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Growth Hormone
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In depth cell signalling and physiology notes, created using synchronous and asynchronous material.
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Growth hormone. Tissue growth can arise as a result of:
Increase in cell size- individual cells get physically bigger.
Increased cell number- increased number of cells within that tissue as a result of cell
division.
A tissue can get physically bigger due to cell differentiation- a cell will differentiate
into a particular cell type, which may have a bigger size.
All of these processes have one common factor: they all require the synthesis of new
material.
Increased cell division, the cell needs to replicate its genome, so it needs to
synthesize more DNA.
In all 3 cases we need to increase cell membrane and the number of cell organelles.
Because all of these processes require the synthesis of new material, they are
anabolic processes.
Growth: environmental regulation.
Growth needs to be highly regulated.
Growth is promoted when the nutritional environment is favorable.
When nutrients are in excess, growth will increase.
Example, birth weight against body mass weight of the mother.
There is a positive correlation, when the mother has a higher body mass index, the
developing fetus is exposed to more nutrients, the resulting birth weight of the baby
will be bigger.
Growth: temporal regulation.
Growth is also regulated as a function of time.
In an average lifespan there are periods of rapid growth in dispersed by periods of
slower growth and even negative growth, during old age.
Periods between 0-20 years growth rates are very rapid for height.
In adulthood there is very minimal growth.
In old age growth starts to decline.
Growth is regulated over time.
Growth: spatial regulation.
We also need to regulate growth in terms of space.
We need growth to occur in all tissues at the same rate, at the same time.
Growth hormones/growth factors.
Growth is tightly regulated by a selection of hormones/growth factors:
Growth hormone (GH)
Insulin-like growth factors (IGFs)
Insulin
Other factors (less common/important, some cell specific
Growth hormone.
Growth hormone is a peptide hormone, so it is transcribed from a gene.
, Gene for growth hormone is located on chromosome 17.
The matured fully processed peptide hormone is 191 amino acids long.
It has a molecular weight of 20KDA.
Average human will produce 500ug of growth hormone everyday.
The amount of growth hormone within the bloodstream are kept quite low 0-
30ng/ml.
Growth hormone has a very short half life. It only lasts around 20 minutes after it is
first synthesized.
Growth hormone release.
Levels of growth hormone within the circulation do not stay constant all the time.
They fluctuate over a 24hr period and this is the diurnal or daily pattern of secretion.
The graph shows:
We plot growth hormone serum levels as a function of time over 2 24 hour periods.
When growth hormone levels are low, they frequently fluctuate.
We can see daily surges of growth hormone secretion, that happen in a regular
pattern.
Growth hormone secretion is stimulated whilst in the sleeping state.
During deep sleep there is a surge in secretion which occurs during first 2 hours of
sleep.
They peak early morning just before awakening.
The lowest levels of growth hormone within the circulation are apparent when you
are awake during the day.
This poses some issues when we are trying to measure growth hormone in the clinic.
peak secretion early morning before awakening
lowest levels when awake during the day
external regulators of growth hormone secretion.
There are a number of external or environmental regulators that will both stimulate
and inhibit growth hormone secretion.
Growth hormones will be stimulated to be released by: Physical stress, Fasting, Deep
sleep.
Growth hormone will be inhibited by: Psychological stress, Feeding, Hypothyroidism,
Growth hormone itself, Ageing.
Fasting and feeding as stimulants and inhibitors of growth hormone may seem
counter intuitive.
Because growth is favored when nutritional environment is rich.
Growth hormone plays an important role in regulating blood sugar levels.
Growth hormone/somatotrophic axis.
Endocrine axis that regulates growth hormone secretion is the growth hormone/
somatotropic axis.
Every endocrine axis starts with some kind of stimulus.
The stimulus will either poke the higher brain centers which will then talk to the
hypothalamus.
Or the stimulus will stimulate the hypothalamus itself.
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