Lecture 17: osmoregulation Flashcards
what is the main determinant of osmolality in the body?
the water content in the body
What is the normal plasma osmolality in the body?
290mOsm/Kg of h20
How much change results in the stimulation of the body’s osmolality regulating mechanisms?
a variation in either direction of 3mOsm/kg H20
what specific receptors detect alterations in the plasma osmolality?
osmoreceptors
What do osmoreceptors regulate?
they regulate the release of ADH from the posterior pituitary gland into the circulation
Where is ADH synthesised and stored?
- ADH is synthesised in the hypothalamus (in the supraoptic nucleus) as part of a large precursor molecule
- the precursor molecule is transported from the hypothalamus to be stored in the posterior pituitary gland
Describe the steps of how the osmoreceptors sense plasma osmolality increase
- increased plasma osmolality ( due to dehydration)
- osmoreceptors in the supraoptic area of the hypothalamus** sense the increase**
- the hypothalamic cells that secrete ADH receive a nueral input from the osmoreceptors
* increase in osmolality stimulates an increase in the rate of ADH secretion
What is the set point of plasma osmolality?
- the set point is the plasma osmolality value at which ADH secretion begins to increase
- note there is always low levels of ADH secretion in the circulation
Describe the mechanism of ADH in the kidney
- ADH released into the circulation
- it binds to the V2 receptors in the basolateral membrane of the principal cells of the collecting duct
- the binding to the V2 receptor activates adenylate cyclase which converts ATP into cAMP.
- cAMP activates a protein kinase which phosphorlyates vesicles containing aqua porin 2 channels
- the water channels shuttle into the apical membrane of the principal cells
What are examples of other factors that alter ADH secretion by the posterior pituitary gland?
- nicotine stimulates ADH secretion
- angiotensin II stimulates
- ethanol inihibits - blocks V2 receptors
How is the volume and pressure of the vascular system assessed?
recap
- through baroreceptors which are stretch receptors
- there are low pressure receptors in the left atrium and pulmonary vessels
- there are high pressure receptors in the aortic arch and carotid sinus
- there are also intrarenal baroreceptors- granular cells of the afferent arteriole secrete renin in response to decreased renal arterial pressure
What does the decrease of baroreceptor activity cause with regards to ADH secretion?
- it increases the release of ADH ( inverse relationship)
Describe the major characteristics between osmoreceptors and baroreceptors
- osmoreceptors - located in the hypothalamus, they measure plasma osmolality, ADH release is timulated by the activation of osmoreceptor
- Baroreceptors- located in the carotid sins and aortic arch, they measure the circulating blood volume and ADH release is stimulated by the supression of the receptor
What occurs in a haemorrhage with regards to ADH release?
- haemorrhage (decreased blood volume) increases ADH release by the volume receptors (baroeceptors) so water is retained - osmoregulation is related to volume regulation in this case
what are examples of malfunctions of the ADH system?
-
central diabetes insipidus - the pituitary gland is unable to secrete ADH
* nephrogenic diabetes insipidus - collecting ducts are unable to respond to ADH because a mutation is present in the V2 receptor
How is water intake regulated - ie thirst signals?
- a decrease in plasma volume is sensed by the baro receptors which increases angiotensin II which causes thirst
- an increase in plasma osmolality is sensed by the osmoreceptors which stimulates thirst
