>>Dr. Ketchum: In this segment you will learn how growth hormone, a peptide hormone, is
synthesized and secreted and how growth hormone synthesis and secretion is turned off. So
to begin, let’s first think about what stimulates growth hormone release.
So we’re thinking about what would cause this system to begin? So if you had a high
plasma amino acid level and a low plasma glucose level, these are two things that could stimulate
growth hormone synthesis and secretion. Now growth hormone is also synthesized and secreted
on a circadian rhythm, meaning that during the nighttime growth hormone is going to be
high. So we are synthesizing and secreting growth hormone while we’re resting, and
during the daytime we’re not synthesizing and secreting growth hormone. So let’s go
ahead and look at how growth hormone is synthesized and secreted. We’re going to begin with
the hypothalamus.
So the hypothalamus is located at about the center of your brain. And we’re going to
focus on some neurons located in the hypothalamus called neurosecretory neurons. So what neurosecretory
neurons are going to be doing in this case is they will be synthesizing and secreting
growth hormone (GH), releasing-hormone, (RH). Now it’s going to be very important for
you to learn both the abbreviation of the—for the hormone names, as well as the full hormone
name as well.
So here, it travels down the axon of these neurosecretory neurons and into the axon terminal.
Growth hormone releasing hormone is then released from the axon terminal. Growth hormone releasing
hormone will then enter into the hypophyseal portal system, which is basically a capillary
system that connects the hypothalamus to the anterior pituitary. Then growth hormone releasing
hormone will bind to receptors on the endocrine cells within the anterior pituitary, and here
are those endocrine cells.
So once growth hormone releasing hormone binds to the receptors on the endocrine cells, that
signals these endocrine cells to synthesize and release growth hormone. Growth hormone
then enters the systemic circulation. Once growth hormone enters into the systemic circulation,
then growth hormone can go off and target certain tissues. Growth hormone, though, will
also target the liver. When growth hormone targets the liver, it targets some cells that
will synthesize and secrete insulin-like growth factor one. So once growth hormone targets
the liver, it causes synthesis and secretion of IGF-1. This is also called a somatomedin
until a growth factor one has two targets. It will target the bone and cartilage, and
it will also target skeletal muscle. When insulin-like growth factor one targets the
bone in the cartilage muscle, it promotes cell division and bone growth. Now IGF-1 also
targets the skeletal muscle, which will promote protein synthesis. So by promoting protein
synthesis, now we can promote the growth of skeletal muscle. So by synthesizing and secreting
growth hormone and then synthesizing and secreting insulin-like growth factor one, we’ve got
bone growth and skeletal muscle growth. Now growth hormone will also target other tissues
as well.
So let’s begin with growth hormone targeting your adipose tissue. And when growth hormone
targets adipose tissue, this causes lipolysis. You know our two products of lipolysis are
glycerol and fatty acids. The glycerol can go into the liver and be converted to glucose.
And then that glucose from the liver can go into circulation, so we will increase our
glucose concentration in the circulation. Remember, one of the stimuli for synthesizing
and secreting growth hormone was a low plasma glucose concentration. So in this case, we’ve
elevated our glucose concentration. Now the fatty acids can also enter into circulation,
and this will increase our plasma level of our fatty acids as well. Now growth hormone
can also target a lot of other types of tissues. And when it does target these tissues, it
decreases glucose utilization. So if we decrease glucose utilization by the cells, meaning
the cells are not going to be using up that glucose, then your glucose level in the circulation
can remain elevated. So what this does then, if we have increased plasma glucose and increased
fatty acid levels in the plasma as well, these can all provide the energy that’s needed
for growth.
So we’re going to begin with the insulin-like growth factor that was synthesized and secreted
by the liver. Insulin-like growth factor, once it’s synthesized and secreted, will
have a negative effect on the anterior pituitary. Specifically, what it does is it inhibits
the responsiveness for growth hormone releasing hormone. So if I inhibiting the responsiveness
for growth hormone releasing hormone on the endocrine cells, then those endocrine cells
will not synthesize and secrete growth hormone. Insulin-like growth factor one can also act
on the hypothalamus and have a negative impact on the hypothalamus as well, because it will
inhibit secretion of growth hormone releasing hormone. Again, IGF-1, insulin like growth
factor, inhibits secretion of growth hormone releasing hormone from the hypothalamus. So
if we don’t release growth hormone releasing hormone from the hypothalamus, then you cannot
stimulate the anterior pituitary to synthesize and secrete growth hormone. So in essence,
we’ve stopped the synthesis and secretion of growth hormone.
There’s also another hormone synthesized and secreted by the hypothalamus that can
help turn off secretion of growth hormone, and this is called growth hormone inhibiting
hormone. So the neurosecretory neurons in the hypothalamus can also synthesize and secrete
growth hormone inhibiting hormone: GHIH. So if growth hormone inhibiting hormone is synthesized
and secreted, it inhibits the anterior pituitary from synthesizing and secreting growth hormone.
So we have two ways in which we can turn off growth hormone synthesis and secretion. One
of those involves the negative feedback loop, utilizing insulin-like growth factor one.
The other is by synthesizing and secreting growth hormone inhibiting hormone.