Endocrinology 9 - Growth Hormone A tissue can get bigger by; Increasing the individual size of the cells in that tissue Number of cells within that tissue Through processes of cell differentiation All of these require the cell to synthesise new material e.g. more cell membrane, cytoplasm, more organelles needs to replicate its DNA So, growth is an anabolic process (synthesising stuff) Growth: Environmental Regulation Growth is promoted when nutritional environment is favourable Do not what to grow if there are not enough nutrients We may not want to initiate growth programs and want to conserve our resources, to initiate survival mechanisms reproduce so that our offspring get through that stage of poor nutrition Higher BMI = higher children birth weight More nutrients for the baby So, this is an example for when growth is matched to the environment Growth: Temporal Regulation Human growth over the average lifespan; Their a period where we undergo rapid growth During old age, we will even see negative growth Growth: Spatial Regulation We need to regulate growth across a whole system The tissues need to talk to one each other Need to maintain similar growth rate across all physiological systems If an animal grows through a stage of starvation, there are two tissues that will continue to grow under these poor nutrition stages Brain and gonad (reproductive organs) Growth Hormones/Factors Growth is tightly controlled by a selection of hormones and factors: - Growth Hormone (GH) - Insulin-like growth factors (IGFs) - Insulin - Other factors Growth Hormones A peptide hormone Quite big: 191 amino acids Molecular weight 20kDa Average human will produce about ~500ug a day
Circulating our body is ~0-30ng/ml Short half-life ~20 minutes When looking for growth hormones in the blood they are often undetectable They also fluctuate over 24 hours They show a diurnal or daily pattern of secretion This depends on your own circadian rhythm 30- GH serum levels (ng/ml) 20- 10- 6 12 18 24 6 12 18 24 Time (hours) 1. Secretion of growth hormones will be stimulated as you got to sleep 2. Surge in secretion during first two hours of sleep 3. Peak secretion early morning before awakening 4. Lowest levels are when awake during the day Hormone Secretion: - It's very sensitive to external stimuli - There are environmental factors that will inhibit or secrete the hormone Stimulants Inhibitors · physical stress (exercise) · fasting (hypoglycaemia) · deep sleep · psychological stress · feeding (hyperglycaemia) · hypothyroidism · growth hormone itself · ageing Feeding? But we grow when nutrients is abundant - When blood sugar drops, it's a signal to the hypothalamus to release GHRH o That's why low blood sugar is going to stimulate GH release - When you eat, this stimulates the release of GHIH This is why feeding is an inhibitor of GH GH secretion is controlled by an endocrinological axis (somatotropic axis) - Start off with the stimulant
o Brain will detect the stimulant and transmit that signal to the hypothalamus