ADH Sensitivity to volume and osmolarity

[Firas]

I answered a UW question wrong
Here's the question
Patient given Mannitol for treatment of intracranial hypertension after trauma.
What will be the immediate effects?
a- Release of ADH because of changes in osmolarity
b- Release of ADH because of changes in blood volume
c- Release of ADH because of changes in blood volume and osmolarity
d- and e- irrelevant answers

UW said the correct answer is C while I say the correct answer is B

What do you think about this, let's talk

[3irfan]

the correct answer is C and no problem with that
mannitol ----> dehydration ---> increased ADH
ADH is released when stretch receptors in heart or osmoreceptors in hypothalamus get stimulated by either increased osmolarity or hypovolemia respectively.
I don't know why you think choice B is correct

[tima]

Mannitol is an Osmotic diuretic. Simply put, it pulls water from blood and therefore, it reduces blood volume = dehydration...so thats why ADH is released, so it can save water from the CT and increase blood volume again....In addition, Mannitol is used to decrease intracranial and intraocular pressure for the same reason because it pulls water (extra fluid) from these regions and therefore decreasing pressure!

Hope it helps!

[rasheed]

Perhaps, Firas's confusion stems from the fact that ADH is more sensitive to volume changes than it's for osmolarity changes. It has been said that ADH is willing to sacrifice the osmolarity in favor of keeping the volume right.
For example, in hyponatremic dehydration the ADH still rises despite the fact that we have hyponatremia!
However, I don't think USMLE or USMLEWorld will be going that far in thought processes. They just want you to know that ADH secretion is sensitive to both.

[Firas]

Originally Posted by rasheed

Perhaps, Firas's confusion stems from the fact that ADH is more sensitive to volume changes than it's for osmolarity changes. It has been said that ADH is willing to sacrifice the osmolarity in favor of keeping the volume right.
For example, in hyponatremic dehydration the ADH still rises despite the fact that we have hyponatremia!
However, I don't think USMLE or USMLEWorld will be going that far in thought processes. They just want you to know that ADH secretion is sensitive to both.

You did exactly touch on what am talking about. But, in the question, specifically, Mannitol will drag water into the vasculature before it causes diuresis, right? [that's why it's never used in heart failure or hemodynamic disturbances] So then it causes hyponatremia and ADH should not be released.
In our case, ADH is released only because of volume depletion and option B should be the correct answer as I think

[rasheed]

Originally Posted by Firas

You did exactly touch on what am talking about. But, in the question, specifically, Mannitol will drag water into the vasculature before it causes diuresis, right? [that's why it's never used in heart failure or hemodynamic disturbances] So then it causes hyponatremia and ADH should not be released.
In our case, ADH is released only because of volume depletion and option B should be the correct answer as I think

hmmmm .... Well ... I think you are right

[DrSeddik]

Originally Posted by Firas

in the question, specifically, Mannitol will drag water into the vasculature before it causes diuresis, right? [that's why it's never used in heart failure or hemodynamic disturbances] So then it causes hyponatremia and ADH should not be released.

That's true, but in general, mannitol won't work unless it increases the osmolarity, even if it does cause hyponatremia, the osmogenic substance will keep the osmolarity higher than normal, dehydrating OVLT cells, and increasing ADH release