Gating mechanism of BK (Slo1) channels: so near, yet so far

A mutually exclusive alternative exon of slo1 codes for a neuronal BK channel with altered function

Large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels are comprised of four pore-forming -subunits (Slo1), whose mRNA is alternatively spliced in a cell-specific manner. Here we report the first case of a correctly spliced mutually exclusive exon in a mammalian (human and mouse) BK channel; an exon coding for the region from S6 to the RCK1 domain is exchanged for an alternative exon of the same length. The slo1 transcript with this novel exon is present in native brain tissues and inclusion of the alternative exon profoundly alters the channel's gating characteristics: faster activation at low Ca(2+) concentrations and greater open probability at resting membrane potential at high Ca(2+) concentrations. The novel gating features conferred by the alternative exon are dominant over those of the commonly described Slo1 variant when coexpressed. The evolutionarily preserved splicing of the Slo1 S6-RCK1 linker segment possess great potential to fine-tune neuronal excitability.

Reciprocal regulation of the Ca2+ and H+ sensitivity in the SLO1 BK channel conferred by the RCK1 domain

Increasing evidence suggests that intracellular H+ directly stimulates large-conductance Ca2+- and voltage-activated K+ (SLO1 BK) channels, thus providing a crucial link between membrane excitability and cell metabolism. Here we report that two histidine residues, His365 and His394, located in the intracellular regulator of conductance for K+ (RCK) 1 domain, serve as the H+ sensors of the SLO1 BK channel. Activation of the channel by H+ requires electrostatic interactions between the histidine residues and a nearby negatively charged residue involved in the channel's high-affinity Ca2+ sensitivity. Reciprocally, His365 and His394 also participate in the Ca2+-dependent activation of the channel, functioning as Ca2+ mimetics once they are protonated. Therefore, a common motif in the RCK1 domain mediates the stimulatory effects of both H+ and Ca2+, and provides a basis for the bidirectional coupling of cell metabolism and membrane electrical excitability.

The RCK1 high-affinity Ca2+ sensor confers carbon monoxide sensitivity to Slo1 BK channels

Carbon monoxide (CO) is a lethal gas, but it is also increasingly recognized as a physiological signaling molecule capable of regulating a variety of proteins. Among them, large-conductance Ca(2+)- and voltage-gated K(+) (Slo1 BK) channels, important in vasodilation and neuronal firing, have been suggested to be directly stimulated by CO. However, the molecular mechanism of the stimulatory action of CO on the Slo1 BK channel has not been clearly elucidated. We report here that CO reliably and repeatedly activates Slo1 BK channels in excised membrane patches in the absence of Ca(2+) in a voltage-sensor-independent manner. The stimulatory action of CO on the Slo1 BK channel requires an aspartic acid and two histidine residues located in the cytoplasmic RCK1 domain, and the effect persists under the conditions known to inhibit the conventional interaction between CO and heme in other proteins. We propose that CO acts as a partial agonist for the high-affinity divalent cation sensor in the RCK1 domain of the Slo1 BK channel.

The RCK2 domain of the human BKCa channel is a calcium sensor

Large conductance voltage and Ca(2+)-dependent K(+) channels (BK(Ca)) are activated by both membrane depolarization and intracellular Ca(2+). Recent studies on bacterial channels have proposed that a Ca(2+)-induced conformational change within specialized regulators of K(+) conductance (RCK) domains is responsible for channel gating. Each pore-forming alpha subunit of the homotetrameric BK(Ca) channel is expected to contain two intracellular RCK domains. The first RCK domain in BK(Ca) channels (RCK1) has been shown to contain residues critical for Ca(2+) sensitivity, possibly participating in the formation of a Ca(2+)-binding site. The location and structure of the second RCK domain in the BK(Ca) channel (RCK2) is still being examined, and the presence of a high-affinity Ca(2+)-binding site within this region is not yet established. Here, we present a structure-based alignment of the C terminus of BK(Ca) and prokaryotic RCK domains that reveal the location of a second RCK domain in human BK(Ca) channels (hSloRCK2). hSloRCK2 includes a high-affinity Ca(2+)-binding site (Ca bowl) and contains similar secondary structural elements as the bacterial RCK domains. Using CD spectroscopy, we provide evidence that hSloRCK2 undergoes a Ca(2+)-induced change in conformation, associated with an alpha-to-beta structural transition. We also show that the Ca bowl is an essential element for the Ca(2+)-induced rearrangement of hSloRCK2. We speculate that the molecular rearrangements of RCK2 likely underlie the Ca(2+)-dependent gating mechanism of BK(Ca) channels. A structural model of the heterodimeric complex of hSloRCK1 and hSloRCK2 domains is discussed.

Mg2+ mediates interaction between the voltage sensor and cytosolic domain to activate BK channels

The voltage-sensor domain (VSD) of voltage-dependent ion channels and enzymes is critical for cellular responses to membrane potential. The VSD can also be regulated by interaction with intracellular proteins and ligands, but how this occurs is poorly understood. Here, we show that the VSD of the BK-type K(+) channel is regulated by a state-dependent interaction with its own tethered cytosolic domain that depends on both intracellular Mg(2+) and the open state of the channel pore. Mg(2+) bound to the cytosolic RCK1 domain enhances VSD activation by electrostatic interaction with Arg-213 in transmembrane segment S4. Our results demonstrate that a cytosolic domain can come close enough to the VSD to regulate its activity electrostatically, thereby elucidating a mechanism of Mg(2+)-dependent activation in BK channels and suggesting a general pathway by which intracellular factors can modulate the function of voltage-dependent proteins.

SLO, SLO, quick, quick, slow: calcium-activated potassium channels as regulators of Caenorhabditis elegans behaviour and targets for anthelmintics

Large-conductance calcium and voltage-activated potassium channels, termed SLO-1 (or BK), are pivotal players in the regulation of cell excitability across the animal phyla. Furthermore, emerging evidence indicates that these channels are key mediators of a number of neuroactive drugs, including the most recent new anthelmintic, the cyclo-octadepsipeptide emodepside. Detailed reviews of the structure, function and pharmacology of BK channels have recently been provided (Salkoff et al. in Nat Rev Neurosci 7:921-931, 2006; Ghatta et al. in Pharmacol Ther 110:103-116, 2006) and therefore these aspects will only briefly be covered here. The purpose of this review is to discuss how SLO-1 channels might function as regulators of neural transmission and network activity. In particular, we focus on the role of SLO-1 in the regulation of Caenorhabditis elegans behaviour and highlight the role of this channel as an effector for pleiotropic actions of neuroactive drugs, including emodepside. On the premise that C. elegans is a 'model nematode' with respect to many aspects of neural function, the intention is that this might inform a broader understanding of the role of these channels in the nematodes and their potential as novel anthelmintic targets.

Na+/Cl- dipole couples agonist binding to kainate receptor activation

Kainate-selective ionotropic glutamate receptors (GluRs) require external Na+ and Cl- as well as the neurotransmitter L-glutamate for activation. Although, external anions and cations apparently coactivate kainate receptors (KARs) in an identical manner, it has yet to be established how ions of opposite charge achieve this. An additional complication is that KARs are subject to other forms of cation modulation via extracellular acidification (i.e., protons) and divalent ions. Consequently, other cation species may compete with Na+ to regulate the time KARs remain in the open state. Here we designed experiments to unravel how external ions regulate GluR6 KARs. We show that GluR6 kinetics are unaffected by alterations in physiological pH but that divalent and alkali metal ions compete to determine the time course of KAR channel activity. Additionally, Na+ and Cl- ions coactivate GluR6 receptors by establishing a dipole, accounting for their common effect on KARs. Using charged amino acids as tethered ions, we further demonstrate that the docking order is fixed with cations binding first, followed by anions. Together, our findings identify the dipole as a novel gating feature that couples neurotransmitter binding to KAR activation.

Gating and inward rectifying properties of the MthK K+ channel with and without the gating ring

In MthK, a Ca2+-gated K+ channel from Methanobacterium thermoautotrophicum, eight cytoplasmic RCK domains form an octameric gating ring that controls the intracellular gate of the ion conduction pore. The binding of Ca2+ ions to the RCK domains alters the conformation of the gating ring, thereby opening the gate. In the present study, we examined the Ca2+- and pH-regulated gating and the rectifying conduction properties of MthK at the single-channel level. The open probability (Po) of MthK exhibits a sigmoidal relationship with intracellular [Ca2+], and a Hill coefficient >1 is required to describe the dependence of Po on [Ca2+], suggesting cooperative Ca2+ activation of the channel. Additionally, intracellular Ca2+ also blocks the MthK pore in a voltage-dependent manner, rendering an apparently inwardly rectifying I-V relation. Intracellular pH has a dual effect on MthK gating. Below pH 7.5, the channel becomes insensitive to Ca2+. This occurs because the gating ring is structurally unstable at this pH and tends to disassemble (Ye, S., Y. Li, L. Chen, and Y. Jiang. 2006. Cell. 126:1161-1173). In contrast, above pH 7.5, a further increase in pH shifts the Po-[Ca2+] relation towards a lower Ca2+ concentration, augments Po at saturating [Ca2+], and activates the channel even in the absence of Ca2+. Channel activity is marked by bursts of rapid openings and closings separated by relatively longer interburst closings. The duration of interburst closing and the burst length are highly Ca2+ and pH dependent, whereas the kinetics of intraburst events is Ca2+ and pH independent. The rapid intraburst openings and closings are also observed with the isolated MthK pore lacking the attached intracellular gating ring. The fast kinetic events, independent of both Ca2+ and pH, therefore appear to be determined by processes occurring within the ion conduction pore, whereas the slow events reflect the gating process controlled by Ca2+ and pH through the gating ring.

Role of cholinergic-activated KCa1.1 (BK), KCa3.1 (SK4) and KV7.1 (KCNQ1) channels in mouse colonic Cl- secretion

AIM

Colonic crypts are the site of Cl- secretion. Basolateral K+ channels provide the driving force for luminal cystic fibrosis transmembrane regulator-mediated Cl- exit. Relevant colonic epithelial K+ channels are the intermediate conductance Ca2+-activated K(Ca)3.1 (SK4) channel and the cAMP-activated K(V)7.1 (KCNQ1) channel. In addition, big conductance Ca2+-activated K(Ca)1.1 (BK) channels may play a role in Ca2+-activated Cl- secretion. Here we use K(Ca)1.1 and K(Ca)3.1 knock-out mice, and the K(V)7.1 channel inhibitor 293B (10 microm) to investigate the role of K(Ca)1.1, K(Ca)3.1 and K(V)7.1 channels in cholinergic-stimulated Cl- secretion.

METHODS

A Ussing chamber was used to quantify agonist-stimulated increases in short circuit current (Isc) in distal colon. Chloride secretion was activated by bl. forskolin (FSK, 2 microm) followed by bl. carbachol (CCH, 100 microm). Luminal Ba2+ (5 mm) was used to inhibit K(Ca)1.1 channels.

RESULTS

K(Ca)1.1 WT and KO mice displayed identical FSK and CCH-stimulated Isc changes, indicating that K(Ca)1.1 channels are not involved in FSK- and cholinergic-stimulated Cl- secretion. CCH-stimulated DeltaIsc was significantly reduced in K(Ca)3.1 KO mice, underscoring the known relevance of this channel in the activation of Cl- secretion by an intracellular Ca2+ increasing agonist. The residual CCH effect observed in K(Ca)3.1 KO mice suggests that yet another K+ channel is driving the CCH-stimulated Cl- secretion. In the presence of the specific K(V)7.1 channel blocker 293B, the residual CCH effect was abolished.

CONCLUSIONS

This demonstrates that both K(Ca)3.1 and K(V)7.1 channels are activated by cholinergic agonists and drive Cl- secretion. In contrast, K(Ca)1.1 channels are not involved in stimulated electrogenic Cl- secretion.

Differential trafficking of carboxyl isoforms of Ca2+-gated (Slo1) potassium channels

The pore-forming subunit of the large-conductance Ca(2+)-dependent K(+) (Slo1) channel is encoded by one gene. However, the functional properties of Slo1 channels are diverse in part because of their numerous regulatory mechanisms including posttranslational modification and alternative splicing. In particular, multiple splice variants of the pore-forming subunit have been reported but their significance is only beginning to be elucidated. Here we examined the cell biological properties of the three common C-terminal isoforms that differ in the last 8 (Slo1_ERL and Slo1_VYR) or 61 residues (Slo1_DEC). We found that Slo1_DEC, the longest isoform, shows dramatically reduced surface expression compared to that of Slo1_ERL or Slo1_VYR. Immunocytochemistry revealed that a large fraction of Slo1_DEC remains localized in endoplasmic reticulum (ER). Using a GST fusion protein containing the Slo1_DEC-specific sequence, affinity purification was carried out to isolate interacting proteins. The identified proteins include protein phosphatase 2A (PP2A-A), actin, and tubulin. The PP2A-A interaction is specific to Slo1_DEC and causes a significant reduction of phosphorylation in Slo1_DEC but not Slo1_ERL or Slo1_VYR. The results together support the notion that Slo1_DEC nucleates isoform-specific protein complexes and possesses a cis element(s) for regulating trafficking of the Slo1 channels.

Nanodomains of single Ca2+ channels contribute to action potential repolarization in cortical neurons

The precise shape of action potentials in cortical neurons is a key determinant of action potential-dependent Ca2+ influx, as well as of neuronal signaling, on a millisecond scale. In cortical neurons, Ca2+-sensitive K+ channels, or BK channels (BKChs), are crucial for action potential termination, but the precise functional interplay between Ca2+ channels and BKChs has remained unclear. In this study, we investigate the mechanisms allowing for rapid and reliable activation of BKChs by single action potentials in hippocampal granule cells and the impact of endogenous Ca2+ buffers. We find that BKChs are operated by nanodomains of single Ca2+ channels. Using a novel approach based on a linear approximation of buffered Ca2+ diffusion in microdomains, we quantitatively analyze the prolongation of action potentials by the Ca2+ chelator BAPTA. This analysis allowed us to estimate that the mean diffusional distance for Ca2+ ions from a Ca2+ channel to a BKCh is approximately 13 nm. This surprisingly short diffusional distance cannot be explained by a random distribution of Ca2+ channels and renders the activation of BKChs insensitive to the relatively high concentrations of endogenous Ca2+ buffers in hippocampal neurons. These data suggest that tight colocalization of the two types of channels permits hippocampal neurons to regulate global Ca2+ signals by a high cytoplasmic Ca2+ buffer capacity without affecting the fast and brief activation of BKChs required for proper repolarization of action potentials.

Intra- and intersubunit cooperativity in activation of BK channels by Ca2+

The activation of BK channels by Ca(2+) is highly cooperative, with small changes in intracellular Ca(2+) concentration having large effects on open probability (Po). Here we examine the mechanism of cooperative activation of BK channels by Ca(2+). Each of the four subunits of BK channels has a large intracellular COOH terminus with two different high-affinity Ca(2+) sensors: an RCK1 sensor (D362/D367) located on the RCK1 (regulator of conductance of K(+)) domain and a Ca-bowl sensor located on or after the RCK2 domain. To determine interactions among these Ca(2+) sensors, we examine channels with eight different configurations of functional high-affinity Ca(2+) sensors on the four subunits. We find that the RCK1 sensor and Ca bowl contribute about equally to Ca(2+) activation of the channel when there is only one high-affinity Ca(2+) sensor per subunit. We also find that an RCK1 sensor and a Ca bowl on the same subunit are much more effective in increasing Po than when they are on different subunits, indicating positive intrasubunit cooperativity. If it is assumed that BK channels have a gating ring similar to MthK channels with alternating RCK1 and RCK2 domains and that the Ca(2+) sensors act at the flexible (rather than fixed) interfaces between RCK domains, then a comparison of the distribution of Ca(2+) sensors with the observed responses suggest that the interface between RCK1 and RCK2 domains on the same subunit is flexible. On this basis, intrasubunit cooperativity arises because two high-affinity Ca(2+) sensors acting across a flexible interface are more effective in opening the channel than when acting at separate interfaces. An allosteric model incorporating intrasubunit cooperativity nested within intersubunit cooperativity could approximate the Po vs. Ca(2+) response for eight possible subunit configurations of the high-affinity Ca(2+) sensors as well as for three additional configurations from a previous study.

Ion channels and calcium signaling in cerebral arteries following subarachnoid hemorrhage

Entry of Ca(2+) through voltage-dependent calcium channels (VDCCs) is critical to the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in vascular smooth muscle and thus the control of cerebral artery diameter. Increased VDCC activity in cerebral artery myocytes may contribute to decreased cerebral blood flow and the accompanying neurological deficits associated with subarachnoid hemorrhage (SAH). This review will focus on the impact of SAH on VDCCs and K(+)-selective ion channels, two important classes of ion channels located in the plasma membrane of cerebral artery myocytes. SAH may act through a variety of direct and indirect mechanisms to increase the activity of VDCCs promoting cerebral artery constriction and reduced cerebral blood flow. Further, SAH may lead to suppression of K(+) channel activity to cause membrane potential depolarization to enhance VDCC activity. The ability of VDCC blockers or K(+) channel activators to alleviate SAH-induced vasospasm will also be examined.

High-conductance potassium channels of the SLO family

High-conductance, 'big' potassium (BK) channels encoded by the Slo gene family are among the largest and most complex of the extended family of potassium channels. The family of SLO channels apparently evolved from voltage-dependent potassium channels, but acquired a large conserved carboxyl extension, which allows channel gating to be altered in response to the direct sensing of several different intracellular ions, and by other second-messenger systems, such as those activated following neurotransmitter binding to G-protein-coupled receptors (GPCRs). This versatility has been exploited to serve many cellular roles, both within and outside the nervous system.

Regulation of the gating of BKCa channel by lipid bilayer thickness

Transmembrane segments of ion channels tend to match the hydrophobic thickness of lipid bilayers to minimize mismatch energy and to maintain their proper organization and function. To probe how ion channels respond to mismatch with lipid bilayers of different thicknesses, we examined the single channel activities of BK(Ca) (hSlo alpha-subunit) channels in planar bilayers of binary mixtures of DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine) with phosphatidylcholines (PCs) of varying chain lengths, including PC 14:1, PC 18:1, PC 22:1, PC 24:1, and with porcine brain sphingomyelin. Bilayer thickness and structure was measured with small angle x-ray diffraction and atomic force microscopy. The open probability (P(o)) of the BK(Ca) channel was finely tuned by bilayer thickness, first decreasing with increases in bilayer thickness from PC 14:1 to PC 22:1 and then increasing from PC 22:1 to PC 24:1 and to porcine brain sphingomyelin. Single channel kinetic analyses revealed that the mean open time of the channel increased monotonically with bilayer thickness and, therefore, could not account for the biphasic changes in P(o). The mean closed time increased with bilayer thickness from PC 14:1 up to PC 22:1 and then decreased with further increases in bilayer thickness to PC 24:1 and sphingomyelin, correlating with changes in P(o). This is consistent with the proposition that bilayer thickness affects channel activity mainly through altering the stability of the closed state. We suggest a simple mechanical model that combines forces of lateral stress within the lipid bilayer with local hydrophobic mismatch between lipids and the protein to account for the biphasic modulation of BK(Ca) gating.

Agonist-activated ion channels

This paper looks at ion channels as an example of the pharmacologist's stock in trade, the action of an agonist on a receptor to produce a response. Looked at in this way, ion channels have been helpful because they are still the only system which is simple enough for quantitative investigation of transduction mechanisms. A short history is given of attempts to elucidate what happens between the time when agonist first binds, and the time when the channel opens.

Reversible binding of heme to proteins in cellular signal transduction

Heme plays critical roles in numerous biological phenomena. Recent evidence has uncovered a new role of heme in cellular signal transduction, and its mechanism involves reversible binding of heme to proteins. This Account highlights the novel function of heme as an intracellular messenger in the regulation of gene expression and ion channel function.

Crystal structures of a ligand-free MthK gating ring: insights into the ligand gating mechanism of K+ channels

MthK is a prokaryotic Ca(2+)-gated K(+) channel that, like other ligand-gated channels, converts the chemical energy of ligand binding to the mechanical force of channel opening. The channel's eight ligand-binding domains, the RCK domains, form an octameric gating ring in which Ca(2+) binding induces conformational changes that open the channel. Here we present the crystal structures of the MthK gating ring in closed and partially open states at 2.8 A, both obtained from the same crystal grown in the absence of Ca(2+). Furthermore, our biochemical and electrophysiological analyses demonstrate that MthK is regulated by both Ca(2+) and pH. Ca(2+) regulates the channel by changing the equilibrium of the gating ring between closed and open states, while pH regulates channel gating by affecting gating-ring stability. Our findings, along with the previously determined open MthK structure, allow us to elucidate the ligand gating mechanism of RCK-regulated K(+) channels.

Modulation of Ca(2+)-activated K+ currents and Ca(2+)-dependent action potentials by exocytosis in goldfish bipolar cell terminals

Retinal bipolar cells convey light-evoked potentials from photoreceptors to ganglion cells and mediate the initial stages of visual signal processing. They do not fire Na(+)-dependent action potentials (APs) but the Mb1 class of goldfish bipolar cell exhibits Ca(2+)-dependent APs and regenerative potentials that originate in the axon terminal. I have examined the properties of Ca(2+)-dependent APs in isolated bipolar-cell terminals in goldfish retinal slices. All recorded terminals fired spontaneous or evoked APs at frequencies of up to 15 Hz. When an AP waveform was used as a voltage stimulus, exocytosis was evoked by single APs, maintained throughout AP trains and modulated by AP frequency. Furthermore, feedback inhibition of the Ca2+ current (I(Ca)) by released vesicular protons reduced depression of exocytosis during AP trains. In the absence of K+ current inhibition, step depolarizations and AP waveforms evoked a rapidly activated outward current that was dependent on Ca2+ influx I(K(Ca). I therefore investigated whether proton-mediated feedback inhibition of I(Ca) affected the activation of I(K(Ca)). A transient inhibition of I(K(Ca)) was observed that was dependent on exocytosis, blocked by high-pH extracellular buffer, of similar magnitude to inhibition of I(Ca) but occurred with a delay of 2.7 ms. In addition, the amplitude of APs evoked under current clamp was inhibited by the action of vesicular protons released by the APs. Protons released via exocytosis may therefore be a significant modulator of Ca(2+)-dependent currents and regenerative potentials in bipolar-cell terminals.

Diversity of Ca2+-activated K+ channel transcripts in inner ear hair cells

Hair cells express a complement of ion channels, representing shared and distinct channels that confer distinct electrophysiological signatures for each cell. This diversity is generated by the use of alternative splicing in the alpha subunit, formation of heterotetrameric channels, and combinatorial association with beta subunits. These channels are thought to play a role in the tonotopic gradient observed in the mammalian cochlea. Mouse Kcnma1 transcripts, 5' and 3' ESTs, and genomic sequences were examined for the utilization of alternative splicing in the mouse transcriptome. Comparative genomic analyses investigated the conservation of KCNMA1 splice sites. Genomes of mouse, rat, human, opossum, chicken, frog and zebrafish established that the exon-intron structure and mechanism of KCNMA1 alternative splicing were highly conserved with 6-7 splice sites being utilized. The murine Kcnma1 utilized 6 out of 7 potential splice sites. RT-PCR experiments using murine gene-specific oligonucleotide primers analyzed the scope and variety of Kcnma1 and Kcnmb1-4 expression profiles in the cochlea and inner ear hair cells. In the cochlea splice variants were present representing sites 3, 4, 6, and 7, while site 1 was insertionless and site 2 utilized only exon 10. However, site 5 was not present. Detection of KCNMA1 transcripts and protein exhibited a quantitative longitudinal gradient with a reciprocal gradient found between inner and outer hair cells. Differential expression was also observed in the usage of the long form of the carboxy-terminus tail. These results suggest that a diversity of splice variants exist in rodent cochlear hair cells and this diversity is similar to that observed for non-mammalian vertebrate hair cells, such as chicken and turtle.

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.

Oxygen sensing in the body

This review is divided into three parts: (a) The primary site of oxygen sensing is the carotid body which instantaneously respond to hypoxia without involving new protein synthesis, and is historically known as the first oxygen sensor and is therefore placed in the first section (Lahiri, Roy, Baby and Hoshi). The carotid body senses oxygen in acute hypoxia, and produces appropriate responses such as increases in breathing, replenishing oxygen from air. How this oxygen is sensed at a relatively high level (arterial PO2 approximately 50 Torr) which would not be perceptible by other cells in the body, is a mystery. This response is seen in afferent nerves which are connected synaptically to type I or glomus cells of the carotid body. The major effect of oxygen sensing is the increase in cytosolic calcium, ultimately by influx from extracellular calcium whose concentration is 2 x 10(4) times greater. There are several contesting hypotheses for this response: one, the mitochondrial hypothesis which states that the electron transport from the substrate to oxygen through the respiratory chain is retarded as the oxygen pressure falls, and the mitochondrial membrane is depolarized leading to the calcium release from the complex of mitochondria-endoplasmic reticulum. This is followed by influx of calcium. Also, the inhibitors of the respiratory chain result in mitochondrial depolarization and calcium release. The other hypothesis (membrane model) states that K(+) channels are suppressed by hypoxia which depolarizes the membrane leading to calcium influx and cytosolic calcium increase. Evidence supports both the hypotheses. Hypoxia also inhibits prolyl hydroxylases which are present in all the cells. This inhibition results in membrane K(+) current suppression which is followed by cell depolarization. The theme of this section covers first what and where the oxygen sensors are; second, what are the effectors; third, what couples oxygen sensors and the effectors. (b) All oxygen consuming cells have a built-in mechanism, the transcription factor HIF-1, the discovery of which has led to the delineation of oxygen-regulated gene expression. This response to chronic hypoxia needs new protein synthesis, and the proteins of these genes mediate the adaptive physiological responses. HIF-1alpha, which is a part of HIF-1, has come to be known as master regulator for oxygen homeostasis, and is precisely regulated by the cellular oxygen concentration. Thus, the HIF-1 encompasses the chronic responses (gene expression in all cells of the body). The molecular biology of oxygen sensing is reviewed in this section (Semenza). (c) Once oxygen is sensed and Ca(2+) is released, the neurotransmittesr will be elaborated from the glomus cells of the carotid body. Currently it is believed that hypoxia facilitates release of one or more excitatory transmitters from glomus cells, which by depolarizing the nearby afferent terminals, leads to increases in the sensory discharge. The transmitters expressed in the carotid body can be classified into two major categories: conventional and unconventional. The conventional neurotransmitters include those stored in synaptic vesicles and mediate their action via activation of specific membrane bound receptors often coupled to G-proteins. Unconventional neurotransmitters are those that are not stored in synaptic vesicles, but spontaneously generated by enzymatic reactions and exert their biological responses either by interacting with cytosolic enzymes or by direct modifications of proteins. The gas molecules such as NO and CO belong to this latter category of neurotransmitters and have unique functions. Co-localization and co-release of neurotransmitters have also been described. Often interactions between excitatory and inhibitory messenger molecules also occur. Carotid body contains all kinds of transmitters, and an interplay between them must occur. But very little has come to be known as yet. Glimpses of these interactions are evident in the discussion in the last section (Prabhakar).

Hair Cells – Beyond the Transducer

OVERVIEW

This review considers the "tween twixt and twain" of hair cell physiology, specifically the signaling elements and membrane conductances which underpin forward and reverse transduction at the input stage of hair cell function and neurotransmitter release at the output stage. Other sections of this review series outline the advances which have been made in understanding the molecular physiology of mechanoelectrical transduction and outer hair cell electromotility. Here we outline the contributions of a considerable array of ion channels and receptor signaling pathways that define the biophysical status of the sensory hair cells, contributing to hair cell development and subsequently defining the operational condition of the hair cells across the broad dynamic range of physiological function.

KCa channel insensitivity to Ca2+ sparks underlies fractional uncoupling in newborn cerebral artery smooth muscle cells

In smooth muscle cells, localized intracellular Ca2+ transients, termed "Ca2+ sparks," activate several large-conductance Ca2+-activated K+ (KCa) channels, resulting in a transient KCa current. In some smooth muscle cell types, a significant proportion of Ca2+ sparks do not activate KCa channels. The goal of this study was to explore mechanisms that underlie fractional Ca2+ spark-KCa channel coupling. We investigated whether membrane depolarization or ryanodine-sensitive Ca2+ release (RyR) channel activation modulates coupling in newborn (1- to 3-day-old) porcine cerebral artery myocytes. At steady membrane potentials of -40, 0, and +40 mV, mean transient KCa current frequency was approximately 0.18, 0.43, and 0.26 Hz and KCa channel activity [number of KCa channels activated by Ca2+ sparksxopen probability of KCa channels at peak of Ca2+ sparks (NPo)] at the transient KCa current peak was approximately 4, 12, and 24, respectively. Depolarization between -40 and +40 mV increased KCa channel sensitivity to Ca2+ sparks and elevated the percentage of Ca2+ sparks that activated a transient KCa current from 59 to 86%. In a Ca2+-free bath solution or in diltiazem, a voltage-dependent Ca2+ channel blocker, steady membrane depolarization between -40 and +40 mV increased transient KCa current frequency up to approximately 1.6-fold. In contrast, caffeine (10 microM), an RyR channel activator, increased mean transient KCa current frequency but did not alter Ca2+ spark-KCa channel coupling. These data indicate that coupling is increased by mechanisms that elevate KCa channel sensitivity to Ca2+ sparks, but not by RyR channel activation. Overall, KCa channel insensitivity to Ca2+ sparks is a prominent factor underlying fractional Ca2+ spark uncoupling in newborn cerebral artery myocytes.

Cysteine oxidation and rundown of large-conductance Ca2+-dependent K+ channels

Gating of Slo1 calcium- and voltage-gated potassium (BK) channels involves allosteric interactions among the channel pore, voltage sensors, and Ca(2+)-binding domains. The allosteric activation of the Slo1 channel is in turn modulated by a variety of regulatory processes, including oxidation. Cysteine oxidation alters functional properties of Slo1 channels and has been suggested to contribute to the decrease in the channel activity following patch excision often referred to as rundown. This study examined the biophysical mechanism of rundown and whether oxidation of cysteine residues located in the C-terminus of the human Slo1 channel (C430 and C911) plays a role. Comparison of the changes in activation properties in different concentrations of Ca(2+) among the wild-type, C430A, and C911A channels during rundown and by treatment with the oxidant hydrogen peroxide showed that oxidation of C430 and C911 markedly contributes to the rundown process.

From molecule to malady

Ion channels are membrane proteins, found in virtually all cells, that are of crucial physiological importance. In the past decade, an explosion in the number of crystal structures of ion channels has led to a marked increase in our understanding of how ion channels open and close, and select between permeant ions. There has been a parallel advance in research on channelopathies (diseases resulting from impaired channel function), and mutations in over 60 ion-channel genes are now known to cause human disease. Characterization of their functional consequences has afforded unprecedented and unexpected insights into ion-channel mechanisms and physiological roles.

Patch Fluorometry: Shedding New Light on Ion Channels

Patch fluorometry has emerged as a new approach to the study of the structure-function relationship in membrane-embedded functional ion channels. Simultaneous fluorescent and electrical recordings are achieved from a small number of channels in a cell-free membrane patch, yielding high recording sensitivities. Further improvement of this approach should permit direct observation of the gating motion of a single-channel protein.

Large-conductance, calcium-activated potassium channels: structural and functional implications

The large-conductance, calcium-activated potassium channels (BK, also termed BK(Ca), Slo, or MaxiK) distributed in both excitable and non-excitable cells are involved in many cellular functions such as action potential repolarization; neuronal excitability; neurotransmitter release; hormone secretion; tuning of cochlear hair cells; innate immunity; and modulation of the tone of vascular, airway, uterine, gastrointestinal, and urinary bladder smooth muscle tissues. Because of their high conductance, activation of BK channels has a strong effect on membrane potential. BK channels differ from all other potassium (K(+)) channels due to their high sensitivity to both intracellular calcium (Ca(2+)) concentrations and voltage. These features make BK channels ideal negative feedback regulators in many cell types by decreasing voltage-dependent Ca(2+) entry through membrane potential hyperpolarization. The current review aims to give a comprehensive understanding of the structure and molecular biology of BK channels and their relevance to various pathophysiological conditions. The review will also focus on the therapeutic potential and pharmacology of the various BK channel activators and blockers.

Structures of the MthK RCK domain and the effect of Ca2+ on gating ring stability

MthK is a Ca2+-gated K+ channel from Methanobacterium autotrophicum. The crystal structure of the MthK channel in a Ca2+-bound open state was previously determined at 3.3 A and revealed an octameric gating ring composed of eight intracellular ligand-binding RCK (regulate the conductance of K+) domains. It was suggested that Ca2+ binding regulates the gating ring conformation, which in turn leads to the opening and closing of the channel. However, at 3.3 AA resolution, the molecular details of the structure are not well defined, and many of the conclusions drawn from that structure were hypothetical. Here we have presented high resolution structures of the MthK RCK domain with and without Ca2+ bound from three different crystals. These structures revealed a dimeric architecture of the RCK domain and allowed us to visualize the Ca2+ binding and protein-protein contacts at atomic detail. The dimerization of RCK domains is also conserved in other RCK-regulated K+ channels and transporters, suggesting that the RCK dimer serves as a basic unit in the gating ring assembly. A comparison of these dimer structures confirmed that the dimer interface is indeed flexible as suggested previously. However, the conformational change at the flexible interface is of an extent smaller than the previously hypothesized gating ring movement, and a reconstruction of these dimers into octamers by applying protein-protein contacts at the fixed interface did not generate enclosed gating rings. This indicated that there is a high probability that the previously defined fixed interface may not be fixed during channel gating. In addition to the structural studies, we have also carried out biochemical analyses and have shown that near physiological pH, isolated RCK domains form a stable octamer in solution, supporting the notion that the formation of octameric gating ring is a functionally relevant event in MthK gating. Additionally, our stability studies indicated that Ca2+ binding stabilizes the RCK domains in this octameric state.

The NH2 terminus of RCK1 domain regulates Ca2+-dependent BK(Ca) channel gating

Large conductance, voltage- and Ca²⁺-activated K⁺ (BK_Ca) channels regulate blood vessel tone, synaptic transmission, and hearing owing to dual activation by membrane depolarization and intracellular Ca²⁺. Similar to an archeon Ca²⁺-activated K⁺ channel, MthK, each of four α subunits of BK_Ca may contain two cytosolic RCK domains and eight of which may form a gating ring. The structure of the MthK channel suggests that the RCK domains reorient with one another upon Ca²⁺ binding to change the gating ring conformation and open the activation gate. Here we report that the conformational changes of the NH₂ terminus of RCK1 (AC region) modulate BK_Ca gating. Such modulation depends on Ca²⁺ occupancy and activation states, but is not directly related to the Ca²⁺ binding sites. These results demonstrate that AC region is important in the allosteric coupling between Ca²⁺ binding and channel opening. Thus, the conformational changes of the AC region within each RCK domain is likely to be an important step in addition to the reorientation of RCK domains leading to the opening of the BK_Ca activation gate. Our observations are consistent with a mechanism for Ca²⁺-dependent activation of BK_Ca channels such that the AC region inhibits channel activation when the channel is at the closed state in the absence of Ca²⁺; Ca²⁺ binding and depolarization relieve this inhibition.

Homology modeling identifies C-terminal residues that contribute to the Ca2+ sensitivity of a BKCa channel

Activation of BK(Ca) channels by direct Ca(2+) binding and membrane depolarization occur via independent and additive molecular processes. The "calcium bowl" domain is critically involved in Ca(2+)-dependent gating, and we have hypothesized that a sequence within this domain may resemble an EF hand motif. Using a homology modeling strategy, it was observed that a single Ca(2+) ion may be coordinated by the oxygen-containing side chains of residues within the calcium bowl (i.e., (912)ELVNDTNVQFLD(923)). To examine these predictions directly, alanine-substituted BK(Ca) channel mutants were expressed in HEK 293 cells and the voltage and Ca(2+) dependence of macroscopic currents were examined in inside-out membrane patches. Over the range of 1-10 microM free Ca(2+), single point mutations (i.e., E912A and D923A) produced rightward shifts in the steady-state conductance-voltage relations, whereas the mutants N918A or Q920A had no effect on Ca(2+)-dependent gating. The double mutant E912A/D923A displayed a synergistic shift in Ca(2+)-sensitive gating, as well as altered kinetics of current activation/deactivation. In the presence of 1, 10, and 80 mM cytosolic Mg(2+), this double mutation significantly reduced the Ca(2+)-induced free energy change associated with channel activation. Finally, mutations that altered sensitivity of the holo-channel to Ca(2+) also reduced direct (45)Ca binding to the calcium bowl domain expressed as a bacterial fusion protein. These findings, along with other recent data, are considered in the context of the calcium bowl's high affinity Ca(2+) sensor and the known properties of EF hands.

Divalent cation sensitivity of BK channel activation supports the existence of three distinct binding sites

Mutational analyses have suggested that BK channels are regulated by three distinct divalent cation-dependent regulatory mechanisms arising from the cytosolic COOH terminus of the pore-forming α subunit. Two mechanisms account for physiological regulation of BK channels by μM Ca²⁺. The third may mediate physiological regulation by mM Mg²⁺. Mutation of five aspartate residues (5D5N) within the so-called Ca²⁺ bowl removes a portion of a higher affinity Ca²⁺ dependence, while mutation of D362A/D367A in the first RCK domain also removes some higher affinity Ca²⁺ dependence. Together, 5D5N and D362A/D367A remove all effects of Ca²⁺ up through 1 mM while E399A removes a portion of low affinity regulation by Ca²⁺/Mg²⁺. If each proposed regulatory effect involves a distinct divalent cation binding site, the divalent cation selectivity of the actual site that defines each mechanism might differ. By examination of the ability of various divalent cations to activate currents in constructs with mutationally altered regulatory mechanisms, here we show that each putative regulatory mechanism exhibits a unique sensitivity to divalent cations. Regulation mediated by the Ca²⁺ bowl can be activated by Ca²⁺ and Sr²⁺, while regulation defined by D362/D367 can be activated by Ca²⁺, Sr²⁺, and Cd²⁺. Mn²⁺, Co²⁺, and Ni²⁺ produce little observable effect through the high affinity regulatory mechanisms, while all six divalent cations enhance activation through the low affinity mechanism defined by residue E399. Furthermore, each type of mutation affects kinetic properties of BK channels in distinct ways. The Ca²⁺ bowl mainly accelerates activation of BK channels at low [Ca²⁺], while the D362/D367-related high affinity site influences both activation and deactivation over the range of 10–300 μM Ca²⁺. The major kinetic effect of the E399-related low affinity mechanism is to slow deactivation at mM Mg²⁺ or Ca²⁺. The results support the view that three distinct divalent-cation binding sites mediate regulation of BK channels.

Heme regulates allosteric activation of the Slo1 BK channel

Large conductance calcium-dependent (Slo1 BK) channels are allosterically activated by membrane depolarization and divalent cations, and possess a rich modulatory repertoire. Recently, intracellular heme has been identified as a potent regulator of Slo1 BK channels (Tang, X.D., R. Xu, M.F. Reynolds, M.L. Garcia, S.H. Heinemann, and T. Hoshi. 2003. Nature. 425:531-535). Here we investigated the mechanism of the regulatory action of heme on heterologously expressed Slo1 BK channels by separating the influences of voltage and divalent cations. In the absence of divalent cations, heme generally decreased ionic currents by shifting the channel's G-V curve toward more depolarized voltages and by rendering the curve less steep. In contrast, gating currents remained largely unaffected by heme. Simulations suggest that a decrease in the strength of allosteric coupling between the voltage sensor and the activation gate and a concomitant stabilization of the open state account for the essential features of the heme action in the absence of divalent ions. At saturating levels of divalent cations, heme remained similarly effective with its influence on the G-V simulated by weakening the coupling of both Ca(2+) binding and voltage sensor activation to channel opening. The results thus show that heme dampens the influence of allosteric activators on the activation gate of the Slo1 BK channel. To account for these effects, we consider the possibility that heme binding alters the structure of the RCK gating ring and thereby disrupts both Ca(2+)- and voltage-dependent gating as well as intrinsic stability of the open state.

Differential effects of beta 1 and beta 2 subunits on BK channel activity

High conductance, calcium- and voltage-activated potassium (BK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (β) subunits. β1 and β2 subunits increase apparent channel calcium sensitivity. The β1 subunit also decreases the voltage sensitivity of the channel and the β2 subunit produces an N-type inactivation of BK currents. We further characterized the effects of the β1 and β2 subunits on the calcium and voltage sensitivity of the channel, analyzing the data in the context of an allosteric model for BK channel activation by calcium and voltage (Horrigan and Aldrich, 2002). In this study, we used a β2 subunit without its N-type inactivation domain (β2IR). The results indicate that the β2IR subunit, like the β1 subunit, has a small effect on the calcium binding affinity of the channel. Unlike the β1 subunit, the β2IR subunit also has no effect on the voltage sensitivity of the channel. The limiting voltage dependence for steady-state channel activation, unrelated to voltage sensor movements, is unaffected by any of the studied β subunits. The same is observed for the limiting voltage dependence of the deactivation time constant. Thus, the β1 subunit must affect the voltage sensitivity by altering the function of the voltage sensors of the channel. Both β subunits reduce the intrinsic equilibrium constant for channel opening (L₀). In the allosteric activation model, the reduction of the voltage dependence for the activation of the voltage sensors accounts for most of the macroscopic steady-state effects of the β1 subunit, including the increase of the apparent calcium sensitivity of the BK channel. All allosteric coupling factors need to be increased in order to explain the observed effects when the α subunit is coexpressed with the β2IR subunit.

Syntaxin-1A Binds to and Modulates the Slo Calcium-Activated Potassium Channel via an Interaction That Excludes Syntaxin Binding to Calcium Channels

From its position in presynaptic nerve terminals, the large conductance Ca2+-activated K+ channel, Slo, regulates neurotransmitter release. Several other ion channels known to control neurotransmitter release have been implicated in physical interactions with the neurotransmitter release machinery. For example, the Cav2.2 (N-type) Ca2+ channel binds to and is modulated by syntaxin-1A and SNAP-25. Furthermore, a close juxtaposition of Slo and Cav2.2 is presumed to be necessary for functional coupling between the two channels, which has been shown in neurons. We report that Slo exhibits a strong association with syntaxin-1A. Robust co-immunoprecipitation of Slo and syntaxin-1A occurs from transfected HEK293 cells as well as from brain. However, despite this strong interaction and the known association between syntaxin-1A and the II–III loop of Cav2.2, these three proteins do not co-immunoprecipitate in a trimeric complex from transfected HEK293 cells. The Slo-syntaxin-1A co-immunoprecipitation is not significantly influenced by [Ca2+]. Multiple relatively weak interactions may sum up to a tight physical coupling of full-length Slo with syntaxin-1A: the C-terminal tail and the S0–S1 loop of Slo each co-immunoprecipitate with syntaxin-1A. The presence of syntaxin-1A leads to reduced Slo channel activity due to an increased V1/2 for activation in 100 nM, 1 μM, and 10 μM Ca2+, reduced voltage-sensitivity in 1 μM Ca2+, and slower rates of activation in 10 μM Ca2+. Potential physiological consequences of the interaction between Slo and syntaxin-1A include enhanced excitability through modulation of Slo channel activity and reduced neurotransmitter release due to disruption of syntaxin-1A binding to the Cav2.2 II–III loop.

Ligand-dependent activation of Slo family channels is defined by interchangeable cytosolic domains

Large-conductance Ca2+- and voltage-regulated K+ channels (Slo1 BK-type) are controlled by two physiological stimuli, membrane voltage and cytosolic Ca2+. Regulation by voltage is similar to that in voltage-dependent K+ channels, arising from positively charged amino acids primarily within the S4 transmembrane helices. The basis for regulation by Ca2+ remains controversial. One viewpoint suggests that the extensive cytosolic C terminus contains the Ca2+ regulatory machinery, whereas another suggests that the pore-forming module contains the Ca2+-sensing elements. To address this issue, we take advantage of another Slo family member, the pH-regulated homolog Slo3. We reason that if the ligand-sensing apparatus is uniquely associated with a particular domain (either the pore or the cytosolic domain), exchange of those domains between Slo1 and Slo3 should result in exchange of ligand dependence in association with the key domain. The results show that the Slo3 cytosolic module confers pH-dependent regulation on the Slo1 pore module, whereas the Slo1 cytosolic module confers Ca2+-dependent regulation on the Slo3 pore module. Thus, ligand-specific regulation is defined by interchangeable cytosolic regulatory modules.

Conserved gating hinge in ligand- and voltage-dependent K+ channels

Ion channels open and close their pore in a process called gating. On the basis of crystal structures of two voltage-independent K(+) channels, KcsA and MthK, a conformational change for gating has been proposed whereby the inner helix bends at a glycine hinge point (gating hinge) to open the pore and straightens to close it. Here we ask if a similar gating hinge conformational change underlies the mechanics of pore opening of two eukaryotic voltage-dependent K(+) channels, Shaker and BK channels. In the Shaker channel, substitution of the gating hinge glycine with alanine and several other amino acids prevents pore opening, but the ability to open is recovered if a secondary glycine is introduced at an adjacent position. A proline at the gating hinge favors the open state of the Shaker channel as if by preventing inner helix straightening. In BK channels, which have two adjacent glycine residues, opening is significantly hindered in a graded manner with single and double mutations to alanine. These results suggest that K(+) channels, whether ligand- or voltage-dependent, open when the inner helix bends at a conserved glycine gating hinge.

Linker-gating ring complex as passive spring and Ca(2+)-dependent machine for a voltage- and Ca(2+)-activated potassium channel

Ion channels are proteins that control the flux of ions across cell membranes by opening and closing (gating) their pores. It has been proposed that channels gated by internal agonists have an intracellular gating ring that extracts free energy from agonist binding to open the gates using linkers that directly connect the gating ring to the gates. Here we find for a voltage- and Ca(2+)-activated K+ (BK) channel that shortening the linkers increases channel activity and lengthening the linkers decreases channel activity, both in the presence and absence of intracellular Ca2+. These observations are consistent with a mechanical model in which the linker-gating ring complex forms a passive spring that applies force to the gates in the absence of Ca2+ to modulate the voltage-dependent gating. Adding Ca2+ then changes the force to further activate the channel. Both the passive and Ca(2+)-induced forces contribute to the gating of the channel.

BK channels: the spring between sensor and gate

K+ channels contain two main functional domains, an ion-selective pore and a sensor that determines whether the cytoplasmic pore gate is open or closed. In this issue of Neuron, Niu et al. provide compelling evidence that the link between sensor and gate is a remarkably simple mechanical spring.

Molecular cloning of a K+ channel from the malaria parasite Plasmodium falciparum

In most living cells, K(+) channels are important for the generation of the membrane potential and for volume regulation. The parasite Plasmodium falciparum, which causes malignant malaria, must be able to deal with large variations in the ambient K(+) concentration: it is exposed to high concentrations of K(+) when inside the erythrocyte and low concentrations when in plasma. In the recently published genome of P. falciparum, we have identified a gene, pfkch1, encoding a potential K(+) channel, which to some extent resembles the big-conductance (BK) K(+) channel. We have cloned the approximately 6000 nucleotide (nt) fragment from cDNA, studied the pattern of expression of pfkch1 throughout the intraerythrocytic part of the parasite's life-cyclus, and characterized the channel on the basis of similarity to other K(+) channels from pro- and eukaryotic organisms. This P. falciparum K(+) channel could be a potential drug target.

Gain-of-function mutation in the KCNMB1 potassium channel subunit is associated with low prevalence of diastolic hypertension

Hypertension is the most prevalent risk factor for cardiovascular diseases, present in almost 30% of adults. A key element in the control of vascular tone is the large-conductance, Ca(2+)-dependent K(+) (BK) channel. The BK channel in vascular smooth muscle is formed by an ion-conducting alpha subunit and a regulatory beta(1) subunit, which couples local increases in intracellular Ca(2+) to augmented channel activity and vascular relaxation. Our large population-based genetic epidemiological study has identified a new single-nucleotide substitution (G352A) in the beta(1) gene (KCNMB1), corresponding to an E65K mutation in the protein. This mutation results in a gain of function of the channel and is associated with low prevalence of moderate and severe diastolic hypertension. BK-beta(1E65K) channels showed increased Ca(2+) sensitivity, compared with wild-type channels, without changes in channel kinetics. In conclusion, the BK-beta(1E65K) channel might offer a more efficient negative-feedback effect on vascular smooth muscle contractility, consistent with a protective effect of the K allele against the severity of diastolic hypertension.

Electrophysiological properties of BK channels in Xenopus motor nerve terminals

Single channel properties of Ca(2+)-activated K(+) (BK or Maxi-K) channels have been investigated in presynaptic membranes in Xenopus motoneurone-muscle cell cultures. The occurrence and density of BK channels increased with maturation/synaptogenesis and was not uniform: highest at the release face of bouton-like synaptic varicosities in contact with muscle cells, and lowest in varicosities that did not contact muscle cells. The Ca(2+) affinity of the channel (K(d)= 7.7 microM at a membrane potential of +20 mV) was lower than those of BK channels that have been characterized in other terminals. Hill coefficients varied between 1.5 and 2.8 at different potentials and open probability increased e-fold per 16 mV change in membrane potential over a range of [Ca(2+)](i) from 1 microM to 1 mM. The maximal activation rate of ensembled single BK channel currents was in the submillisecond range at > or =+20 mV. The activation rate increased approximately 10-fold in response to a [Ca(2+)](i) increase from 1 to 100 microM, but increased only approximately 2-fold with a voltage change from +20 to +130 mV. The fastest activation kinetics of BK channels in cell-attached patches resembled that in inside-out patches with [Ca(2+)](i) of 100 microM or more, suggesting that many BK channels are located very close to calcium channels. Given the low Ca(2+) affinity and rapid Ca(2+) binding/unbinding properties, we conclude that BK channels in this preparation are adapted to play an important role in regulation of neurotransmitter release, and they are ideal reporters of local [Ca(2+)] at the inner membrane surface.