lecture 26 - The pancreas, the hypothalamus and pituitary, growth hormone Flashcards
Exocrine gland
secrete substances out and external to the body (the gut is consider the outside of the body)
e.g. cells of the pancreatic acini secrete digestive enzymes
Endocrine gland
secrete their products, hormones, directly into the blood.
Beta cells
Produces and secretes the hormone insulin. Insulin lowers blood glucose levels by increasing the rate of glucose uptake and use by most body cells, and by increasing glycogen synthesis in skeletal muscles and liver
Alpha cells
Produces and secretes the hormone glucagon. Glucagon raises blood glucose levels by increasing the rates of glycogen breakdown and glucose release by the liver
Pancreas
The pancreatic islets secrete hormones regulating the rate of glucose uptake and utilisation by body tissues
Pancreatic islets
Endocrine gland
Blood glucose concentration
Must be maintained within a narrow range at all times for normal functions. If it is too high for too long, diabetes develops. If too low, hypoglycaemia occurs. The brain must be supplied with glucose at all times, as glucose is the only fuel that the brain uses but blood glucose concentration changes throughout the day as we use fuel continuously but we eat intermittently
Fed state
Cellular uptake of nutrients and anabolic metabolism (synthesis of glycogen, protein and fat)
Mediated by insulin
Fasting state
Mobilisation of nutrients and catabolic metabolism (breakdown of glycogen, protein and fat)
Mediated by glucagon
Two metabolic states during the day
Fed state and fasting state
How is blood glucose regulated?
Hormonal regulation - insulin and glucagon maintain blood glucose concentration between 70-110 mg dL^-1 (reference range)
Insulin secretion is increased when blood glucose concentration increases and glucagon secretion increases when blood glucose concentration decreases.
Insulin
Insulin is a peptide hormone therefore target cells have membrane receptors
The increase in blood glucose concentration stimulated the pancreatic islet beta cells to secrete insulin into the bloodstream (beta cells are the sensor and control centre)
This hormone in the target cells …. In muscle and adipose cells it stimulates the uptake of glucose (storage of glucose), increases amino acid uptake, glycogen and protein synthesis and adipose tissue is going to utilise the glucose to form fat. In the liver cells the glucose output stops and this stimulus stimulates the increase in net glucose uptake which causes glycogen and fat synthesis
Overall this leads to decrease in blood glucose concentration and this stimulus acts as negative feedback to inhibit the loop from continuing despite the stimulus of high blood glucose concentration now being gone (response reduces the stimulus)
Note that insulin is the only hormone that can decrease the blood glucose concentration
Insulin overall causes
Insulin overall increases fuel storage (glycogen)
Glucagon
Glucagon is a peptide hormone therefore target cells have membrane receptors
The decrease in blood glucose concentration stimulates the pancreatic islet alpha cells to secrete glucagon into the bloodstream. (alphas cells are the sensors and the control centre)
This hormone in the liver cell (target cell) stimulates the increase breakdown of glycogen (glycogenolysis), increase in glucose synthesis (gluconeogensis) and increase in ketone synthesis(ketones are used by some cells to create energy)
Overall this leads to an increase in blood glucose concentration and an increasing in blood ketone concentration (this happens so that the blood glucose can be directed primarily towards the brain) . This stimulus acts as a negative feedback to inhibit the loop from continuing since the stimulus of low blood glucose is gone (response reduces the stimulus)
Not that growth hormone, adrenaline and cortisol can also increase blood glucose concentration
Glucagon overall causes
Increased fuel release
Glycogenolysis
Breakdown of glycogen
Gluconeogenesis
Glucose synthesis
Glycogen
Stored form of glucose
Pituitary gland
Located at the base of the brain and is attached to the hypothalamus. The hypothalamus controls the secretion of the pituitary hormones (when activated by neural input, the hypothalamus stimulates the pituitary glad to secrete hormones
Some pituitary hormones stimulate target cells and some stimulate the secretion of hormones by other endocrine glands
Has two parts - anterior lobe and posterior lobe and each part has a distinctive mechanism for hormone secretion
Hypothalamus
Controls the secretion of the pituitary glands
Anterior lobe of the pituitary gland
Connected to the hypothalamus by blood vessels
Neural input to hypothalamus triggers the release of inhibiting or releasing hormones into blood vessels
Hormones travel via blood vessels to anterior pituitary
Hormones bind to receptors in anterior pituitary
Triggers release of another hormone into blood circulation
Posterior lobe of the pituitary gland
Connected to the hypothalamus by neurons and secretes hormones into the blood
Connected to the hypothalamus directly through neurons
Cell bodies in hypothalamus and axon terminals in posterior pituitary
When action potentials reach the axon terminal, they do not synapse onto another neuron but instead release their own hormones into the blood
How does the hypothalamus communicate with the posterior pituitary?
The hypothalamus is connected to the posterior lobe by neurons - cell bodies in the hypothalamus and axons terminate in the posterior lobe.
Posterior lobe hormones are made in the hypothalamus (cell body of neuron), travel down the axon and are stored at the axon endings until requires (peptide hormones)
The hypothalamus uses neural communication with the posterior lobe to release hormones into the blood
Hormones released by the posterior pituitary?
ADH (antidiuretic hormone) promotes water retention in kidneys (stimulates the kidneys to reabsorb water, the kidneys conserve water when the body dehydrates)
Oxytocin stimulates uterine muscle contraction during childbirth (an example of positive feedback) and milk release
Both of these hormones are made in the hypothalamus and stored in the posterior pituitary until required
How does the hypothalamus communicate with the anterior pituitary?
Stimulus is the neural input within the hypothalamus.
The hypothalamus signals the anterior pituitary to release hormones by hormonal stimulation, secreting stored releasing hormones (or inhibiting hormones)
Hormone binds to receptor of membrane of a specific cell type and a specific peptide hormone is secreted e.g. prolactin, growth hormone
Feedback regulation of the anterior pituitary
In the hypothalamus the release of a releasing hormone stimulates the anterior pituitary gland to release the pituitary hormone which stimulated the target organ to release its hormone
There are negative feedback loads which prevents too much releasing hormone being release (either by the pituitary hormone or the releasing hormone itself in some cases)
Anterior pituitary hormones
Growth hormone, Thyroid stimulating hormone, prolactin, ACTH
Growth hormone
Growth hormone (GH), also called somatotropin or human growth hormone, peptide hormone secreted by the anterior lobe of the pituitary gland. It stimulates the growth of essentially all tissues of the body, including bone.
Direct and indirect effects of growth hormone
Direct effects
Muscle - stimulates protein synthesis and inhibits cellular uptake of glucose (long term)
Liver - stimulates glucose synthesis (short term)
Fat- increases triglyceride breakdown in adipose tissue (short term)
Indirect effects of GH
Promotes the growth of bones, muscles and other tissues by causes the release of somatomedin C, which promotes cell division (long term)
Summary - long term effects on growth and short term effects on metabolism
Feedback regulation of growth hormone
The hypothalamus has somatostatin neurons and GHRH neurons.
GHRH stimulates the release of GH from the anterior pituitary
Somatostatin inhibits the release of GH from the anterior pituitary.
GH feeds back to the hypothalamus preventing the release of more GH (negative feedback)
Somatomedin C/IGF1 produced by the liver feeds back to inhibit GHRH and promote somatostatin release from the hypothalamus
GH has effects one the muscle, liver and fat as well as indirect effects.
GH concentrations over a day and a lifetime
Have peaks and troughs throughout the day and GH concentrations are highest during sleep
GH concentrations are higher in children than in adults, with a peak period during puberty. GH secretion declines with ageing therefore worsening wound repair for adults.
Somatomedin C
A hormonal growth factor
Effects- feedsback to the hypothalamus to inhibit further release of GHRH, interestingly also feedback to the somatostatin neurons which releases GHIH
Posterior lobe versus anterior lobe of the pituitary
The hypothalamus is connected to the posterior lobe by axons and to the anterior lobe by blood vessels
Posterior pituitary hormones are made in the hypothalamus but stored in the posterior pituitary. Action potentials cause the release of posterior pituitary hormones.
Anterior pituitary hormones are made in the anterior pituitary by specific cells (different cells produce different hormones). The hypothalamus communicated with the anterior lobe using releasing hormones and inhibiting hormones.
Pituitary hormones bind to receptors on the membrane of specific cells and a specific response is exhibited by those cells (this may be a direct cellular response or the release of a further hormone)
Too much GH
Likely to see more SS and less GHRH
Likely to get additional signalling to the liver, muscle and fat cells. Get additional formation of somatomedin C
Too little GH
Not enough somatomedin C (or signalling to liver, muscle and fat cells)
