The foundation of our
understanding of metabolic
physiology is built on
discoveries in fundamental, but
isolated model systems.
Results from genes to
organelles and cells may belie
the physiognome.
A mechanism is only as
important as its functional
impact in the whole
organism.
Well-Controlled Animal Models Bridge
Cell Biology to
the Physiology of Exercise
The Study of Glucoregulation
Provocative or Sensitizing
Tests
• Physical Exercise
• Hormone and Metabolic Challenges (e.g.
hyperinsulinemic, euglycemic glucose clamps)
• Etcetera
Four Grams of Glucose
Maintaining 4 Grams of Glucose in the Blood
Sedentary, Postabsorptive
Brain
Fat
Liver
Glucose
~4 grams
Blood
Liver
Muscle
Maintaining 4 Grams of Glucose in the Blood
Feeding
Brain
Suppression
(Insulin)
Fat
Liver
Glucose
~4 grams
Blood
Liver
GI Tract
Stimulus
(Insulin)
Muscle
Maintaining 4 Grams of Glucose in the Blood
Exercise
Brain
Stimulus
Fat
Liver
Glucose
~4 grams
Blood
Liver
Stimulus
Muscle
Why don’t we get hypoglycemic when we exercise?
Exercise
6
Glucose
Utilization
mg･kg-1･min-1
If the liver does not
release more glucose
during exercise
0
6
Hepatic Glucose
Production
mg･kg-1･min-1
0
100
Arterial Plasma
Glucose
mg·dl-1
. . . Hypoglycemia
rapidly ensues
0
-30
0
Time (minutes)
60
Five Guiding Principles to Study of
Metabolism in vivo
• Glucose metabolism is all about flux control.
• Glucose flux control is distributed amongst distinct
systems that require an in vivo model to be fully
understood.
• Glucose fluxes are most sensitively regulated and
therefore best studied in the conscious state.
• Novel animal models can be used to bridge basic
and clinical research.
• Provocative tests are often necessary to precipitate
phenotypes and reveal functional limitations.
Endocrine and Sympathetic Nerve
Response to Exercise
Exercise
16
120
Glucagon
Arterial
Glucagon
80
12
pg·ml-1
40
8
Insulin
0
0
300
Norepinephrine
Arterial
Catecholamines
200
pg·ml-1
100
Epinephrine
0
-60
-30
0
30
60
90
Time (min)
120
150
Arterial
Insulin
µU·ml-1
A Minimal Overview of the Circulation
Investigator
sees…
Liver
sees…
arterial
head
and
upper
extremities
heart
and
lungs
pancreas
liver
gut
portal vein
venous
trunk
and
lower
extremities
Chronically-Catheterized Conscious Dog Model
Investigator
sees…
Liver
sees…
arterial
head
and
upper
extremities
trunk
and
lower
extremities
pancreas
heart
and
lungs
liver
gut
portal vein
venous
Exercise
Basal
Exercise
Basal
300
300
Portal Vein
Hepatic Vein
Plasma 200
Glucagon
(pg·ml-1)
100
Plasma 200
Epinephrine
(pg·ml-1)
100
Arterial
Portal Vein
Artery
0
-50
0
50
Time (min)
Hepatic Vein
100
150
0
-50
0
50
Time (min)
100
150
Protocols: Role of Glucagon
-120 min
-40
Equilibration
150
0
Basal
Moderate Treadmill Exercise
Somatostatin + [3-3H]glucose + [U-14C]alanine
Basal Intraportal Insulin
Saline
Exercise-Simulated Intraportal Insulin
Variable Glucose
Protocol A
Basal Intraportal Glucagon
Protocol B
Basal Intraportal Glucagon
Exercise-Simulated Intraportal Glucagon
Exercise as a model to study glucagon action
Exercise
150
Simulated Glucagon
Arterial Glucagon
100
pg/ml
50
Basal Glucagon
0
15
Arterial Insulin
Basal Glucagon
10
µU/ml
5
Simulated Glucagon
0
-60
-30
0
30
60
90
Time (min)
120 150
Exercise-induced Increment in Glucagon
Stimulates Hepatic Glucose Production
Exercise
120
Simulated Glucagon
Arterial
Plasma
Glucose
80
mg·dl-1
40
Basal Glucagon
0
Hepatic
Glucose
Production
10
mg·kg-1·min-1
4
Simulated
Glucagon
8
6
Basal Glucagon
2
0
-40
0
30
60
90
120
150
Exercise-induced Increment in Glucagon
Stimulates Gluconeogenesis from Alanine
Exercise
400
Simulated Glucagon
Gluconeogenesis
from Alanine
(% Basal)
300
200
100
Basal Glucagon
0
400
Intrahepatic
Gluconeogenic
Efficiency
from Alanine
(% Basal)
Simulated Glucagon
300
200
100
0
-60
Basal Glucagon
-30
0
30
60
Time (min)
90
120 150
6
5
Increase in
Endogenous
Glucose
Production
(mg·kg-1·min-1)
4
Comparison of the
Effects of Similar
Increases in
Glucagon at Rest
and during
Exercise
3
2
1
0
Rest
Exercise
Why is Glucagon so Effective during Exercise?
Brain
Autonomic
Nerve
Activity
Adrenal
Working
Muscle
Epi
Intestine
?
Adipose
Glycerol
NEFA
Pancreas
IL6
RBP4
Glucagon
Insulin
Amino
Acids
GNG
Lactate
Amino Acids
Glucose
4 grams
Substrates
Signals
Gly
Liver
Why is Glucagon so Effective during Exercise?
Brain
Autonomic
Nerve
Activity
Adrenal
Body is in a ‘Gluconeogenic Mode’
Epi
Intestine
?
Adipose
Glycerol
NEFA
Pancreas
IL6
RBP4
Glucagon
Insulin
Working
Muscle
Amino
Acids
GNG
Lactate
Amino Acids
Glucose
4 grams
Substrates
Signals
Gly
Liver
Why is Glucagon so Effective during Exercise?
Brain
Autonomic
Nerve
Activity
Adrenal
Body is in a ‘Gluconeogenic Mode’
Epi
Intestine
?
Adipose
Glycerol
NEFA
Pancreas
IL6
RBP4
Glucagon
Insulin
Effects are Potentiated
by the Fall in Insulin
Working
Muscle
Amino
Acids
GNG
Lactate
Amino Acids
Glucose
4 grams
Substrates
Signals
Gly
Liver
Why is Glucagon so Effective during Exercise?
Brain
Autonomic
Nerve
Activity
Adrenal
Body is in a ‘Gluconeogenic Mode’
Epi
Intestine
?
Adipose
Glycerol
NEFA
Pancreas
IL6
RBP4
Glucagon
Insulin
Effects are Potentiated
by the Fall in Insulin
Working
Muscle
Amino
Acids
GNG
Lactate
Amino Acids
Glucose
4 grams
 Glucose Uptake
Prevents
Hyperglycemia
Substrates
Signals
Gly
Liver
Protocol: Study of Splanchnic Amino
Acid Metabolism during Exercise
-120 min
-30
Equilibration
0
Basal
150
Treadmill Exercise
[5-15N]Glutamine + [1-13C]Leucine
The Exercise-induced Glucagon Response is Essential
to the Increment in Hepatic Glutamine Extraction
Simulated Glucagon
0.60
Hepatic
Fractional
Glutamine
Extraction
Basal Glucagon
*
*
0.40
*
†
0.20
0.00
†
Basal
25-50
75-100
125-150
Exercise Duration
(min)
Basal
25-50
75-100
125-150
Exercise Duration
(min)
The Exercise-induced Glucagon Response Drives Urea
Formation in the Liver
Simulated Glucagon
20
Net Hepatic
Urea Output
(mol·kg-1･min-1)
Basal Glucagon
*
*
15
*
10
5
0
Basal
25-50
75-100
125-150
Exercise Duration (min)
Basal
25-50
75-100
125-150
Exercise Duration (min)
The Exercise-induced Glucagon Response is Required
for the Accelerated transfer of Glutamine Amide
Nitrogen to Urea in the Liver
3.0
Formation of Urea from
2.0
Glutamine Amide Nitrogen
during Exercise
(mol·kg-1･min-1)
1.0
0.0
Simulated
Glucagon
Basal
Glucagon
Energy State and the Liver
Energy State in the Liver is Controlled by Glucagon
*
Studies using the
PhloridzinEuglycemic Clamp
further Illustrate the
Role of Glucagon in
Liver Energy Balance
Blood is Regulated like a
Homeostat
Liver is the Battery
(rechargeable)
Substrates and Signals Implicated in Control of
Glucose Fluxes to Working Muscle during Exercise
Brain
Sensors
Carotid Sinus
Liver/Portal Vein
Working Muscle
Autonomic
Nerve
Feedforward
Activity
Feedback
Chemical
Mechanical
Adrenal
Epi
Intestine
IL6
Adipose
Pancreas
IL6
RBP4
Glucagon
Insulin
Working
Muscle
Glycerol
NEFA
Amino
Acids
GNG
Gly
Liver
Lactate
Amino Acids
Glucose
4 grams
Substrates
Signals
What about the Famous Catecholamine
Response to Exercise?
• Epinephrine plays little to no role in control of glucose
production during exercise. Moates et al Am J Physiol 255:
E428-E436, 1988.
• Hepatic nerves are not necessary for the exercise-induced
rise in glucose production. Wasserman et al Am J Physiol
259: E195-E203, 1990.
• Liver specific blockade of both - and -adrenergic
receptors do not attenuate the increase in glucose
production during exercise. Coker et al Am J Physiol 273:
E831-E838, 1997. Coker et al Am J Physiol 278: 444-451, 2000.
Catecholamines
Essential, in association with the fall in insulin, for
extrahepatic substrate mobilization during exercise.
Muscle glycogenolysis
Adipose tissue lipolysis
NEFA Flux is Accelerated during Moderate Exercise by
Increased Lipolysis and Decreased Re-esterification
ATP
TG
FFA
G3P
Glucose
FFA
FFA
NE
TG
TG
TG
Glycerol
Glycerol
Glycerol
NEFA Flux is Accelerated during Moderate Exercise by
Increased Lipolysis and Decreased Re-esterification
ATP
TG
FFA
G3P
Glucose
FFA
FFA
NE
TG
TG
TG
Glycerol
Glycerol
Glycerol
Four Grams of Glucose
Controlling Rate of Removal
Extracellular
Membrane
glucose
6-phosphate
glucose
• blood flow
• capillary recruitment
• spatial barriers
Intracellular
• transporter #
• transporter activity
• hexokinase #
• hexokinase
compartmentation
• spatial barriers
Strategy
Selectively remove sites of resistance to MGU in
conscious mice by using transgenic mice or
pharmacological methods.
Ohm’s Law Applied to Glucose Influx
Current (I)
V1
Resistor1
V1 = I · Resistor1
V2
Resistor2
V3
V2 = I · Resistor2
Resistor3
V4
V3 = I · Resistor3
Glucose Influx (Ig)
Ga
RExtracell
Gextracell= Ig · Rextracell
Ge
RTransport
Gi
Gtransport= Ig · Rtransport
RPhosp
Gphos = Ig · RPhosp
0
Ohm’s Law to Determine Sites of Resistance
to Muscle Glucose Uptake
Glucose Influx
Ga
Ge
Gi
0
WT
Transgenics
GLUT4Tg
HKTg
GLUT4Tg
HKTg
Chronically Catheterized,
Conscious Unstressed Mouse
Sample
[3-3H]Glc
Blood
Insulin
Glucose
[2-14C]DG
Vein
Artery
From: Glucose Clamping the Conscious Mouse by Vanderbilt MMPC 2005
ptf 2002/jea 2005
Metabolic Control Analysis of MGU
• Control Coefficient ( C ) = lnRg/ln[E]
• Sum of Control Coefficients in a
Defined Pathway is 1
i.e. Cd + Ct + Cp = 1
Control Coefficients for MGU by Mouse Muscle
Comprised of Type II Fibers
Delivery
Transport
Phosphorylation
Rest
0.1
0.9
0.0
Insulin
(~80 µU/ml)
0.5
0.1
0.4
Exercise Protocol
-90
0
Acclimation
5
30 min
Sedentary or Exercise
[2-3H]DG
Bolus
Excise
Tissues
Sedentary and Exercising Mice
Exercise
Sedentary
250
200
Blood
Glucose 150
*
(mg·dl-1)
*
*
*
*
*
*
100
*
*
*
50
0
0
5
10
15
20
Time (min)
WT
HKTg
25
30
0
5
10 15 20
Time (min)
25
30
GLUT4Tg
HKTg + GLUT4Tg
Fueger et al. Am J Physiol; 286: E77-84, 2004
Sedentary and Exercising Mice
Gastrocnemius
†
†
40
†
20
0
Muscle Glucose
Uptake
(mol·100g-1·min-1)
20
SVL
10
†
†
†
0
†
Soleus
†
100
Sedentary
Exercise
50
0
WT
GLUT4Tg
HKTG
HKTg
+ GLUT4Tg
Fueger et al. Am J Physiol; 286: E77-84, 2004
Control Coefficients for MGU by Mouse Muscle
Comprised of Type II Fibers
Rest
Insulin
(~80 µU/ml)
Exercise
Delivery
Transport
Phosphorylation
0.1
0.9
0.0
0.5
0.1
0.4
0.2
0.0
0.8
Distributed Control of
Muscle Glucose Uptake
• Transport is clearly the primary barrier to muscle glucose uptake in the
fasted, sedentary state.
• Transport is so effectively regulated by exercise and insulin that the
membrane is no longer the primary barrier to muscle glucose uptake.
• The resistance to insulin-stimulated muscle glucose uptake with high
fat feeding is due, in large part, to defects in the delivery of glucose to
the muscle.
The vast majority of the literature on the regulation of glucose uptake
is comprised of studies in isolated muscle tissue or cells that are blind
to fundamental control mechanisms involved in muscle glucose uptake.
Four Grams of Glucose
Liver
Extracellular
gluconeogenic
precursors
glycogen
glucose
Intracellular
Extracellular
Membrane Membrane Intracellular
glucose
6-phosphate
glucose
6-phosphate
The distributed control of blood glucose allows for more precise
control of glucose homeostasis, multiple mechanisms of glucose flux
control, and multiple targets to correct dysregulation of metabolism
such as is seen in diabetes
Carefully conducted studies in the whole animal are necessary to
ascribe function to putative controllers of glucose homeostasis.