Step I Aldosterone and hypochloremia

[beeitchness]

Page 513 of 2012 FA under electrolyte disturbances.

Low serum Cl is "secondary to metabolic alkalosis, hypokalemia, hypovelemia, increased aldosterone"

Why would increased aldosterone cause hypochloremia? I searched online, and all I could find was that decreased aldosterone causes hypochloremia.....seems contradictory

 

[lemonade90]

Aldo retains HCO3- (by dumping H+). To maintain macroscopic neutrality, you have to lose negative charge. Since Cl and HCO3- are the main negative charges in the body, you will end up losing Cl-.

 

[Phloston]

Lemonade, that's a good point about electrical neutrality, but that makes it seem as though the Cl- loss is a direct result of the HCO3- retention. Although there are physiologic HCO3-/Cl- exchangers, the PCTs are responsible for 65% of Cl- reabsorption, and they don't have those transporters.

What's the actual mechanism?

As far as I'm aware, there's a PCT apical Cl-/organic base antiporter that is directly upregulated by aldosterone. In RTA type-IV, this pump is under-active. Normally, the Cl- is pumped into the tubule cell and the organic base into the lumen. The base (e.g. oxalate, formate) recombines with H+ to form a non-polar entity, which is passively reabsorbed. With the pump under-active, less H+ can combines with organic base to be reabsorbed (pH of urine decreases); instead, the H+ is left to interact with Cl-. Since HCl is a strong acid, it fully dissociates, and H+ + Cl- are flushed out. Therefore, at the level of the PCT, chloride is lost with decreased aldosterone.