Quantification of fatty acid uptake with14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid
([18F]FTHA) and PET
Long-chain free fatty acids (FFA) are transported in blood bound to albumin.
Passive diffusion and facilitated transport via specific membrane-associated and
cytosolic fatty acid binding proteins (FABP) are involved in entry into the cells
(Frohnert and Bernlohr, 2000).
In cells acyl-CoA synthetase (ACS) activates FFA to the fatty acyl-CoA derivative.
This is necessary for the major metabolic pathways of FFA metabolism, including
β-oxidation and esterification, and therefore net flux through this reaction
represents total fatty acid utilization.
The long-chain fatty acid analogue
14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid
([18F]FTHA, DeGrado et al., 1991) enters the cells by the same mechanism
natural fatty acids and undergoes partial metabolism in mitochondria before being
trapped.
Tissue accumulation of 18F represents total fatty acid utilization,
including both storage and β-oxidation.
[18F]FTHA net influx rate (Ki) or fractional uptake rate
(FUR) must be multiplied by FFA concentration in (arterial) plasma to calculate
the FFA uptake (utilization) rate.
There is a 100-fold range of possible FFA plasma concentrations.
To account for the different kinetics of
[18F]FTHA and palmitate and other FFAs, the rate index should also be
be divided by "lumped constant" (LC).
Lumped constant (LC)
LC represents the ratio of the probability that arterial tracer molecule,
[18F]FTHA, will be activated to [18F]FTHA-CoA,
to the probability that an arterial long-chain fatty acid (palmitate and other
FFAs) molecule will undergo activation to fatty acyl-CoA.
If the kinetics of the tracer and of the average native compound would be identical,
LC would equal 1.
Because LC is difficult to measure in human studies, it is often assumed to equal 1.
This may cause bias in results and should be taken into account in interpretation
of the results.
Fatty acids in plasma and blood
FFAs in plasma are normally more than 99.99% bound to circulating plasma proteins.
Covalently bound fatty acids in erythrocytes slowly exchange (half-life 30 min
or more) with fatty acids in plasma.
Multiple-time graphical analysis for irreversible uptake (Gjedde-Patlak plot)
with metabolite corrected plasma input can be used to measure FFA oxidation
and storage rate in the fasting state (Iozzo et al., 2003a;
Hannukainen et al., 2007; Viljanen et al., in press).
The uptake kinetics of [18F]FTHA are not
fully irreversible in the liver, but the slight downward curvature of
Gjedde-Patlak plots does not prevent line-fitting to plot data measured
10-32 min after injection (Iozzo et al., 2003a).
Uptake rate can be calculated
regionally
or a Ki
image can be computed. If only static image or late scan with few frames is
available, fractional
uptake rate (retention index) image can be computed.
Mäki et al., 1998; Turpeinen et al., 1999.
Using [18F]FTHA and Gjedde-Patlak graphical analysis has been
validated in a pig study, suggesting that in skeletal muscle
[18F]FTHA depicts FFA uptake, but not specifically β-oxidation
(Takala et al., 2002). [18F]FTHA uptake has been quantitated also
from a single late PET frame (Hannukainen et al., 2006 and 2007).
Regulation of fatty acid utilization
It is not known what exactly are the roles of passive diffusion and
facilitated transport
via specific membrane-associated and cytosolic fatty acid binding proteins in
entry into cells; at least in certain conditions the fatty acid uptake
displays saturation kinetics.
Regulation of fatty acid utilization in skeletal muscle during exercise may lie
mainly within the entrance into the mitochondria or metabolism within the
mitochondria (Kiens and Roepstorff, 2003), although there is evidence that
plasmalemmal fatty acid transport in heart and skeletal muscle is regulated
for example by contraction, insulin, leptin (Bonen et al., 2003).
It has been shown in humans that a marked storage of lipids in skeletal muscle
can be increased by the combination of hyperinsulinemia and elevation of
circulating FFAs, but not by one of these conditions alone (Brechtel et al.,
2001).
Entry of long-chain fatty acids into adipocytes is mediated by facilitated
transport.
Experimental validation studies have shown correlation between trapping of
[18F]FTHA and fatty acid oxidation in various conditions
(Ebert et al., 1994; Stone et al., 1998; Takala et al., 2002).
However, in hypoxic conditions of myocardium [18F]FTHA may not be
optimal for measuring changes in β-oxidation (Renstrom et al., 1998).
Gjedde-Patlak plot has been used to calculate the [18F]FTHA net
influx rate (Ki), and further by assuming LC=1, it has been used to
estimate myocardial β-oxidation rate of long chain fatty acids without
(Taylor et al., 2001; Wallhaus et al., 2001) or with (Ebert et al., 1994;
Mäki et al. 1997, 1998; Takala et al., 1999, 2002; Turpeinen et al., 1999;
Hannukainen et al., 2007; Tuunanen et al., 2007) plasma metabolite correction.
A good review on fatty acid incorporation in the brain is written by Robinson et
al. (1992).
The equilibrium unbound fraction of unacylated palmitate in plasma is less than
0.007%, and fatty acid-albumin complex cannot cross the blood-brain barrier.
Yet, in rat brain the single-pass extraction of palmitate is about 5%.
This means that fatty acids must be released from albumin during the passage through
capillaries, and therefore protein-bound fraction is not needed to quantitate the
fatty acid uptake in PET studies.
Relatively low first-pass extraction also means that fatty acid uptake in the brain
is independent of cerebral blood flow.
See also:
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