CATEGORY: biology/anabolism
TECHNICAL: ***
SUMMARY:
This document talks about muscle growth, which by definition,
is what anabolism means. As you read through this material, take
note of a few of the facts. Specifically, that muscle cells can
only hypertrophy (grow) and not hyperplasia (split into two). Also
note the role of hGH, and IGF's. They are the key components of
muscle anabolism, and will be discussed in greater detail in the future.
"Muscle Memory" is one of the reasons I tell people to train.
Because, no matter how out of shape you are (or how old), you'll
be able to work yourself back to your best fitness level. You can
even go beyond, if you work at it..

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Date: Mon, 22 Jul 1996 12:47:15 -0400
Subject: Muscle Growth

Muscle Growth

Muscle growth is a specialized form of protein synthesis. As we saw
above, a steroid hormone (testosterone) enters the muscle cell by
diffusing directly across the cell membrane, combines with a receptor in
the cell and then stimulates gene transcription and protein formation via
the DNA -> mRNA -> tRNA -> protein pathway. Specific receptors and genes
are involved.

Muscle cells, as mentioned before, are long cells called myofibrils.
They differ from most other cells in that when muscles grow, the
individual cells simply become thicker and longer instead of dividing into
entirely new cells. Muscle cells also differ from most other body cells
in that muscle cells are multinucleated. A myofibril may increase in size
up to 28 times its initial size.

The interesting questions come in as we start looking at exactly how
and when this process occurs. Human growth hormone (hGH) and insulin-like
growth factors (IGFs) seem to play an important, though somewhat unclear,
role.

hGH is released from the anterior pituitary and travels through the
blood. It acts on the liver to release IGFs. Both IGFs and hGH are
peptide hormones; IGFs are structurally very similar to a large section
of the insulin molecule - hence their name.

What precisely happens at the muscle cell is not known, but we can
make some fairly well-informed speculation. Since IGFs are similar to
insulin, it makes sense to think that they would also have a similar
function. So IGFs probably work to increase uptake of amino acids and
glucose into muscle cells. It is not clear whether muscle cells have
receptors for hGH, but if they do, then it could be that hGH increases
nuclear division in muscle without triggering cellular division (mitosis).

We have seen how DNA and RNA are critical to protein synthesis, so it
is clear that having more nucleii within muscle would be very beneficial
for more rapid protein synthesis (muscle growth). It turns out that each
nucleus has a sort of effective "range". When the muscle grows, it can
only grow as far as the nucleii will "reach". So the number of nucleii
control the ultimate size of the muscle fiber. One of the major functions
of hGH is to stimulate cell division. Now, if there are hGH receptors in
muscle, but muscle cells lack the ability to divide, and hGH has an
anabolic effect on muscle, it stands to reason that hGH is increasing the
nuclear division process (and thus the total number of available nucleii
in the muscle), but the cytoplasmic separation process never kicks in.
Perhaps the mechanism for it that is found most cells has been lost over
time in muscle as an evolutionary adaption. (There is no doubt that
muscles are very important to survival!)

It seems then that hGH and IGFs might have complementary functions in
stimulating muscle growth. hGH could be instructing the muscle cells to
"build more factories" for muscle while IGFs could be stimulating the
cells to take in more "building blocks" for protein synthesis. Both hGH
and IGFs may affect other important components in the process as well -
such as increasing the production of hormone receptors or tRNA or
activating enzymes that accelerate transcription.

Multinucleation might explain the longstanding anecdotal phenomenon
most bodybuilders call "muscle memory". Muscle memory is recognized when
someone who has had a substantial muscular mass and then lost it due to
injury or layoffs from training, returns to training and regains the
majority of the mass in a much shorter time than was initially required to
develop it. What could be happening is this: the specific muscle
proteins in the muscle were cannibalized by the body for energy production
during non-use. However, the muscle retains the higher than average
number of nucleii that the previous exercise stress caused the body to
create. When presented with exercise and proper nutrients, new protein
synthesis can occur at an accelerated rate.


:cool: TJ :cool: