I am still new in learning this method at present.

If what I understand is right, to test the relative ion permeability of one type of ion channels, we initially use the Goldman equation. We set the permeability of one ion (like K+) as 1 normalizing other ions permeability to it.

Say like we are tesitng relative permeability of [Na+], technically we need values of [Na+]i, [Na+]o, [K+]o, [K+]i and Vm to apply to the equation and calculate the value of Pna/Pk.

Quesiton is: should the Vm here resting membrane potential (current=0)? In my opinion the Vm should be the balanced membrane potential when there are only two ions Na+ and K+ in the solution of both side. I am not sure which is right.

If I understand correctly, you should measure the membrane potential (Vm), and then you should apply the value of membrane potential (Vm), [K]o, [K]i, {Na}o, [Na]i to the Goldman equation.

From that, you can calculate the permeability (Pna/Pk) which is multiplied to [Na]i and [Na]o.

Thus, Vm is the measured membrane potential.

Hope this makes sense.

one thing I still dont understand.

If what I know is right, the membrane potential (Vm) is a combination of reversal potential of all ions (while Pk+ is dominant). So if Vm means the cellular membane potential, all the ion cencentration should be included in the GHK equation, like Cl-, Ca2+, H+, etc., but not only Na+ and K+.

Pcl, Pca, etc. are ignorable compare with Pna in the resting potential.

You can see the GHK equation on lemi-log graph. In the range of lower [K]o concentration, the curve gets deviation from the Nernst equation for K ion ( a straight line) because of the Pna.

Hope this makes sense.

so does it mean when we detect the permeability of Cl-, we should apply [Na]i and [Na]o in the equation?

If you detect the Cl permeability in resting membrane, then you apply Pcl/Pk to GHK equation.

This website is explaining all that you should calculate.

FYI

http://www.physiologyweb.com/calculators/ghk_equation_calculator.html