Effect of Bay K 8644 (−) and the β_2a Subunit on Ca²⁺-dependent Inactivation in α_1C Ca²⁺ Channels

Ca²⁺ currents recorded from Xenopus oocytes expressing only the α_1C pore-forming subunit of the cardiac Ca²⁺ channel show Ca²⁺-dependent inactivation with a single exponential decay. This current-dependent inactivation is not detected for inward Ba²⁺ currents in external Ba²⁺. Facilitation of pore opening speeds up the Ca²⁺-dependent inactivation process and makes evident an initial fast rate of decay. Facilitation can be achieved by (a) coexpression of the β_2a subunit with the α_1C subunit, or (b) addition of saturating Bay K 8644 (−) concentration to α_1C channels. The addition of Bay K 8644 (−) to α_1Cβ_2a channels makes both rates of inactivation faster. All these maneuvers do not induce inactivation in Ba²⁺ currents in our expression system. These results support the hypothesis of a mechanism for the Ca²⁺-dependent inactivation process that is sensitive to both Ca²⁺ flux (single channel amplitude) and open probability. We conclude that the Ca²⁺ site for inactivation is in the α_1C pore-forming subunit and we propose a kinetic model to account for the main features of α_1Cβ_2a Ca²⁺ currents.