Download Effects of physiological hyperinsulinemia on systemic, renal, and

Effects of physiological hyperinsulinemia on
systemic, renal, and hepatic substrate metabolism
CHRISTIAN MEYER,1 JEAN DOSTOU,1 VEENA NADKARNI,1 AND JOHN GERICH1,2
Departments of 1Medicine and 2Physiology and Pharmacology,
University of Rochester School of Medicine, Rochester, New York 14642
liver; gluconeogenesis; free fatty acids
IT IS NOW WELL ESTABLISHED that, in postabsorptive
humans, both liver and kidney release glucose into the
circulation (26, 37). The liver is responsible for 70–80%
of overall systemic glucose release under these conditions (5, 26, 37). However, because virtually all of the
glucose released by the kidney is due to gluconeogenesis (46), the contributions of the liver and kidney to
overall gluconeogenesis are roughly comparable (37).
Both hepatic glucose release (HGR) and renal glucose
release have been shown to be influenced by a variety of
hormones under in vitro conditions (11, 35). Insulin
suppresses these processes in both organs, whereas
other hormones, such as glucagon, epinephrine, vaso-
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.
pressin, and parathyroid hormone, can stimulate them
to varying degrees. Studies in normal volunteers, using
the euglycemic clamp technique, have demonstrated
complete suppression of systemic glucose release with
physiological hyperinsulinemia (49). These results imply that, in humans, both renal glucose release and
HGR are under physiological control by insulin. However, it is unclear to what extent and with what relative
sensitivity insulin acts on these organs.
Certain observations suggest that there may be
differences between hepatic and renal glucose release.
Much of the glucose release by the liver is the result of
glycogenolysis, a process that appears to be more
sensitive to insulin than gluconeogenesis (6), whereas
glucose release by the kidney is almost exclusively due
to gluconeogenesis (46). Acidosis stimulates renal gluconeogenesis while suppressing hepatic gluconeogenesis
(46). Although gluconeogenesis in both organs is stimulated by free fatty acids (FFA) (21, 44), liver and kidney
differ in their use of gluconeogenic precursors, e.g.,
alanine is almost exclusively used by the liver (2) and
glutamine is predominantly used by the kidney (27).
Moreover, epinephrine appears to augment hepatic
gluconeogenesis indirectly by increasing precursor availability (34), whereas it probably acts on the kidney
through a direct mechanism (35). Finally, although
insulin does not increase splanchnic (or, presumably,
hepatic) glucose uptake under euglycemic conditions
(10), infusion of insulin into a renal artery has been
shown to increase the glucose uptake of the infused
kidney (5), suggesting insulin’s direct renal effect.
These observations are of particular interest in view of
recent studies providing evidence that insulin may
predominantly suppress HGR via indirect peripheral
effects (1, 24).
The present studies were therefore undertaken to
assess the relative effects of physiological hyperinsulinemia in suppressing hepatic and renal glucose release
and gluconeogenesis from glutamine. In addition, we
also assessed the effects of physiological hyperinsulinemia on renal and systemic glucose, glutamine, and
FFA disposal.
METHODS
Subjects. We obtained informed written consent from nine
normal volunteers (4 men and 5 women) after the protocol
had been approved by the University of Rochester Institutional Review Board. The subjects were 31 6 3 yr of age
(means 6 SE), weighed 75 6 5 kg (body mass index of 24.9 6
1.2 kg/m2 ), had normal glucose-tolerance tests according to
World Health Organization criteria, and had no family history of diabetes mellitus. For 3 days before the study, all
subjects were kept on a weight-maintaining diet containing
at least 200 g carbohydrate and had abstained from alcohol.
0363-6127/98 $5.00 Copyright r 1998 the American Physiological Society
F915
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.3 on August 9, 2017
Meyer, Christian, Jean Dostou, Veena Nadkarni, and
John Gerich. Effects of physiological hyperinsulinemia on
systemic, renal, and hepatic substrate metabolism. Am. J.
Physiol. 275 (Renal Physiol. 44): F915–F921, 1998.—To determine the effect of physiological hyperinsulinemia on renal
and hepatic substrate metabolism, we assessed systemic and
renal glucose release and uptake, systemic and renal gluconeogenesis from glutamine, and certain aspects of systemic and
renal glutamine and free fatty acid (FFA) metabolism. These
were assessed under basal postabsorptive conditions and
during 4-h hyperinsulinemic euglycemic clamp experiments
in nine normal volunteers using a combination of isotopic
techniques and renal balance measurements. Hepatic glucose
release (HGR) and glutamine gluconeogenesis were calculated as the difference between systemic and renal measurements. Infusion of insulin suppressed systemic glucose release and glutamine gluconeogenesis by ,50% during the last
hour of the insulin infusion (P , 0.001). Renal glucose release
and glutamine gluconeogenesis decreased from 2.3 6 0.4 to
0.9 6 0.2 (P , 0.002) and from 0.52 6 0.07 to 0.14 6 0.03
µmol · kg21 · min21 (P , 0.001), respectively. HGR and glutamine gluconeogenesis decreased from 8.7 6 0.4 to 4.5 6 0.5
(P , 0.001) and from 0.35 6 0.02 to 0.27 6 0.03 µmol ·
kg21 · min21 (P , 0.002), respectively. Renal glucose uptake
(RGU) increased from 1.61 6 0.19 to 2.18 6 0.25 µmol · kg21 ·
min21 (P 5 0.029) but accounted for only ,5% of systemic
glucose disposal (40.6 6 4.3 µmol · kg21 · min21 ). Both systemic and renal FFA clearance increased approximately
fourfold (P , 0.001 for both). Nevertheless, renal FFA uptake
decreased (P 5 0.024) and was inversely correlated with RGU
(r 5 20.582, P 5 0.011). Finally, insulin increased systemic
glutamine release (P 5 0.007), uptake (P , 0.005), and
clearance (P , 0.001) but left renal glutamine uptake and
release unaffected (P . 0.4 for both).
F916
EFFECT OF INSULIN ON KIDNEY METABOLISM
(arterial [6-3H]glucose concentration 2 renal vein [6-3H]glucose concentration)/(arterial [6-3H]glucose concentration).
Renal glucose uptake (RGU) was calculated as RBF 3
arterial glucose concentration 3 FX. Renal glucose net balance (NB) was calculated as RBF 3 (arterial glucose concentration 2 renal vein glucose concentration). Renal glucose
release was calculated as RGU 2 NB. Analogous equations
were used for [14C]glutamine and [3H]palmitate, except that
RPF was used. Renal NB of other substrates was calculated
as described for glucose, except that RPF was used for alanine
and glycerol. HGR was calculated as the difference between
systemic glucose release and renal glucose release.
Renal gluconeogenesis from glutamine was calculated as
RBF 3 renal vein [14C]glucose concentration 2 (1 2 FX) 3
(arterial [14C]glucose concentration)/1.2 3 (renal vein [14C]glutamine SA) (5), where FX is the renal fractional extraction of
[3H]glucose.
Hepatic gluconeogenesis was calculated as the difference
between systemic and renal glutamine gluconeogenesis. Assumptions and limitations of the combined NB and the
isotopic approach for determining glucose release and glutamine gluconeogenesis by liver and kidney have been previously discussed in detail (26, 38).
Statistical analysis. Unless otherwise stated, data are
expressed as means 6 SE. Paired two-tailed Student’s t-tests
were used to compare the means of three baseline determinations with mean data obtained from the last two time points
of the euglycemic clamp. P , 0.05 was considered statistically
significant.
RESULTS
Arterial insulin, glucose, C-peptide, and glucagon
concentrations. During infusion of insulin, plasma insulin concentrations averaged ,220 pM, and plasma
glucose concentrations were maintained at 4.97 6 0.03
mM (coefficient of variation of 4.1 6 0.4%) (Fig. 1).
Plasma C-peptide and glucagon concentrations both
decreased by 20% (P 5 0.014 and P , 0.001, respectively).
Systemic, renal, and hepatic glucose metabolism.
Infusion of insulin decreased overall systemic glucose
release by ,50% from 11.0 6 0.4 to 5.37 6 0.51
µmol · kg21 · min21 during the last hour (P , 0.001) (Fig.
2). Renal glucose release decreased from 2.29 6 0.45 to
0.90 6 0.20 µmol · kg21 · min21 (P , 0.002) but was
nearly maximally suppressed within the first 80 min.
HGR decreased less abruptly from 8.71 6 0.42 to 4.47 6
0.46 µmol · kg21 · min21 (P , 0.001). Although the absolute decrement in glucose release was greater for liver
than for kidney, insulin reduced HGR to a lesser extent
than renal glucose release in terms of percent suppression (47.3 6 6.0 vs. 60.7 6 4.3%, P 5 0.027).
Systemic glucose disposal increased nearly fourfold
from 11.0 6 0.4 to 40.6 6 4.3 µmol · kg21 · min21 (P ,
0.001) (data not shown). RBF (Table 1) increased by
,15% by the end of the infusion of insulin (P , 0.001)
and RGU increased by ,30% (P 5 0.029), although
renal glucose FX was not significantly increased (P 5
0.506).
Systemic and renal glutamine metabolism. Arterial
glutamine concentrations (Fig. 3) decreased by ,15%
during the insulin infusion (P 5 0.001), despite the fact
that the systemic release of glutamine (Fig. 4) in-
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.3 on August 9, 2017
Some of the baseline data of six subjects were included in a
previous publication (26).
Protocol. Subjects were admitted to the University of
Rochester General Clinical Research Center between 6:00
and 7:00 PM of the evening before the experiments. They
consumed a standard meal (10 kcal/kg: 50% carbohydrate,
35% fat, and 15% protein) between 6:30 and 8:00 PM and
were fasted overnight until experiments were completed.
At approximately 5:30 AM, an antecubital vein was cannulated and primed; continuous infusions of [6-3H]glucose (,30
µCi, ,0.3 µCi/min; Amersham International, UK) and of
[U-14C]glutamine (,20 µCi, ,0.2 µCi/min; Amersham International) were started. At approximately 8:00 AM, an infusion of p-aminohippuric acid (12 mg/min) was started to
determine renal blood flow (RBF). Between 8:00 and 9:00 AM,
a renal vein was catheterized under fluoroscopy. At ,9:00
AM, an infusion of [9,10-3H]palmitate (,0.2 µCi/min; Amersham International) was initiated and a dorsal hand vein was
cannulated and kept in a thermoregulated Plexiglas box at
65°C for sampling arterialized venous blood. Beginning ,1 h
later, after allowing ,4 h for isotopic steady state to be
achieved, three blood samples were collected from the dorsal
hand vein and the renal vein simultaneously at 30-min
intervals (260, 230, and 0 min) to determine glucose, glutamine, glutamate, alanine, lactate, glycerol, FFA, insulin,
glucagon, and p-aminohippuric acid concentrations and to
assess [3H]- and [14C]glucose, [14C]glutamine, and [3H]palmitate specific activities. At 0 min, a continuous infusion of
insulin (0.6 mU · kg21 · min21 ) was started via the antecubital
infusion line. Subsequently, blood glucose concentrations
were maintained at ,5 mM throughout the study, using the
glucose-clamp technique and a 20% glucose infusion enriched
with [6-3H]glucose to minimize changes in plasma glucose
specific activity (13, 49). Arterialized venous blood glucose
was measured every 5 min. Blood was collected, as described,
at 40-min intervals for the first 2 h (40, 80, and 120 min) and
then at 30-min intervals for the last 2 h of the study (150, 180,
210, and 240 min).
Analytical procedures. Whole blood glucose was immediately determined in triplicate with a glucose analyzer (Yellow
Springs Instrument, Yellow Springs, OH). Plasma glutamine,
glutamate, alanine, and FFA concentrations, as well as [3H]and [14C]glucose and [14C]glutamine specific activities, were
determined by high-performance liquid chromatography (17,
28, 30). Lactate and glycerol concentrations were measured
by microfluorometric methods. Insulin C-peptide and glucagon concentrations were determined by standard radioimmunoassays (26).
Calculations. Systemic (total) release and removal of glucose, glutamine, and palmitate from the circulation were
determined with steady-state equations under basal conditions (47) and, subsequently, during the euglycemic clamp
with the non-steady-state equations of DeBodo et al. (9),
which were modified for glucose as described by Finegood et
al. (13).
The proportion of systemic glucose release in the steady
state due to whole body glutamine gluconeogenesis was
calculated as (arterial [14C]glucose SA/arterial [14C]glutamine SA) 3 100/1.2, where SA is specific activity. Total
glutamine gluconeogenesis was calculated as the proportion
of glucose release due to glutamine multiplied by glucose
release. During the non-steady-state condition (i.e., during
insulin infusion), whole body glutamine gluconeogenesis was
calculated using the equation of Chiasson et al. (7).
Renal plasma flow (RPF) and RBF were determined by the
p-aminohippuric acid-clearance technique (3). Fractional extraction (FX) of glucose across the kidney was calculated as
F917
EFFECT OF INSULIN ON KIDNEY METABOLISM
Table 1. Renal and hepatic glucose release and renal
substrate exchange
Insulin
P
1,358 6 118
1,548 6 110
,0.001
20.68 6 0.50
1.87 6 0.25
1.61 6 0.19
1.29 6 0.37
2.02 6 0.19
2.18 6 0.25
,0.002
0.506
0.029
0.32 6 0.07
8.44 6 2.09
0.60 6 0.12
0.027 6 0.07
0.64 6 0.08
20.34 6 0.09
0.00 6 0.04
2.22 6 0.34
0.56 6 0.09
0.23 6 0.04
5.93 6 1.03
0.46 6 0.09
0.023 6 0.07
0.17 6 0.04
20.44 6 0.11
20.09 6 0.07
3.60 6 0.50
0.09 6 0.01
0.306
0.364
0.414
0.582
,0.001
,0.03
0.402
,0.009
,0.001
0.32 6 0.04
6.36 6 0.89
0.34 6 0.05
0.02 6 0.04
0.23 6 0.02
26.9 6 4.2
0.24 6 0.02
0.00 6 0.00
0.024
,0.001
50.026
0.669
Values are means 6 SE. Renal blood flow (RBF) values are in
ml/min; fractional extraction (FX) values are in %; all other values
are in µmol · kg21 · min21. Baseline values are means taken at 260,
230, and 0 min. Insulin values are means taken at 210 and 240 min.
NB, net balance; FFA, free fatty acids.
Fig. 1. Arterial insulin, glucose, C-peptide, and glucagon concentrations.
creased (P 5 0.007). This was probably due to the fact
that, during the early part of the insulin infusion,
systemic glutamine disposal increased to a slightly
greater extent than its release. Systemic glutamine
clearance increased by ,25% from 10.0 6 0.8 to 12.8 6
0.6 ml · kg21 · min21 (P , 0.001). Renal glutamine NB,
FX, uptake, and release were unchanged (Table 1).
Systemic, renal, and hepatic glutamine gluconeogenesis. Before infusion of insulin, renal (0.52 6 0.07 µmol ·
kg21 ·min21 ) and hepatic (0.35 6 0.03 µmol·kg21 ·min21 )
glutamine gluconeogenesis accounted for 60.6 6 2.8
and 39.4 6 2.8% of systemic glutamine gluconeogenesis, respectively (0.87 6 0.08 µmol · kg21 · min21 ) (Fig.
4). Infusion of insulin decreased systemic glutamine
gluconeogenesis by ,50% to 0.41 6 0.03 µmol · kg21 ·
min21, P , 0.001. Renal and hepatic glutamine gluconeogenesis decreased to 0.14 6 0.03 (P , 0.001) and
0.27 6 0.03 µmol · kg21 · min21 (P , 0.002), respectively,
during the last hour. As was the case for glucose
release, insulin suppressed renal glutamine gluconeogenesis to a greater extent than hepatic glutamine
gluconeogenesis (71.6 6 5.2 vs. 24.7 6 4.9%, P , 0.001).
Moreover, the time course of changes in renal and
hepatic gluconeogenesis differed; renal glutamine gluconeogenesis decreased within 40 min and was maximally suppressed by 120 min, whereas hepatic glutamine gluconeogenesis increased during the initial 80
min before finally decreasing below baseline values.
Arterial concentrations and renal NBs of glutamate,
alanine, lactate, and glycerol. Arterial glutamate concentrations decreased by ,20% during the last hour of
the insulin infusion (P , 0.001) despite an increase of
the renal net release of glutamate. Arterial alanine
Fig. 2. Systemic (total), hepatic, and renal rates of glucose release.
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.3 on August 9, 2017
RBF
Renal glucose
NB
FX
Uptake
Renal glutamine
NB
FX
Uptake
Release
Conversion to Glucose
Glutamate NB
Alanine NB
Lactate NB
Glycerol NB
Renal FFA
NB
FX
Uptake
Release
Baseline
F918
EFFECT OF INSULIN ON KIDNEY METABOLISM
unchanged (51.5 6 6.8 vs. 55.3 6 1.7% basal, P 5
0.617).
Systemic and renal FFA metabolism. Arterial FFA
concentrations (Fig. 4) decreased by ,80% (P , 0.001)
during the last hour of the insulin infusion. Systemic
FFA release (Fig. 5) decreased by ,40% (P , 0.002).
Despite the fact that systemic FFA disposal decreased
(P , 0.002), systemic FFA clearance increased approximately fourfold (P , 0.001). Renal FFA FX increased
(P , 0.001) and was positively correlated with systemic
FFA clearance (r 5 0.889, P , 0.001). Because there
was no significant renal FFA release, renal FFA NB
closely approximated renal FFA uptake (Table 1). This
was inversely correlated with RGU (r 5 20.582, P 5
0.011).
Fig. 3. Arterial glutamine, glutamate, alanine, lactate, glycerol, and
free fatty acid (FFA) concentrations.
concentrations and renal NB remained essentially unchanged. In contrast, arterial lactate concentrations
and renal lactate net uptake both increased (P , 0.01).
Arterial glycerol concentrations decreased by 75% (P ,
0.001). This could wholly account for the reduced renal
glycerol uptake because renal glycerol FX remained
Fig. 4. Rates of systemic glutamine release and disposal (top) and
rates of systemic, hepatic, and renal glutamine gluconeogenesis
(bottom).
In the present studies, infusion of insulin in normal
postabsorptive volunteers, which produced physiological hyperinsulinemia (,200 pM) comparable to values
observed postprandially (29), suppressed overall kidney and liver glucose release as well as their production
of glucose from glutamine. Moreover, the percent suppression of renal glucose release and glutamine gluconeogenesis was greater than that of HGR and glutamine gluconeogenesis, indicating that the kidney was
more sensitive to insulin. Physiologically, this would
make sense because the arterial insulin concentrations
to which the kidney is exposed are considerably lower
than the portal venous insulin levels to which the liver
is exposed.
Fig. 5. Rates of systemic FFA release, disposal, and clearance.
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.3 on August 9, 2017
DISCUSSION
EFFECT OF INSULIN ON KIDNEY METABOLISM
gluconeogenesis by the kidney (38). Because its uptake
by the kidney was unaltered during infusion of insulin
and because renal uptake of lactate (another major
renal gluconeogenic precursor) increased during infusion of insulin, it seems likely that insulin suppressed
renal gluconeogenesis by altering intrarenal disposal of
gluconeogenic precursors rather than by altering their
transport into the kidney. This could have occurred
through inhibition of gluconeogenic enzymes and/or by
stimulation of alternate pathways for disposal of potential gluconeogenic precursors.
Our data cannot provide insight into the relative
importance of these potential mechanisms. However, it
is of note that the activity of several renal gluconeogenic enzymes is increased in insulin-deficient diabetic
animals, implying that insulin can suppress the activity of those enzymes (23). Moreover, in present studies
before insulin infusion, nearly all renal glutamine
uptake could be accounted for by the conversion of
glutamine to glucose. During insulin infusion, only
about one-third of glutamine uptake could be accounted
for by conversion to glucose. Some of the glutamine
carbon might have been diverted to glutamate, the
renal release of which was increased during infusion of
insulin, and some of the glutamine could have been
used as an oxidative fuel in place of FFA, the renal
uptake of which was diminished.
Regarding glucose uptake, we found that insulin
stimulated RGU to a moderate extent (,20%). Because
whole body glucose uptake increased to a much greater
extent (,400%), it would appear that the human
kidney is not a major site of insulin-mediated glucose
disposal. In the studies of Cersosimo et al. (5), in which
insulin was infused locally into the renal artery of dogs
without causing systemic hyperinsulinemia, glucose
uptake by the infused kidney increased by ,75%.
Because arterial insulin levels were probably increased
to a greater extent in our studies, these differences
could indicate that the RGU may be more sensitive to
insulin in dogs than in humans.
In the present studies, renal glucose FX was not
significantly altered by insulin. About 60% of the
increase in RGU could be accounted for by an increase
in RBF induced by insulin. A similar effect of insulin on
RBF has been observed in previous studies (12, 31).
Moreover, in the present studies, plasma FFA levels
and renal FFA uptake decreased. There was a significant inverse correlation between renal FFA and glucose
uptake, with ,35% of the variation in RGU being
explained by the variation in renal fatty acid uptake
(r 5 20.582, P 5 0.011). These latter observations are
consistent with the concept of a renal glucose-fatty acid
cycle (26) and, taken together with the changes in RBF,
suggest that infusion of insulin may increase RGU in a
manner other than by directly stimulating renal glucose transport.
Although in the present study insulin reduced renal
and systemic FFA uptake, both renal FFA FX and
systemic FFA clearance increased significantly. The
latter is consistent with previous reports of an increase
in systemic FFA clearance during hyperinsulinemic
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.3 on August 9, 2017
It is of interest that both renal glucose release and
renal glutamine gluconeogenesis followed similar time
courses, being maximally suppressed by ,120 min.
This is consistent with the fact that renal glucose
release is largely due to gluconeogenesis. In contrast,
hepatic glutamine gluconeogenesis and HGR did not
parallel one another and took a longer time to reach
maximal suppression. HGR decreased progressively
over the initial 120 min, whereas hepatic glutamine
gluconeogenesis actually increased during the initial
80 min.
An explanation for this seemingly paradoxical increase in hepatic glutamine gluconeogenesis is unclear
but could involve hepatic autoregulation (16), wherein
a decrease in hepatic glycogenolysis causes an increase
in hepatic gluconeogenesis analogous to the situation
in which an increase in hepatic gluconeogenesis causes
a reduction in hepatic glycogenolysis. Another possibility is the insulin stimulation of hepatic amino acid
transport (36). The hepatic amino acid A-transporter
system, which is partly responsible for glutamine uptake, is stimulated by insulin, whereas it is not in the
kidney (36). Consistent with this, we found no change
in renal glutamine FX and uptake, whereas there was a
significant increase in systemic glutamine uptake and
clearance, which could have involved the liver. Still
another possibility is that some of the glutamine gluconeogenesis actually represented gluconeogenesis from
[14C]glutamate formed from [14C]glutamine in the kidney or other tissues. Finally, it is also possible that our
assumption of equivalent changes in hepatic and renal
Krebs cycle carbon exchange may not be valid. If Krebs
cycle carbon exchange decreased in the liver, then this
would have led to an overestimation of hepatic glutamine gluconeogenesis.
Regardless of the cause of the initially increased
hepatic glutamine gluconeogenesis, if the changes in
hepatic glutamine gluconeogenesis reflected hepatic
gluconeogenesis in general, then this would imply that
the observed early suppression of HGR was entirely
attributable to suppression of glycogenolysis. This would
be consistent with observations of Chiasson et al. (6)
demonstrating that, in dogs, hepatic glycogenolysis is
more sensitive than gluconeogenesis to insulin suppression.
The demonstration in the present study, that renal
glucose release is suppressed by physiological insulin
concentrations, indicates that insulin is a physiological
regulator of renal glucose release. One might therefore
expect that renal glucose release might be increased in
insulin-resistant conditions such as type II diabetes
(25). Indeed, increased renal glucose release has been
demonstrated to occur in insulin-resistant animals
such as alloxan or streptozotocin diabetic rats (19, 22,
39) and glucocorticoid-treated rats (39, 43).
Insulin suppression of renal glucose release should
largely reflect suppression of renal gluconeogenesis.
Renal glycogen stores are generally negligible, and
cells that could store glycogen lack glucose-6-phosphatase and cannot release glucose via glycogenolysis (46).
Glutamine is the predominant amino acid used for
F919
F920
EFFECT OF INSULIN ON KIDNEY METABOLISM
Finally, some comment is warranted regarding Krebs
cycle carbon exchange. It is well established that
carbon exchange in the Krebs cycle can cause an
underestimation of isotopic determination of systemic
gluconeogenesis as performed in the present studies
(18). We have assumed that such underestimation
would be comparable for liver and kidney, as suggested
from in vitro studies (20), and thus would not disproportionately affect hepatic glutamine gluconeogenesis calculated as the difference between systemic and renal
glutamine gluconeogenesis. However, there is little
support for this assumption. It could be argued that
during infusion of insulin, when glutamine flux was
increased, Krebs cycle carbon exchange would not have
been reduced. This might not have been operative in
kidney, where glutamine uptake was unaltered, but
may have occurred in liver, where glutamine uptake
might have increased. If so, this would have resulted in
a relative overestimate of hepatic glutamine gluconeogenesis.
In summary, the present studies indicate that, in
humans, physiological concentrations of insulin exert
numerous actions on renal substrate homeostasis and
extrarenal glutamine metabolism. Suppression of glutamine gluconeogenesis is more sensitive to insulin in the
kidney than in the liver, whereas stimulation of RGU
plays a minor role in insulin-stimulated glucose disposal and may in fact represent an indirect action of
insulin mediated by changes in RBF and renal FFA
uptake rather than a direct effect on renal glucose
transport.
We thank the staff of the General Clinical Research Center for
excellent technical help and Mary Little for superb editorial support.
The present work was supported in part by National Institutes of
Health Grants 5M01-RR-00044 and DK-20411.
Address for reprint requests: J. E. Gerich, Univ. of Rochester
School of Medicine, 601 Elmwood Ave., Box MED/CRC, Rochester, NY
14642
Received 24 April 1998; accepted in final form 20 August 1998.
REFERENCES
1. Ader, M., and R. Bergman. Peripheral effects of insulin
dominate suppression of fasting hepatic glucose production.
Am. J. Physiol. 258 (Endocrinol. Metab. 21): E1020–E1032,
1990.
2. Björkman, O., P. Felig, and J. Wahren. The contrasting
responses of splanchnic and renal glucose output to gluconeogenic substrates and to hypoglucagonemia in 60-h-fasted humans. Diabetes 29: 610–616, 1980.
3. Brun, C. A rapid method for the determination of paraaminohippuric acid in kidney function tests. J. Lab. Clin. Med.
37: 955–958, 1951.
4. Campbell, P., M. Carlson, J. Hill, and N. Nurjhan. Regulation of free fatty acid metabolism by insulin in humans: role of
lipolysis and reesterification. Am. J. Physiol. 263 (Endocrinol.
Metab. 26): E1063–E1069, 1992.
5. Cersosimo, E., R. Judd, and J. Miles. Insulin regulation of
renal glucose metabolism in conscious dogs. J. Clin. Invest. 93:
2584–2589, 1994.
6. Chiasson, J., J. Liljenquist, F. Finger, and W. Lacy. Differential sensitivity of glycogenolysis and gluconeogenesis to insulin
infusions in dogs. Diabetes 25: 283–291, 1976.
7. Chiasson, J., J. Liljenquist, W. Lacy, A. Jennings, and A.
Cherrington. Gluconeogenesis: methodological approaches in
vivo. Fed. Proc. 36: 229–235, 1977.
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.3 on August 9, 2017
euglycemic clamp experiments in normal postabsorptive humans (14, 42). Insulin suppresses systemic FFA
oxidation while stimulating overall FFA reesterification (4). One might expect a similar mechanism to be
operative in the kidney, especially because the insulininduced increase in RGU should reduce renal FFA
oxidation and increase availability of glycerol-3-phosphate for esterification of FFA. This would be consistent with findings of in vitro studies indicating that
triglyceride synthesis is an important route of FFA
disposal in the kidney (45).
The design of the present studies also permitted
evaluation of the effects of insulin on systemic and
renal glutamine metabolism, which have not been
previously studied. Infusion of insulin increased systemic glutamine turnover (both uptake and release)
without significantly altering renal FX, uptake, and
release. Thus the systemic effects of insulin on glutamine metabolism are attributable to extrarenal tissues.
Although the increase in systemic glutamine disposal
might have been anticipated from the known effects of
insulin to stimulate tissue-amino acid transport and
protein synthesis (33, 40), it was surprising to also
observe an increase in systemic glutamine release. The
effect of insulin to suppress proteolysis (15, 40) would
have been expected to reduce systemic glutamine release if this were the major factor influencing systemic
glutamine release in the circulation.
An increase in the conversion of plasma glucose to
glutamine, the so-called glucose-glutamine cycle (32),
could provide a possible explanation for the increased
systemic glutamine release during insulin infusion.
Under postabsorptive conditions, ,12% of glutamine
carbons entering the circulation originate from plasma
glucose (32); this represents ,5% of systemic glucose
disposal. In the present study, infusion of insulin
increased systemic glucose disposal approximately fourfold. If a comparable proportion of the increased glucose
disposal involved the conversion of glucose to glutamine, this would have increased systemic glutamine
release by about 2 µmol · kg21 · min21, which is nearly
threefold more than the observed increase in glutamine
release (,0.7 µmol · kg21 · min21 ).
Thus an increase in the glucose-glutamine cycle
would have been able to offset a decrease in glutamine
release due to suppression of proteolysis and would
result in a net increase in glutamine release into the
circulation. This postulate of an insulin-induced increase in the glucose-glutamine cycle seems plausible
because increases in the Cori (glucose-lactate) cycle
occur during euglycemic hyperinsulinemic clamp experiments (8, 41, 48). Physiologically, the operation of these
cycles would permit anabolic actions of insulin (protein
and glycogen formation) while sustaining appropriate
glucose release into the circulation via provision of
gluconeogenic precursors. It is of note, however, that in
the present study there was increased net renal uptake
of lactate and no change in renal glutamine release
during the infusion of insulin. Thus it seems unlikely
that the kidney was involved in increases in these
cycles.
EFFECT OF INSULIN ON KIDNEY METABOLISM
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
output and diminished early insulin release in impaired glucose
tolerance. N. Engl. J. Med. 326: 22–29, 1992.
Nurjhan, N., F. Kennedy, A. Consoli, C. Martin, J. Miles,
and J. Gerich. Quantification of the glycolytic origin of plasma
glycerol as an index of lipolysis in vivo. Metabolism 37: 371–377,
1988.
Pelikánová, T., I. Smrcková, J. Krı́zová, J. Strı́brná, and V.
Lánská. Effects of insulin and lipid emulsion on renal haemodynamics and renal sodium handling in IDDM patients. Diabetologia 39: 1074–1082, 1996.
Perriello, G., R. Jorde, N. Nurjhan, M. Stumvoll, G. Dailey,
T. Jenssen, D. Bier, and J. Gerich. Estimation of the glucosealanine-lactate-glutamine cycles in postabsorptive man: role of
the skeletal muscle. Am. J. Physiol. 269 (Endocrinol. Metab. 32):
E443–E450, 1995.
Posner, B., P. Kelly, R. Shiu, and H. Friesen. Studies of
insulin, growth hormone and prolactin binding: tissue distribution, species variation and characterization. Endocrinology 95:
521–531, 1974.
Sacca, L., C. Vigorito, M. Cicala, G. Corso, and R. Sherwin.
Role of gluconeogenesis in epinephrine-stimulated hepatic glucose production in humans. Am. J. Physiol. 245 (Endocrinol.
Metab. 8): E294–E302, 1983.
Schoolwerth, A., B. Smith, and R. Culpepper. Renal gluconeogenesis. Miner. Electrolyte Metab. 14: 347–361, 1988.
Shotwell, M., M. Kilberg, and D. Oxender. The regulation of
neutral amino acid transport in mammalian cells. Biochim.
Biophys. Acta 737: 267–284, 1983.
Stumvoll, M., U. Chintalapudi, G. Perriello, S. Welle, O.
Gutierrez, and J. Gerich. Uptake and release of glucose by the
human kidney: postabsorptive rates and responses to epinephrine. J. Clin. Invest. 96: 2528–2533, 1995.
Stumvoll, M., C. Meyer, G. Perriello, M. Kreider, S. Welle,
and J. Gerich. Human kidney and liver gluconeogenesis: evidence for organ substrate selectivity. Am. J. Physiol. 274 (Endocrinol. Metab. 37): E817–E826, 1998.
Teng, C. Studies on carbohydrate metabolism in rat kidney
slices II effects of alloxan diabetes and insulin administration on
glucose uptake and formation. Arch. Biochem. Biophys. 48:
415–423, 1954.
Tessari, P., R. Barazzoni, M. Zanetti, M. Vettore, S. Normand, D. Bruttomesso, and B. Beaufrere. Protein degradation and synthesis measured with multiple amino acid tracers in
vivo. Am. J. Physiol. 271 (Endocrinol. Metab. 34): E733–E741,
1996.
Virkamaki, A., I. Puhakainen, N. Nurjhan, J. Gerich, and
H. Yki-Jarvinen. Measurement of lactate formation from glucose using [6-3H]- and [6-14C]glucose in humans. Am. J. Physiol.
259 (Endocrinol. Metab. 22): E397–E404, 1990.
Webber, J., J. Taylor, H. Greathead, J. Dawson, P. Buttery,
and I. MacDonald. Effects of fasting on fatty acid kinetics and
on the cardiovascular, thermogenic and metabolic responses to
the glucose clamp. Clin. Sci. (Colch.) 87: 697–706, 1994.
White, L., and B. Landau. Effect of glucocorticoids on metabolism of carbohydrate by kidney cortex. Am. J. Physiol. 211:
449–456, 1966.
Williamson, J., R. Kreisberg, and P. Felts. Mechanism for the
stimulation of gluconeogenesis by fatty acids in perfused rat
liver. Proc. Natl. Acad. Sci. USA 56: 247–254, 1966.
Wirthensohn, G., and W. Guder. Renal lipid metabolism.
Miner. Electrolyte Metab. 9: 203–211, 1983.
Wirthensohn, G., and W. Guder. Renal substrate metabolism.
Physiol. Rev. 66: 469–497, 1986.
Wolfe, R. Radioactive and Stable Isotope Tracers in Biomedicine:
Principles and Practice of Kinetic Analysis. New York: WileyLiss, 1992.
Yki-Järvinen, H., C. Bogardus, and J. Foley. Regulation of
plasma lactate concentration in resting human subjects. Metabolism 39: 859–864, 1990.
Yki-Järvinen, H., A. Consoli, N. Nurjhan, A. Young, and J.
Gerich. Mechanism for underestimation of isotopically determined glucose disposal. Diabetes 38: 744–751, 1989.
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.3 on August 9, 2017
8. Consoli, A., N. Nurjhan, J. Gerich, and L. Mandarino.
Skeletal muscle is a major site of lactate uptake and release
during hyperinsulinemia. Metabolism 41: 176–179, 1992.
9. DeBodo, R., R. Steele, A. Dunn, and J. Bishop. On the
hormonal regulation of carbohydrate metabolism: studies with
14C glucose. Recent Prog. Horm. Res. 19: 445–448, 1963.
10. DeFronzo, R., E. Ferrannini, R. Hendler, R. Felig, and J.
Wahren. Regulation of splanchnic and peripheral glucose uptake by insulin and hyperglycemia in man. Diabetes 32: 35–45,
1983.
11. Exton, J. Mechanisms of hormonal regulation of hepatic glucose
metabolism. Diabetes Metab. Rev. 3: 163–183, 1987.
12. Ferrannini, E., J. Wahren, O. Faber, P. Felig, C. Binder, and
R. DeFronzo. Splanchnic and renal metabolism of insulin in
human subjects: a dose-response study. Am. J. Physiol. 244
(Endocrinol. Metab. 7): E517–E527, 1983.
13. Finegood, D., R. Bergman, and M. Vranic. Estimation of
endogenous glucose production during hyperinsulinemic-euglycemic glucose clamps. Comparison of unlabeled and labeled exogenous glucose infusates. Diabetes 36: 914–924, 1987.
14. Groop, L., R. Bonadonna, M. Shank, A. Petrides, and R.
DeFronzo. Role of free fatty acids and insulin in determining
free fatty acid and lipid oxidation in man. J. Clin. Invest. 87:
83–89, 1991.
15. Jensen, M., J. Miles, J. Gerich, P. Cryer, and M. Haymond.
Preservation of insulin effects on glucose production and proteolysis during fasting. Am. J. Physiol. 254 (Endocrinol. Metab. 17):
E700–E707, 1988.
16. Jenssen, T., N. Nurjhan, A. Consoli, and J. Gerich. Failure of
substrate-induced gluconeogenesis to increase overall glucose
appearance in normal humans. Demonstration of hepatic autoregulation without a change in plasma glucose concentrations. J.
Clin. Invest. 86: 489–497, 1990.
17. Jenssen, T., N. Nurjhan, G. Perriello, A. Bucci, I. Toft, and
J. Gerich. Determination of [14C]glutamine specific activity in
plasma. J. Liq. Chromatogr. 17: 1337–1348, 1994.
18. Katz, J. Determination of gluconeogenesis in vivo with 14Clabeled substrates. Am. J. Physiol. 248 (Regulatory Integrative
Comp. Physiol. 17): R391–R399, 1985.
19. Kida, K., S. Nakago, F. Kamiya, Y. Ttoyama, N. Takashi, and
H. Nakagawa. Renal net glucose release in vivo and its contribution to blood glucose in rats. J. Clin. Invest. 62: 721–726, 1978.
20. Krebs, H., R. Hems, M. Weidemann, and R. Speake. The fate
of isotopic carbon in kidney cortex synthesizing glucose from
lactate. Biochem. J. 101: 242–249, 1966.
21. Krebs, H., R. Speake, and R. Hems. Acceleration of renal
gluconeogenesis by ketone bodies and fatty acids. Biochem. J. 94:
712–720, 1965.
22. Landau, B. Gluconeogenesis and pyruvate metabolism in rat
kidney in vitro. Endocrinology 67: 744–751, 1960.
23. Lemieux, G., M. Aranda, P. Fournel, and C. Lemieux. Renal
enzymes during experimental diabetes mellitus in the rat. Role
of insulin, carbohydrate metabolism, and ketoacidosis. Can. J.
Physiol. Pharmacol. 62: 70–75, 1984.
24. Lewis, G., M. Vranic, P. Harley, and A. Gracca. Fatty acids
mediate the acute extrahepatic effects of insulin on hepatic
glucose production in humans. Diabetes 46: 1111–1119, 1997.
25. Meyer, C., V. Nadkarni, and J. Gerich. Demonstration that
renal and hepatic glucose release are both increased in noninsulin dependent diabetes (NIDDM). Diabetes 46, Suppl. 1: 85A,
1997.
26. Meyer, C., V. Nadkarni, M. Stumvoll, and J. Gerich. Human
kidney free fatty acid and glucose uptake: evidence for a renal
glucose-fatty acid cycle. Am. J. Physiol. 273 (Endocrinol. Metab.
36): E650–E654, 1997.
27. Meyer, C., M. Stumvoll, U. Chintalapudi, O. Gutierrez, M.
Kreider, G. Perriello, S. Welle, and J. Gerich. Alanine and
glutamine: selective markers for hepatic and renal gluconeogenesis in humans. Diabetes 45, Suppl. 2: 255A, 1996.
28. Miles, J., M. Ellman, K. McLean, and M. Jensen. Validation
of a new method for determination of free fatty acid turnover.
Am. J. Physiol. 252 (Endocrinol. Metab. 15): E431–E438, 1987.
29. Mitrakou, A., D. Kelley, T. Veneman, T. Pangburn, J. Reilly,
and J. Gerich. Role of reduced suppression of hepatic glucose
F921