Estimation of relative Ca2+ permeability

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Estimation of relative Ca2+ permeability

1. Why do we use Cs+ to calculate relative Ca2+ permeability for a Ca2+ channel. Is it possible to use Na+? why would someone choose Cs+ over other ions?

2.  Is there a basic reference to understand how bi-ionic conditions are used to calculate relative Ca2+ permeability?

I understand that it can be calculated using the reversal potential of Ca2+ and Cs+ in this case using modified lewis equation i.e. Vrev(Ca)-Vrev(Cs) = (RT/F) ln( (4Pca *[Ca]o/Pcs)/([Cs]o* (1+exp(Vrev(Ca)/ (RT/F))) (equation 8 in the following reference:

I tried deriving this equation but could not get the exact RHS.

I assumed that Vrev(Ca) = RT/2F ln (4Pca* [Ca]o/Pcs*[Cs]i) and Vrev (Cs) = RT/F ln ([Cs]o/[Cs]i)

Given: While measuring Vrev (Ca), [Ca]o = 1.8 mM and [Cs]i = 146 mM
While measuring Vrev (Cs), [Cs]o = 143 mM and [Cs]i = 146 mM