FIGURE 8.

Computer modeling of Ca²⁺ current deactivation and inactivation in the absence of Ca²⁺ agonist. The kinetic scheme used is shown in A. One closed (C) and two open states (O₁ and O₂) are sequentially linked. Only the C and O₁ states have their corresponding inactivated states, I₀ and I₁. The k_(s) represent the rate constants of transition (listed in Table 2). In B, simultaneous differential equations based on the scheme A were numerically solved to draw the deactivation time courses seen in the repolarization step at −40 (a), −60 (b), and −80 mV (c) after conditioning depolarization that can convert the conformation of all Ca²⁺ channels into O₂. To compare deactivation time courses at the three repolarizing potentials, the amplitude of the tail current was expressed relative to that at t = 0. In C, the voltage dependence of steady-state inactivation was calculated as the ratio of channel number in the C, O₁, and O₂ states to the total channel number after conditioning depolarization for 4 s (from −100 to +100 mV). The drawing in D is a computer calculation for Ca²⁺ channel current evoked by a step depolarization to 0 mV.