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

Abstract

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.