Evaluation of the expanded measurement uncertainty in the determination of the net flux of nutrients and carbon atoms through the liver in ruminants

Abstract

In in vivo metabolic studies, quantifying the flux of a nutrient and/or the carbon atoms through the liver implies the analytical determination of nutrient concentrations before and after crossing the liver and of blood or plasma flux. There is a measurement uncertainty (MU) associated with each input quantity, which may compromise accurate physiological interpretations of the results. The objective of this work was to calculate the expanded MU of net fluxes of nutrients and carbon atoms through the liver of ruminants and to identify the main components that affect MUs. To this end, all quantities (i.e., nutrient concentrations, para-aminohippuric acid concentrations, hematocrit, time of infusion, and others) that influenced the final results were identified and quantified. Their associated MUs were calculated with Type A or Type B approaches. An expanded MU model was developed by combining these two approaches. In general, the relative expanded MU of the net flux of nutrients calculated in plasma was lower than that in blood. For both plasma and blood fluxes, the main components that increased MUs were identified. The net hepatic flux of carbon atoms, for example, was estimated to be (63 ± 229) mmol min⁻¹. This high expanded MU was mainly a result of acetate concentration measurements in hepatic and portal veins. Priority strategies to reduce MUs, such as selecting an adequate matrix or using more accurate measurement methods, were identified.

Glossary

HA

Hepatic artery

HV

Hepatic vein

MA

Mesenteric artery

MV

Mesenteric vein

PV

Portal vein

RV

Ruminal vein

AAN

Amino acid nitrogen

BOH

β-hydroxybutyrate

NEFA

Non-esterified fatty acids

pAH

Para-aminohippuric acid

VFA

Volatile fatty acids

β

Net carbon atoms flux through the liver (mmol L⁻¹), calculated as F _β + G _β

C _i (i = inf, PV, HV or MA)

Para-aminohippuric acid concentration in infused solution (g L⁻¹) or in PV, HV or MA (mg L⁻¹)

f _i (i = PV or HV)

Plasma flux in PV or HV (L min⁻¹)

F ^(j)

Plasma hepatic flux of a nutrient j (mmol min⁻¹)

F _β

Plasma hepatic flux of carbon atoms (mmol min⁻¹)

g _i (i = PV or HV)

Blood flux in PV or HV (L min⁻¹)

G ^(j’)

Blood hepatic flux of a nutrient j’ (mmol min⁻¹)

G _β

Blood hepatic flux of carbon atoms (mmol min⁻¹)

H

Hematocrit

N _i (i = PV, HV or MA)

Nutrient concentration in PV, HV or MA (mmol L⁻¹)

Q _i (i = PV, HV or MA)

Carbon atoms plasma concentrations (present as AAN, NEFA and triglycerides) in PV, HV or MA (mmol L⁻¹)

R _i (i = PV, HV or MA)

Carbon atoms blood concentrations (present as acetate, butyrate, BOH, CO₂, glucose, lactate and propionate) in PV, HV or MA (mmol L⁻¹)

s(X)

Standard deviation of an input X, estimated for the Type A approach

SEM

Standard error of the mean

T _inf

Time of para-aminohippuric acid infusion (min)

u(X)

standard uncertainty of an input X, estimated for the Type B approach

u _c (Y)

Combined standard uncertainty of a quantity Y

U(Y)

Expanded uncertainty of a quantity Y

U _rel

Relative expanded uncertainty of a measurement result (%)

W _inf

Total weight of infused volume in each syringe (expressed in mL, where the density of the solution is 1 g mL⁻¹)