Urinary K+ excretion is increased by all of the following EXCEPT:

[arc5005]

Urinary K+ excretion is increased by all of the following EXCEPT:

A. vasopressin
B. hyperkalemia
C. an increase in the GFR
D. a chronic elevation in plasma aldosterone levels

C) an increase in the GFR.

So I was able to eliminate B and D immediately, since Hyperkalemia would increase aldosterone levels which would increase K+ urinary excretion.

A bit confused why vasopressin increases urinary K+ excretion, and why the answer is C.

So ADH, increases blood pressure through water retention and vasoconstriction. So, because of water retention, are more ions being excreted via urine? or what exactly is the correlation here?

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[Lawpy]

Okay you're right that high potassium levels in the blood lead to increased aldosterone levels, which in turn stimulate potassium excretion in the urine. Aldosterone also stimulates sodium reabsorption into the blood. Water is also retained because of the osmotic pressure generated from 3 sodium ions reabsorbed for every 2 potassium ions secreted. This is why aldosterone antagonists like spironolactone are used to treat fluid buildup (edema) and high blood pressure, while also treating hypokalemia by reducing urinary potassium excretion.

Now when we are discussing electrolyte and solute levels in the blood and urine, we are talking about their concentrations (Potassium: Reference Range, Interpretation, Collection and Panels ). Vasopressin/ADH acts to increase water reabsorption in the collecting duct, thereby making the urine more concentrated. This in turn increases the concentration of potassium excreted in the urine. The number of moles of potassium ions is unchanged but because the volume is decreased by water reabsorption, potassium concentration increases.

Changes in glomerular filtration rate (GFR) are compensated by secretion and reabsorption processes. Potassium excretion begins at the distal convoluted tubule and continues into the collecting duct. GFR measures the filtration of a solute, but excretion also depends on reabsorption and secretion:

Amount excreted = amount filtered - amount reabsorbed + amount secreted

So increasing GFR can be compensated by increasing potassium reabsorption and/or decreasing potassium secretion, keeping the net potassium excretion constant. When you have a solute that is neither reabsorbed nor secreted, the net excretion of the solute would equal to the filtration rate/GFR. That's why inulin can be used to measure GFR, since inulin is neither reabsorbed nor secreted by the kidneys, and so inulin clearance = GFR (although creatinine clearance is used in the clinical setting to estimate GFR.)

https://www.kidney.org/sites/default/files/02-10-6785_HBE_Hyperkalemia_Bulletin.pdf

Processes of the Kidneys

 

[Ole_Toe]

I just want to point out for future reference, that if this is the Berkeley Review question from section 3 of Bio Book 1, the correct answer is actually A, most likely for the reason that Lawper provides above.