7 Flashcards
Hormones of posterior pituitary
oxytocin and vasopressin (ADH)
Describe the make up of Posterior pituitary
Posterior pituitary is a collection of axons terminals arising from 2 main nuclei of hypothalamus
these 2 nuclei are secretory in nature
these secretions from neurones do not end up in synaptic gap (which normally happens)- their secretions end up in the circulation instead
the axon terminals are called in neural endophysis/posterior pituitary as it’s nothing but neurones
What are the hypothalamic nuclei who’s neural axons make up the posterior pituitary?
PVN- makes OCT
SON- maked ADH
Describe the structure of Posterior pituitary hormones
- Are structurally similair, but have very different functions
- Both are nonapeptides- made of 9 AA
- Formation of ring via disulfide bridge-> between 1st and 6 AA-> creates ring structure
ring structure is present in both hormones - Highly conserved amino acid sequences among varied species. Exception: Pigs have lysine-vasopressin at 8th position, whereas humans have arginine
Oxytocin function
Contraction of smooth muscle cells:
- Myoepithelial cells of the alveoli
- Smooth muscle cells of the uterus during labour
Arginine Vasopressin (antidiuretic hormone) function
- H2O retention by the kidney
- Contraction of blood vessels (arterioles)
- regulation of blood volume and pressure
Vasopressin/antidiuretic hormone (ADH) receptor
Two different receptors used named V1 and V2
Describe V1 ADH receptor
V1a and V1b
V1a is found in vascular smooth muscle, platelets, hepatocytes and myometrium
Function: vasoconstriction, myocardial hypertrophy, platelet aggregation, glycogenolysis, uterine contraction
Acts via GPCR-> IP3
V1b is found in anterior pituitary
Function: ACTH release
Describe V2 ADH receptor
Found: endothelium, smooth muscle, collecting kidney tubule
Function: involved in water reabsorption and epithelium cells where it is involved in blood clotting
Vasopressin function
Regulation of water retention and thirst – primary regulator of blood osmolality
Osmolarity vs osmolality
osmolarity= concentration of solute in a volume of solvent (per L)
vs
osmolality-concentration of solute in a mass of solvent (per kg)
Where is osmolality control centre found? How does it analyze it? what are the functions?
osmostat in hypothalamus expresses osmoreceptors- resulting in an osmostat- > reads the osmotic pressure of the blood
functions:
- Control/conservation of water
- Regulation of Na concentrations in plasma
- Pressure-volume (involves baroreceptors)
What does the regulation of thirst involve?
Involves renin-angiotensin system and aldosterone
What are the 2 systems used by the vasopressin?
osmoreceptors in the hypothalamus and baroreceptors in blood vessels
Is the osmostat receptor very sensitive
Yes, Osmostat responds to a little as 1 % change in osmolality
What is the receptor of the osmostat system? Describe its function and location
Vanilloid Receptor-related Osmotically Activated Channel (VR-OAC are the candidate osmoreceptor)
ligand activated channel that is connected to the cytoskeleton of the cells
allows water to go in and out; as it is connected to the cytoskeleton, it allows cells to change in size-> increase or decrease
they seem to be spread around in different nucleus of the hypothalamus
Isotonic vs hypotonic vs hypertonic
Hypotonic- increased volume, decreased ionic strength
Isotonic- have a certain shape
Hypertonic- decreased volume, increased ionic strength
Homeostatic response to hypertonicity
Increased Natriuresis, (Na excretion). Decreased Na intake (behaviour), increased thirst (behaviour), increased vasopressin (water retention)
Homeostatic response to hyportonicity
Decreased Natriuresis, (Na retention). Increased Na intake (behaviour), Decreased thirst (behaviour), Decreased vasopressin (water excretion)
What 2 things involved homeostatic response to changes in tonicity
behavioural changes, vasopressin
What are the factors that affect vasopressin release?
Angiotensin II
Hyperosmolarity
Decreased atrial receptor firing
Sympathetic stimulation
Allow for the secretion of vasopressin
What are the 2 main effects of vasopressin and what do they lead to overall
1) Vasoconstriction-> Increased atrial pressure
2) Renal Fluid reabsorption-> increased blood volume-> Increased atrial pressure
Where is the vasopressin receptor found in the kidney? What is the type of this receptor?
How does it function
Vasopressin receptor is found in the basal cells of distal tubules
GPCR receptor -> cyclic AMP-> result in opening of aquaporin channels-> allow for water movement form the filtrate in the lumen of the tubule into the blood
specific aquporins for __ site, others for __ side
specific aquporins for luminal site, others for basal side
Can urine production be terminated
no, can only be minimized
What is Insensible water loss
basal level of urine formation
Describe thirst response and how its triggered
It’s a behaviour -> allows fr water intake and stops water loss
- Defense mechanism
- Triggered by changes in osmolality or volume of blood
- Strongly triggered by hypovolemia (reduced blood volume) and decrease in blood pressure
Do we drink enough fluid
generally people ingest excess fluid
How are changes in blood pressure and osmotic pressure sensed?
hypothalamic neurones can recognize osmotic pressure through osmoreceptors
blood pressure is recognized through baroreceptros
What happens in blood pressure and osmotic pressure decreases?
Hypothalamic neurones are stimulated to release vasopressin and to send thirst signals
- Vasopressin results in arteriolar vasoconstriction-> Increased atrial pressure
- Vasopressin also results in increased permeability of distal basal cells-> reabsorption of water-> increased blood volume and decreased osmolarity-> Increased ECF volume and decreased osmolarity
These changes are detected by the hypothalamus-> vasopressin release ceases
Pregnancy and vasopressin release
- Increased blood volume, increased size of organs
- Osmostat is reset (less sensitive) and vol/pressure reset
- As a result of vasodilation (expanded vol. is sensed as “normal”)
– Vasopressin responds to reset point, increased blood volume doesn’t dramatically effect vasopressin - Placenta produces enzyme to degrade vasopressin - wk 20-40 —> increased urination and water intake
- Can lead to diabetes insipidus of pregnancy if over-expressed
elderly and vasopressin release
- By age 80 total body water declines to as low as 50 % of adult
- decrease in kidney filtration rate
- collecting duct less responsive to Vasopressin
- decreased response to dehydration
- reduced ability to excrete water load
- elderly susceptible to both hypo and hypernatremia—> decreased or increased urine output
What is Diabetes Insipidus
excretion of a large volume of urine (diabetes) that is hypotonic, dilute and tasteless (insipid)_
What are the causes of Diabetes insipidus
- lack of vasopressin (trauma, tumour etc)–> no water reabsorption
- lack of response to vasopressin in kidney
reduced CAMP and PKA-> reduced aquaporin
also can be due to aquaporin defects - receptor defect or aquaporin defect
- Rapid metabolism of vasopressin
- Pregnancy i.e. transient diabetes insipidus
What is polydipsia and what does it lead to?
individual drinks too much-> Polyurea
Where are baroreceptors found?
they are present in aortic arch where aorta comes out of the heart (blood to the body), carotid sinus in the neck (blood to the brain)
What are the changes that trigger AVP release?
8 % decrease (volume)
5 % decrease (pressure)
Very sensitive!!
What is vasoconstriction caused by?
Increased myogenic activity Increased oxygen Decreased CO2 and other metabolites Increased sympathetic stimulation Vasopressin, angiotensin II Cold
What is vasodilation caused by?
Decreased myogenic activity Decreased oxygen Increased CO2 and other metabolites Decreased sympathetic stimulation Histamine Heat
Describe Physiological regulation of oxytocin secretion
Complicated
Differences among species
What are the cites of oxytocin production apart from posterior pituitary?
Ovaries (corpus luteum) – involved in luteolysis
Uterus in some species
What is the classical regulatory mechanism of oxytocin production?
Regulated by suckling stimuli
How is milk let down triggered by oxytocin?
Oxytocin binding to Oxytocin receptors
-> Contraction of myoepithelial layer; surround each of alveoli-> alveoli contracts
How does oxytocin participate in Contraction of smooth muscle around uterus during parturition
- Acts upon uterine myometrium
- Relaxed during pregnancy to allow for fetal development
- This relaxed state is maintained due Progesterone (placenta/corpus luteum) and relaxin (hormone from cervix) which gets produced as pregnancy advances
- also remains relaxed due to less oxytocin receptors being expressed
How does oxytocin signalling change to give birth?
Become responsive to oxytocin as parturition approaches
- > Increased number of receptors
- > Formation of gap junctions (synchronous contraction) between smooth muscles cells in myometrium-> synchronized contraction of muscle cells
- Works in concert with prostaglandin F2a; helps in contraction-> increased pressure that allows parturition to occur
How does oxytocin affected by social behaviour
lot’s of oxytocin-> monogamous
few-> polygamous
Which receptors are used by oxytocin when determining social behaviour?
Action via receptors in the brain
