To this end, a steep concentration gradient between pipette and bath solution was imposed (250 mM versus 12.5 mM monovalent cation concentration). In another combination of solutions, monovalent cations were completely replaced by Ca2 +. The composition of the various solutions used in this study is summarized in Table 1. Free Ca2 + concentrations in the pipette solutions were calculated using the program ‘calcium’ written by Führ et al.