Splanchnic free fatty acid kinetics
Michael D.
Jensen1,
Sylvain
Cardin2,
Dale
Edgerton2, and
Alan
Cherrington2
1 Endocrine Research Unit, Mayo Clinic, Rochester,
Minnesota 55905; and 2 Department of
Molecular Physiology and Biophysics, and Diabetes Research and
Training Center, Vanderbilt University School of Medicine,
Nashville, Tennessee 37232
 |
ABSTRACT |
These studies were conducted to assess the
relationship between visceral adipose tissue free fatty acid (FFA)
release and splanchnic FFA release. Steady-state splanchnic bed
palmitate ([9,10-3H]palmitate) kinetics were determined
from 14 sampling intervals from eight dogs with chronic indwelling
arterial, portal vein, and hepatic vein catheters. We tested a model
designed to predict the proportion of FFAs delivered to the liver from
visceral fat by use of hepatic vein data. The model predicted that
15 ± 2% of hepatic palmitate delivery originated from visceral
lipolysis, which was greater (P = 0.004) than the
11 ± 2% actually observed. There was a good relationship
(r2 = 0.63) between the predicted and
observed hepatic palmitate delivery values, but the model overestimated
visceral FFA release more at lower than at higher palmitate
concentrations. The discrepancy could be due to differential uptake of
FFAs arriving from the arterial vs. the portal vein or to release of
FFAs in the hepatic circulatory bed. Splanchnic FFA release measured
using hepatic vein samples was strongly related to visceral adipose
tissue FFA release into the portal vein. This finding suggests that
splanchnic FFA release is a good indicator of visceral adipose tissue lipolysis.
isotope tracers; lipolysis; kinetic model
 |
INTRODUCTION |
INCREASED VISCERAL
FAT, especially in the setting of obesity, is associated with a
greater risk of dyslipidemia, insulin resistance, and type 2 diabetes
mellitus. It has been proposed that active visceral adipose tissue
lipolysis mediates the physiology of this association by
disproportionately increasing the delivery of free fatty acids (FFAs)
to the liver (4). This would occur because the venous
drainage of omental and mesenteric (visceral) fat is directly into the
portal venous circulation, and the FFAs released by visceral fat
undergo no further uptake before reaching the liver. Increased hepatic
FFA delivery results in insulin resistance with regard to the
suppression of hepatic glucose output (3) and increased
VLDL triglyceride production (8), some cardinal features
of upper body/visceral obesity. We have suggested that the increased
hepatic FFA delivery in upper body/visceral obesity is due to increased
FFA release from upper body subcutaneous fat, not from visceral adipose
tissue FFA release (5). Unfortunately, without better
means of assessing intrasplanchnic FFA kinetics, it is difficult to
be certain. Understanding the pathophysiology of obesity could be
enhanced by improved assessment of visceral adipose tissue lipolysis in humans.
The lack of direct information regarding visceral adipose tissue
lipolysis relates to the impracticality of accessing the portal vein,
except in surgical patients (16). It is possible to
measure splanchnic FFA balance (net uptake or release based upon
arteriovenous concentration difference) and also new splanchnic FFA
release in humans by using hepatic venous blood sampling and isotope
dilution techniques. Unfortunately, these values are not widely
believed to be a suitable reflection of visceral lipolysis. FFAs are
delivered to the liver via both the hepatic artery and the portal vein.
Portal venous FFAs are a mixture of those released directly from
visceral fat together with those that enter the splanchnic bed via the
celiac, superior, and inferior mesenteric arteries that escape
extraction by nonhepatic splanchnic tissues. Further FFA extraction
occurs in the liver, subjecting portal venous FFAs of an arterial
origin to a second extraction process. Because FFAs released from
visceral adipose tissue are subject only to this second extraction
(hepatic), but not to the nonhepatic splanchnic extraction, knowledge
of the total (hepatic plus nonhepatic splanchnic extraction) from
hepatic vein catheterization plus isotopic techniques does not allow
calculation of visceral adipose tissue FFA release. Instead, knowledge
of "hepatic" fractional FFA extraction (which requires portal
vein catheterization) is needed to determine visceral adipose tissue
FFA release with use of splanchnic FFA release data obtained from
isotope dilution techniques and hepatic vein catheterization. In this
case, visceral adipose tissue FFA release = splanchnic FFA
release
(1
fractional hepatic FFA extraction).
We hypothesized that it might be possible to determine the portion of
hepatic FFA delivery that originates from visceral fat by using hepatic
vein catheterization data. Herein, we propose and test a model
(APPENDIX A) to predict the fraction of hepatic FFA
delivery that arises from visceral adipose tissue lipolysis. We
reasoned that, if it were possible to predict the fraction of hepatic
FFA delivery that arises from visceral fat, this could help address the
question of the relative importance of visceral fat in affecting
hepatic metabolism. For example, if a greater fraction of hepatic FFA
delivery derives from visceral fat in conditions such as upper body
obesity, this would argue for a more important role for visceral
lipolysis than we have previously suggested.
An assumption of the proposed model is that the fractional hepatic
uptake of FFAs is the same regardless of whether FFAs reach the liver
via the portal vein or the hepatic artery; this assumption has not been
tested. If the fractional uptakes of FFA delivered to the liver via the
hepatic artery and via the portal vein are different, the proposed
model may not be applicable.
To address this issue, splanchnic palmitate kinetics were measured in
chronically catheterized, conscious dogs during studies that involved
collecting blood samples from the arterial, portal venous, and hepatic
venous circulation. Samples were collected under conditions that
resulted in a wide range of plasma FFA concentrations to test the model
under a variety of conditions. Measures of regional FFA concentration
and specific activity (SA), combined with specific measures of regional
blood flow, allowed us to test the proposed model for predicting the
proportion of hepatic FFA delivery that originates from visceral fat.
We also examined the relationship between direct measures of FFA
release into the portal vein and FFAs appearing in the hepatic vein.
 |
MATERIALS AND METHODS |
Preparation of infusates.
[9,10-3H]palmitate (New England Nuclear, Boston, MA) was
prepared for intravenous infusion by binding it to albumin in plasma collected from the dogs used in the studies.
Assays.
The concentration and SA of plasma palmitate were determined by a
modification (7) of a previously published HPLC method (11). Portal venous and hepatic arterial blood flows were
measured using Doppler flow probes (12).
Experimental design.
Experiments were conducted in a total of 16 dogs participating in three
different protocols for the collection of data over a wide range of FFA
concentrations. Permanent, indwelling catheters were surgically placed
in the hepatic vein, portal vein, and femoral artery in each dog ~17
days before study. The catheters were tunneled to a pouch under the
skin for future access. Doppler flow probes were placed around the
portal vein and hepatic artery during surgery. No omental or mesenteric
fat was removed. The dogs were allowed to recover fully before the
experiments. These studies were approved by the Vanderbilt
Institutional Animal Care and Use Committee and were designed primarily
to address issues of glucose metabolism; the FFA tracer studies were a
supplement to the protocols. In order not to impede the conduct of the
glucose metabolism studies, no experimental design modifications were
made specifically to achieve steady-state FFA concentrations and SA.
Thus, in only a portion of the studies were satisfactory FFA
steady-state conditions achieved. [3H]palmitate was
infused into a peripheral vein at a rate of ~0.3 µCi/min in each
animal throughout the duration of the study. The palmitate
tracer infusion was begun 30 min before collection of the first
blood sample to achieve isotopic steady state. For each of the 16 dogs
studied, a series of blood samples was collected over 20-60 min
during two study intervals. Approximately 1 ml of plasma was collected
from arterial, hepatic venous, and portal venous blood for assay at
each time point. Blood samples were immediately placed on ice, and the
plasma was separated by spinning in a refrigerated centrifuge. The
plasma was stored below or at
20°C until assayed. We have found
that these procedures prevent ex vivo lipolysis of lipoprotein
triglyceride provided that heparin is administered only in amounts
sufficient to maintain catheters patent (0.01 U
· kg
1 · min
1).
Steady state was considered to be present if arterial palmitate concentrations were stable in two to three samples over the 20- to
60-min period. Satisfactory steady-state conditions were obtained in a
total of eight animals (14 time intervals).
Problems with non-steady-state conditions prevented the use of data
from the other animals and other time intervals. In our previous
regional FFA kinetics studies (15), we found that even moderate deviations from steady-state conditions create
nonphysiological values for calculated rates of FFA uptake and release.
We have therefore limited the analysis in subsequent human studies and in this study to truly steady-state conditions. The coefficients of
variation of plasma palmitate concentrations and SA in these studies
were 5 ± 1 and 7 ± 1%, respectively, for the steady-state intervals.
There were three different protocol conditions for the animals included
in this study. The dogs in protocol A (a glycogen phosphorylase inhibitor study) underwent an 18-h fast before the study.
Concurrent with the glycogen phosphorylase inhibitor infusion, somatostatin was infused to suppress pancreatic hormone secretion and,
simultaneously, glucagon and basal insulin infusions were administered
into the portal vein. In dogs A-1 and A-3, basal insulin was given together with sufficient glucose to maintain euglycemia. In dog A-2 the study protocol was the same;
however, after the first sample collection interval, the insulin
infusion was discontinued, and therefore FFA concentrations were
increased due to insulin deficiency. The dog from protocol B
was also fasted for 18 h before the study, having received
intracerebroventricular leptin on the previous day. As in
protocol A, somatostatin was infused to suppress pancreatic
hormone secretion and, simultaneously, glucagon (0.5 ng · kg
1 · min
1)
and basal insulin (250 µU · kg
1 · min
1)
were administered into the portal vein together with glucose to
maintain euglycemia. After an initial sampling interval, the insulin
infusion was increased to 0.6 mU · kg
1 · min
1,
and the glucose infusion rate was adjusted to maintain euglycemia. The
dogs in protocol C were 18 h fasted and, after
collection of the initial series of blood samples, glucose was infused
into the portal vein at a rate of 2.5 mg · kg
1 · min
1.
Endogenous insulin secretion was allowed to increase naturally. There
were 1- to 2-h intervals between sample collection time points to allow
for reequilibration.
Calculations and statistical analysis.
The mean plasma palmitate concentration and SA in the arterial plasma,
portal venous plasma, and hepatic venous plasma for each time interval
were used for the calculations of regional palmitate uptake and
release. Hepatic arterial blood flow and portal blood flow values were
converted to plasma flow by use of the measured hematocrit for each
animal. Splanchnic plasma flow was the sum of the portal vein and
hepatic artery plasma flow.
The following definitions all relate to subsequent Eqs.
1-16.
arterial[palmitate] |
Concentration of palmitate (µmol/l) in arterial plasma
|
HV[palmitate] |
Concentration of palmitate (µmol/l) in hepatic venous plasma
|
PV[palmitate] |
Concentration of palmitate (µmol/l) in portal venous plasma
|
arterial[3H]palmitate |
Concentration of 3H palmitate (dpm/ml) in arterial plasma
|
HV[3H]palmitate |
Concentration of 3H palmitate (dpm/ml) in hepatic venous
plasma
|
PV[3H]palmitate |
Concentration of 3H palmitate (dpm/ml) in portal venous
plasma
|
PFHA |
Plasma flow in hepatic artery
|
PFPV |
Plasma flow in the portal vein
|
PFSPL |
Splanchnic plasma flow
|
Palmitate delivery to the liver (µmol/min) from the hepatic
artery is
|
(1)
|
Palmitate delivery to the liver (µmol/min) from the portal
vein is
|
(2)
|
Total hepatic palmitate delivery is the sum of Eqs. 1 and 2. The net uptake of palmitate by the splanchnic bed was
calculated as
|
(3)
|
The net hepatic palmitate uptake was calculated as
|
(4)
|
The fractional net hepatic palmitate uptake is
|
(5)
|
[3H]palmitate delivery to the liver (dpm/min) from
the hepatic artery is
|
(6)
|
[3H]palmitate delivery to the liver (dpm/min) from
the portal vein is
|
(7)
|
Total hepatic [3H]palmitate delivery is the sum of
Eqs. 6 and 7. The uptake of
[3H]palmitate by the splanchnic bed was calculated as
|
(8)
|
The hepatic [3H]palmitate uptake was calculated as
|
(9)
|
The fractional hepatic [3H]palmitate extraction is
|
(10)
|
Comparison of the net fractional palmitate uptake with
fractional [3H]palmitate extraction is used to assess the
validity of the proposed model. The fractional hepatic extraction of
[3H]palmitate can also be used to estimate the amount of
palmitate derived from visceral adipose tissue lipolysis that should
appear in hepatic venous plasma
|
(11)
|
The net release of palmitate by nonhepatic splanchnic tissues
(presumably visceral adipose tissue) was calculated as
|
(12)
|
Splanchnic palmitate uptake measured isotopically (as opposed to
net uptake) was calculated as
|
(13)
|
This calculation assesses the uptake of palmitate by
both hepatic and nonhepatic splanchnic tissues. Splanchnic palmitate release was calculated as
|
(14)
|
The uptake of palmitate by nonhepatic splanchnic tissues was
calculated as
|
(15)
|
Hepatic palmitate uptake is calculated by subtracting nonhepatic
splanchnic palmitate uptake from splanchnic palmitate uptake.
Visceral adipose tissue palmitate release was calculated as
|
(16)
|
Release of palmitate into the hepatic circulation was calculated
by subtracting the predicted hepatic vein palmitate appearance derived
from visceral fat (Eq. 11) from splanchnic palmitate release (Eq. 14).
All values are expressed as means ± SE unless otherwise
indicated. The model described in APPENDIX A was used to
estimate the percentage of palmitate delivered to the liver from
visceral adipose tissue lipolysis. Statistical comparisons between the model-predicted percent delivery of palmitate to the liver from visceral fat vs. the observed visceral palmitate delivery were made
with a paired t-test. Linear regression analysis was used to
assess the relationship between plasma palmitate concentrations and the
model-derived error in the proportion of palmitate delivered to the
liver from visceral adipose tissue lipolysis.
 |
RESULTS |
The average steady-state palmitate concentrations and SA in
arterial plasma, hepatic venous plasma, and portal venous plasma for
the 14 time intervals in the 8 dogs are presented in Table 1. Arterial concentrations were higher
than hepatic venous concentrations and tended to be lower than portal
palmitate concentrations.
The average splanchnic plasma flow was 410 ± 16 ml/min. Portal
venous plasma flow averaged 330 ± 17 ml/min, and hepatic arterial plasma flow was 81 ± 8 ml/min. The body weight and portal and hepatic artery plasma flow values are provided in Table
2.
Net palmitate balance.
Splanchnic palmitate delivery from the arterial circulation was
71.4 ± 10.5 µmol/min, and splanchnic palmitate exit via the hepatic vein was 62.6 ± 9.4 µmol/min, for a net splanchnic
palmitate uptake of 8.9 ± 1.7 µmol/min. Palmitate delivery to
the liver via the hepatic artery and portal vein averaged 13.5 ± 1.9 and 62.3 ± 10.2 µmol/min, respectively; total hepatic
palmitate delivery was 75.8 ± 11.6 µmol/min. The net hepatic
palmitate uptake was 13.4 ± 2.3 µmol/min, for a net fractional
hepatic palmitate extraction of 16 ± 2%. The net release of
palmitate across visceral fat was 4.2 ± 1.8 µmol/min.
Tracer-determined intrasplanchnic palmitate kinetics.
Total splanchnic palmitate uptake was 17.4 ± 2.3 µmol/min (mean
of all 14 values), of which the 4.3 ± 0.8 µmol/min portion was
nonhepatic splanchnic uptake and the 13.1 ± 2.3 µmol/min
portion was hepatic uptake. Individual values are provided in Table
3. Twenty-five ± 1% of
[3H]palmitate was taken up traversing the splanchnic bed,
18 ± 2% in the hepatic bed and 8 ± 2% in nonhepatic
splanchnic tissues. The fractional extraction of
[3H]palmitate across the liver was 21 ± 2%, which
was greater (P = 0.003) than the fractional extraction
of unlabeled palmitate (from arteriovenous concentration differences
alone) across the liver of 16 ± 2%.
Individual values for visceral and splanchnic palmitate release are
provided in Table 4. The palmitate
release from visceral adipose tissue was 7.9 ± 1.8 µmol/min,
accounting for 11 ± 2% of the palmitate delivered to the liver.
Splanchnic palmitate release was 8.7 ± 1.5 µmol/min. Because
~79% of [3H]palmitate delivered to the liver via the
portal vein and hepatic artery appeared in the hepatic vein, we
anticipated that palmitate release from the splanchnic bed would be
6.1 ± 1.4 µmol/min. This predicted value was less
(P < 0.01) than the observed splanchnic palmitate
release rate. To account for this discrepancy, 2.5 ± 0.6 µmol/min of palmitate would have been released into the hepatic circulation if there had been no differences in FFA extraction between
the portal and arterial circulation.
Visceral adipose tissue palmitate release ranged from ~0 to 21 µmol/min (Table 3), and splanchnic palmitate release ranged from 0.6 to 22 µmol/min. There was a good agreement between visceral adipose
FFA release measured by use of portal vein sampling and splanchnic FFA
release measured with hepatic vein sampling (Fig. 1).

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Fig. 1.
Visceral adipose tissue palmitate release is plotted vs.
splanchnic palmitate release measured during 14 study intervals from 8 dogs. Solid line, regression line between visceral and splanchnic
palmitate release; dotted line, line of identity (shown for comparison
purposes only). , , and
, Data from protocols A, B, and
C, respectively.
|
|
Model-predicted vs. measured hepatic palmitate delivery.
For all time intervals for which steady-state data were available, the
model (APPENDIX A) predicted that 15 ± 2% of hepatic
palmitate delivery was occurring from visceral adipose tissue
lipolysis. This is most readily apparent from Table 1; the hepatic vein
palmitate SA averages 15 ± 2% less than the matching arterial
palmitate SA. The model-predicted fractional delivery from visceral
lipolysis was greater (P = 0.004) than the percentage of palmitate delivery to the liver from visceral adipose tissue lipolysis that was measured with the portal vein and hepatic artery data (11 ± 2%). Despite the discrepancy, there was a generally good relationship between the predicted percent visceral FFA release from adipose tissue and the actual percent visceral FFA delivery to the
liver (Fig. 2). The relationship between
the model-predicted and observed values for percent hepatic palmitate
delivery was as follows: intercept = 3.6% [standard error of the
estimate (SEE) = 2.8%];
-coefficient = 1.11 (SEE = 0.25). Because the intercept of this relationship was not significantly
different from 0, but the slope was >1, we examined factors that might
relate to the error in the predicted hepatic palmitate delivery
to the liver. Figure 3 describes the
relationship between arterial plasma palmitate concentrations and the
error in the model-predicted hepatic palmitate delivery. There was a
significant (P < 0.05) inverse correlation between
arterial plasma palmitate concentrations. At low plasma palmitate
concentrations, the model significantly overpredicted the percentage of
hepatic palmitate delivery originating from visceral fat, whereas at
mid- and high-physiological concentrations there was reasonably good
agreement between the model-predicted and the observed percentages.

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Fig. 2.
Actual %hepatic palmitate delivery originating from
visceral adipose tissue free fatty acid (FFA) release is plotted vs.
%hepatic palmitate delivery from visceral fat predicted on the basis
of the model presented in APPENDIX A. Dashed line, line of
identity (shown for comparison purposes only). ,
, and , Data from protocols A,
B and C, respectively.
|
|

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Fig. 3.
Error of model-predicted hepatic palmitate delivery from
visceral fat is plotted as a function of the arterial palmitate
concentration. Error is calculated by subtracting the observed
%hepatic palmitate delivery from visceral fat from observed %hepatic
palmitate delivery predicted by the model presented in APPENDIX
A.
|
|
 |
DISCUSSION |
These studies were undertaken to assess the relationship between
visceral adipose tissue FFA release into the portal vein and splanchnic
FFA release by use of hepatic vein sampling. One goal was to determine
whether it is possible to predict the percentage of FFAs being
delivered to the liver from visceral adipose tissue lipolysis. We
tested a model that relied on the assumption that the FFAs delivered to
the liver from the hepatic artery and the portal venous circulation are
extracted equally and that there is no FFA released across the hepatic
bed. If this model were correct, there should have been no difference
between the observed and the model-predicted percentages of FFAs
delivered to the liver originating from visceral fat. Instead, we found
that the model overestimated the portion of visceral adipose tissue
FFAs being delivered to the liver. The error was greatest at lower FFA,
with errors of 4-14% at plasma palmitate concentrations below 100 µmol/l.
There appear to be at least two possible general explanations for the
discrepancy between the observed proportion of hepatic FFA delivery
originating from visceral adipose tissue lipolysis and that predicted
by the model. If FFAs delivered to the liver via the hepatic artery are
cleared more readily than those delivered via the portal circulation,
the observed splanchnic palmitate release would exceed that predicted
by the measured visceral adipose tissue palmitate uptake and hepatic
[3H]palmitate uptake data (see APPENDIX
B). Alternatively, FFA release directly into intrahepatic
circulation would give the same result. FFAs being released directly
from the liver would be surprising, given that the liver does not seem
equipped as a lipolytic organ. We could find no reports of expression
of hormone-sensitive lipase in adult liver tissue. Likewise, liver does
not express some of the other proteins important in the regulation of
trafficking and modulation of fatty acid release (e.g., perilipin-A).
An alternative possibility is that the hydrolysis of triglyceride-rich
lipoproteins by hepatic lipase is not accompanied by quantitative
uptake of the resulting fatty acids into hepatocytes. Some of the fatty acids generated by this process may escape into the hepatic venous circulation. We have observed the entry of chylomicron triglyceride fatty acids into the systemic FFA pool (15), indicating
that such a phenomenon is possible. We cannot distinguish between these possibilities given the present data.
APPENDIX B uses a numeric example to depict a scenario of a
model overestimate of the relative contribution of visceral adipose
tissue lipolysis to hepatic FFA delivery. If the relative uptake of
fatty acids delivered via the hepatic artery is greater than the uptake
of fatty acids delivered via the portal vein, the model overestimates
the percentage of FFAs delivered to the liver from visceral adipose
tissue lipolysis. This error could not be distinguished from
intrahepatic FFA release, and it is the pattern we observed
in the present study. We also considered the possibility that
differences in the ratio of arterial to total hepatic blood flow or
errors in blood flow measurement could account for the apparent model
errors. By use of regression analysis approaches similar to those
described to generate Fig. 3, there was no statistically significant
association between relative source of hepatic blood flow and the model error.
Although the model significantly overestimated the relative
contribution of visceral adipose tissue lipolysis to hepatic FFA delivery, it is reassuring to note that the error was primarily observed at low plasma FFA concentrations. With plasma FFA
concentrations closer to the overnight postabsorptive range, the
systematic error was much reduced (see Fig. 2). A perhaps surprising
observation is the good agreement between the FFAs released from
visceral adipose tissue lipolysis and splanchnic FFA release (Fig. 1). The data points fall around the line of identity, except perhaps at
very low rates of visceral FFA release, where splanchnic FFA release
was systematically greater than visceral FFA release. We expected that
splanchnic FFA release would be systematically less than visceral
adipose tissue FFA release. We take some comfort from the fact that the
model-derived values of the percentage of FFAs delivered to the liver
from visceral fat was highly correlated with the actual percentage of
FFAs delivered to the liver from visceral adipose tissue lipolysis.
Thus the splanchnic FFA release information obtained from hepatic vein
catheterization studies seems to provide good, albeit not perfect,
information about visceral adipose tissue FFA release, except at a time
of marked suppression of visceral lipolysis.
In this regard, we must reconsider results of some of our published
hepatic vein catheterization studies. We studied regional suppression
of FFA release by insulin (10) and by meals
(6) in humans (13). In some cases, we found
less relative suppression of splanchnic FFA release than of systemic
FFA release and concluded that visceral adipose tissue lipolysis was
more resistant to suppression than leg or upper body subcutaneous
adipose tissue. The results of the present study lead us to question
this assertion. Although splanchnic FFA release may account for a
greater share of systemic FFA release under insulin-suppressed
conditions, we cannot be certain that the "excess" FFAs actually
arise from visceral adipose tissue lipolysis. Despite this reservation,
the finding that the splanchnic bed becomes a net producer of FFAs
under hyperinsulinemic conditions (10) cannot be
attributed to differences in the hepatic uptake of FFAs delivered from
the hepatic artery vs. the portal vein. Only less suppression of
visceral adipose tissue lipolysis or substantial generation of FFAs
within the hepatic circulation could account for this observation.
Our results are in general concordance with the elegant studies
of intrasplanchnic FFA metabolism published by Basso and Havel (2). An exception is their conclusion that, because
hepatic extraction was not different between the
14C-labeled FFAs and titratable FFAs, there was no
release of FFAs across the hepatic circulatory bed. It is likely that
the greater precision of HPLC, which was used to measure both palmitate
concentration and SA in the present study, allowed us to detect the
relatively small discrepancy between these two values.
These studies involved a number of different protocols and insulin
conditions that allowed us to test the model over a range of FFA
availability. One concern, however, is that the experimental design
could affect factors that influence the FFA results, such as hepatic
lipase activity. For example, leptin administration to ob/ob
mice increases hepatic lipase activity (9), and one of the
dogs received intracerebroventricular leptin before the study. The data
points from this animal fell on the same regression lines (see Figs. 1
and 2) as those of the other animals, making it unlikely that the
leptin intervention had an effect in this leptin-sufficient animal.
Hepatic lipase activity has also been reported to change after 2 wk of
growth hormone (GH) therapy in GH-deficient adult humans
(14), but the short-term nature of these experiments makes
is unlikely that changes in hepatic lipase would have occurred via a GH
mechanism. It is also possible that the glycogen phosphorylase
inhibitor affected hepatic lipase activity or fractional palmitate
uptake from portal vs. arterial sources. The data presented in Figs. 1
and 2 do not indicate that animals in this protocol were different from
those that received only a glucose infusion. Other studies have shown
that insulin is not different from saline infusion as regards hepatic
lipase activity (1), indicating that the insulin infusions
likely did not affect this aspect of our study.
In summary, we performed studies of intrasplanchnic FFA kinetics in
dogs with hepatic vein, portal vein, and arterial catheters to
determine whether it is possible to model the fraction of FFAs delivered to the liver originating from visceral lipolysis. Studies were selected to include a wide range of FFA release rates to test the
model over the range of FFA concentrations encountered in metabolic
studies. We found that the model significantly overestimated the
portion of FFAs delivered to the liver from visceral lipolysis. Fortunately, there was a good correlation between the
model-predicted and observed visceral contribution to hepatic FFA
delivery. In addition, we found that splanchnic FFA release rates were
in good agreement with visceral adipose FFA release rates, except when lipolysis was suppressed. Under these circumstances, splanchnic FFA
release may overestimate visceral adipose tissue FFA release due either
to generation of a small amount of FFAs in the intrahepatic circulation
or differential hepatic extraction of FFAs from the arterial vs. portal
venous circulation, which are factors that do not appear to be
proportional to the prevailing plasma FFA concentration. Awareness of
this limitation to the result of hepatic vein catheterization studies
should help avoid overinterpretation of results.
 |
APPENDIX A |
Known values are
A |
Arterial FFA entering the splanchnic bed (µmol/min)
|
A* |
Arterial FFA exiting the splanchnic bed (µmol/min)
|
V* |
Visceral FFA exiting the splanchnic bed (µmol/min)
|
A is the product of arterial[palmitate] (µmol/l)
and splanchnic plasma flow (l/min), both measured values.
A* is known, because the arterial palmitate SA (dpm/µmol) entering
the splanchnic bed is measured and is not changed as FFAs pass through
the splanchnic bed (there is no addition of 3H or selective
removal of labeled or nonlabeled FFAs). Therefore, the appearance of
[3H]palmitate (dpm/min) into the hepatic vein divided by
the arterial [3H]palmitate SA (dpm/µmol) permits the
calculation of arterial palmitate exiting the splanchnic bed
(µmol/min).
V* is the FFAs (palmitate) exiting the splanchnic bed over and above
those passing through from the arterial circulation. This value is
equal to the exit of palmitate from the splanchnic bed
(HV[palmitate] × splanchnic plasma flow)
A* .
Unknown values are
A' |
Arterial FFAs delivered to the liver
|
V' |
Visceral FFAs delivered to the liver
|
Let y = the fraction of arterial FFAs taken up
by extrahepatic splanchnic tissues.
Let z = the fraction of FFA taken up by the liver.
It follows that
The proportion of FFAs delivered to the liver originating from
visceral fat equals
Because both V* and A* are known values, the proportion of FFAs
delivered to the liver that originates from visceral fat can
theoretically be determined.
More simply stated, this model assumes that the SA (or
enrichment in studies using stable isotopes) of FFAs appearing in the hepatic vein is equivalent to the average SA of FFAs delivered to the
liver from the portal vein and hepatic artery relative to their
respective plasma flows. Thus the fractional reduction in hepatic vein
FFA SA relative to the arterial FFA SA reflects the fraction of hepatic
FFA delivery originating from visceral lipolysis.
 |
APPENDIX B |
A numeric example is provided in Fig. B1 to depict how the model
proposed in APPENDIX A might perform if the fractional extraction of FFAs delivered from the hepatic artery were greater than
that delivered from the portal vein.
In this example, the arterial FFA SA is 1.0 dpm/nmol, and total
splanchnic FFA delivery is 1,000 nmol/min (200 nmol/min from the
hepatic artery and 800 nmol/min from the celiac, superior, and inferior
mesenteric arteries). The nonhepatic splanchnic tissues take up 200 nmol/min (and thus 200 dpm/min) of the 800 nmol/min to which they are
exposed, allowing 600 nmol/min and 600 dpm/min of arterially delivered
FFAs to enter into the portal vein. In this example, visceral adipose
tissue releases 600 nmol/min of FFAs (no radioactive FFAs) into the
portal vein.
In this hypothetical example, hepatic uptake of arterially delivered
FFAs is greater (100%, i.e., 200 nmol/min and 200 dpm/min) than portal
vein FFA uptake (33%, i.e., 400 nmol/min and 200 dpm/min). In this
case, the model outlined in APPENDIX A will overestimate the proportion of FFAs delivered to the liver from visceral adipose tissue. In this numeric example, total hepatic FFA delivery is 1,400 nmol/min, i.e., 800 from arterial sources and 600 (42.9%) from
visceral lipolysis. The appearance of total FFAs in the hepatic vein is
800 nmol/min, and the appearance of radioactive FFAs in the hepatic
vein is 400 dpm/min. The model would predict that 50% of hepatic FFA
delivery is of visceral adipose tissue origin and would thus be incorrect.
 |
ACKNOWLEDGEMENTS |
This study was supported by National Institute of Diabetes and
Digestive and Kidney Diseases Grants DK-40484 and DK-14507 and by the
Mayo Foundation.
 |
FOOTNOTES |
Address for reprint requests and other correspondence:
M. D. Jensen, Endocrine Research Unit, 5-194 Joseph,
Mayo Clinic, Rochester, MN 55905 (E-mail:
jensen.michael{at}mayo.edu).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpendo.00268.2002
Received 18 June 2002; accepted in final form 18 February 2003.
 |
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