Figure 8.

Predictions of the allosteric model. (A) Q _OFF component amplitudes determined after pulses to +240 mV in 0 Ca²⁺ are plotted versus pulse duration. The fast component is reduced to <10% of the total OFF charge after a 20-ms pulse. The relaxation of all three components is fit by exponential functions (solid lines) with a τ = 0.91 ms. (B) The decay of Q _OFF–Q _ss is plotted on a semilog scale after 0.1- or 20-ms pulses to +160 mV in 60 μM Ca²⁺ (HP = −80). The 0.1-ms trace is fit by a triple exponential function (solid line, τ_F = 23.8 μs, τ_M = 150 μs, τ_S = 822 μs) and the 20-ms trace is fit with a double-exponential (τ_M = 150 μs, τ_S = 822 μs), indicating that the fast component is eliminated when most channels are opened. Dashed lines represent the two components of the 20-ms fit and the fast component of the 0.1-ms fit. (C) Normalized Q _OFF component amplitudes and total OFF charge (Q _p) are plotted versus pulse duration for pulses to +160 mV in 0 Ca²⁺. OFF components were measured upon repolarization to −80 mV and are normalized to the fast component of ON charge (Q _fast) at +160 mV. (D) When Q _OFF is measured upon repolarization to 0 mV, the Fast component and Q _p are unchanged. However, the Medium component decreases and the Slow component increases in a complementary manner. (E) The charge distributions predicted by the allosteric model for Closed (Q _C) and Open channels (Q _O) are plotted versus voltage (z _J = 0.55 e, V_h(J) = 155 mV, L = 2 × 10⁻⁶, z _L = 0.4 e, D = 17). Arrows indicate the predicted amplitudes of Medium and Slow OFF components at repolarization voltages of −80 and 0 mV after a pulse to +160 mV (V_P).