Effects of a conditioning depolarization on the t tubule voltage sensor. (A) Intramembranous charge movement currents I_Q in test depolarizations to a variable level V_m applied from rest or after a conditioning depolarization (diagram at top). (B) Charge displacement Q_m versus V_m in the range 0–40 mV for the same cell, with displacement during the ON (Q_ON) represented by closed symbols, Q_OFF values by open symbols, and the conditioned values in red. Q_OFF values become progressively greater than Q_ON at V_m > 0 mV, which is an indication that large test voltages activate ionic currents that interfere with the measurement of Q_m. The continuous curves plot Boltzmann fits Q_ON = Q_max/(1 + exp(−(V_m − V_T)/K)). In reference (black), the parameter values were Q_max = 24.0 nC/μF, V_T = −16.4 mV, and K = 16.5 mV. After conditioning (red) Q_max = 19.0 nC/μF, V_T = −20.3 mV, and K = 16.3 mV. ID: 1066E. (C) Q_m averaged over 10 experiments at test voltages up to 0 mV. Symbols and colors are as described for B. Error bars represent SEM. In this range, the averages of Q_ON and Q_OFF were not significantly different, and the changes induced by the conditioning depolarization were not significantly different from zero. ID: 1059E through 1069E (excludes 1062E).