Effects of divalent cations on the activation of a calcium-dependent potassium channel in hippocampal neurons

The Large-Conductance, Calcium-Activated Potassium Channel: A Big Key Regulator of Cell Physiology

Large-conductance Ca²⁺-activated K⁺ channels facilitate the efflux of K⁺ ions from a variety of cells and tissues following channel activation. It is now recognized that BK channels undergo a wide range of pre- and post-translational modifications that can dramatically alter their properties and function. This has downstream consequences in affecting cell and tissue excitability, and therefore, function. While finding the “silver bullet” in terms of clinical therapy has remained elusive, ongoing research is providing an impressive range of viable candidate proteins and mechanisms that associate with and modulate BK channel activity, respectively. Here, we provide the hallmarks of BK channel structure and function generally, and discuss important milestones in the efforts to further elucidate the diverse properties of BK channels in its many forms.

BK channels: multiple sensors, one activation gate

Ion transport across cell membranes is essential to cell communication and signaling. Passive ion transport is mediated by ion channels, membrane proteins that create ion conducting pores across cell membrane to allow ion flux down electrochemical gradient. Under physiological conditions, majority of ion channel pores are not constitutively open. Instead, structural region(s) within these pores breaks the continuity of the aqueous ion pathway, thereby serves as activation gate(s) to control ions flow in and out. To achieve spatially and temporally regulated ion flux in cells, many ion channels have evolved sensors to detect various environmental stimuli or the metabolic states of the cell and trigger global conformational changes, thereby dynamically operate the opening and closing of their activation gate. The sensors of ion channels can be broadly categorized as chemical sensors and physical sensors to respond to chemical (such as neural transmitters, nucleotides and ions) and physical (such as voltage, mechanical force and temperature) signals, respectively. With the rapidly growing structural and functional information of different types of ion channels, it is now critical to understand how ion channel sensors dynamically control their gates at molecular and atomic level. The voltage and Ca²⁺ activated BK channels, a K⁺ channel with an electrical sensor and multiple chemical sensors, provide a unique model system for us to understand how physical and chemical energy synergistically operate its activation gate.

Intracellular Mg2+ influences both open and closed times of a native Ca2+-activated BK channel in cultured human renal proximal tubule cells

Effects of intracellular Mg2+ on a native Ca(2+)-and voltage-sensitive large-conductance K+ channel in cultured human renal proximal tubule cells were examined with the patch-clamp technique in the inside-out mode. At an intracellular concentration of Ca2+ ([Ca2+](i)) of 10(-5)-10(-4) M, addition of 1-10 mM: Mg2+ increased the open probability (P(o)) of the channel, which shifted the P(o) -membrane potential (V(m)) relationship to the negative voltage direction without causing an appreciable change in the gating charge (Boltzmann constant). However, the Mg(2+)-induced increase in P(o) was suppressed at a relatively low [Ca2+](i) (10(-5.5)-10(-6) M). Dwell-time histograms have revealed that addition of Mg2+ mainly increased P(o) by extending open times at 10(-5) M Ca2+ and extending both open and closed times simultaneously at 10(-5.5) M Ca2+. Since our data showed that raising the [Ca2+](i) from 10(-5) to 10(-4) M increased P(o) mainly by shortening the closed time, extension of the closed time at 10(-5.5) M Ca(2+) would result from the Mg(2+)-inhibited Ca(2+)-dependent activation. At a constant V(m), adding Mg2+ enhanced the sigmoidicity of the P(o)-[Ca2+](i) relationship with an increase in the Hill coefficient. These results suggest that the major action of Mg2+ on this channel is to elevate P(o) by lengthening the open time, while extension of the closed time at a relatively low [Ca2+](i) results from a lowering of the sensitivity to Ca2+ of the channel by Mg2+, which causes the increase in the Hill coefficient.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Tuning magnesium sensitivity of BK channels by mutations

Intracellular Mg(2+) at physiological concentrations activates mSlo1 BK channels by binding to a metal-binding site in the cytosolic domain. Previous studies suggest that residues E374, Q397, and E399 are important in Mg(2+) binding. In the present study, we show that mutations of E374 or E399 to other amino acids, except for Asp, abolish Mg(2+) sensitivity. These results further support that the side chains of E374 and E399 are essential for Mg(2+) coordination. To the contrary, none of the Q397 mutations abolishes Mg(2+) sensitivity, suggesting that its side chain may not coordinate to Mg(2+). However, because Q397 is spatially close to E374 and E399, its mutations affect the Mg(2+) sensitivity of channel gating by either reducing or increasing the Mg(2+) binding affinity. The pattern of mutational effects and the effect of chemical modification of Q397C indicate that Q397 is involved in the Mg(2+)-dependent activation of BK channels and that mutations of Q397 alter Mg(2+) sensitivity by affecting the conformation of the Mg(2+) binding site as well as by electrostatic interactions with the bound Mg(2+) ion.

Tedisamil blocks BK-type Ca(2+)-dependent K(+) channels and modulates action potentials in rat hippocampal neurons

Tedisamil, a bradycardic compound in heart, also acts on K(+) channels in neurons. We determined the actions of tedisamil on action potentials in CA1 pyramidal neurons in hippocampal slices and on BK-type Ca(2+)-activated K(+) channel activity in inside-out patches excised from hippocampal neurons. In slices, tedisamil (5 microM) attenuated the fast afterhyperpolarization (AHP) and prolonged the repolarization phase of the action potential. Additionally, the compound induced burst-firing activity and enhanced the slow AHP that follows a train of action potentials. The single channel data showed tedisamil actions to be consistent with open channel blockade of the BK-type of K(+) channel. Together, the results are consistent with the possibility that prolongation of the action potential by tedisamil is mediated by a tetraethylammonium-like effect of the agent to block BK-type Ca(2+)-activated K(+) channels. The study also points to a number of effects that may contribute to the known nervous system toxicity induced by tedisamil.

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

Calcium dependence of C-type natriuretic peptide-formed fast K(+) channel

The lipid bilayer technique was used to characterize the Ca(2+) dependence of a fast K(+) channel formed by a synthetic 17-amino acid segment [OaCNP-39-(1-17)] of a 39-amino acid C-type natriuretic peptide (OaCNP-39) found in platypus (Ornithorhynchus anatinus) venom (OaV). The OaCNP-39-(1-17)-formed K(+) channel was reversibly dependent on 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-buffered cis (cytoplasmic) Ca(2+) concentration ([Ca(2+)](cis)). The channel was fully active when [Ca(2+)](cis) was >10(-4) M and trans (luminal) Ca(2+) concentration was 1.0 mM, but not at low [Ca(2+)](cis). The open probability of single channels increased from zero at 1 x 10(-6) M cis Ca(2+) to 0.73 +/- 0.17 (n = 22) at 10(-3) M cis Ca(2+). Channel openings to the maximum conductance of 38 pS were rapidly and reversibly activated when [Ca(2+)](cis), but not trans Ca(2+) concentration (n = 5), was increased to >5 x 10(-4) M (n = 14). Channel openings to the submaximal conductance of 10.5 pS were dominant at >/=5 x 10(-4) M Ca(2+). K(+) channels did not open when cis Mg(2+) or Sr(2+) concentrations were increased from zero to 10(-3) M or when [Ca(2+)](cis) was maintained at 10(-6) M (n = 3 and 2). The Hill coefficient and the inhibition constant were 1 and 0.8 x 10(-4) M cis Ca(2+), respectively. This dependence of the channel on high [Ca(2+)](cis) suggests that it may become active under 1) physiological conditions where Ca(2+) levels are high, e.g., during cardiac and skeletal muscle contractions, and 2) pathological conditions that lead to a Ca(2+) overload, e.g., ischemic heart and muscle fatigue. The channel could modify a cascade of physiological functions that are dependent on the Ca(2+)-activated K(+) channels, e.g., vasodilation and salt secretion.

pH modulation of Ca2+ responses and a Ca2+-dependent K+ channel in cultured rat hippocampal neurones

1. The effects of changes in extra- and intracellular pH (pHo and pHi, respectively) on depolarization-evoked rises in intracellular free Ca2+ concentration ([Ca2+]i) and the activity of a Ca2+-dependent K+ channel were investigated in cultured fetal rat hippocampal neurones. 2. In neurones loaded with 2', 7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein (BCECF), changes in pHo evoked changes in pHi. At room temperature, the ratio DeltapHi : DeltapHo (the slope of the regression line relating pHi to pHo) was 0.37 under HCO3-/CO2-buffered conditions and 0.45 under Hepes-buffered conditions; corresponding values at 37 C were 0.71 and 0.79, respectively. The measurements of changes in pHi evoked by changes in pHo were employed in subsequent experiments to correct for the effects of changes in pHi on the Kd of fura-2 for Ca2+. 3. In fura-2-loaded neurones, rises in [Ca2+]i evoked by transient exposure to 50 mM K+ were reduced and enhanced during perfusion with acidic and alkaline media, respectively, compared with control responses at pHo 7.3. Fifty percent inhibition of high-[K+]o-evoked rises in [Ca2+]i corresponded to pHo 7.23. In the presence of 10 microM nifedipine, 50 % inhibition of high-[K+]o-evoked responses corresponded to pHo 7.20, compared with a pHo of 7.31 for 50% inhibition of [Ca2+]i transients evoked by N-methyl-D-aspartate. 4. Changes in pHi at a constant pHo were evoked by exposing neurones to weak acids or bases and quantified in BCECF-loaded cells. Following pH-dependent corrections for the Kd of fura-2 for Ca2+, rises in [Ca2+]i evoked by high-[K+]o in fura-2-loaded cells were found to be affected only marginally by changes in pHi. When changes in pHi similar to those observed during the application of weak acids or bases were elicited by changing pHo, reductions in pH inhibited rises in [Ca2+]i evoked by 50 mM K+ whereas increases in pH enhanced them. 5. The effects of changes in pH on the kinetic properties of a BK-type Ca2+-dependent K+ channel were investigated. In inside-out patches excised from neurones in sister cultures to those used in the microspectrofluorimetric studies, with internal [Ca2+] at 20 microM, channel openings at an internal pH of 6.7 were generally absent whereas at pH 7.3 (or 7.8) the open probability was high. In contrast, channel activity in outside-out patches was not affected by reducing the pH of the bath (external) solution from 7.3 to 6.7. In inside-out patches with internal [Ca2+] at 0.7 microM, a separate protocol was applied to generate transient activation of the channel at a potential of 0 mV following a step from a holding level of -80 mV. In this case open probabilities were 0.81 (at pH 7.8), 0.57 (pH 7.3), 0.19 (pH 7.0) and 0.04 (pH 6.7). Channel conductance was not affected by changes in internal pH. 6. The results indicate that, in fetal rat hippocampal neurones, depolarization-evoked rises in [Ca2+]i mediated by the influx of Ca2+ ions through dihydropyridine-sensitive and -resistant voltage-activated Ca2+ channels are modulated by changes in pHo. The effects of pHo cannot be accounted for by changes in pHi consequent upon changes in pHo. However, changes in pHi affect the unitary properties of a Ca2+-dependent K+ channel. The results support the notion that pHo and/or pHi transients may serve a modulatory role in neuronal function.

Ion channels of human microglia in culture

Macroscopic and microscopic currents have been recorded using human microglia isolated from fetal human brains (12-20 weeks gestation). Within a period of two days following plating of cells, inward K+ currents were small (mean amplitude of 0.3 nA at -100 mV) and outward K+ currents were not observed. For periods in excess of five days after adherence to substrate, an inactivating outward K+ current, sensitive to 4-aminopyridine, was expressed. A slowly rising current, blocked by tetraethylammonium, was also evident in a small population of human microglia. This current was activated with cell depolarization positive to +10 mV and had properties similar to those recently described for a proton current in mouse cells. In early adherent cells (days 1 or 2 after plating), treatment of microglia with interferon-gamma led to the expression of outward K+ current which was lacking in the absence of the treatment. In excised, inside-out patches, two high conductance channels were identified. A calcium-dependent K+ channel (unitary conductance of 106 pS with physiological levels of K+ across the patch) had an open probability of 0.5 with internal Ca2+ at 7 microM and the patch potential at 0 mV. In addition, an anion channel (unitary conductance of 280 pS) was transiently activated with depolarizing or hyperpolarizing steps applied from 0 mV. Characterization of the macroscopic and unitary properties of currents in microglia will have relevance to a description of putative cell functions in the human CNS. In particular, modification of cell electrophysiological properties by various activating stimuli may contribute to signalling processes in CNS pathology.

Potassium currents in motoneurones

The measurements of intrinsic unitary and macroscopic properties of potassium channels in motoneurones constitutes an important step in the characterization of cellular function. A number of potassium currents contribute to the integrated function of motoneurones and this review considers the properties of the inward rectifier (Ih-type), calcium-dependent (BK- and SK-types), delayed rectifier, transient outward, and leak currents. The modification of channel properties by agents such as serotonin and thyrotropin-releasing hormone is likely relevant to modulation of motoneurone function. Indeed, even during the course of normal cell development, the expression and properties of particular potassium channels can undergo considerable changes, which would be manifested as altered firing patterns and excitability of motoneurones. Brainstem and spinal cord slices have considerable utility for electrophysiological studies and have been used in recordings from spinal, hypoglossal and facial motoneurones. New procedures have also been developed which would enhance the purification of cultured motoneurones for patch-clamp experiments.

Effects of intracellular Mg2+ on the properties of large-conductance, Ca(2+)-dependent K+ channels in rat cerebrovascular smooth muscle cells

The effects of intracellular magnesium ions, Mg2+i on large-conductance, Ca(2+)-dependent K+ channels (BK channels) of adult rat cerebrovascular smooth muscle cells (CVSMCs) were studied using patch clamp techniques and cells enzymatically dispersed from basilar, middle, and posterior cerebral arteries. Recordings used inside-out membrane patches and took place at 20-24 degrees C. One millimeter [Mg2+]i produced a fast block of BK channel currents, as well described by the Woodhull model of channel occlusion by a charged species. However, the affinity and voltage-sensitivity of Mg2+i block were dependent on the concentration of free intracellular calcium ions, [Ca2+]i. Calcium ions may stabilize a channel conformation in which Mg2+i binding sites are relocated closer to the inner membrane surface. In the presence of 1 microM [Ca2+]i, 0.5 mM [Mg2+]i shifted the Boltzmann curve relating BK channel open probability, Po, to membrane voltage leftward on the voltage axis, without any change in its slope. The enhancing effect of Mg2+i on Po was, therefore, insensitive to membrane potential. Quantitative considerations suggest that physiological levels of Mg2+i tonically facilitate BK channel activation. Alterations of [Mg2+]i during hyper- or hypomagnesemia may contribute to the dilation or contraction of cerebral vessels seen under these two conditions.

Inactivation of a high conductance calcium dependent potassium current in rat hippocampal neurons

Inactivating, high conductance BK-type currents have been recorded from inside-out patches (internal and external K+ of 140 mM and 5 mM, respectively), obtained from cultured rat hippocampal neurons. The presence of prominent inactivation, not normally associated with BK channel activity, was dependent on two factors: a depolarizing step to 0 mV from a holding level of -80 mV and internal calcium at a concentration of 0.7 microM. Without the prior conditioning step to a negative potential, unitary currents were not evident at 0 mV; in addition, such currents were not elicited with the stimulus protocol if the internal Ca2+ was reduced to a level of 0.3 microM. Concomitant with current inactivation was the finding of a delayed activation of BK currents following the depolarizing step. Higher internal calcium, at 100 microM, led to persistent and sustained channel activity at 0 mV which was not dependent on a prior step to -80 mV. These results may be relevant to the complex nature of the repolarizing neuronal current Ic which is the macroscopic analogue of the unitary BK current.