (radial artery) and venous (contralateral brachial vein) blood samples were drawn simultaneously at 15-min intervals for 45 min. Mivacurium isomers and metabolite plasma concentrations were determined by stereospecific high-performance liquid chromatography. Using the corresponding arterial and venous concentrations, the tissue extraction coefficient as well as total body clearance were calculated. Results During steady state conditions, the venous concentrations of the and isomers were 34 +/- 13% and 42 +/- 11% (mean +/- SD) lower than the corresponding arterial concentrations (P &lt; 0.05), respectively. For the isomer, the difference between venous and arterial concentrations was 3 +/- 4% (P = 0.063). Total body clearances of the and isomers were greater when based on venous sampling (P &lt; 0.05). Conclusion Pharmacokinetic parameters derived from a constant infusion of mivacurium depend heavily on the sampling site (arterial or venous) for the rapidly hydrolyzed isomers. These results strongly suggest a significant metabolism of mivacurium within muscle tissue that may account for the large interpatient variability in response to mivacurium.

scholarly journals Neuroreceptor Quantitation in vivo by the Steady-State Principle Using Constant Infusion or Bolus Injection of Radioactive Tracers

1992 ◽  
Vol 12 (5) ◽  
pp. 709-716 ◽  
Author(s):  
Niels A. Lassen
Keyword(s):  
Steady State ◽  
Bolus Injection ◽  
Single Photon ◽  
Photon Emission ◽  
Animal Experiments ◽  
Emission Tomography ◽  
Constant Infusion ◽  
Receptor Sites ◽  
Long Time

The approaches hitherto used for measuring the kinetic constants Kd and Bmax of neuroreceptors in vivo all violate the steady state of the system. This complicates the kinetic analysis as approximations must be made, introducing errors of unknown magnitude. The present study presents the theory for designing experiments in which the steady state is preserved. It is based on maintaining a constant degree of receptor binding (occupancy) throughout the experiment. This is achieved by administering by prolonged intravenous infusion the non-radioactive ligand one wishes to study. The fraction of receptor sites not occupied by the “cold” ligand is measured by using trace amounts of a radioactive ligand binding to the same receptor. A minimum of two studies at different occupancies must be performed. In this presentation it is proposed to make the second study at essentially zero receptor occupancy by administering the tracer alone. The pair of tracer studies, the one without and the other with infusion of cold ligand, allows calculation of the cold ligand's equilibrium dissociation constant Kd. In the special case when tracer and cold ligands are chemically identical, then Bmax can also be calculated. Two different modes of tracer administration can be used. If the tracer is also infused at a constant rate for a long time, then the occupancy of receptor sites by the cold ligand can be calculated by measuring the equilibrium tracer concentrations in brain and plasma. If the tracer is administered as an intravenous bolus injection, then the area under the brain and plasma radioactivity curves or compartmental analysis must be used. The bolus injection approach, described in this paper for the first time, has the highest overall counting efficiency and should therefore be particularly suited for studies in man using positron emission tomography (PET) or single photon emission tomography (SPECT). Tracer infusion is the method of choice for animal experiments, as only one set of values are needed, those at long time, as can be obtained post vivo by counting samples of brain or by using autoradiographic techniques. The steady-state principle shows that ligands with very low Kd values, i.e., with very high affinity, are not suited for receptor quantitation.

Whole Body Nitrogen Kinetics in Man: Determination from Plasma [Guanidino-15N]Arginine

1984 ◽  
Vol 66 (3) ◽  
pp. 337-342 ◽  
Author(s):  
Marc Yudkoff ◽  
Itzhak Nissim ◽  
Mark Glassman ◽  
Stanton Segal
Keyword(s):  
Protein Synthesis ◽  
Steady State ◽  
Molecular Medicine ◽  
Whole Body ◽  
Constant Infusion ◽  
Isotopic Enrichment ◽  
Healthy Young Adult ◽  
Metabolic Pool ◽  
15N Urea ◽  
Oral Doses

1. Whole body nitrogen turnover and protein synthesis were calculated by the method of D. Picou & T. Taylor-Roberts [Clinical Science and Molecular Medicine (1969) 36, 283–296] except that plateau plasma enrichment of [guanidino-15N]arginine was used in place of the [15N]urea enrichment after a constant infusion of [15N]-glycine. With this approach metabolic pool turnover and protein synthesis were 637.2 ± 73.0 mg of N day−1 kg−1 and 2964.0 ± 409.5 mg of protein day−1 kg−1 respectively. 2. Virtually identical isotopic enrichment in [guanidino-15N]arginine and [15N]urea were observed in a healthy young adult who took repeated oral doses of [15N]glycine for a period of 60 h: 0.47 (arginine) and 0.48 (urea) atom% excess. 3. The turnover of glycine nitrogen and of urea, determined from the constant infusion of [15N]glycine and [13C]urea, was 66.2 ± 3.3 mg of N day−1 kg−1 and 156.2 ± 4.3 mg of urea day−1 kg−1 respectively. The ratio of steady-state enrichment in arginine to that in glycine, reflecting the fraction of arginine derived from glycine, was 10.5%. By using the [guanidino-15N]arginine enrichment as representative of the expected enrichment in [15N]urea at plateau, it was calculated that approximately 25% of glycine N flux is directed toward the synthesis of urea, with the remainder directed to protein and quantitatively minor products like haem and creatinine. 4. Unlike steady-state [15N]urea labelling, which is achieved only after infusion of [15N]-glycine for several days, plateau isotopic abundance in [guanidino-15N]arginine was attained after only 1–2 h of [15N]glycine infusions, thereby allowing estimation of whole body nitrogen kinetics and the rate of transfer of glycine nitrogen to urea in a relatively brief experiment.

Regulation of urea production by glucose infusion in vivo

1987 ◽  
Vol 253 (5) ◽  
pp. E543-E550 ◽  
Author(s):  
F. Jahoor ◽  
R. R. Wolfe
Keyword(s):  
Amino Acid ◽  
Urea Cycle ◽  
Amino Nitrogen ◽  
Suppressive Effect ◽  
Glucose Infusion ◽  
Constant Infusion ◽  
Fixed Nitrogen ◽  
Urea Production ◽  
Direct Suppression

We have investigated the acute in vivo regulation of urea production in normal postabsorptive volunteers by administering a primed constant infusion of 15N2-urea to measure urea production during the constant intravenous infusion of equivalent molar quantities of exogenous nitrogen, given as alanine or glutamine, either with or without a simultaneous infusion of glucose at 4 mg.kg-.min-1. These responses were compared with the response to the infusion of glucose alone. Both amino acid infusions elicited significant (P less than 0.05) and identical (26%) increases in urea production over 4 h. When the glucose infusion was added to the amino acid infusions, urea production remained constant, despite the comparable increases in plasma total nonessential amino nitrogen, as were observed with the amino acid infusions alone. Glucose infused alone elicited a significant (P less than 0.05) reduction (18%) in urea production but no corresponding change in plasma total amino nitrogen. We conclude that 1) infused glucose or its hormonal response suppresses urea production by blunting the normal hepatic ureagenic response to a fixed nitrogen load, 2) this suppressive effect is not mediated via a reduction in substrate (nitrogen) supply, and 3) the inhibition of hepatic gluconeogenesis from amino acids represents one component of this suppressive effect, and direct suppression of urea cycle activity probably represents another component.

Isotopic determination of amino acid-urea interactions in exercise in humans

1984 ◽  
Vol 56 (1) ◽  
pp. 221-229 ◽  
Author(s):  
R. R. Wolfe ◽  
M. H. Wolfe ◽  
E. R. Nadel ◽  
J. H. Shaw
Keyword(s):  
Amino Acids ◽  
Essential Amino Acids ◽  
Plasma Urea ◽  
Urea Production ◽  
The Difference ◽  
Previous Assumption ◽  
Exercise In Humans ◽  
Alanine Production ◽  
Rest And Exercise

We recently reported that in light exercise (30% VO2max) the oxidation of [1-13C]leucine was significantly increased but the rate of urea production was unchanged (J. Appl. Physiol: Respirat. Environ. Exercise Physiol. 52: 458–466, 1982). We have therefore tested three possible explanations for this apparent incongruity. 1) We infused NaH13CO3 throughout rest and exercise and found that, although altered bicarbonate kinetics in exercise resulted in greater recovery of 13CO2, the difference between rest and recovery was small compared with the increase in the rate of 13CO2 excretion during exercise when [1-13C]leucine was infused. 2) We infused [15N]leucine and isolated plasma urea N to determine directly the rate of incorporation of the 15N. During exercise there was no increase in the rate of 15N incorporation. Simultaneously, we infused [2,3-13C]alanine and quantified the rate of incorporation of 15N in alanine. We found that [15N]alanine production from [15N] leucine more than doubled in exercise, and by deduction, alanine production from other amino acids also doubled. 3) We tested our previous assumption that [1-13C]leucine metabolism in exercise was representative of the metabolism of other essential amino acids by infusing [1-13C] and [alpha-15N]lysine throughout rest and exercise. We found that the rate of breakdown of lysine during exercise was not increased in a manner comparable to that of leucine. Thus, these data confirm our original findings that leucine decarboxylation is enhanced in light exercise but urea production is unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel 13C NMR method to assess intracellular glucose concentration in muscle, in vivo

1998 ◽  
Vol 274 (2) ◽  
pp. E381-E389 ◽  
Author(s):  
Gary W. Cline ◽  
Beat M. Jucker ◽  
Zlatko Trajanoski ◽  
Alexander J. M. Rennings ◽  
Gerald I. Shulman
Keyword(s):  
Glucose Concentration ◽  
Glycogen Synthesis ◽  
Constant Infusion ◽  
Muscle Glycogen Synthesis ◽  
Extracellular Glucose ◽  
Internal Concentration ◽  
The Difference ◽  
Reduced Rate ◽  
Intracellular Glucose

Intracellular glucose concentration in skeletal muscle of awake rats was determined under conditions of hyperglycemic (10.2 ± 0.6 mM) hyperinsulinemia (∼1,200 pM) and hyperglycemic (20.8 ± 1.5 mM) hypoinsulinemia (<12 pM) by use of13C nuclear magnetic resonance (NMR) spectroscopy during a prime-constant infusion of [1-13C]glucose and [1-13C]mannitol with either insulin (10 mU ⋅ kg−1 ⋅ min−1) or somatostatin (1.0 μg ⋅ kg−1 ⋅ min−1). Intracellular glucose was calculated as the difference between the concentrations of total tissue glucose (calculated from the in vivo13C NMR spectrum with mannitol as an internal concentration standard) and extracellular glucose, corrected by the ratio of intra- and extracellular water space. Extracellular concentration was corrected for an interstitial fluid-to-plasma glucose concentration gradient of 0.83 ± 0.07, determined by open-flow microperfusion. The mean ratio of intra- to extracellular glucose space, determined from the relative NMR signal intensities and concentrations of mannitol and total creatine, was 9.2 ± 1.1 (hyperglycemic hyperinsulinemia, n = 10), and 9.0 ± 1.7 (hyperglycemic hypoinsulinemia, n= 7). Mean muscle intracellular glucose concentration was <0.07 mM under hyperglycemic-hyperinsulinemic conditions ( n = 10) and 0.32 ± 0.06 mM under hyperglycemic-hypoinsulinemic conditions ( n = 7). This method is noninvasive and should prove useful for resolving the question of whether glucose transport or phosphorylation is responsible for the reduced rate of muscle glycogen synthesis observed in diabetic subjects.

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