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Mg & K

Posted Dec 07 2012 12:00am

It is well known that hypokalemia does not correct easily if it is accompanied by hypomagnesemia. A medical student I met looked into this topic and found a “Science in Renal Medicine” article.  According to this article, one of the mechanisms through which hypokalemia occurs in a hypomagnesemic state is through renal potassium wasting. Several observations have shown that magnesium infusion decreases renal K secretion in the distal nephron.
A study from Nature found that ROMK (aka Kir), one of two potassium channels in the distal nephron, is responsible for the distal renal K wasting in hypomagnesemia. The mechanism is that the intra-cellular free Mg blocks the pore of the ROMK channel and limits potassium secretion in a concentration-dependent manner; therefore low intracellular Mg level increases potassium secretion.
Some renal Mg wasting disorders (e.g. Mg channel TRPM6 mutation) do not always present with hypokalemia. Why is that? The reason is that you need 2 components for potassium excretion. One is increased K permeability of the ROMK, and the other is a driving force to secrete K like increased distal Na delivery or an elevated aldosterone level (via enhanced Na reabsorption in the distal nephron). It seems in these disorders you don’t have the second determinant for K secretion.
Another fascinating renal physiology article! Now we are still confused but have a better understanding of hypomagnesemia in a case of hypokalemia. 
Posted by Tomoki Tsukahara
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