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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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Hyperpolarization cannot turn off currents activated by Ca2+. Excised patches were exposed to 500 nM Ca2+ (A and B) or 1 μM Ca2+ (C and D). The membrane potential was held at various values between −200 and 0 mV. The instantaneous currents at +120 mV were measured to determine the conductance activated at the preceding voltage. At both [Ca2+], hyperpolarization to −200 mV was not able to inactivate the Ca2+-activated current. Only selected traces are shown in A and C for clarity.
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Figure 5: Hyperpolarization cannot turn off currents activated by Ca2+. Excised patches were exposed to 500 nM Ca2+ (A and B) or 1 μM Ca2+ (C and D). The membrane potential was held at various values between −200 and 0 mV. The instantaneous currents at +120 mV were measured to determine the conductance activated at the preceding voltage. At both [Ca2+], hyperpolarization to −200 mV was not able to inactivate the Ca2+-activated current. Only selected traces are shown in A and C for clarity.

Mentions: Fig. 4 has shown that at 2.2 μM Ca2+ the channel cannot be closed by voltages as negative as −120 mV and that at ∼10 nM Ca2+ the channel cannot be opened by voltages as positive as +120 mV. This suggests that channel opening is Ca2+ dependent and that the voltage-sensitive step is after Ca2+ binding. Fig. 5 extends the voltage range and shows that the channel cannot be closed by voltages even as negative as −200 mV. An excised patch was held at negative potentials for 10 s, and then stepped to +120 mV to measure the instantaneous current, to determine whether negative potentials could deactivate the current completely. At 500 nM Ca2+ (Fig. 5A and Fig. B), a small, but significant outward current (30 pA) was recorded upon stepping from −100 to +120 mV. Increasing the holding potential to −200 mV had no significant effect on the magnitude of the instantaneous current. Thus, even at this intermediate [Ca2+], it was not possible to close the channels by strong depolarization. At 1 μM Ca2+ (Fig. 5C and Fig. D), the currents were larger, but their amplitudes were not significantly reduced by increasing the holding potential from −160 to −200 mV. From these results, it is clear that channel opening is not voltage gated and that voltage only modulates the current amplitude. Rather, channel opening is strictly dependent on Ca2+ and the voltage sensitivity must occur at a later step.


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

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

Hyperpolarization cannot turn off currents activated by Ca2+. Excised patches were exposed to 500 nM Ca2+ (A and B) or 1 μM Ca2+ (C and D). The membrane potential was held at various values between −200 and 0 mV. The instantaneous currents at +120 mV were measured to determine the conductance activated at the preceding voltage. At both [Ca2+], hyperpolarization to −200 mV was not able to inactivate the Ca2+-activated current. Only selected traces are shown in A and C for clarity.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC1887779&req=5

Figure 5: Hyperpolarization cannot turn off currents activated by Ca2+. Excised patches were exposed to 500 nM Ca2+ (A and B) or 1 μM Ca2+ (C and D). The membrane potential was held at various values between −200 and 0 mV. The instantaneous currents at +120 mV were measured to determine the conductance activated at the preceding voltage. At both [Ca2+], hyperpolarization to −200 mV was not able to inactivate the Ca2+-activated current. Only selected traces are shown in A and C for clarity.
Mentions: Fig. 4 has shown that at 2.2 μM Ca2+ the channel cannot be closed by voltages as negative as −120 mV and that at ∼10 nM Ca2+ the channel cannot be opened by voltages as positive as +120 mV. This suggests that channel opening is Ca2+ dependent and that the voltage-sensitive step is after Ca2+ binding. Fig. 5 extends the voltage range and shows that the channel cannot be closed by voltages even as negative as −200 mV. An excised patch was held at negative potentials for 10 s, and then stepped to +120 mV to measure the instantaneous current, to determine whether negative potentials could deactivate the current completely. At 500 nM Ca2+ (Fig. 5A and Fig. B), a small, but significant outward current (30 pA) was recorded upon stepping from −100 to +120 mV. Increasing the holding potential to −200 mV had no significant effect on the magnitude of the instantaneous current. Thus, even at this intermediate [Ca2+], it was not possible to close the channels by strong depolarization. At 1 μM Ca2+ (Fig. 5C and Fig. D), the currents were larger, but their amplitudes were not significantly reduced by increasing the holding potential from −160 to −200 mV. From these results, it is clear that channel opening is not voltage gated and that voltage only modulates the current amplitude. Rather, channel opening is strictly dependent on Ca2+ and the voltage sensitivity must occur at a later step.

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