UNC-8(G387E) calcium and magnesium sensitivity. (A) Example of ionic currents recorded in an oocyte injected with UNC-8(G387E) and perfused with the divalent cation–free plus EGTA NaCl solution. The voltage protocol from Fig. 3 A was used here. (B) The same oocyte was perfused with the NaCl solution containing 10 µM CaCl2. (C) Calcium dose–response curve. Currents were recorded at −100 mV in oocytes perfused with increasing concentrations of extracellular calcium. Currents in the presence of calcium were normalized for the current recorded in the same oocyte in divalent cation–free plus EGTA NaCl solution. Before normalization, all currents were leak subtracted. Data points were fitted with a sigmoidal curve for a Ki of 6 µM (n = 8). (D) Voltage dependence of calcium block. The Ki for calcium was plotted against the voltage. Data points were fitted using the Woodhull model; the δ value was 0.06 (n = 7). (E) Same as in A, except that the divalent-free solution did not contain EGTA. (F) The same oocyte shown in E was perfused with the NaCl solution containing 500 µM MgCl2. (G) Magnesium dose–response curve. Data points were derived as in C. Fitting with a sigmoidal curve gave a Ki value of 598 µM (n = 11). (H) The Ki for magnesium block was plotted against the voltage. Data points were fitted by the Woodhull model; the δ value was 0.13 (n = 9).
