Relative permeabilities for the major ions of the patch clamp solutions are accessible from intersection potentials (i.e. reversal potentials of the current voltage relations associated with field induced pores) by the Goldman-Hodgkin-Katz equation. The original version of this equation only takes monovalent ions into account. However, for this study the Ca2 + permeability of the pores was of major importance. Therefore, an extended version of the equation developed by Piek [11] that includes Ca2 + permeability was used here: