Insulin regulation of renal glucose metabolism in humans
Eugenio
Cersosimo,
Peter
Garlick, and
John
Ferretti
Departments of Medicine, Surgery and Radiology, State University
of New York at Stony Brook, Stony Brook, New York 11794
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ABSTRACT |
Eighteen healthy subjects had arterialized hand and renal veins
catheterized after an overnight fast. Systemic and renal glucose and
glycerol kinetics were measured with
[6,6-2H2]glucose
and [2-13C]glycerol
before and after 180-min peripheral infusions of insulin at 0.125 (LO)
or 0.25 (HI)
mU · kg
1 · min
1
with variable
[6,6-2H2]dextrose
or saline (control). Renal plasma flow was determined by plasma
p-aminohippurate clearance. Arterial
insulin increased from 37 ± 8 to 53 ± 5 (LO) and to 102 ± 10 pM (HI, P < 0.01) but not in
control (35 ± 8 pM). Arterial glucose did not change and averaged
5.2 ± 0.1 (control), 4.7 ± 0.2 (LO), and 5.1 ± 0.2 (HI) µmol/ml; renal vein glucose decreased from 4.8 ± 0.2 to 4.5 ± 0.2 µmol/ml (LO) and from 5.3 ± 0.2 to 4.9 ± 0.1 µmol/ml
(HI) with insulin but not saline infusion (5.3 ± 0.1 µmol/ml).
Endogenous glucose production decreased from 9.9 ± 0.7 to 6.9 ± 0.5 (LO) and to 5.7 ± 0.5 (HI)
µmol · kg
1 · min
1;
renal glucose production decreased from 2.5 ± 0.6 to 1.5 ± 0.5 (LO) and to 1.2 ± 0.6 (HI)
µmol · kg
1 · min
1,
whereas renal glucose utilization increased from 1.5 ± 0.6 to 2.6 ± 0.7 (LO) and to 2.9 ± 0.7 (HI)
µmol · kg
1 · min
1
after insulin infusion (all P < 0.05 vs. baseline). Neither endogenous glucose production (10.0 ± 0.4),
renal glucose production (1.1 ± 0.4), nor renal glucose utilization
(0.8 ± 0.4) changed in the control group. During insulin infusion,
systemic gluconeogenesis from glycerol decreased from 0.67 ± 0.05 to 0.18 ± 0.02 (LO) and from 0.60 ± 0.04 to 0.20 ± 0.02 (HI)
µmol · kg
1 · min
1
(P < 0.01), and renal
gluconeogenesis from glycerol decreased from 0.10 ± 0.02 to 0.02 ± 0.02 (LO) and from 0.15 ± 0.03 to 0.09 ± 0.03 (HI)
µmol · kg
1 · min
1
(P < 0.05). In contrast, during
saline infusion, systemic (0.66 ± 0.03 vs. 0.82 ± 0.05 µmol · kg
1 · min
1)
and renal gluconeogenesis from glycerol (0.11 ± 0.02 vs. 0.41 ± 0.04 µmol · kg
1 · min
1)
increased (P < 0.05 vs. baseline).
We conclude that glucose production and utilization by the kidney are
important insulin-responsive components of glucose metabolism in humans.
kidney; carbohydrate; fuel homeostasis; turnover; glycerol kinetics
 |
INTRODUCTION |
THE ROLE of the kidney in glucose metabolism is thought
to be minor under most circumstances. Cahill and co-workers (11, 12)
demonstrated that net renal glucose output was negligible in the
postabsorptive state, but it increased substantially with prolonged
fasting, contributing nearly one-half of daily systemic glucose
appearance after 7-10 wk of fasting in humans. More recently, however, although studies in postabsorptive dogs (13) and in humans
(39) have confirmed that net glucose output is minimal, partition of
glucose production and utilization across the kidney indicates that
renal glucose production equals glucose utilization and accounts for
~15-25% of endogenous glucose production. These recent in vivo
experiments are in agreement with the original observations under
fasting conditions (22, 32) and further suggest that glucose released
by the kidney in the postabsorptive state appears to reflect renal
gluconeogenesis, primarily from lactate (16), glycerol (13), and
circulating amino acids (38). They are also consistent with the
abundant in vitro evidence indicating that the kidney is capable of
simultaneous glucose production and utilization. Several years ago,
Krebs et al. (28) demonstrated that the biochemical machinery is in
place in cells of the proximal convoluted tubule to efficiently convert
3-carbon precursors to glucose. At the same time, cells of the distal
nephron and those in the interstitial medulla are very active in
glucose uptake and oxidation (43, 44). The mechanisms that regulate
glucose production and utilization by the kidney, however, are
presently unknown.
Most in vitro experiments have led us to believe that renal glucose
production is hormone insensitive and regulated primarily by substrate
availability (44). In contrast, recent studies in dogs demonstrate that
physiological hyperinsulinemia can simultaneously suppress glucose
production and stimulate glucose utilization by the kidney (13).
Insulin-induced hypoglycemia is associated with a twofold increase in
renal glucose production that cannot be abolished by normalization of
renal plasma glucose (15). These observations strongly suggest that
glucose production by the kidney, analogous to the liver, is inhibited
by insulin and stimulated by counterregulatory hormones, particularly
catecholamines. In support, epinephrine infusion in postabsorptive
humans has recently been shown to enhance renal glucose production
(39). The regulatory role of insulin on renal glucose metabolism in humans, however, has not yet been investigated. If, similar to our
observations in dogs (13), renal glucose production and utilization in
humans are divergently regulated, these processes would be of potential
significance. The present studies were therefore undertaken to
determine whether renal glucose production and utilization are
responsive to physiological hyperinsulinemia in postabsorptive humans,
using arteriovenous balance measurements combined with a tracer technique.
 |
MATERIALS AND METHODS |
Subjects. Informed written consent was
obtained from 18 healthy volunteers after the protocol had been
approved by our local institutional review board. All subjects (Table
1) had normal fasting glucose and no
personal or family history of diabetes, hypertension, or renal disease.
For 3 days before the study, all had been on a weight-maintaining diet
containing
200 g of carbohydrate and had abstained from alcohol.
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Table 1.
Characteristics of eighteen subjects studied in postabsorptive
state and after peripheral insulin infusion of either saline (control
group) or insulin
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Protocol. Subjects were admitted to
the University Hospital General Clinical Research Center at State
University of New York at Stony Brook after an overnight fast between
6:00 PM and 7:00 AM in the morning of the experiments. An antecubital
vein was cannulated, and a primed continuous infusion of
[6,6-2H2]glucose
(24 µmol/kg, 0.24 µmol · kg
1 · min
1;
Cambridge Isotope Laboratories, Andover, MA) and
[2-13C]glycerol (45 µmol/kg, 0.45 µmol · kg
1 · min
1;
Cambridge Isotope Laboratories) and a continuous infusion of p-aminohippurate (12 mg/min; Merck,
West Point, PA) were started. Tracer infusion rates were chosen and
changed accordingly during each experiment to produce steady-state
arterial and renal vein plasma glucose and glycerol enrichment levels
to permit detection of a difference as low as 5% across the kidney
(13, 15). Subsequently, a dorsal hand vein was cannulated retrogradely
and kept in a thermoregulated Plexiglas box at 65°C for sampling
arterialized venous blood (14). During the 150-min equilibration
period, subjects had left (n = 16) or
right (n = 2) renal vein catheterized
through the right femoral vein under fluoroscopy, and the position of
the catheter tip was ascertained by injecting a small amount of
iodinated contrast material. The catheter was then continuously infused
with a heparinized saline solution (4.0 U/min) to maintain patency.
During the baseline period (
30 to 0 min), three consecutive
blood samples were collected simultaneously from the dorsal hand vein
and the renal vein at 15-min intervals for the determination of
p-aminohippurate, insulin, free fatty
acid, glucose, and glycerol concentrations and plasma glucose and
glycerol enrichments. At 0 min, on completion of baseline collections,
subjects were randomized to receive a 180-min continuous peripheral
infusion of insulin at the rate of either 0.125 (n = 6) or 0.250 (n = 8)
mU · kg
1 · min
1
with a concomitant variable infusion of
[6,6-2H2]dextrose
(2% atoms % excess, dextrose in water, 10% solution) to maintain
plasma glucose concentration and enrichment constant, assuming a
reduction in endogenous glucose production between 25 and 50% (10,
23). These insulin infusion rates were selected to produce elevations
in plasma insulin concentration comparable to low and high postprandial
hyperinsulinemia in humans and to reduce presumed hepatic glucose
production by <50% on the basis of previous
publications by Katz et al. (27). Four additional individuals were
infused with normal saline (0.9% NaCl) at the rate of 50 ml/h
throughout the entire 180-min experimental period, and these
individuals represent the control group. Blood samples were collected
from the dorsal hand and renal veins at 30-min intervals from 0 to 150 min and at 15-min intervals from 150 to 180 min.
Analytic techniques. Plasma glucose at
the bedside was measured with the Beckman II glucose analyzer
(Fullerton, CA); p-aminohippurate concentration was determined by a colorimetric method (9), and insulin
was determined by radioimmunoassay (24). Plasma free fatty acid
concentration was determined by the colorimetric method described by
Bergman et al. (6) with a commercially available kit (Wako, Osaka,
Japan). Plasma concentration and enrichment of
[2H2]glucose,
[13C1]glucose,
and
[13C1]glycerol
were measured by gas chromatography-mass spectrometry. In brief, 150 µl of plasma were added to 150 µl of glucose internal standard
solution (5 mmol/l
[U-13C]glucose).
Samples were deproteinized with acetonitrile and evaporated to dryness.
Derivatization was carried out with butane boronic acid in pyridine and
acetic anhydride (40). The glucose derivative was quantified by
selective ion monitoring at mass-to-charge ratios (m/z)
297, 298, 299, and 303 for natural
[12C1]-,
[13C1]-,
[2H2]-,
and [U-13C]glucose,
respectively. Two sets of standard were measured containing known
amounts of
[2H2]
and
[13C1]glucose.
Isotope enrichments were calculated by multiple linear regression (41).
A set of standards containing 0 to 10 mmol of glucose and 5 mmol of
[U-13C]glucose
internal standard was used to calculate plasma concentration of
glucose. To determine plasma glycerol enrichment and concentration, 100 µl of plasma were added to 100 µl of glycerol internal standard solution (60 µmol
[2H5]glycerol).
After samples were deproteinized and dried, derivatization was carried out with acetic anhydride (7), and the glycerol derivative
was quantified in the positive ion chemical ionization mode with
methane as the reagent gas. Separation was achieved with selective ion
monitoring at
m/z
159, 160, and 164 for natural [12C1]-,
[13C1]-,
and
[2H5]glycerol,
respectively. Isotope enrichments were calculated by multiple linear
regression (41). A set of standards containing 0-350 µmol/l of
glycerol and 60 µmol/l of
[2H5]glycerol
internal standard was used to calculate plasma concentration of glycerol.
Calculations. Renal plasma flow was
calculated by p-aminohippurate
clearance with the equation
|
(1)
|
where
RPF is renal plasma flow in milliliters per minute, INF is
p-aminohippurate infusion rate in
milligrams per minute, [PAH] is plasma
p-aminohippurate concentrations in
milligrams per minute, subscript rv is renal vein, and subscript a is
artery. Whole body glucose rate of appearance
(Ra) was calculated with the
steady-state formula
|
(2)
|
where
INF is the rate of
[6,6-2H2]glucose
infusion in micromoles per kilogram per minute, and
[2H2]PEa
is the percentage of the arterial plasma glucose enriched with
[2H2]glucose.
During the experimental period (0-180 min), INF represents the
time-varying rate of infusion of
[6,6-2H2]dextrose
in micromoles per kilogram per minute at each time point, according to
the "Hot-GINF" method (23). Underestimation of the
Ra of unlabeled glucose in the
systemic circulation related to deficiencies in the monocompartmental
equations was minimized by maintenance of isotopic steady state during
the entire experiment (see RESULTS).
Endogenous glucose production (EGP) rate was calculated by subtracting
the rate of exogenous dextrose infusion from
Ra in
Eq. 2. Systemic glycerol
Ra was calculated by an equation
similar to Eq. 2, except that the infusion rate of
[2-13C]glycerol was
divided by arterial plasma enrichment of
[13C]glycerol. Net
renal glucose balance was calculated by the product of the
arteriovenous glucose concentration difference and renal plasma flow.
Renal fractional extraction of glucose
(FEg) was calculated with the
following formula
|
(3)
|
where
[Glc] is plasma glucose concentration, and PE refers to the
[2H2]glucose
plasma enrichment. Renal glucose uptake (RGU) was calculated with the
following formula
|
(4)
|
Because
glycosuria was not present, renal glucose utilization was assumed to be
equal to glucose uptake. Renal glucose production (RGP) was calculated
with the following formula
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(5)
|
Because
glucose is extracted into whole blood and there is rapid equilibration
between red cell and plasma glucose concentration, Eqs.
4 and 5 will underestimate renal glucose
production and utilization. The percentage of systemic
glucose production derived from glycerol was calculated by the
formula
|
(6)
|
where
[13C]Glc
PEa and
[13C]glycerol
PEa represent arterial plasma
enrichment of
[13C]glucose and
[13C]glycerol,
respectively. This is a standard product-precursor calculation that
takes into account the fact that 2 moles of glycerol are required to
produce 1 mole of glucose. The rate of systemic glycerol-derived
gluconeogenesis (GNGs) was
calculated with the following formula
|
(7)
|
The
contribution of the kidney to the
Ra of
[13C]glucose derived
from glycerol (GNGk) was
obtained from the following formula
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(8)
|
The
numerator in the formula represents the renal vein plasma
[13C]glucose
enrichment in excess of that anticipated from the known arterial plasma
[13C]glucose
enrichment and the fractional extraction of
[2H2]glucose,
i.e., that
[13C]glucose that has
been newly generated in the kidney. The denominator in the formula is
the precursor pool (glycerol) percent enrichment, again taking into
account the fact that 2 moles of glycerol are required to generate 1 mole of glucose. This formula will underestimate actual renal
gluconeogenesis from glycerol to the extent that the kidney metabolizes
newly synthesized
[13C]glucose. The use
of arteriovenous balance measurements and tracer techniques in the
above equations to assess renal glucose and glycerol kinetics has been
previously validated in animals (13, 15).
Statistics. All values obtained in
each study in the baseline and study periods were used in the
calculations and are expressed as means ± SE. Mean data at baseline
in each group were compared with those from the last 30 min of study
periods with paired t-tests. To
ascertain that steady-state conditions had been reached, changes within
each group over 30 min in the baseline and the last 30 min in each
experimental period were assessed by one-way ANOVA. Results obtained
during the experimental periods were compared among groups with
repeated-measures two-way ANOVA. All P
values <0.05 were considered statistically significant (36).
 |
RESULTS |
Arterial plasma insulin concentration increased from 37 ± 8 to an
average of 53 ± 5 and 102 ± 10 pmol/l
(P < 0.01) after insulin infusion at
the rates of 0.125 and 0.250 mU · kg
1 · min
1,
respectively, but it did not change in the saline control group (30 ± 7 vs. 35 ± 8 pmol/l). Renal plasma flow was 10.0 ± 1.6 and 10.8 ± 1.5 ml · kg
1 · min
1
at baseline and remained constant during the study period, 10.6 ± 1.7 and 11.3 ± 1.7 ml · kg
1 · min
1
(P = nonsignificant), in the low and
high insulin infusion groups, respectively; renal plasma flow did not
change during saline infusion (9.40 ± 1.2 vs. 9.10 ± 1.4 ml · kg
1 · min
1).
Figure 1 shows plasma glucose concentration
during baseline and 180-min experimental periods. Arterial plasma
glucose concentration averaged 5.23 ± 0.10 and 5.22 ± 0.05 µmol/ml in the saline control group, 4.70 ± 0.10 and 4.50 ± 0.20 µmol/ml in the low insulin infusion group, and 5.20 ± 0.20 and 5.05 ± 0.15 µmol/ml in the high insulin infusion group,
respectively, during baseline and in the final 30 min of the infusion
period. Renal vein glucose concentration did not change during saline
infusion (5.25 ± 0.10 vs. 5.27 ± 0.10 µmol/ml) but decreased
from 4.80 ± 0.20 at baseline to 4.52 ± 0.20 µmol/ml
(P < 0.05 vs. baseline) in
the last 30 min of the study period in the low insulin group and from
5.27 ± 0.23 to 4.90 ± 0.12 µmol/ml
(P < 0.05 vs. baseline) in the high insulin group. As a consequence, although net glucose balance did not
change significantly during saline infusion (
0.19 ± 0.10 vs.
0.46 ± 0.30 µmol · kg
1 · min
1),
it changed from a net output of
1.00 ± 0.40 to net uptake of
1.06 ± 0.50 µmol · kg
1 · min
1
(P < 0.05 vs. baseline) after low
insulin infusion and from a net output of
0.74 ± 0.15 to net
uptake of 1.70 ± 0.20 µmol · kg
1 · min
1
(P < 0.05 vs. baseline) after high
insulin infusion. As shown in Fig. 2,
arterial plasma enrichment of
[2H2]glucose
was stable at 2.83 ± 0.03% in the saline group, at 2.32 ± 0.03% in the low insulin group, and at 2.28 ± 0.03% in the high
insulin group during the entire experimental period and was
consistently higher than in the renal vein (2.75 ± 0.03%
in the saline group, 2.20 ± 0.02% in the low insulin group, and
2.25 ± 0.02% in the high insulin group). Endogenous glucose
production (Fig. 3) remained constant at
10.0 ± 0.4 µmol · kg
1 · min
1
during the entire saline infusion period. After insulin infusion, however, it decreased from 9.9 ± 0.7 at baseline to 6.9 ± 0.5 µmol · kg
1 · min
1
during the last 30 min (P < 0.01) of
the low dose and from 9.5 ± 0.7 at baseline to 5.7 ± 0.5 µmol · kg
1 · min
1
(P < 0.01) during the last 30 min of
the high dose. Whole body glucose disappearance rates were 10.8 ± 0.60 and 11.12 ± 0.80 µmol · kg
1 · min
1
in the last 30 min of the low and high insulin doses, respectively. Renal glucose production (Fig. 3) remained constant at 1.1 ± 0.4 µmol · kg
1 · min
1
during the entire saline infusion period, but when insulin was infused
at the rate of 0.125 mU · kg
1 · min
1,
renal glucose production decreased from 2.5 ± 0.6 to an average of
1.5 ± 0.5 µmol · kg
1 · min
1
(P < 0.05) in the last 30 min of the
experimental period. Similarly, when insulin was infused at the rate of
0.250 mU · kg
1 · min
1,
renal glucose production decreased from 2.6 ± 0.6 to an average of
1.2 ± 0.6 µmol · kg
1 · min
1
(P < 0.05) in the last 30 min of the
experimental period. Mean renal glucose utilization did not change
during saline infusion (0.91 ± 0.40 vs. 0.65 ± 0.30 µmol · kg
1 · min
1),
but it increased from 1.50 ± 0.60 to 2.60 ± 0.70 µmol · kg
1 · min
1
(P < 0.05) in the low insulin group
and from 1.60 ± 0.50 to 2.90 ± 0.70 µmol · kg
1 · min
1
(P < 0.05) in the high insulin
group, respectively, at baseline and in the last 30 min of the
experimental period.

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Fig. 1.
Plasma glucose concentration in µmol/ml in artery (solid symbols) and
in renal vein (open symbols) during baseline ( 30 to 0 min) and
study (0-180 min) periods, after either saline
(upper) or insulin infusion at the
rates of 0.125 mU · kg 1 · min 1
(middle) and 0.250 mU · kg 1 · min 1
(lower). Arterial plasma glucose was
maintained constant in all experiments, whereas renal vein glucose
concentration decreased in the last 30 min of study period
(* P < 0.05 vs. baseline) in
both insulin, but not saline, infusion groups.
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Fig. 2.
Plasma
[2H2]glucose
enrichment (%) in artery (solid symbols) and in renal vein (open
symbols) during baseline ( 30 to 0 min) and study (0-180
min) periods, after either saline
(upper) or insulin infusion at the
rates of 0.125 mU · kg 1 · min 1
(middle) and 0.250 mU · kg 1 · min 1
(lower). Steady state had been
achieved both at baseline and during the last 30 min in all experiments
(ANOVA). Plasma
[2H2]glucose
enrichment in renal vein was consistently lower than in artery at
baseline, whereas in the last 30 min of insulin infusion study periods
these became equivalent. Plasma
[2H
2]glucose enrichment in renal vein was equal or
lower than in artery during entire 180-min saline infusion period.
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Fig. 3.
Endogenous (solid symbols) and renal (open symbols) glucose production
rates in
µmol · kg 1 · min 1
during baseline ( 30 to 0 min) and study (0-180 min)
periods, after either saline (upper)
or insulin infusion at the rates of 0.125 mU · kg 1 · min 1
(middle) and 0.250 mU · kg 1 · min 1
(lower). Endogenous glucose
production decreased by ~30 and ~40%
(* P < 0.05, last 30 min vs.
baseline), respectively, in low and high insulin infusion groups, but
it did not change during saline infusion. Decrease in endogenous
glucose production is comparable in both insulin infusion groups, but
greater than saline controls at all time points
(P < 0.01, by two-way ANOVA). Renal
glucose production was ~40 and ~50% lower in the last 30 min of
study period than at baseline
(* P < 0.05) after low and
high insulin infusion, respectively, but it did not change during
saline infusion. Changes in renal glucose production were comparable in
all 3 groups, except that in the last 30 min of insulin infusion, the
decrease in renal glucose production was equal with both insulin doses
and different from saline group (P < 0.05 by two-way ANOVA).
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Table 2 summarizes data on systemic and
renal glycerol appearance and on gluconeogenesis from glycerol. Plasma
[13C]glucose and
[13C]glycerol
enrichments were stable during the baseline and in the last 30 min of
study periods in all experiments, indicating steady-state conditions
had been achieved. In the saline control study, plasma enrichment of
[13C]glucose increased
from 1.20 ± 0.03 to 1.45 ± 0.08%
(P < 0.05) in the artery and from
1.21 ± 0.02 to 1.52 ± 0.02% in the renal vein
(P < 0.05), whereas after insulin
infusion at the rate of 0.125 mU · kg
1 · min
1,
it decreased from 1.50 ± 0.02 to 0.93 ± 0.02% in the artery (P < 0.01) and from 1.45 ± 0.01 to 0.89 ± 0.02% in the renal vein. Similarly, after insulin
infusion at the rate of 0.250 mU · kg
1 · min
1,
plasma enrichment of
[13C]glucose decreased
from 1.23 ± 0.03 to 0.92 ± 0.02% in the artery (P < 0.01) and from 1.26 ± 0.02 to 0.94 ± 0.02% (P < 0.01) in the renal vein. Arterial plasma enrichment of
[13C]glycerol did not
change significantly during saline infusion (9.5 ± 0.4 vs. 8.9 ± 0.2%, P = 0.10), but it
increased from an average of 12.3 ± 0.2 to 16.3 ± 0.3%
(P < 0.01) and from 8.8 ± 0.4 to
15.1 ± 0.2% (P < 0.01),
respectively, in the low and high insulin groups. Arterial glycerol
concentration increased from 105 ± 11 in the baseline to 134 ± 3 µmol · kg
1 · min
1
after 180 min of saline infusion (P < 0.05), whereas it decreased from 60 ± 5 to 34 ± 10 µmol/l
and from 71 ± 6 to 38 ± 5 µmol/l (P < 0.01) after insulin infusion at
low and high doses, respectively. Similarly, arterial plasma free fatty
acid concentration increased from 771 ± 40 to 917 ± 34 µmol/l
(P < 0.05) after 180 min of saline infusion, but it
decreased from 710 ± 85 to 248 ± 22 µmol/l and from 692 ± 71 to 282 ± 37 µmol/l (all P < 0.01) after insulin infusion, respectively, at low and high doses.
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Table 2.
Whole body glycerol Ra and systemic and renal GNG in
baseline and last 30 min of experimental period during either saline or
insulin infusion
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 |
DISCUSSION |
The present studies confirm previous findings in dogs (13) and humans
(39) indicating that renal glucose production approximates glucose
utilization and accounts for ~10-25% of endogenous glucose production in postabsorptive healthy subjects, and they further document a reversal in renal glucose balance to net uptake during physiological hyperinsulinemia. Net uptake of glucose by the kidney is
due to simultaneous suppression of renal glucose production and
stimulation of glucose utilization by insulin. Low and high physiological insulin infusions reduce endogenous glucose production by
~30 and ~40% and suppress renal glucose production by ~40 and 50%, respectively, whereas renal glucose utilization nearly doubles after either insulin dose. Whether insulin suppresses renal glucose production in a dose-dependent fashion, however, analogous to its
effect on hepatic (endogenous) glucose production (10), cannot be
determined in these studies. The fact that neither endogenous or renal
glucose production nor renal glucose utilization changes significantly
during saline infusion confirms that these are not merely
time-dependent effects but secondary to insulin infusion. Plasma free
fatty acid, glycerol concentration, and glycerol turnover decrease by
~50%, and systemic and renal glycerol conversion to glucose are
substantially reduced with either insulin dose. On the basis of these
findings with a combination of arteriovenous balance and tracer
dilution across the kidney, it appears that a more complicated model of
glucoregulation, which takes into account possible different effects on
the kidney and liver of the large number of hormones (17, 35, 37) and
substrates (2, 18, 33) known to affect systemic glucose metabolism, is required.
The combination of arteriovenous balance and tracer dilution with
stable isotopes can effectively partition uptake and release in a
tissue bed and has previously been applied to the study of glucose (5,
15, 25), lipids (26), and amino acid metabolism (4). It should be
recognized, however, that differences in both glucose concentrations
and plasma enrichments across the kidney are small and may lead to
error. In these experiments, for example, rates of renal fractional
extraction of glucose measured with
[2H2]glucose
show considerable interindividual variation, ranging from 0 to 6.11%
in the postabsorptive period and during saline infusion and from 1.81 to 9.57% during the euglycemic-hyperinsulinemic clamp period
(individual data not presented). As a consequence, renal glucose
production, i.e., tracer dilution across the kidney, is detected in
some, but not all, individuals studied. The potential for error is
further amplified when arteriovenous differences are multiplied by
renal blood (plasma) flow, which represents ~20% of cardiac output
(~16
ml · kg
1 · min
1,
in our series). In addition, by assuming that negative fractional extraction rates are equal to zero in 4 of 18 postabsorptive subjects, we have introduced a bias and overestimated mean fractional extraction and renal glucose utilization rates in these studies by ~4% (mean renal glucose production is not significantly affected). Thus our
estimated rates of renal glucose production and utilization should not
be interpreted in absolute terms but as a near-quantitative assessment
of true rates. Nonetheless, even though the observations that the
kidney contributes to tracer-determined glucose production and is
insulin sensitive contrast with the prevailing notion that the liver is
the only significant source of glucose production under insulin
regulation in postabsorptive humans (10, 29), our findings are entirely
supported by the available data (3, 21, 42). The possibility that the
kidney makes a contribution to gluconeogenesis in the postabsorptive
state is further supported by the fact that nearly 20% of systemic
glycerol conversion to glucose (see Table 2) can be detected by
measuring [13C]glucose
percent enrichment in the renal vein in postabsorptive individuals.
Our studies provide strong evidence that the kidney is responsible for
a small and variable, albeit significant, fraction of systemic glucose
turnover in postabsorptive humans and that insulin suppresses renal
glucose production and stimulates glucose utilization under euglycemic
conditions. Maintenance of arterial plasma glucose and enrichment
constant during prolonged insulin infusion periods with the Hot-GINF
method (23) enables evaluation of the rate of onset as well as the
eventual maximal response to a given insulin concentration of glucose
production and utilization rates. Comparable to recent studies in
healthy subjects (10), it is apparent from Fig. 3 that the rate of fall
of endogenous glucose production at both insulin doses is greater
during the 1st h than in the subsequent 2 h. Considering that the two
insulin infusion doses were used in small samples of different subjects and that there is variability in insulin action even in lean normal individuals, however, one should use caution in comparing the effects
of the two insulin doses in these experimental conditions. Although
changes in renal glucose production show considerable variation during
the initial 120 min of insulin infusion, steady values lower than
baseline in the final 30 min of the hyperinsulinemic period were
achieved when each insulin dose has had its full effect in suppressing
endogenous glucose production. It must be emphasized that insulin
action on renal glucose metabolism is readily apparent from the
arteriovenous concentration difference data alone, although the use of
tracers provides additional information on individual rates of glucose
production and utilization. The observations that glucose balance
switches to net uptake and that renal glucose utilization, which
accounts for ~15% of systemic glucose disposal at baseline, becomes
responsible for ~25% after either insulin dose indicate that changes
in glucose handling by the kidney should be taken into consideration
when systemic glucose kinetics are evaluated during insulin infusion
studies in humans. Whether these effects are due to direct or indirect
insulin action (2, 29) and the extent to which the initial reduction in
endogenous (hepatic) glucose production attributed to insulin-induced
suppression of glucagon secretion (10, 30) is limited to the liver,
without affecting the kidney (40), were not evaluated. The findings that plasma free fatty acid and glycerol concentrations and systemic glycerol turnover, indexes of adipose tissue lipolysis, are equally reduced by ~50% and that systemic and renal gluconeogenesis from glycerol also decreases by ~50% at either insulin dose are
consistent with the notion that insulin suppresses gluconeogenesis, at
least in part, via inhibition of adipose tissue lipolysis (2, 27, 29).
Moreover, in agreement with previous data in hyperinsulinemic dogs (13)
and postabsorptive humans (31) and analogous to muscle (34),
although at a smaller scale, doubling of renal glucose utilization
after either insulin dose may have been secondary to a reduction in
uptake of circulating free fatty acid by the kidney. Because glucose
utilization by the kidney reflects both oxidative and nonoxidative
glucose disposal, it is also conceivable that renal glucose oxidation
enhances with acute stimulation of renal electrolyte transport (19,
20). Whether glycogen storage (8) and lactate formation in the distal
nephron (1, 44) are also stimulated by insulin infusion, and the
potential intermediary role of numerous other renal glucoregulators,
such as glucose itself and amino acids, in hyperinsulinemic conditions,
will require additional studies.
In summary, we have demonstrated that the kidney is responsible for
~10-25% of systemic glucose turnover in postabsorptive humans.
Renal glucose production is suppressed and glucose utilization is
stimulated during physiological hyperinsulinemia, which results in net
renal glucose uptake. Concomitant and proportional reductions in
circulating free fatty acid and in systemic and renal gluconeogenesis from glycerol suggest that insulin exerts its peripheral effects on
systemic glucose production and utilization, in part, by reducing renal
glucose production and enhancing renal glucose utilization via
inhibition of adipose tissue lipolysis. We conclude that glucose production and utilization by the kidney are important
insulin-responsive components of glucose metabolism in humans.
 |
ACKNOWLEDGEMENTS |
We thank E. Hayes, I. Zaitseva, S. Pacheco, and J. Vasek for
excellent technical assistance and J. Boshard for editorial help.
 |
FOOTNOTES |
This work was supported in part by grants from the American Diabetes
Association and National Institute of Diabetes and Digestive and Kidney
Diseases (DK-49861).
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. §1734 solely to indicate this fact.
Address for reprint requests: E. Cersosimo, Dept. of Medicine, Division
of Endocrinology, Health Science Center T15-060, SUNY at Stony
Brook, Stony Brook, NY 11794-8154.
Received 6 July 1998; accepted in final form 14 September
1998.
 |
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