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When extracellular $K^+$ concentration increase by a certain amount, excitability of cells is higher because the resting potential shifts toward a higher equilibrium potential of $K^+$, therefore causing depolarization. However, when there concentration of extracellular $Ca^{2+}$ increases, excitability of cells decreases. If $Ca^{2+}$ and $K^+$ are both positively charged ions, what is the cause behind their opposite physiological effects?

I'm not a biology student, so detailed answers would be kindly appreciated. Thank you!

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Hyperkalaemia (high potassium levels) actually paradoxically decreases the excitability of cells due to increased channel inactivation –  Rory M Oct 22 '13 at 22:56
Rory M - that's not always true about decreasing excitability. Case and point: > 25 mM K+ is a standard depolarizing agent in cell culture that triggers action potential firing. Also, a high K+ gradient across the membrane still leaks some K+ into the cell. Strictly speaking, this depends on the complement of voltage-gated and voltage-insensitive ion channels expressed on the cell surface. –  leonardo Aug 30 at 0:20

2 Answers 2

If extracellular $Ca^{2+}$ concentration increased, it makes the extracellular membrane more positive then it used to be, then it would take the membrane potential to lower level then the excitability of the cells will be decreased

if extracellular $K^+$ increased more than the concentration inside the cell than it will make the $K^+$ on extracellular space move inside the call down the concentration gradient, it would take the potential membrane to higher state make it easier to reach the threshold (easier to be excitable)

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Well, the answer which explains the difference between calcium and potassium is quite simple. Do we agree that charge separation (between the two sides of the membrane) is the thing which creates the potential? meaning, if the charge concentrations at the two sides were the same, then the membrane potential was zero. right? So here is the thing. You have much more Ca outside the cell than inside and you have much more potassium inside than outside. So now, if you add Ca outside, you increase the difference between the two sides (the separation of Calcium ions) so that Ca will be in a greater state of unrest, the system will be less stable and the potential will be higher. On the other hand, when you add K outside, you narrow the gap between its two concentrations on the two sides (decrease its gradient), so now it'll experience less tension, less unrest, hence contribute less potential to the membrane. Remember that in our cells, the ion pumps are the ones which serve as the battery that keeps the potential fixed. When do you think the pumps will have to work harder and to do more work (voltage times charge is work), when you add more K outside, or when it's the Ca which is being added?

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Welcome to Biology SE! Can you add more sources to this answer. –  Devashish Das Aug 30 at 5:42

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