Equation for calculation of P
. The GHK can rearranged to give the above linear form, with the assumptions given in the text (A). Where P = PNa/PK, M is the total cation concentration outside the membrane (Na+ and K+). When the left term from equation A is plotted against the test (K+) cation concentration, equation A is simplified to equation B. Where b is the slope. B can then be rewritten as C where a is the intercept.