>>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.