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Selected Hormonal Issues
Relating to Exercise and
Substrate Use
Remember This Stuff???
ATP used up by muscle contraction can be regenerated through 4 processes:
1-MK
2-CPK
3-Glycolysis
4-Oxidative Phosphorylation
. . . With processes #3 & #4 being dependent on availability of glucose, fatty
acids, and amino acids . . .
We now need to look at estimates of body “stores”
of energy substrates
Tissue Stores
Adipose TG - all
muscles
~g
9,000
~ Days
Starvation
34
~ Days
Walking
~ Minutes
Marathon
Running
10.8
4,018
7 - 14 g/kg @ ~ 20 - 30 kg muscles = 140g – 420 g
0.25 – 0.75
90 - 270
Liver Glycogen
90
0.15
0.05
18
Muscle Glycogen
350
0.6
0.2
71
Blood Glucose
20
0.03
0.01
4
Regulation of glucose levels in the blood is very important
Normal Fasting (Serum) ~ 70 – 100 mg/dl
Elevated = Diabetes
Low = Hypoglycemia
Insulin from pancreas stimulates uptake of glucose into muscle
cells by activating the GLUT4 transporter. When serum levels
of glucose decrease too much, then the pancreas releases
glucagon to stimulate the liver and kidney to break down
glycogen to glucose and release glucose into the blood
(glycogenolysis). Adrenals will release cortisol if levels stay too
low to enhance breakdown of protein in muscles to release
amino acids so the liver can pick up the amino acids and make
them into glucose (gluconeogenesis).
Insulin binds to
the insulin
receptor which
then activates a
series of
phosphorylase
& proteinprotein enzyme
reactions ending
up with the
translocation of
the GLUT4
protein to the
cell membrane.
Notice that at rest FFA provide the
majority of the cells energy
requirements . . . and insulin can
greatly increase uptake of glucose
into the cell . . .
Because the energy needs of the cells have
not changed all that much after a meal,
most of the additional uptake of glucose is
handled in the storage pathways while
some is metabolized at the expense of FFA
(or glucagon binding to
the glucagon receptor)
At rest, epinephrine can
activate glycogen and TG
breakdown in muscles . . .
it is important to note that
P’ase b also can be
activated by calcium and
by ADP . . . AND . . . that
Triglyceride Lipase, aka
hormone sensitive lipase,
can be activated by PKC
and ERK.
In the liver and kidney,
glucagon will bind to the
glucagon receptors &
activate PKA to enhance
glycogen breakdown in
these tissues for export to
the blood and FFA
metabolism to inhibit
glycolysis . . .
Growth hormone is released in large quantities during
prolonged periods of fast growth, such as during early
childhood and adolescence. Stressful exercise also
stimulates a transient increase in release of GH.
Note that GH “directly” activates STAT, PLC, IRS1&2
and SOS/Grb-2. In addition, both typical PKC (via IP3
and DAG) and atypical PKC (via PDK-1) are activated.
These effects result in the activation of an array of
promoter molecules as well as glucose uptake and TG
breakdown.
Muscle contraction can activate glucose
transport independent of insulin and
activate TG breakdown independent of
epinephrine, greatly increasing glucose
uptake and FFA availability . . .
As a result of muscle contraction, all
metabolic pathways of ATP generation
increase while those of storage and
synthesis decrease . . . Because the
glycolysis pathway increases so much
faster than the others, glycogen stores in
the muscle will decrease very quickly
compared to TG stores . . .
Moderately stressful exercise:
As a result of continuous muscle contraction, all metabolic pathways
of ATP generation increase while those of storage and synthesis
decrease . . . Because the glycolysis pathway increases so much faster
than the others, glycogen stores in the muscle will decrease more
quickly than TG stores . . . increasing the dependence of muscles on
plasma supplies of substrates . . . a hormonal issue . . .
- Time to exhaustion:
~ 2 - 3 hours
Subjective feelings of fatigue appear to coincide with an
increase in glucagon and cortisol levels. These increased
glucogenic hormones are often associated with a decline in
serum glucose - resulting predominantly from glycogen
depletion in the liver (and muscles) or associated with stress;
resulting from anything that induces a state of anxiety or
tension (such as being sore? . . . tired? . . . annoyed?)
Declining glycogen stores means a greater reliance on the less
efficient fuel: FFA (you have to slow down).
Consuming carbohydrate during exercise should blunt the
endocrine response as well as delay fatigue . . .
-Time to exhaustion:
~ 2.3 – 3.3 hours
One more endocrine issue is the effect of repeated exercise
on the endocrine response to acute exercise.