Bimodal Control of a Ca²⁺-Activated Cl⁻ Channel by Different Ca²⁺ Signals

Ca²⁺-activated Cl⁻ channels play important roles in a variety of physiological processes, including epithelial secretion, maintenance of smooth muscle tone, and repolarization of the cardiac action potential. It remains unclear, however, exactly how these channels are controlled by Ca²⁺ and voltage. Excised inside-out patches containing many Ca²⁺-activated Cl⁻ channels from Xenopus oocytes were used to study channel regulation. The currents were mediated by a single type of Cl⁻ channel that exhibited an anionic selectivity of I⁻ > Br⁻ > Cl⁻ (3.6:1.9:1.0), irrespective of the direction of the current flow or [Ca²⁺]. However, depending on the amplitude of the Ca²⁺ signal, this channel exhibited qualitatively different behaviors. At [Ca²⁺] < 1 μM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current–voltage relationship was strongly outwardly rectifying. At higher [Ca²⁺], the currents did not rectify and were time independent. This difference in behavior at different [Ca²⁺] was explained by an apparent voltage-dependent Ca²⁺ sensitivity of the channel. At +120 mV, the EC₅₀ for channel activation by Ca²⁺ was approximately fourfold less than at −120 mV (0.9 vs. 4 μM). Thus, at [Ca²⁺] < 1 μM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca²⁺ binding. The voltage-dependent Ca²⁺ sensitivity was explained by a kinetic gating scheme in which channel activation was Ca²⁺ dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca²⁺ concentration jumps were voltage sensitive, but that the activation time constants were Ca²⁺ sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca²⁺ concentrations adds a new dimension to Ca²⁺ signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca²⁺ signal.