Figure 7.

Ion selectivity of the r-InsP₃R-3. (A) The I-V curve of the InsP₃R-3 in the presence of solutions containing asymmetric K⁺ concentrations. The pipette contained the low K⁺ solution. The bath solution was the standard 140 mM KCl solution with 0 mM Mg²⁺. V_app values were corrected for the liquid junction potential (Neher, 1992) between the asymmetric solutions across the membrane patch. Channel currents were obtained as in Fig. 3. • are data points for r-InsP₃R-3 fitted with the dashed line, giving V_rev = 25.2 ± 0.7 mV. ○ are data points for X-InsP₃R-1 fitted with the solid line, giving V_rev = 25.3 ± 0.2 mV. (B) The I-V curve of InsP₃R-3 in the presence of a [Ca²⁺] gradient. The pipette contained the standard 140 mM KCl solution with 0 mM Mg²⁺ and [Ca²⁺]_i = 24.7 nM. To avoid exposing the oocyte nucleus to high [Ca²⁺], which interferes with formation of giga-ohm seals (Mak and Foskett 1994), an excised patch with InsP₃R activities was first obtained from a nucleus in the standard 140 mM KCl solution (0 mM Mg²⁺). The bath solution was then replaced with the high Ca²⁺ solution by perfusion. V_app values were corrected for liquid junction potential (Neher, 1992) between the asymmetric solutions across the membrane patch. Channel currents were obtained as in Fig. 3. The solid curve is a fifth order polynomial fitted to the data points, giving V_rev = 20.6 ± 0.5 mV.