I was reading the hypoaldosteronism section in the "cecil essentials of medicine" by Adreoli et al. Regarding hyponatremia that occurs in secondary hypoaldosteronsim, the authors say it occurs because of increased ADH secretion resulting from volume depletion..I don't quite understand why one would have volume depletion in the presence of secondary hypoaldosteronism. ok, there is lack of cortisol but cortisol increases free water clearance so shouldn't there be volume expansion, if anything, with cortisol deficiency? considering mineralocorticoid activity is not affected in secondary aldosteronism..
I'm extremely confused..please HELP!!

Gotta admit, Doc, this is not your ordinary AP question -- but we take all comers! It's been so long since I studied this area of physiology that I can't even remember if I've forgotten it. According to Harrison's Principles of Internal Medicine, 14th Ed. (1998), p. 2052 (the section on primary adrenocortical deficiency, but should apply here as well):

quote:

The hyponatremia is due both to loss of sodium into the urine (due to aldosterone deficiency) and to movement into the intracellular compartment. This extravascular sodium loss depletes extracellular fluid volume and accentuates hypotension. Elevated plasma vasopressin and angiotensin II levels may contribute to the hyponatremia by impairing fee water clearance.

Note that vasopressin is also known as ADH (antidiuretic hormone) or AVP (arginine vasopressin).

As usual, in trying to disentangle the roles of multiple homeostatic feedback mechanisms operating simultaneously, the trick is to sort out cause from effect.

I'm not sure I agree with your statement that "mineralocorticoid activity is not affected in secondary aldosteronism". Aldosterone is by far the most important mineralocorticoid, so whether aldosterone deficiency is primary (adrenal pathology) or secondary (renal pathology), you still have Na+ loss, causing decreased extracellular volume causing hypovolemia causing increased ADH secretion.

Take a look at the diagram here as well (a page on hypoaldost. that seems to be erroneously titled hyperaldost., from a tutorial on regulation of sodium and ECF, from Medical College of Georgia).

If this doesn't help, I doubt I can be of further assistance. You'll have to consult an expert. Good luck!

thank you sir for your comprehensive answer

I didn't understand how your answer was relevant to my question initially though
and then I realised I posted the queston wrong..
I meant hypocortisolism not hypoaldosteronism..my mistake
My original question was meant to be
why in the presence of low cortisol level, there was volume depletion, (considering cortisol is meant to increases free water excretion and mineralocorticoids are not affected in hypocortisolism etc etc)

I think I understand you but it is somewhat unclear. Low cortisol would decrease sodium reabsoption at the tubule and thus lead to volume depletion, NOT expansion. Moreover, if the cause of the low glucocorticoids is caused by something like Addison's, then you also have the low aldosterone problem concurrently... which also causes volume depletion.

exactly my point!!!
the textbook i mentioned above stil says there is volume expansion in hypercortisolism due to increased secretion of ADH..
what's ADH got to do with hypersortisolism...arrggggg