Effect of raising extracellular K+

**brianfall** · 26-02-2006, 08:31 PM

My brain has stopped working again..... any tips gratefully received. Feel free to hurl abuse...

We've been told that raising interstital (extracellular) K+ in cells that have become ischaemic (inadequate blood flow due to heart attack) depolarises the membrane potential.

I've been staring at the same page now and can't get it to make sense because I thought that

1) The resting membrane potential was due to a build up of anions (-'ve) just inside the membrane, and a build up of cations (+'ve) just outside the membrane.

and

2) Resting membrane potential is normally around -70mV because the inside of the cell is negative relative to the outside.

If interstital K+ goes up, wouldn't that make the inside more negative relative to the outside, making membrane potential more negative - i.e. more polarised rather than less polarised?

Thanks in advance,
Brian

**stargazer** · 26-02-2006, 08:43 PM

Hmmm - i have no idea! sorry. I'm sticking to anatomy today. Acland is beginning to get on my nerves!! I think he should produce some relaxation tapes - his voice would be ideal.

**brianfall** · 26-02-2006, 08:45 PM

good luck!

**stargazer** · 26-02-2006, 08:47 PM

You reckon we'll get our marks back immediately like at Xmas. i need to know where I screwed up.

**Sipadan** · 26-02-2006, 08:49 PM

It's counterintuitive, but you need K+ to exit the cell in order for repolarisation to occur. If you increase the extracellular concentration of K+ this reduces the concentration gradient, thereby reducing K+ expulsion from the cell, keeping it in a state of depolarisation and preventing further action potentials from occuring.

**hunni** · 26-02-2006, 08:53 PM

Oh I'm confused - I will have to read through that again to make sense of it............

**brianfall** · 26-02-2006, 08:54 PM

Originally Posted by Sipadan

It's counterintuitive, but you need K+ to exit the cell in order for repolarisation to occur. If you increase the extracellular concentration of K+ this reduces the concentration gradient, thereby reducing K+ expulsion from the cell, keeping it in a state of depolarisation and preventing further action potentials from occuring.

Cheers Sipadan, so is there initially more K+ inside the cell (at resting potential) and the buildup on the outside just stops these from coming out?

**Sipadan** · 26-02-2006, 09:01 PM

Yes! Review intra vs. extracellular ion concentrations, the standard AP diagram, what direction the ions move in and the concept of the refractory period and all will be revealed. DO NOT think about electrogenic gradients or you'll be back to sqaure one