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Intracellular Mg(2+) enhances the function of BK-type Ca(2+)-activated K(+) channels.

Shi J, Cui J - J. Gen. Physiol. (2001)

Bottom Line: Intracellular Mg(2+) also modulates BK channels in multiple ways with opposite effects on channel function.We have confirmed this result by studying macroscopic currents of the mslo1 channel.Quantitative computation of these effects reveals that the overall effect of Mg(2+) under physiological conditions is to enhance BK channel function.

View Article: PubMed Central - PubMed

Affiliation: Cardiac Bioelectricity Research and Training Center, Case Western Reserve University, Cleveland, OH 44106-7207, USA.

ABSTRACT
BK channels modulate neurotransmitter release due to their activation by voltage and Ca(2+). Intracellular Mg(2+) also modulates BK channels in multiple ways with opposite effects on channel function. Previous single-channel studies have shown that Mg(2+) blocks the pore of BK channels in a voltage-dependent manner. We have confirmed this result by studying macroscopic currents of the mslo1 channel. We find that Mg(2+) activates mslo1 BK channels independently of Ca(2+) and voltage by preferentially binding to their open conformation. The mslo3 channel, which lacks Ca(2+) binding sites in the tail, is not activated by Mg(2+). However, coexpression of the mslo1 core and mslo3 tail produces channels with Mg(2+) sensitivity similar to mslo1 channels, indicating that Mg(2+) sites differ from Ca(2+) sites. We discovered that Mg(2+) also binds to Ca(2+) sites and competitively inhibits Ca(2+)-dependent activation. Quantitative computation of these effects reveals that the overall effect of Mg(2+) under physiological conditions is to enhance BK channel function.

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Mentions: The apparent Kd from the Hill equation fits clearly shows a voltage dependence (Fig. 4 D). At 0 mV, the apparent Kd is 242.3 mM, whereas at 180 mV, it is 1.6 mM. Such apparent voltage dependence appears to be in contrast to the results in Fig. 1 B and Fig. 2 that the shifts of G-V relations caused by Mg2+ are insensitive to voltage. In other words, the results in Fig. 1 B and Fig. 2 indicate that the binding of Mg2+ is not directly dependent on voltage, whereas the apparent Kd in Fig. 4 indicates that it is influenced by voltage. These results can be reconciled by concluding that the binding of Mg2+ must be dependent on the conformation of the channel but not on voltage per se. The Mg2+ affinity is higher at the open conformation than at the closed. At more positive voltages, more channels are open, therefore, the apparent Kd decreases with voltage. This mechanism of cooperative Mg2+ binding is described by the model for allosteric transitions (Fig. 1), which is similar to the mechanism of Ca2+-dependent activation of the channel (Cox et al. 1997a; Cui et al. 1997).


Intracellular Mg(2+) enhances the function of BK-type Ca(2+)-activated K(+) channels.

Shi J, Cui J - J. Gen. Physiol. (2001)

© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2233844&req=5

Mentions: The apparent Kd from the Hill equation fits clearly shows a voltage dependence (Fig. 4 D). At 0 mV, the apparent Kd is 242.3 mM, whereas at 180 mV, it is 1.6 mM. Such apparent voltage dependence appears to be in contrast to the results in Fig. 1 B and Fig. 2 that the shifts of G-V relations caused by Mg2+ are insensitive to voltage. In other words, the results in Fig. 1 B and Fig. 2 indicate that the binding of Mg2+ is not directly dependent on voltage, whereas the apparent Kd in Fig. 4 indicates that it is influenced by voltage. These results can be reconciled by concluding that the binding of Mg2+ must be dependent on the conformation of the channel but not on voltage per se. The Mg2+ affinity is higher at the open conformation than at the closed. At more positive voltages, more channels are open, therefore, the apparent Kd decreases with voltage. This mechanism of cooperative Mg2+ binding is described by the model for allosteric transitions (Fig. 1), which is similar to the mechanism of Ca2+-dependent activation of the channel (Cox et al. 1997a; Cui et al. 1997).

Bottom Line: Intracellular Mg(2+) also modulates BK channels in multiple ways with opposite effects on channel function.We have confirmed this result by studying macroscopic currents of the mslo1 channel.Quantitative computation of these effects reveals that the overall effect of Mg(2+) under physiological conditions is to enhance BK channel function.

View Article: PubMed Central - PubMed

Affiliation: Cardiac Bioelectricity Research and Training Center, Case Western Reserve University, Cleveland, OH 44106-7207, USA.

ABSTRACT
BK channels modulate neurotransmitter release due to their activation by voltage and Ca(2+). Intracellular Mg(2+) also modulates BK channels in multiple ways with opposite effects on channel function. Previous single-channel studies have shown that Mg(2+) blocks the pore of BK channels in a voltage-dependent manner. We have confirmed this result by studying macroscopic currents of the mslo1 channel. We find that Mg(2+) activates mslo1 BK channels independently of Ca(2+) and voltage by preferentially binding to their open conformation. The mslo3 channel, which lacks Ca(2+) binding sites in the tail, is not activated by Mg(2+). However, coexpression of the mslo1 core and mslo3 tail produces channels with Mg(2+) sensitivity similar to mslo1 channels, indicating that Mg(2+) sites differ from Ca(2+) sites. We discovered that Mg(2+) also binds to Ca(2+) sites and competitively inhibits Ca(2+)-dependent activation. Quantitative computation of these effects reveals that the overall effect of Mg(2+) under physiological conditions is to enhance BK channel function.

Show MeSH