-activated K Channels

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. key words: magnesium • calcium • BK channel • ion channel gating • competitive inhibition I N T R O D U C T I O N Intracellular free Mg 2 concentration has been measured to be between 0.4 and 3 mM under normal physiological conditions (Flatman, 1984; Gupta et al., 1984; Corkey et al., 1986; Flatman, 1991). At such concentrations, Mg 2 modulates a variety of Ca 2 and K channels to affect the excitability or excitation-contraction coupling in neurons, cardiac myocytes, and smooth muscle cells (Altura et al., 1987; Matsuda et al., 1987; Vandenberg, 1987; White and Hartzell, 1988; Altura and Gupta, 1992; Chuang et al., 1997; Romani et al., 2000). After central nervous system injury, [Mg 2 ] i is significantly reduced, contributing to a number of factors including increased neurotransmitter release and oxidative stress that initiate an autodestructive cascade of biochemical and pathophysiological changes, known as secondary injury, that ultimately results in irreversible tissue damage (Vink and Cernak, 2000). Pharmacological studies have shown that Mg 2 may be an effective therapeutic agent after neurotrauma to improve survival and motor outcome and to alleviate cognitive deficits (Vink and Cernak, 2000). Magnesium supplements are also important in the prevention and management of cardiovascular diseases that predispose to hypertension or congestive heart failure (Laurant and Touyz, 2000; Seelig, 2000). The activation of large conductance Ca 2 activated K channels (BK channels)* depends on both voltage and intracellular calcium (Marty, 1981; Pallotta et al., 1981). Because of this property, BK channels are uniquely suited to play a role in biological processes that involve both calcium signaling and voltage changes. These include neurotransmitter release (Robitaille et al., 1993; Yazejian et al., 1997), electric tuning of cochlear hair cells (Hudspeth and Lewis, 1988a,b; Wu et al., 1995), and vascular smooth muscle contraction (Nelson et al., 1995; Brenner et al., 2000; Pluger et al., 2000). The function of BK channels is further modulated by intracellular Mg 2 , resulting in a reduced single-channel conductance (Ferguson, 1991; Zhang et al., 1995; Morales et al., 1996; Wachter and Turnheim, 1996), an increased open probability at certain [Ca 2 ] i (Squire and Petersen, 1987; Zamoyski et al., 1989; McLarnon and Sawyer, 1993; Zhang et al., 1995; Bringmann et al., 1997), and an increased apparent cooperativity of Ca 2 in activating the channel (Golowasch et al., 1986; Oberhauser et al., 1988; Trieschmann and Isenberg, 1989). These Mg 2 effects on BK channel function may contribute significantly to its physiological and pathophysiological roles. A series of previous studies have focused on the Mg 2 block of BK channels. These studies have suggested that Mg 2 reduces the single-channel conductance by binding to a site inside the pore with fast kinetics and blocking the channel (Ferguson, 1991; Laver, 1992; Address correspondence to Dr. Jianmin Cui, Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106-7207. Fax: (216) 368-4969; E-mail: [email protected] * Abbreviations used in this paper: BK channels, Ca 2 -activated K channels; MWC, Monod-Wyman-Changeux; P o , open probability. on F ebuary 2, 2013 jgp.rress.org D ow nladed fom Published November 2, 2001