The endocrine system Flashcards
what is homeostasis
a state of balance among all body systems needed for the body to survive and function correctly
what 4 factors does homeostatic control rely on
- sensor: constant monitoring
- integration center: to coordinate
- response system: to change
- Negative feedback
homeostasis furnace analogy
house temp falls - thermostat detects it (sensory system) - furnace turns on (response system) - heat is produced - house temp rises - thermostat detects - furnace turned off
homeostasis and blood pressure: lying down to standing up
stimulus: blood pressure falls
sensor: blood pressure receptors respond
effector: heart rate increases
Negative feedback response: rise in blood pressure
basic endocrine dysfunctions
hyper-function: too much hormone
hypo-function: too little hormone
resistance: too little effect of hormone
connections between the endocrine system and disease statistics
- diabetes Mellitus is the 6th leading cause of death in Canada
- thyroid disorders affect around 5% of the population, increasing with age
- endocrine ovarian disorders affect around 6% of the female population and are the e#1 cause of infertility
what is a hormone
a regulatory molecule secreted into the blood by endocrine glands
what is an endocrine gland
a tissue that releases a substance into the bloodstream
- the substance then travels via the blood stream to influence target cell
what did banting and best identify
- contributed to insulin
- an antidiabetic substance in pancreatic extracts, injecting this extract prevents symptoms of diabetes
what is insulin
a peptide hormone produced by beta cells in the pancreas
- promotes the absorption of glucose from blood to skeletal muscle and fat tissue
active vs inactive form of insulin
active: a monomer
inactive: a heximer - made of zinc held together by histidine residues
what are the 4 types of hormones
- polypeptides and proteins (most common)
- steroids (cholesterol derivatives)
- amines (catecholamines)
- amines (thyroid)
3 levels of effect of hormones
- autocrine: secretory cell is also the target cell
- paracrine: secretory cell is adjacent to its target cells
- endocrine: secretory cell send the molecule through the bloodstream to reach the target cell
characteristics of peptide hormones
synthesis: in advance
storage: in secretory vesicles
release from cell: exocytosis
transport in blood: dissolved in blood plasma
half life: short
example: insulin
characteristics of steroid hormones
synthesis: on demand
release from cell: diffusion
transport in blood: bound to carrier proteins
half life: long
example: estrogen/androgen
characteristics of amine hormones (Cat)
synthesis: in advance
storage: secretory vesicles
release from cell: exocytosis
transport in blood: dissolved in plasma
half life: short
example: epinephrine/norepinephrine
characteristics of amine hormones (thyroid)
synthesis: in advance
storage: secretory vesicles
release from cell: diffusion
transport in blood: bound to carrier proteins
half life: long
example: T4
where are the receptors of hormones located
most hormones: transmembrane
thyroid hormones: nucleus
steroid hormones: the cytoplasm
specific binding of hormones to receptors
- hormones have specific receptors they bind to at their target cells (don’t actually enter the cell)
- non-specific binding is known as hormone “overspill”
- there is a continuous turnover of the receptor-hormone complex
transmembrane receptor binding
- hormone binds the extracellular domain of the receptor and activates one or more cytoplasmic signalling pathways
- many pathways involve phosphorylation and enzyme activation
- some pathways lead to the DNA/mRNA/protein pathway response and others just have a local effect in the target
Adenylate cyclase (AC) pathway
- hormone attaches to the receptor and G-proteins dissociate
- alpha subunit activates AC
- AC turns ATP in the cytosol to cAMP
- cAMP binds to inhibitor protein on protein kinase and releases them
- protein kinase activates many other molecules (hormonal response)
phospholipase C-Ca2+ (PIPLC) pathway
- hormones attach to receptor
- G-proteins dissociate and activates phospholipase C (PLC)
- the phosphate bearing head-group is cleaved from the membrane to get IP3
- IP3 binds endoplasmic reticulum and releases stored Ca2+ into the cytoplasm
- Ca2+ activates other molecules (hormonal response)
beta-adrenergic vs alpha-adrenergic receptors
activate AC (via Gq) , mediate vasodialation and smooth muscle relaxation = beta-adrenergic
activate PIPLC (via Gs), mediate smooth muscle contraction and vasoconstriction= alpha-adrenergic
- G-alpha subunits fall into several subtypes (Gq-alpha and Gs-alpha)
Cytoplasmic receptor binding: steroid hormones
- steroid hormone is transported bound to plasma carrier protein
- steroid hormone binds to receptor in the cells cytoplasm
- hormone translocates to the nucleus and binds to DNA
- transcription happens then protein synthesis
- steroid hormone releases response
nucleus receptor binding: thyroid hormone
- T4 (thyroxine) binds to a carrier protein in the blood plasma
- T4 enters the target cell and becomes T3 (triiodothyronine)
- T3 uses binding proteins to enter the nucleus and get to the receptor
- hormone bind to the receptor which binds to DNA
- transcription makes a new mRNA then a protein
- thyroid hormone response
how does the hypothalamus control hormones
- hormones that start in the hypothalamus must travel through blood vessels to the anterior pituitary then go through the circulation to reach the target tissue
6 major hormones from the anterior pituitary and their target tissues
- prolactin - mammary gland (breasts)
- thyroid stimulating hormone (TSH) - acts on the adrenal cortex
- Adendocorticotropic hormone (ACTH) - acts on the adrenal cortex
- growth hormone (GH) - acts on bone, muscle and adipose tissue
- folicle-stimulating hormone (FSH) - acts on the ovaries and testes
- luteinizing hormone (LH) - acts on the ovaries and testes
what do inhibitory hormones do
provide an alternate pathway - signals are prevented instead of produced
what are the 3 parts of the anterior pituitary
- pars tuberalis
- pars intermedia (skin colouring, other things)
- pars distalis
7 major hormones from the hypothalamus and what they regulate
- Dopamine (PIH) - inhibits secretion of prolactin
- Prolactin-releasing hormone (PRH) - stimulates the release of
- thyrotropin-releasing hormone (TRH) - regulates the secretion of TSH
- Corticotropin-releasing hormone (CRH) - regulates the secretion of ACTH
- Growth hormone inhibiting hormone (GHIH) - inhibits secretion of GH
- Growth hormone releasing hormone (GHRH) - stimulates secretion of GH
- Gonadotropin-releasing hormone (GnRH) - regulates secretion of LH and FSH
adrenal cortex axis
hypothalamus - releases CRH - acts on anterior pituitary - releases ACTH - acts on adrenal cortex - secretes cortisol - acts on many tissues
CRH synthesis and release from the hypothalamus
- produced by paraventricular cells in the nucleus
- central stimulatory control of the hypothalamus is noradrenergic which stimulates pre-pro CRH gene protein expression
- pre-pro CRH is processed from 196 AA to 41 AA which is released in a pulsatile manor
- released at the median eminence from their neurosecretory nerve terminals into the blood vessels
inhibitory influences of CRH release
- ## physiological levels of cortisol inhibit the release of CRH (possibly inhibit pre-pre CRH gene expressions)
ACTH synthesis and release from the anterior pituitary
- ACTH comes from the POMC family and regulates adrenal cortex function
- convertases cleave POMC to give rise to ACTH
hormones from the adrenal cortex
synthesizes 3 main types of steroid hormones
1. glucocorticoids (cortisol) - controlled by ACTH
2. mineralocorticoids (aldosterone) - not controlled by ACTH
3. sex steroids (testosterone) - controlled by ACTH
3 regions of the adrenal cortex
- Z glomerulosa
- Z fasciculata (secretes cortisol)
- Z reticularis
how does cholestrol get converted to cortisol in humans vs rodants
humans: cholesterol - 17 hydroxypregnenolone - 17 hydroxyprogesterone - deoxycortisol - cortisol
rodants: cholesterol - pregnenolone - prohesterone - deoxycorticosterone - corticosterone
why is cortisol essential
- protects against hypoglycemia (low blood sugar)
- protects gluconeogenesis ( increased blood sugar)
- plays a role in the immune system, bone, muscle, etc.
why can having too much cortisol be bad?
- breaks down skeletal muscle (for gluconeogenesis)
- suppresses the immune system
- catabolic on bone
- affects brain function (mood memory, learning)
Cushings syndrome: primary
- high levels of corticosteroids in the blood
- prolonged exposure to high levels of cortisol
- caused by taking glucocorticoid drugs, or diseases that result in excess cortisol, ACTH or CRH levels
Cushing’s disease: secondary
- the cause is pituitary-dependent, a tumour in the pituitary gland produces large amounts of ACTH, causing adrenals to make excess cortisol
- ACTH levels are higher in the secondary form
What are the effects of Cushing’s syndrome
- changes carbohydrate and protein metabolism, hyperglycemia, hypertension, muscular weakness
- metabolic problems give rise to puffy appearance and CNS disorders (depression, decreased learning, memory etc.)
treatments for Cushing’s syndrome
- surgery to remove pituitary or adrenal gland
- medical management of signs and symptoms
- if not treated, disease worsens and any other health problems will also get worse
Why might cortisol be too low?
Addison’s disease is the primary cause of hypocortisolism
- adrenal insufficiency: many causes including genetic, autoimmune destruction of adrenal cortex
- high-dose steroids > 1 week begins to suppress adrenal glands by suppressing CRH and ACTH (and feedback pathways)
how does adrenal cortex secretion work?
- continuous: in the blood 24v hours a day
- pulsatile: released in little spurts
- circadian rhythm: highest when you wake, lowest in deep sleep
the adrenal cortex: Insomnia and the HPA axis
people with insomnia secrete more cortisol around sleep time, elevated levels make it harder to sleep
PPID - pituitary pars intermedia dysfunction
- Cushing’s disease in horses
- mostly affects older
signs and causes of PPID in hoses
Signs: hypertrichosis, abnormal hair coat, muscle atrophy, excessive sweating, fat pads on top of neck tail head and eyes, pot-bellied appearance
causes: impaired pituitary gland - hyperlasia and hypertrophy in the pars intermedia, results in high blood glucose and suppression of the immune system
diagnosis, treatment and management of PPID in horses
Diagnosis: measuring resting (basal) ACTH and fasting insulin
Treatment: Drug (pergolide) that acts on the pituitary gland to decrease circulating ACTH
Management: exercise, weight loss (if obese), limit starch/sugar in diet