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.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. We are synthesizing and secreting growth hormone while we’re resting, and during the daytime we’re not synthesizing and secreting growth hormone. Let’s go ahead and look at how growth hormone is synthesized and secreted. We’re going to begin with the hypothalamus.
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. 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.
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. 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.
Let’s begin with growth hormone targeting your adipose tissue. And when growth hormone targets adipose tissue, this causes lipolysis. Remember, one of the stimuli for synthesizing and secreting growth hormone was a low plasma glucose concentration. 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. 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. 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. 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. 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. The neurosecretory neurons in the hypothalamus can also synthesize and secrete growth hormone inhibiting hormone: GHIH. If growth hormone inhibiting hormone is synthesized and secreted, it inhibits the anterior pituitary from synthesizing and secreting growth hormone. 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.