In our final lecture on muscle, we're going to look at what changes take place
in muscle in response to various types of exercise.
When muscles are activated during exercise, we need to think of some of the
changes that are taking place, how they're detected, and what might be some
of the consequences that mediate these adaptations to exercise.
So, if we think about the various stimuli, that might activate pathways
that lead to muscle adaptation there are a number of obvious candidates.
A number of times, we've spoken about calcium.
Calcium is fundamental to muscle contraction.
And it activates a number of processes within the muscle cell.
Changes in energy status as the muscle breaks down ATP and other important
fuels. That's, they are potentially signals that
could lead to adaptation. Changes in the redox state, the oxidation
and the reducation reactions that occur within the muscls.
Changes in muscle temperature, muscle tension, changes in metabolites, changes
in circulating hormones. All of these can change during exercise
and impact on the muscle. In recent years, there's been a lot of
work on the various key enzymes and kinases phosphatase and various proteins
that detect these signals. The calcium sensitive enzymes include
calcium calmodulin-dependent kinase, calcineurin.
Energy status is monitored by the AMP activated protein kinase.
There are mitogen activated protein kinases that seem to play a role in
growth adaptations. An important enzyme involved in protein
synthesis known as emptor. And certainly with the advances in
molecular biology and cell biology we understand a lot more about these
molecular responses. And so how do these signaling pathways
impact ultimately on gene expression, messenger RNA expression, protein
expression. Various transcription factors MEF2,
GLUT-4 enhancement factor, PGC-1 alpha, which is thought to play an important
role in mitochondrial biogenesis. Our all transcription factors that are
been in implicated in the adoptive response of muscles to exercise.
And more recently interest in small non-coding pieces of RNA called micro
RNAs, which have been shown to influence the expression and transcription of genes
and the expression of proteins. And ultimately, all of these changes are
coordinated so that there is either a, or there's a change in either the expression
and or the activity of various proteins, which are fundamental to muscle
performance. And effect, ultimately, the functional
properties of that muscle. So, here in graphical form and nicely
summarized we can see various signals that are acting on a contracting muscle.
Either in an endurance type prolonged exercise, high intensity exercise but
with dynamic exercise. Through two more resistance type exercise
where the adaptations perhaps are more related to muscle mass or what we can
refer to hypertrophy. And you can see all of the signals here,
the circulating factors in the bloodstream, the metabolites that we're
going to talk about in the fuels lectures.
And how these can impact on various signalling kinases to either increase
mitochondria or to increase muscle mass. If we focus on some of the adaptations to
endurance type exercise, these are largely focused on increasing the
oxidative capacity of the muscle. The mitochondria who are the oxidated
powerhouse of the muscle cell. They increase in their total volume in
response to exercise and that has important metabolic consequences.
We see large increases in mitochondrial density and oxidated enzymes to response
to endurance type exercise. We also see an increase in capillary
density. And you can see in this slide here, the
capillaries that circle muscle fibers and there are more of them after training.
And that facilitates the delivery of oxygen to the muscle and needs to occur
in parallel with the increases in oxidative capacity within the muscle.
There's an increase in the glucose transport protein GLUT 4, and that helps
facilitate the storage of glycogen. There's an increase in the sodium
potassium ATPase activity. And that is an enzyme that's critically
involve in the activation of the muscle in limiting the loss of potassium from
contracting muscle. And question mark on fiber type changes.
I mentioned in a previous lecture, it's been ongoing discussion whether prolonged
endurance type exercise can induce fiber type changes.
And there are some studies suggesting that that can occur, and certainly in
animal studies with chronic long term electrical stimulation.
You see quite profound transformation of the characteristics of skeletal muscles.
In terms of the functional consequences, we see reduced reliance on carbohydrates,
reduced lactate production and increased fat oxidation during exercise after
endurance type training. And we'll talk more about this in the
next module on fuels for exercise. From a health perspective, endurance
training also improves the action of insulin.
And that's thought to be very important in minimizing the risk and indeed
managing metabolic diseases such as type II diabetes.
In terms of specific adaptations, let me give you just one example.
And my apologies for being a bit parochial.
But, this is one of our research interests here at the University of
Melbourne. How does endurance training impact on the
expression of the GLUT4 transport protein.
And here's one of our early studies where we examined cross sectionally the
differences between untrained and trained subjects in terms of their oxidative
capacity as measured by the maximal activity of an enzyme in the mitochondria
called citrate synthase. And the glucose transport protein GLUT4.
And you can see that the trained subjects had higher muscle oxidative capacity and
higher levels of GLUT4 compared with the untrained subjects.
When we asked these trained subjects to stop their regular training for ten days,
and report back to the lab. You can see that there was a reduction in
their oxidative capacity and a reduction in their GLUT4 expression.
So, cessation of training reduced or diminished some of these adaptive
responses. You'll note that they didn't fall all the
way down to untrained levels. Whether that would take a longer period
of detraining, or whether these athletes have some genetic characteristics, which
endow them with slightly higher muscle oxidative capacity.
Or GLUT4 again, is one of those questions we may ask later in the course.
If we look at the factors that regulate GLUT4 expression, and this seems like a
complex slide. But it really reprises the thing in the
very first slide about signals, enzymes that detect those signals and functional
consequences. So, in the case of GLUT4 expression there
are two main signals that are thought to be important.
One is the increase in calcium, which activates the calcium calmodulin
independent protein kinase and the other changes in the high energy metabolites.
The change in the energy status of the muscle cell, activates the AMP activated
protein kinase. Both of these enzymes can target various
transcription factors, which bind to the GLUT4 gene.
And ultimately changes in their localization and their activity increase
GLUT4 transcription so that you see increase GLUT4 MR.
And I after exercise in skeletal muscle and ultimately is incresed in MRI is
translated into a increased GLUT4 protein.
This increased GLUT4 protein means that you're better able to take up glucose in
response to insulin and in response to contractions.
In our next module, we'll talk about glucose uptake during exercise and the
importance of GLUT4 for that process. Interesting observation that's been made
for a number of years is that intensity can often be a very powerful stimulus.
And here's a recent study looking, comparing two types of exercise training,
designed to increase the functional capacity of muscle.
And they measure the oxidation capacity again using the enzyme the activity of
enzyme, the activity of the enzyme citrus synthesize.
And there are two types of exercise training implied one was ver high
intensity exercise that you can see here, involved repeated 30 sec springs, three
per week over about a six week period. The other was a more traditional
endurance type training program, 40 to 60 minutes of cycling, five times per week.
You can see here, over the entire training period, the high intensity
exercise for a total about ten minutes at a very high intensity and the endurance
training group for a total of four and a half hours.
And yet when you looked at the increases, in citrating those, they were somewhat
similar. There was no statistically significant
difference between the increase. Whether you would see changes in the long
term remains to be seen, but it makes the point that intensity can be a powerful
stimulus. And in time, when people are increasingly
time-poor, there has been some interest in using these high intensity training
programs in health and disease to try and promote these muscle adaptations.
The other important adaptation that it can occur in muscle is an increase in its
size and one of the hallmark adaptations to prolong resistance type exercise,
weight lifting type exercise, is an increase in muscle mass.
You can see in this slide relatively short exercise period quiet a large
increase in maximal voluntary contraction ,the force generating capacity of the
muscle with relatively modest changes in the muscle cross sectional area of the
mark of size of the muscle. So, if you measure the EMG or the
electrical activity, you can see that much of the early increase in strength,
functional strength, following resistance training is due to neurological
adaptation. But over time you do see increases in
muscle mass. Conversely, if a muscle is subject to
disuse because of injury, sickness subjects that confined to bed, like have
a cast, a, a range of reasons that might mean they're unable to fully activate the
muscles fully on a regular basis. You can see that the myofibrillar protein
synthesis decreases quiet quickly and in times this results in cross section of
muscle cross sectional surface area. Again, its been a lot of interest in
regulation of muscle mass because of the importance of muscle mass in health and
disease. For athletes, it is important because
it's the force generating tissue. In normal, healthy individuals an
adequate muscle mass is important because of the role that skeletal muscle has in
supporting movement, supporting body functions and supporting metabolism.
And one of the consequences of aging and a number of disease state is a loss of
muscle mass, and that can impact on functional capacity and the quality of
life. So, there's been a lot of interest in the
regulation of muscle mass. In terms of building muscle, an important
enzyme appears to be emptor. and this has an important role in protein
synthesis. It's influenced by a number of growth
factors acting through insulin-like pathways.
On the other side, factors which influence the degradation of muscle or
muscle atrophy. There's been a lot of interest in the
so-called ubiquitin ligases, which are involved in breaking down muscle and how
that might be regulated. In relation to exercise effects on one of
those signaling proteins, emptor, what we see is that exercise activates that
enzyme. At least as measured by its
phosphorylation status, which is the usual way that we, we measure the
activation of these proteins. If you look at protein synthesis, and
here is the measure, the fractional synthetic rate for protein, during
exercise the protein synthesis rate actually goes down.
And given that exercise is an energy demanding process and protein synthesis
requires energy, it makes sense to perhaps turn that off while you are
exercising. But you can see after exercising, an
increase in protein synthesis. And so with regular bouts of exercise.
Regular activation of emptor and various other proteins synthetic pathways.
You get an increase, a cumulative increase of protein, myofibrillar
protein. Muscle gets bigger and the muscle gets
stronger. And finally on interesting aspect of
muscle adaptation exercise that's really been studied in great detail in the last
decade or so. And that's the observation that
contracting muscles in fact release biologically active substances.
And these have been termed myokines, and don't worry about the detail in this
slide in terms of specific myokines. But you can see a number of them are
released and they have a fix on adipose tissue, on the liver, perhaps on the
pancreas, on the bone, on blood vessels. And this raises the interesting
possibility and likelihood that these biologically active compounds that are
released from contracting muscles have effects on tissues distant from the
muscle. And myokine at least partly for the
systemic benefits of radialer muscular exercise in promoting health and well
being.