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Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals.

Kuruma A, Hartzell HC - J. Gen. Physiol. (2000)

Bottom Line: At higher [Ca(2+)], the currents did not rectify and were time independent.The deactivation time constants increased linearly with the log of membrane potential.The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, Emory University School of Medicine, Atlanta, Georgia 30322-3030, USA.

ABSTRACT
Ca(2+)-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 Ca(2+) and voltage. Excised inside-out patches containing many Ca(2+)-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 [Ca(2+)]. However, depending on the amplitude of the Ca(2+) signal, this channel exhibited qualitatively different behaviors. At [Ca(2+)] < 1 microM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current-voltage relationship was strongly outwardly rectifying. At higher [Ca(2+)], the currents did not rectify and were time independent. This difference in behavior at different [Ca(2+)] was explained by an apparent voltage-dependent Ca(2+) sensitivity of the channel. At +120 mV, the EC(50) for channel activation by Ca(2+) was approximately fourfold less than at -120 mV (0.9 vs. 4 microM). Thus, at [Ca(2+)] < 1 microM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca(2+) binding. The voltage-dependent Ca(2+) sensitivity was explained by a kinetic gating scheme in which channel activation was Ca(2+) dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca(2+) concentration jumps were voltage sensitive, but that the activation time constants were Ca(2+) sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

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Average data for deactivation of Ca2+-activated Cl− currents. The experiments were performed as in Fig. 7. (A) The data were averaged for 280 nM Ca2+ (n = 7), 460 nM Ca2+ (n = 6), 680 nM Ca2+ (n = 5), and 1.1 μM Ca2+ (n = 5). The data were fitted to the equation τdeact = Ae q FV/RT + b. (B) The q values calculated from the fits in A were plotted vs. [Ca2+].
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Figure 8: Average data for deactivation of Ca2+-activated Cl− currents. The experiments were performed as in Fig. 7. (A) The data were averaged for 280 nM Ca2+ (n = 7), 460 nM Ca2+ (n = 6), 680 nM Ca2+ (n = 5), and 1.1 μM Ca2+ (n = 5). The data were fitted to the equation τdeact = Ae q FV/RT + b. (B) The q values calculated from the fits in A were plotted vs. [Ca2+].

Mentions: To test this model further, we examined in detail the kinetics of current activation and deactivation at different [Ca2+]. The deactivation was determined by exponential fitting of the tail current decay at various potentials after a depolarizing step to +120 mV (Fig. 7). At all [Ca2+] examined, the deactivating tail currents observed upon repolarization to different potentials from +120 mV were well fitted by single exponentials (superimposed, but hard to discern in Fig. 7). τdeact increased with depolarization and increased with increasing [Ca2+] within the submicromolar range (Fig. 8 A). The equivalent off gating charge movement, calculated by fitting plots of τdeact vs. Vm to the equation τdeact = Ae q FV/RT + b, was 0.35 at 280 nM Ca2+, 0.27 at 460 nM, 0.26 at 680 nM, and 0.12 at 1 μM Ca2+ (Fig. 8 B). These data are consistent with a model in which there was a dominant rate-limiting transition in the backward direction from the open to the closed states, which was voltage sensitive at low [Ca2+]. At higher [Ca2+], the voltage sensitivity became less and deactivation was incomplete because the forward voltage-independent reaction shifted the equilibrium strongly towards open states, regardless of the backward voltage-dependent reaction.


Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals.

Kuruma A, Hartzell HC - J. Gen. Physiol. (2000)

Average data for deactivation of Ca2+-activated Cl− currents. The experiments were performed as in Fig. 7. (A) The data were averaged for 280 nM Ca2+ (n = 7), 460 nM Ca2+ (n = 6), 680 nM Ca2+ (n = 5), and 1.1 μM Ca2+ (n = 5). The data were fitted to the equation τdeact = Ae q FV/RT + b. (B) The q values calculated from the fits in A were plotted vs. [Ca2+].
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC1887779&req=5

Figure 8: Average data for deactivation of Ca2+-activated Cl− currents. The experiments were performed as in Fig. 7. (A) The data were averaged for 280 nM Ca2+ (n = 7), 460 nM Ca2+ (n = 6), 680 nM Ca2+ (n = 5), and 1.1 μM Ca2+ (n = 5). The data were fitted to the equation τdeact = Ae q FV/RT + b. (B) The q values calculated from the fits in A were plotted vs. [Ca2+].
Mentions: To test this model further, we examined in detail the kinetics of current activation and deactivation at different [Ca2+]. The deactivation was determined by exponential fitting of the tail current decay at various potentials after a depolarizing step to +120 mV (Fig. 7). At all [Ca2+] examined, the deactivating tail currents observed upon repolarization to different potentials from +120 mV were well fitted by single exponentials (superimposed, but hard to discern in Fig. 7). τdeact increased with depolarization and increased with increasing [Ca2+] within the submicromolar range (Fig. 8 A). The equivalent off gating charge movement, calculated by fitting plots of τdeact vs. Vm to the equation τdeact = Ae q FV/RT + b, was 0.35 at 280 nM Ca2+, 0.27 at 460 nM, 0.26 at 680 nM, and 0.12 at 1 μM Ca2+ (Fig. 8 B). These data are consistent with a model in which there was a dominant rate-limiting transition in the backward direction from the open to the closed states, which was voltage sensitive at low [Ca2+]. At higher [Ca2+], the voltage sensitivity became less and deactivation was incomplete because the forward voltage-independent reaction shifted the equilibrium strongly towards open states, regardless of the backward voltage-dependent reaction.

Bottom Line: At higher [Ca(2+)], the currents did not rectify and were time independent.The deactivation time constants increased linearly with the log of membrane potential.The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, Emory University School of Medicine, Atlanta, Georgia 30322-3030, USA.

ABSTRACT
Ca(2+)-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 Ca(2+) and voltage. Excised inside-out patches containing many Ca(2+)-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 [Ca(2+)]. However, depending on the amplitude of the Ca(2+) signal, this channel exhibited qualitatively different behaviors. At [Ca(2+)] < 1 microM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current-voltage relationship was strongly outwardly rectifying. At higher [Ca(2+)], the currents did not rectify and were time independent. This difference in behavior at different [Ca(2+)] was explained by an apparent voltage-dependent Ca(2+) sensitivity of the channel. At +120 mV, the EC(50) for channel activation by Ca(2+) was approximately fourfold less than at -120 mV (0.9 vs. 4 microM). Thus, at [Ca(2+)] < 1 microM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca(2+) binding. The voltage-dependent Ca(2+) sensitivity was explained by a kinetic gating scheme in which channel activation was Ca(2+) dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca(2+) concentration jumps were voltage sensitive, but that the activation time constants were Ca(2+) sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

Show MeSH
Related in: MedlinePlus