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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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Excised patch currents are Cl− currents. The reversal potential of the Ca2+-activated currents recorded in inside-out patches was determined by measuring the instantaneous current at different potentials following a depolarizing step to +120 mV (voltage protocol is shown above B). The pipet solution contained either 160 (A) or 40 (B) mM Cl−. The bath solution contained 160 mM Cl−. (C) Instantaneous current–voltage relationship. The amplitudes of the tail currents were plotted versus the membrane potential for symmetric 160 mM Cl− (○) or for 40 mM Clo–160 mM Cli (•). The reversal potential shifted from 0 to +38.1 mV with the reduction in extracellular Cl. The shift for a Cl-selective channel predicted by the Goldman-Hodgkin-Katz equation is +35.2 mV.
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Figure 2: Excised patch currents are Cl− currents. The reversal potential of the Ca2+-activated currents recorded in inside-out patches was determined by measuring the instantaneous current at different potentials following a depolarizing step to +120 mV (voltage protocol is shown above B). The pipet solution contained either 160 (A) or 40 (B) mM Cl−. The bath solution contained 160 mM Cl−. (C) Instantaneous current–voltage relationship. The amplitudes of the tail currents were plotted versus the membrane potential for symmetric 160 mM Cl− (○) or for 40 mM Clo–160 mM Cli (•). The reversal potential shifted from 0 to +38.1 mV with the reduction in extracellular Cl. The shift for a Cl-selective channel predicted by the Goldman-Hodgkin-Katz equation is +35.2 mV.

Mentions: These currents were carried by Cl− ions (Fig. 2). In this experiment, the instantaneous current–voltage relationship of the current was determined by measuring the amplitude of tail currents at different potentials after a depolarizing step to +120 mV with 160 mM Cl− on both sides of the membrane (Fig. 2 A) or with 40 mM Cl− in the bath and 160 mM Cl− in the pipet (Fig. 2 B). In this experiment, the reversal potential of the current shifted +38.1 mV upon reducing extracellular [Cl−]. On average, the reversal potential shifted +38.0 mV (symmetric Cl− Erev = 0.1 ± 0.43 mV, n = 18; asymmetric Cl− Erev = +39.0 ± 0.27 mV, n = 9). This shift was very close to the +36.3-mV shift predicted by the Goldman-Hodgkin-Katz equation. We conclude that this current in the excised patch corresponds to the Ca2+-activated Cl− current ICl1-S we have described in intact oocytes because they are both activated by Ca2+, carried by Cl−, and have similar waveforms and steady state current–voltage relationships.


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

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

Excised patch currents are Cl− currents. The reversal potential of the Ca2+-activated currents recorded in inside-out patches was determined by measuring the instantaneous current at different potentials following a depolarizing step to +120 mV (voltage protocol is shown above B). The pipet solution contained either 160 (A) or 40 (B) mM Cl−. The bath solution contained 160 mM Cl−. (C) Instantaneous current–voltage relationship. The amplitudes of the tail currents were plotted versus the membrane potential for symmetric 160 mM Cl− (○) or for 40 mM Clo–160 mM Cli (•). The reversal potential shifted from 0 to +38.1 mV with the reduction in extracellular Cl. The shift for a Cl-selective channel predicted by the Goldman-Hodgkin-Katz equation is +35.2 mV.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 2: Excised patch currents are Cl− currents. The reversal potential of the Ca2+-activated currents recorded in inside-out patches was determined by measuring the instantaneous current at different potentials following a depolarizing step to +120 mV (voltage protocol is shown above B). The pipet solution contained either 160 (A) or 40 (B) mM Cl−. The bath solution contained 160 mM Cl−. (C) Instantaneous current–voltage relationship. The amplitudes of the tail currents were plotted versus the membrane potential for symmetric 160 mM Cl− (○) or for 40 mM Clo–160 mM Cli (•). The reversal potential shifted from 0 to +38.1 mV with the reduction in extracellular Cl. The shift for a Cl-selective channel predicted by the Goldman-Hodgkin-Katz equation is +35.2 mV.
Mentions: These currents were carried by Cl− ions (Fig. 2). In this experiment, the instantaneous current–voltage relationship of the current was determined by measuring the amplitude of tail currents at different potentials after a depolarizing step to +120 mV with 160 mM Cl− on both sides of the membrane (Fig. 2 A) or with 40 mM Cl− in the bath and 160 mM Cl− in the pipet (Fig. 2 B). In this experiment, the reversal potential of the current shifted +38.1 mV upon reducing extracellular [Cl−]. On average, the reversal potential shifted +38.0 mV (symmetric Cl− Erev = 0.1 ± 0.43 mV, n = 18; asymmetric Cl− Erev = +39.0 ± 0.27 mV, n = 9). This shift was very close to the +36.3-mV shift predicted by the Goldman-Hodgkin-Katz equation. We conclude that this current in the excised patch corresponds to the Ca2+-activated Cl− current ICl1-S we have described in intact oocytes because they are both activated by Ca2+, carried by Cl−, and have similar waveforms and steady state current–voltage relationships.

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