Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0615
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The aim of this study was to determine the effect of high levels of free fatty acids (FFA) and/or hyperglycemia on hepatic glycogenolysis and gluconeogenesis. Intralipid was infused peripherally in 18-h-fasted conscious dogs maintained on a pancreatic clamp in the presence (FFA + HG) or absence (FFA + EuG) of hyperglycemia. In the control studies, Intralipid was not infused, and euglycemia (EuG) or hyperglycemia (HG) was maintained. Insulin and glucagon were clamped at basal levels in all four groups. The arterial blood glucose level increased by 50% in the HG and FFA + HG groups. It did not change in the EuG and FFA + EuG groups. Arterial plasma FFA increased by ~140% in the FFA + EuG and FFA + HG groups but did not change significantly either in the EuG or HG groups. Arterial glycerol levels increased by ~150% in both groups. Overall (3-h) net hepatic glycogenolysis was 196 ± 26 mg/kg in the EuG group. It decreased by 96 ± 20, 82 ± 16, and 177 ± 22 mg/kg in the HG, FFA + EuG, and FFA + HG groups, respectively. Overall (3-h) hepatic gluconeogenic flux was 128 ± 22 mg/kg in the EuG group, but it was suppressed by 30 ± 9 mg/kg in response to hyperglycemia. It was increased by 59 ± 12 and 56 ± 10 mg/kg in the FFA + EuG and FFA + HG groups, respectively. In conclusion, an increase in plasma FFA and glycerol significantly inhibited hepatic glycogenolysis and markedly stimulated hepatic gluconeogenesis.
free fatty acid; hyperglycemia; glycogenolysis; gluconeogenesis
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
AN INCREASE IN FREE FATTY ACIDS (FFA) stimulates hepatic gluconeogenesis (10, 14, 37). In vitro studies have shown that perfusion of rat liver with lipid increases gluconeogenesis (36, 37) and that an increase in FFA oxidation stimulates the activities of key enzymes in the gluconeogenic pathway (3, 26). In accord with this, Boden and Jadali (5) reported that a rise in plasma FFA increased hepatic glucose production (HGP) in normal human subjects. In addition, Saloranta et al. (30) showed that an increase in FFA and glycerol availability, brought about by Intralipid infusion, increased gluconeogenesis and glucose production in type 2 diabetic patients. In contrast, in a recent study, Roden et al. (27) did not detect a change in HGP in response to an increase in FFA in normal humans. Likewise, Johnston et al. (18) reported that an acute increase in the plasma FFA level did not change HGP in type 2 diabetic subjects. In agreement with the latter, Puhakainen and colleagues (24, 25) showed that a decrease in plasma FFA, although it reduced gluconeogenesis, did not change HGP in patients with type 2 diabetes. Most recently, Boden et al. (4) showed in both normal and type 2 diabetic subjects that increasing and decreasing plasma FFA stimulated and inhibited gluconeogenesis, respectively, but did not alter glucose production. In another recent study, Stingl et al. (35) showed that in normal humans HGP and glycogenolysis both decreased in response to an increase in plasma FFA level. It is obvious from the above discordance that the effect of an increase in FFA availability on HGP in vivo, as well as its gluconeogenic and glycogenolytic components, requires further investigation.
Previous studies by Shulman et al. (32) and Chu et al. (7) showed that hyperglycemia significantly inhibited HGP in the dog, even when basal levels of insulin and glucagon were maintained. In accord with this, Sacca et al. (29) reported that hyperglycemia had an inhibitory effect on glucose production in the presence of fixed basal pancreatic hormone concentrations in the overnight-fasted human.
Using the rat and dog, respectively, Rossetti et al. (28) and Sindelar et al. (34) showed that hyperglycemia inhibits glucose production through an effect on hepatic glycogenolysis. In line with this, Petersen et al. (23) reported that hyperglycemia significantly inhibited net hepatic glycogenolysis in the presence of a pancreatic clamp in humans. The available data, therefore, indicate clearly that hyperglycemia per se can inhibit hepatic glycogenolysis. An earlier study by Shulman et al. (32) in the dog and a recent study by Hellerstein et al. (17) in the rat indicated that hyperglycemia can also decrease hepatic gluconeogenesis. On the other hand, recent studies by Rossetti et al. in the rat and Sindelar et al. in the dog could not confirm this conclusion. Data from in vitro studies are similarly conflicting. A study in isolated rat hepatocytes by Sanchez-Gutierrez et al. (31) indicated that hyperglycemia significantly inhibited gluconeogenesis. A study by Davidson (12), on the other hand, suggested that hyperglycemia did not do so. It is also of interest, therefore, to investigate the effect of hyperglycemia on hepatic gluconeogenesis.
It is known that chronic hyperlipidemia increases insulin resistance in various animal models and humans. An increase in plasma FFA decreases the effect of insulin on the liver and peripheral tissues (muscle and adipose tissues), which in turn results in hyperglycemia in these subjects. In fact, patients with type 2 diabetes have long been known to exhibit hyperglycemia and hyperlipidemia concurrently. Because hyperlipidemia stimulates HGP while hyperglycemia inhibits it, it is of interest to examine the interaction between high plasma FFA levels and hyperglycemia in controlling hepatic glycogenolysis and gluconeogenesis in vivo.
The aims of the present study, therefore, were to determine the effects of hyperlipidemia alone, hyperglycemia alone, and hyperlipidemia plus hyperglycemia on hepatic glycogenolysis and gluconeogenesis in vivo in the presence of fixed basal levels of insulin and glucagon.
![]() |
METHODS AND MATERIALS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Experiments were carried out on 24 18-h-fasted conscious mongrel dogs (20-30 kg) of either sex that had been fed a standard diet of meat and chow described elsewhere (7-9). The animals were housed in a facility that met American Association for the Accreditation of Laboratory Animal Care guidelines, and the protocols were approved by the Vanderbilt University Medical Center Animal Care Committee.
A laparotomy was performed 16-18 days before each experiment to implant catheters and Doppler flow probes into or around appropriate blood vessels, as described elsewhere (7-9). Each dog was used for only one experiment. All dogs studied had 1) a leukocyte count <18,000/mm3, 2) a hematocrit >35%, 3) a good appetite, and 4) normal stools.
Each experiment consisted of a 100-min tracer equilibration and hormone
adjustment period (140 to
40 min), a 40-min basal period (
40 to 0 min), and a 180-min test period (0-180 min). In all studies, a
priming dose of purified [3-3H]glucose (42 µCi) was
given at
140 min, followed by a constant infusion of
[3-3H]glucose (0.35 µCi/min),
[U-14C]alanine (0.35 µCi/min), and indocyanine green
(ICG, 0.1 mg · m
2 · min
1).
An infusion of somatostatin (0.8 µg · kg
1 · min
1) was
started at
130 min to inhibit endogenous insulin and glucagon secretion. Concurrently, intraportal replacement infusions of insulin
(300 µU · kg
1 · min
1) and
glucagon (0.5 ng · kg
1 · min
1) were
started. The plasma glucose level was monitored every 5 min, and
euglycemia was maintained by adjusting the rate of insulin infusion.
The final alteration in the insulin infusion rate was made at least 30 min before the start of the basal period, and the rate of insulin
infusion (mean of 235 µU · kg
1 · min
1) remained
unchanged thereafter. The study included four groups: EuG, FFA + EuG,
hyperglycemia (HG), and FFA + HG (Fig.
1). Intralipid (0.02 ml · kg
1 · min
1, 20% fat
emulsion; Pharmacia & Upjohn) and heparin (0.5 U · kg
1 · min
1) were
infused during the test period via the right saphenous vein in the
FFA + EuG and FFA + HG groups. Dextrose (20%) was infused to clamp
the arterial blood glucose level at 120-130 mg/dl (equal to
165-178 mg/dl plasma glucose) via the right cephalic vein in the
HG and FFA + HG groups. In the EuG group, saline was infused
peripherally during the test period. Data from five of the six dogs in
each of the EuG and HG groups were published previously in the context
of another study (7, 33). The data are reproduced here to
facilitate comparison of the groups.
|
Plasma and blood glucose, plasma [3H]- and
[14C]glucose, blood lactate, glycerol,
-hydroxybutyrate (
-OHB), alanine, glutamine, glutamate, glycine,
serine, threonine, and plasma FFA were determined using previously
described methods (7-9). The levels of insulin, glucagon, cortisol, epinephrine, and norepinephrine were also determined as described elsewhere (7-9). Transonic
flow probes and ICG were used to measure total hepatic blood flow
(7-9). Because a high level of Intralipid interferes
with the measurement of ICG in plasma, the net hepatic balance and
fractional extraction of metabolites were calculated using
Transonic-determined flow. The net hepatic balance and fractional
extraction of blood glucose, lactate, glycerol,
-OHB, alanine, other
gluconeogenic amino acids, and plasma FFA in the present study were
calculated using arteriovenous difference (a-v) methods described
elsewhere (7-9).
Total glucose production and utilization were determined using
both one- and two-compartment models, as previously described (7-9). The results were similar regardless of which
approach was employed because the deviations from steady state were
minimal. The glucose production and utilization data shown in Figs.
1-9 and Tables
1-3
are those calculated using the two-compartment model. It
should also be noted, since the kidneys produce a small amount of
glucose, that the rate of endogenous glucose production determined by
the tracer method slightly (0.2 mg · kg1 · min
1)
overestimates total hepatic glucose release (20).
Gluconeogenic efficiency was assessed using a double-isotope technique
described elsewhere (7-9). Because the conversion of
[14C]alanine to [14C]glucose by the kidney
is minimal (21), [14C]glucose production in
our study was almost exclusively attributable to the liver. The hepatic
gluconeogenic flux from circulating gluconeogenic precursors was
calculated using the methods described previously
(7-9). Briefly, the net hepatic balances of the
gluconeogenic precursors alanine, glycine, serine, threonine,
glutamine, glutamate, lactate, and glycerol were measured in the
present study. The net hepatic balance of pyruvate was assumed to be
10% of lactate balance. The hepatic gluconeogenic flux rate was
calculated by dividing the above uptake rates by two to account
for the incorporation of the C-3 precursors into the C-6 glucose
molecule. Net hepatic glycogenolysis was calculated as follows
(7-9)
![]() |
![]() |
|
|
|
|
|
|
|
|
|
|
|
Statistical analysis. All statistical comparisons were made using repeated-measures ANOVA with post hoc analysis by univariate F-tests or the paired Student's t-test where appropriate. Statistical significance was accepted at P < 0.05. Data are expressed as means ± SE.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Hormone levels and hepatic blood flow. The arterial plasma levels of insulin and glucagon remained unchanged and at basal levels in all groups (Fig. 2) as did the arterial plasma levels of epinephrine, norepinephrine, and cortisol (Fig. 2). Hepatic blood flow also remained unchanged and equivalent in all groups (Table 1).
Arterial blood levels, net hepatic balances, and fractional
extractions of FFA, -OHB, and glycerol.
The arterial plasma level of FFA increased from 746 ± 69 to
1,850 ± 245 (P < 0.05) and from 746 ± 131 to 1,817 ± 189 (P < 0.05) µmol/l in the
FFA + EuG and FFA + HG groups, respectively, during Intralipid
infusion (Fig. 3). FFA levels remained unchanged in the EuG and HG
groups. The net hepatic uptake of FFA increased from 2.5 ± 0.3 to
4.5 ± 0.5 (P < 0.05) and from 2.6 ± 0.5 to
4.5 ± 0.9 (P < 0.05)
µmol · kg
1 · min
1 in the
FFA + EuG and FFA + HG groups, respectively (Fig. 3). It did not
change significantly in the EuG and HG groups. Net hepatic fractional
extraction of FFA did not change significantly in any group (Fig. 3).
Glucose kinetics.
The arterial blood glucose level increased from 79 ± 4 to
121 ± 2 and 78 ± 3 to 131 ± 1 mg/dl in the HG and
FFA + HG groups, respectively (Fig. 5). It remained unchanged in the
EuG and FFA + EuG groups. Net hepatic glucose output did not change
significantly in the EuG and FFA + EuG groups (Fig. 5). In response
to hyperglycemia alone, net hepatic glucose output decreased by
1.5 ± 0.4 mg · kg1 · min
1 (from
2.2 ± 0.3 to 0.7 ± 0.5 mg · kg
1 · min
1;
P < 0.05) by 90 min and remained reduced thereafter.
In response to infusion of Intralipid and glucose, net hepatic glucose
output decreased by 1.7 ± 0.3 mg · kg
1 · min
1
(P < 0.05; Fig. 5). The changes in tracer-determined
glucose production paralleled those in net hepatic glucose balance
(Fig. 5). Glucose utilization in the EuG and FFA + EuG groups did not change significantly (Table 2). On the other hand, glucose utilization increased moderately from 2.4 ± 0.3 to 3.4 ± 0.4 (P < 0.05) and from 2.8 ± 0.3 to 3.7 ± 0.4 (P < 0.05)
mg · kg
1 · min
1 by 120 min
in the HG and FFA + HG groups, respectively (Table 2). Glucose
clearance decreased slightly from 2.5 ± 0.2 to 2.0 ± 0.1 (P < 0.05) and from 2.7 ± 0.2 to 2.2 ± 0.2 (P < 0.05)
ml · kg
1 · min
1,
respectively, by 90 min in the FFA + EuG and FFA + HG groups (Table
2), but it did not change significantly in EuG and HG groups.
Arterial blood levels and net hepatic balances of lactate, alanine,
glutamate, glutamine, glycine, serine, and threonine.
The arterial blood lactate level did not change significantly in any
group (Fig. 6). Net hepatic lactate balance slowly decreased by
2.4 ± 1.3 µmol · kg1 · min
1 in the
EuG group (Fig. 6). In response to Intralipid infusion in the absence
of hyperglycemia, net hepatic lactate balance decreased significantly
(7.1 ± 2.3 µmol · kg
1 · min
1,
P < 0.05). In the presence of hyperglycemia alone, net
hepatic lactate balance increased (5.4 ± 1.6 µmol · kg
1 · min
1). When
plasma FFA levels rose in the presence of hyperglycemia, net hepatic
lactate balance decreased significantly (5.9 ± 1.6 µmol · kg
1 · min
1).
Gluconeogenesis and glycogenolysis.
The hepatic gluconeogenic flux rate did not change significantly
(0.6 ± 0.1 to 0.8 ± 0.2 and 0.8 ± 0.1 to 0.7 ± 0.2 in the EuG and HG groups, respectively; Fig. 7A) in the
absence of increased FFA. In response to Intralipid infusion, the
gluconeogenic flux rate increased from 0.6 ± 0.1 to 1.3 ± 0.2 (P < 0.05) and from 0.6 ± 0.1 to 1.2 ± 0.2 (P < 0.05)
mg · kg1 · min
1 in the
FFA + EuG and FFA + HG groups, respectively (Fig. 7A). The overall amount of glucose produced by hepatic gluconeogenic flux
during the 3-h test period was 128 ± 22 mg/kg in the EuG group
(Fig. 7A). In response to hyperglycemia, the overall amount of glucose produced by hepatic gluconeogenic flux was reduced by 30 ± 9 mg/kg (the area between HG and EuG curves in Fig.
7B; not significant) or 23% inhibition (Fig. 8). Elevated
plasma FFA increased overall gluconeogenic flux by 59 ± 12 mg/kg
(P < 0.05) or 46% stimulation (Fig. 8). In response
to combined hyperglycemia and hyperlipidemia, overall gluconeogenic
flux was increased by 56 ± 10 mg/kg (P < 0.05)
or 44% (Fig. 8).
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In the present study, we were able to directly examine the effect of an increase in plasma FFAs and glycerol alone or in the presence of hyperglycemia on hepatic glycogenolysis and gluconeogenesis in vivo. The arterial levels of insulin, glucagon, epinephrine, norepinephrine, and cortisol remained at basal values in all groups, thereby simplifying data interpretation. The increments in plasma FFAs and glycerol were indistinguishable in the FFA + EuG and FFA + HG groups, as were the increments in arterial glucose concentrations in the HG and FFA + HG groups.
Under euglycemic conditions in the control group, net hepatic glucose
output and net hepatic glycogenolysis decreased slightly over the
course of the study. The overall contribution of net hepatic
glycogenolysis to HGP was 196 ± 26 mg/kg over the 3-h test
period. Gluconeogenesis did not change significantly during the
study, and overall hepatic gluconeogenic flux was 128 ± 22 mg/kg
(~40% of HGP). Hyperglycemia alone significantly decreased net
hepatic glucose output from 2.2 ± 0.3 to 0.7 ± 0.5 mg · kg1 · min
1
(P < 0.05) as the result of a 49% reduction in net
hepatic glycogenolysis and a 23% reduction in hepatic gluconeogenic
flux. Our data therefore demonstrate that moderate hyperglycemia per se
can significantly inhibit hepatic glycogenolysis while having only a
modest effect on gluconeogenesis. In previous studies, Rossetti et al.
(28), Chu and colleagues (7, 9), and Sindelar
et al. (34), among others, showed that hyperglycemia can
inhibit glucose production by means of a decrease in hepatic
glycogenolysis. The effect of hyperglycemia on gluconeogenesis
has been more controversial. An earlier study by Shulman et al.
(32) showed that, in the 36-h fasted dog with fixed basal
insulin and glucagon, hyperglycemia significantly decreased the
conversion of [14C]alanine and [14C]lactate
to [14C]glucose. Net hepatic lactate uptake was
significantly inhibited (>80%) by the hyperglycemia, whereas net
hepatic alanine uptake was not changed significantly. Because the net
hepatic lactate uptake accounted for ~55% of net hepatic
gluconeogenic flux in the study by Shulman et al., it is obvious that
hyperglycemia decreased net hepatic gluconeogenic flux ~45%. In a
recent study, Hellerstein et al. (17) reported that, in
20-h fasted rats (no hormone data shown), hyperglycemia (glucose
infusion of 35 mg · kg
1 · min
1 iv)
significantly inhibited HGP (10.3 ± 0.6 vs. 3.6 ± 0.5 mg · kg
1 · min
1; control
group vs. iv glucose infusion) and gluconeogenesis (8.0 ± 0.6 vs.
3.4 ± 0.5 mg · kg
1 · min
1; control
group vs. iv glucose infusion). Using isolated rat hepatocytes, Sanchez-Gutierrez et al. (31) demonstrated a significant
positive correlation between the glucose level in the medium and the
inhibition of gluconeogenesis.
Rossetti et al. (28), on the other hand, reported that, in
6-h fasted rats maintained with basal levels of insulin and glucagon, hyperglycemia (plasma glucose 240 mg/dl) significantly decreased HGP
and glycogenolysis but had no effect on gluconeogenesis. Sindelar et
al. (34) showed that, in overnight-fasted dogs, modest
hyperglycemia (blood glucose
140 mg/dl) abolished net hepatic
glucose output but did not inhibit the gluconeogenic rate. In the study
by Rossetti et al., gluconeogenesis was estimated only at the last time
point (120 min) using the PEP specific activity/uridine
diphosphoglucose specific activity method. In the study by
Sindelar et al., the net hepatic balances of gluconeogenic amino acids
were only measured in the last 30 min of test period, thereby
preventing the contribution of gluconeogenesis to HGP over the
whole test period from being calculated. It is possible, therefore,
that an earlier inhibitory effect of hyperglycemia on the overall
contribution of hepatic gluconeogenesis to HGP could have been
overlooked in those two studies.
In response to an increase in plasma FFA and glycerol, net hepatic
glucose output did not change significantly, but net hepatic glycogenolysis decreased from 2.0 ± 0.2 to 1.3 ± 0.2 mg · kg1 · min
1
(P < 0.05, a 35% inhibition). The overall
contribution of net hepatic glycogenolysis to HGP over 3 h
decreased significantly by 82 ± 16 mg/kg (P < 0.05; a 42% inhibition). Gluconeogenic flux, on the other
hand, increased significantly from 0.6 ± 0.1 to 1.3 ± 0.2 mg · kg
1 · min
1
(P < 0.05; a 2-fold stimulation). The overall
contribution of hepatic gluconeogenic flux increased by 59 mg/kg
(P < 0.05; a 46% stimulation). The combination of
hyperglycemia and an increase in plasma FFA and glycerol completely
abolished net hepatic glycogenolysis (1.7 ± 0.2 to
0.1 ± 0.2 mg · kg
1 · min
1,
P < 0.05) such that it decreased by 177 ± 22 mg/kg (P < 0.05; 90% inhibition). Gluconeogenic flux,
on the other hand, increased from 0.6 ± 0.1 to 1.2 ± 0.2 mg · kg
1 · min
1
(P < 0.05; 2-fold stimulation), and the overall
contribution of hepatic gluconeogenic flux increased by 56 ± 10 mg/kg (P < 0.05; a 44% stimulation). It should be
noted that the increased FFA levels may have decreased glucose
oxidation in the liver. To the extent they did so we would have
overestimated net hepatic glycogenolysis since we assumed a fixed
glucose oxidation rate. Given the low basal rate of glucose
oxidation, this effect would have been very small (<0.2
mg · kg
1 · min
1). If
this occurred, the inhibition of glycogenolysis that occurred in the
FFA and FFA + hyperglycemia protocols may in reality have been
slightly greater than estimated. In both Intralipid protocols, the
blood glycerol level rose by more than twofold. This gave rise to
increases in net hepatic glycerol uptake of
1.8
µmol · kg
1 · min
1, which
could have accounted for an increase in gluconeogenesis of 0.16 mg · kg
1 · min
1. Assuming
that all of the glycerol taken up by the liver was converted to
glucose, the rise in glycerol uptake could account for no more than
40% of the increase in gluconeogenesis. It seems most likely,
therefore, that the rise in gluconeogenesis was primarily attributable
to the rise in net hepatic FFA uptake. A similar conclusion was reached
in the study by Stingl et al. (35). Taken together, the
above findings demonstrate that an acute simulated rise in lipolysis
that increased plasma FFA and glycerol significantly inhibited hepatic
glycogenolysis but markedly stimulated gluconeogenesis. High
FFA, glycerol, and hyperglycemia have an additive effect in inhibiting
hepatic glycogenolysis. Interestingly, the stimulatory effects of FFA
and glycerol on gluconeogenesis appeared to overcome the small
inhibitory effect of hyperglycemia on the process.
In the present study, net hepatic lactate balance decreased slowly over time under euglycemic conditions. Hyperglycemia, on the other hand, caused an increase in net hepatic lactate output. The likely explanation for this finding is that, in response to hyperglycemia, hepatic glucose uptake increased, and as a result the intracellular glucose and glucose 6-phosphate levels rose, which in turn increased glycolytic flux. On the other hand, in the presence of euglycemia, high plasma FFA levels resulted in an increase in net hepatic lactate uptake consistent with a stimulation of gluconeogenesis. Elevation of plasma FFA levels completely eliminated the ability of hyperglycemia to cause net hepatic lactate output, again suggesting that an increase in plasma FFA levels stimulated hepatic gluconeogenesis and inhibited glycolysis.
Earlier studies in vitro (14, 36, 37) showed that FFA could stimulate gluconeogenesis in the perfused rat liver. The increase in FFA availability elevated FFA oxidation by the liver. This in turn stimulated gluconeogenesis via increases in the NADH-to-NAD+ ratio, in acetyl-CoA production, and in ATP production. NADPH generates reducing equivalents for the gluconeogenesis process. Acetyl-CoA stimulates hepatic gluconeogenesis through an activation of pyruvate carboxylase and an increase in citrate concentration. The latter can inhibit phosphofructokinase and thereby inhibit glycolysis (11, 22). Several earlier in vivo studies (5, 10, 13, 30) have shown that an elevated FFA level resulting from Intralipid infusion can increase gluconeogenesis in the human.
Although it has been generally accepted that an increase in FFA stimulates gluconeogenesis, the direct effects of plasma FFA on hepatic glycogenolysis have not been determined. As a result, the effects of plasma FFAs on overall HGP have been controversial. Clore et al. (10) and Johnston et al. (18) reported that an increase in plasma FFA level resulting from the infusion of Intralipid did not change HGP in normal humans or type 2 diabetic subjects. Puhakainen and colleagues (24, 25) and Lee et al. (19) reported that, in the absence of pancreatic clamp, a decrease in the FFA level by infusion of acipimox reduced hepatic gluconeogenesis but did not change HGP. These authors attributed their observations to a hepatic autoregulatory mechanism, which resulted in an increase in glycogenolysis that offset the fall in gluconeogenesis. An alternative explanation for the above results would be that FFAs have a direct inhibitory effect on hepatic glycogenolysis.
Ferrannini et al. (13) reported that, in normal humans in the presence of a hyperglycemic hyperglucagonemic hypoinsulinemic condition, HGP was higher in the presence of elevated FFA levels than in the presence of normal FFA levels. These authors suggested that FFAs stimulate gluconeogenesis and increase HGP. It should be noted that, since no insulin was infused and the glucagon level was clamped, the combination of these changes in the pancreatic hormones would have significantly stimulated hepatic glycogenolysis, which would itself have caused an increase in HGP. Therefore, the inhibitory effect of a high plasma FFA concentration on hepatic glycogenolysis could have been masked by the effects of changes in the insulin and glucagon levels. A recent study by Roden et al. (27) showed that, in normal humans infused with somatostatin, basal insulin, and glucagon, HGP did not change significantly in the presence or absence of high plasma FFA levels, but the gluconeogenic contribution to glucose production increased. In another recent study, Stingl et al. (35) reported that an increase in plasma FFA decreased glycogenolysis and glucose production in healthy men in the absence of a pancreatic clamp. Because in this study the plasma insulin level in the high FFA group was twofold greater than in the saline group, the decrease in glycogenolysis and glucose production observed in the high FFA group could have been attributable to the inhibitory effect of elevated insulin rather than the increase to FFA per se.
The present study showed that net hepatic glycogenolysis was inhibited by 48, 43, and 94% in response to hyperglycemia, high levels of FFA and glycerol, and hyperglycemia + high levels of FFA and glycerol, respectively. These data indicate that there is an additive effect of hyperglycemia, high FFA, and glycerol concentrations on the inhibition of hepatic glycogenolysis. They also indicate that the inhibition caused by hyperglycemia and increased lipolysis might occur via different mechanisms. Studies in vitro (1, 2) and in vivo (6) showed that hyperglycemia per se induces a translocation of hepatic glucokinase to the cytosol, thereby increasing the activity of glucokinase, which in turn increases HGP levels and glycogen synthase activity (15) and increases glycogen synthesis. Hyperglycemia can also work through a mass action mechanism to push more glucose into the hepatocytes. The inhibitory effects of high FFA and glycerol on net hepatic glycogenolysis appear to be mediated via their stimulatory effects on gluconeogenesis and/or an inhibitory effect on glycolysis.
In conclusion, 1) an increase in plasma FFA and glycerol significantly inhibits net hepatic glycogenolysis and markedly stimulates gluconeogenic flux, without changing HGP, 2) hyperglycemia significantly inhibits net hepatic glycogenolysis and stimulates hepatic glycolysis, but modestly inhibits gluconeogenesis, 3) simultaneous increases in plasma FFA, glycerol, and glucose have an additive effect on the inhibition of net hepatic glycogenolysis, and 4) an increase in plasma FFA and glycerol can overcome the inhibitory effect of hyperglycemia on hepatic gluconeogenic flux.
![]() |
ACKNOWLEDGEMENTS |
---|
We appreciate assistance from Jon Hastings, Melanie Scott, Wanda Snead, and Paul Flakoll.
![]() |
FOOTNOTES |
---|
Part of this work was presented at the 57th Annual Meeting of the American Diabetes Association, San Antonio, TX, in June 2000.
This work was supported in part by National Institute of Diabetes and Digestive and Kidney Diseases Grants 2RO1 DK-18243 and 5P60 DK-20593 (Diabetes Research and Training Center).
Address for reprint requests and other correspondence: A. D. Cherrington, Dept. of Molecular Physiology and Biophysics, 702 Light Hall, Vanderbilt Univ. School of Medicine, Nashville, TN 37232-0615 (E-mail: alan.cherrington{at}mcmail.vanderbilt.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.00136.2001
Received 20 March 2001; accepted in final form 28 September 2001.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Agius, L,
and
Peak M.
Intracellular binding of glucokinase in hepatocytes and translocation by glucose, fructose and insulin.
Biochem J
296:
785-796,
1993[ISI][Medline].
2.
Agius, L,
Peak M,
Newgard CB,
Gomez-Foix AM,
and
Guinovart JJ.
Evidence for a role of glucose-induced translocation of glucokinase in the control of hepatic glycogen synthesis.
J Biol Chem
271:
30479-30486,
1996
3.
Bahl, JJ,
Matsuda M,
DeFronzo RA,
and
Bressler R.
In vitro and in vivo suppression of gluconeogenesis by inhibition of pyruvate carboxylase.
Biochem Pharmacol
53:
67-74,
1997[ISI][Medline].
4.
Boden, G,
Chen XH,
Capulong E,
and
Mozzoli M.
Effects of free fatty acids on gluconeogenesis and autoregulation of glucose production in type 2 diabetes.
Diabetes
50:
810-816,
2001
5.
Boden, G,
and
Jadali F.
Effects of lipid on basal carbohydrate metabolism in normal men.
Diabetes
40:
686-692,
1991[Abstract].
6.
Chu, CA,
Igawa K,
Fujimoto Y,
Pan A,
Cherrington D,
and
Shiota M.
Hyperglycemia plays a dominant role in the regulation of hepatic glucokinase translocation.
Diabetes
48, Suppl:
A49,
1999[ISI].
7.
Chu, CA,
Sindelar DK,
Neal DW,
Allen EJ,
Donahue EP,
and
Cherrington AD.
Comparison of the direct and indirect effects of epinephrine on hepatic glucose production.
J Clin Invest
99:
1044-1056,
1997
8.
Chu, CA,
Sindelar DK,
Neal DW,
and
Cherrington AD.
Direct effects of catecholamines on hepatic glucose production in conscious dog are due to glycogenolysis.
Am J Physiol Endocrinol Metab
271:
E127-E136,
1996
9.
Chu, CA,
Sindelar DK,
Neal DW,
and
Cherrington AD.
The direct effect of norepinephrine on hepatic glucose production in conscious dog.
Am J Physiol Endocrinol Metab
274:
E162-E171,
1998
10.
Clore, JN,
Glickman PS,
Nestler JE,
and
Blackard WG.
In vivo evidence for hepatic autoregulation during FFA-stimulated gluconeogenesis in normal man.
Am J Physiol Endocrinol Metab
261:
E425-E429,
1991
11.
Colombo, G,
Tate PW,
Girotti AW,
and
Kemp RG.
Interaction of inhibitors with muscle phosphofructokinase.
J Biol Chem
250:
9404-9412,
1975[Abstract].
12.
Davidson, MB.
Autoregulation by glucose of hepatic glucose balance: permissive effect of insulin.
Metabolism
30:
279-284,
1981[ISI][Medline].
13.
Ferrannini, E,
Barrett EJ,
Bevilacqua S,
and
DeFronzo RA.
Effect of fatty acids on glucose production and utilization in man.
J Clin Invest
72:
1737-1747,
1983[ISI][Medline].
14.
Friedman, B,
Goodman EH,
and
Weinhouse S.
Effects of insulin and fatty acids on gluconeogenesis in the rat.
J Biol Chem
242:
3620-3627,
1967
15.
Gomis, RR,
Ferrer JC,
and
Guinovart JJ.
Shared control of hepatic glycogen synthesis by glycogen synthase and glucokinase.
Biochem J
351:
811-816,
2000[ISI][Medline].
16.
Hamilton, KS,
Gibbons FK,
Bracy DP,
Lacy DB,
Cherrington AD,
and
Wasserman DH.
Effect of prior exercise on the partitioning of an intestinal glucose load between splanchnic bed and skeletal muscle.
J Clin Invest
98:
125-135,
1996
17.
Hellerstein, M,
Neese R,
Schwarz J,
Turner S,
Faix D,
and
Wu K.
Altered fluxes responsible for reduced hepatic glucose production and gluconeogenesis by exogenous glucose in rats.
Am J Physiol Endocrinol Metab
272:
E163-E172,
1997
18.
Johnston, P,
Hollenbeck C,
Sheu W,
Chen YD,
and
Reaven GM.
Acute changes in plasma non-esterified fatty acid concentration do not change hepatic glucose production in people with type 2 diabetes.
Diabet Med
7:
871-875,
1990[ISI][Medline].
19.
Lee, K-U,
Park JY,
Kim CH,
Hong SK,
Suh KI,
Park KS,
and
Park SW.
Effect of decreasing plasma free fatty acids by acipimox on hepatic glucose metabolism in normal rats.
Metabolism
45:
1408-1414,
1996[ISI][Medline].
20.
McGuinness, OP,
Fujiwara T,
Murrell S,
Bracy D,
Neal DW,
O'Connor D,
and
Cherrington AD.
Impact of chronic stress hormone infusion on hepatic carbohydrate metabolism in the conscious dog.
Am J Physiol Endocrinol Metab
265:
E314-E322,
1993
21.
Meyer, C,
Stumvoll M,
Chintalapudl U,
Gutierrez O,
Kreider M,
Perriello G,
Welle S,
and
Gerich J.
Alanine and glutamine: selective markers for hepatic and renal gluconeogenesis in humans (Abstract).
Diabetes
45, Suppl1:
945,
1996[ISI].
22.
Newsholme, EA,
Sugden PH,
and
Williams T.
Effect of citrate on the activities of 6-phosphofructokinase from nervous and muscle tissues from different animals and its relationships to the regulation of glycolysis.
Biochem J
166:
123-129,
1977[ISI][Medline].
23.
Petersen, KF,
Laurent D,
Rothman DL,
Cline GW,
and
Shulman GW.
Mechanism by which glucose and insulin inhibit net hepatic glycogenolysis in humans.
J Clin Invest
101:
1203-1209,
1998
24.
Puhakainen, I,
Koivisto VA,
and
Yki-Jarvinen H.
No reduction in total hepatic glucose output by inhibition of gluconeogenesis with ethanol in NIDDM patients.
Diabetes
40:
1319-1327,
1991[Abstract].
25.
Puhakainen, I,
and
Yki-Jarvinen H.
Inhibition of lipolysis decreases lipid oxidation and gluconeogenesis from lactate but not fasting hyperglycemia or total hepatic glucose production in NIDDM.
Diabetes
42:
1694-1699,
1993[Abstract].
26.
Randle, PJ,
Priestman DA,
Mistry SC,
and
Arsland A.
Glucose fatty acid interactions and the regulation of glucose disposal.
J Cell Biochem
55S:
1-11,
1994[ISI][Medline].
27.
Roden, M,
Stingl H,
Chandramouli V,
Schumann W,
Hofer A,
Landau B,
Nowotny P,
Waldhausel W,
and
Shulman G.
Effects of free fatty acid elevation on postabsorptive endogenous glucose production and gluconeogenesis in humans.
Diabetes
49:
701-707,
2000[Abstract].
28.
Rossetti, L,
Giaccari A,
Barzilar N,
Howard K,
Sebel G,
and
Hu M.
Mechanism by which hyperglycemia inhibits hepatic glucose production in conscious rats: implications for the pathophysiology of fasting hyperglycemia in diabetes.
J Clin Invest
92:
1126-1134,
1993[ISI][Medline].
29.
Sacca, L,
Vitale D,
Cicala M,
Trimarco B,
and
Ungaro B.
The glucoregulatory response to intravenous glucose infusion in normal man: roles of insulin and glucose.
Metabolism
30:
457-461,
1981[ISI][Medline].
30.
Saloranta, C,
Franssila-Kallunki A,
Ekstrand A,
Taskinen MR,
and
Groop L.
Modulatiuon of hepatic glucose production by non-esterified fatty acids in type 2 (non-insulin-dependent) diabetes mellitus.
Diabetologia
34:
409-415,
1991[ISI][Medline].
31.
Sanchez-Gutierrez, J,
Lechuga C,
Sanchez-Arias J,
Samper B,
and
Feliu J.
Impairment of the modulation by glucose of hepatic gluconeogenesis in the genetically obese (fa/fa) Zucker rat.
Endocrinology
136:
1877-1884,
1995[Abstract].
32.
Shulman, G,
Lacy W,
Liljenquist J,
Keller U,
Williams P,
and
Cherrington AD.
Effect of glucose, independent of changes in insulin and glucagon secretion, on alanine metabolism in the conscious dog.
J Clin Invest
65:
496-505,
1980[ISI][Medline].
33.
Sindelar, DK,
Chu CA,
Rohlie M,
Neal DW,
Swift LL,
and
Cherrington AD.
The role of fatty acids in mediating the effects of peripheral insulin on hepatic glucose production in the conscious dog.
Diabetes
46:
187-196,
1997[Abstract].
34.
Sindelar, DK,
Chu CA,
Venson P,
Donahue EP,
Neal DW,
and
Cherrington AD.
Basal hepatic glucose production is regulated by the portal vein insulin concentration.
Diabetes
47:
523-529,
1998[Abstract].
35.
Stingl, H,
Krssak M,
Krebs M,
Bischof MG,
Nowotny P,
Furnsinn C,
Shulman GI,
and
Waldhausl W.
Lipid-dependent control of hepatic glycogen stores in healthy humans.
Diabetologia
44:
48-54,
2001[ISI][Medline].
36.
Williamson, JR,
Browing ET,
and
Scholz R.
Control mechanisms of gluconeogenesis and ketogenesis. I. Effects of oleate on gluconeogenesis in perfused rat liver.
J Biol Chem
244:
4607-4616,
1969
37.
Williamson, JR,
Kreisberg RA,
and
Felts PW.
Mechanism for the stimulation of gluconeogenesis by fatty acids in perfused rat liver.
Proc Natl Acad Sci USA
56:
247-254,
1966[ISI][Medline].