Cloning and expression of the large-conductance Ca(2+)-activated K+ channel from colonic smooth muscle

Modelling Human Colonic Smooth Muscle Cell Electrophysiology

The colon is a digestive organ that is subject to a wide range of motility disorders. However, our understanding of the etiology of these disorders is far from complete. In this study, a quantitative single cell model has been developed to describe the electrical behaviour of a human colonic smooth muscle cell (hCSMC). This model includes the pertinent ionic channels and intracellular calcium homoeostasis. These components are believed to contribute significantly to the electrical response of the hCSMC during a slow wave. The major ion channels were constructed based on published data recorded from isolated human colonic myocytes. The whole cell model is able to reproduce experimentally recorded slow waves from human colonic muscles. This represents the first biophysically-detailed model of a hCSMC and provides a means to better understand colonic disorders.

Age-Dependent Susceptibility to Alcohol-Induced Cerebral Artery Constriction

BACKGROUND

Age has been recognized as an important contributor into susceptibility to alcohol-driven pathology.

PURPOSE

We aimed at determining whether alcohol-induced constriction of cerebral arteries was age-dependent.

STUDY DESIGN

We used rat middle cerebral artery (MCA) in vitro diameter monitoring, patch-clamping and fluorescence labeling of myocytes to study an age-dependent increase in the susceptibility to alcohol in 3 (50 g), 8 (250 g), and 15 (440 g) weeks-old rats.

RESULTS

An age-dependent increase in alcohol-induced constriction of MCA could be observed in absence of endothelium, which is paralleled by an age-dependent increase in both protein level of the calcium-/voltage-gated potassium channel of large conductance (BK) accessory β1 subunit and basal BK channel activity. Ethanol-induced BK channel inhibition is increased with age.

CONCLUSIONS

We demonstrate an increased susceptibility of MCA to ethanol-induced constriction in a period equivalent to adolescence and early adulthood when compared to pre-adolescence. Our work suggests that BK β1 constitutes a significant contributor to age-dependent changes in the susceptibility of cerebral arteries to ethanol.

Single-channel biophysical and pharmacological characterizations of native human large-conductance calcium-activated potassium channels in freshly isolated detrusor smooth muscle cells

Recent studies have demonstrated the importance of large-conductance Ca(2+)-activated K(+) (BK) channels in detrusor smooth muscle (DSM) function in vitro and in vivo. However, in-depth characterization of human native DSM single BK channels has not yet been provided. Here, we conducted single-channel recordings from excised patches from native human DSM cells. Inside-out and outside-out recordings in high K(+) symmetrical solution (containing 140 mM KCl and ~300 nM free Ca(2+)) showed single-channel conductance of 215-220 pS, half-maximum constant for activation of ~+75 to +80 mV, and low probability of opening (P o) at +20 mV that increased ~10-fold at +40 mV and ~60-fold at +60 mV. Using the inside-out configuration at +30 mV, reduction of intracellular [Ca(2+)] from ~300 nM to Ca(2+)-free decreased the P o by ~85 %, whereas elevation to ~800 nM increased P o by ~50-fold. The BK channel activator NS1619 (10 μM) enhanced the P o by ~10-fold at +30 mV; subsequent application of the selective BK channel inhibitor paxilline (500 nM) blocked the activity. Changes in intracellular [Ca(2+)] or the addition of NS1619 did not significantly alter the current amplitude or single-channel conductance. This is the first report to provide biophysical and pharmacological profiles of native human DSM single BK channels highlighting their importance in regulating human DSM excitability.

Involvement of P2Y1 and P2Y11 purinoceptors in parasympathetic inhibition of colonic smooth muscle

Purinergic signaling was first recognized in the guinea pig (Cavia porcellus) taenia coli, where relaxation of smooth muscle by nerve-released ATP may involve the activation of P2Y(1) and P2Y(11) receptors, and where transcripts for both genes have been found. A partial sequence for P2Y(11) protein was identified; the full-length P2Y(1) sequence has already been described. P2Y(1) and P2Y(11) proteins were localized by immunohistochemistry in smooth muscle cells. P2X(2) and P2X(3) proteins were also localized in motoneurons of the myenteric plexus. alphabeta-Methylene-ATP (alphabetameATP) and dibenzoyl-ATP (BzATP) evoked fast relaxations in the taenia, and they were inhibited by the P2Y(1) receptor antagonist 2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate (MRS2179). However, alphabetameATP and BzATP may stimulate neuronal P2X receptors to release ATP, which then acts on P2Y(1) receptors. In accordance, fast relaxations evoked by alphabetameATP and BzATP were inhibited by the P2X(3) and P2X(2/3) receptor antagonist 5-({[3-phenoxybenzyl][(1S)-1,2,3,4-tetrahydro-1-naphthalenyl] amino} carbonyl)-1,2,4-benzene-tricarboxylic acid (A317491). When P2Y(1), P2X(3), and P2X(2/3) receptors were blocked and adenosine was removed enzymatically, alphabetameATP and BzATP evoked slow relaxations that were inhibited by Reactive Red. Fast and slow relaxations involve small and large conductance calcium-activated potassium channels; the latter are dependent on intracellular cyclic AMP levels, which altered the duration and amplitude of relaxations. alphabetameATP and BzATP were confirmed as agonists, and Reactive Red as an antagonist, of human P2Y(11) receptors. In summary, G(q)-coupled P2Y(1) receptors are involved mainly in fast relaxations, whereas G(q)and G(s)-coupled P2Y(11) receptors are involved in both fast and slow relaxations. These P2Y receptor subtypes, plus neuronal P2X receptors, may explain the phenomenon of parasympathetic inhibition first described by Langley (1898).

Role of K+ channels in the regulation of electrical spontaneous activity of the mouse small intestine

The roles of K(+) channels in the regulation of slow waves and pacemaker potentials recorded from mouse small intestine were investigated using intracellular recording techniques in the presence of nifedipine. Iberiotoxin (0.1 microM) and charybdotoxin (0.1 microM) had no effect on the generation of slow waves recorded from circular smooth muscle cells. Apamin (0.3 microM) depolarized the membrane and decreased the amplitude of early, rapid repolarization of slow waves, without altering the amplitude, frequency, duration, or maximum rate of rise of the initial upstroke phase (dV/dt(max)). The early, rapid repolarization was enhanced by phenylephrine (15 microM). 4-Aminopyridine (4-AP, 5 mM) depolarized the membrane and increased the amplitude and dV/dt(max) of slow waves. Both apamin and 4-AP depolarized the membrane and decreased the amplitude and dV/dt(max) of pacemaker potentials recorded from interstitial cells of Cajal distributed in the myenteric region (ICC-MY). Membrane depolarization with a high-K(+) solution decreased the amplitude and dV/dt(max) of slow waves. These results suggest that apamin-sensitive K(+) conductance and 4-AP-sensitive K(+) conductance may contribute to the resting membrane potential of circular smooth muscle cells. The early, rapid repolarization of slow waves appears to result from the opening of apamin-sensitive K(+) conductance. 4-AP-sensitive K(+) conductance is likely to be activated in the initial upstroke component (primary component) of slow waves. In ICC-MY, membrane depolarization induced by apamin or 4-AP may result from electrotonic spread from smooth muscle cells.

BK channel beta1-subunit regulation of calcium handling and constriction in tracheal smooth muscle

The large-conductance, Ca2+-activated K+ (BK) channels are regulators of voltage-dependent Ca2+ entry in many cell types. The BK channel accessory beta1-subunit promotes channel activation in smooth muscle and is required for proper tone in the vasculature and bladder. However, although BK channels have also been implicated in airway smooth muscle function, their regulation by the beta1-subunit has not been investigated. Utilizing the gene-targeted mice for the beta1-subunit gene, we have investigated the role of the beta1-subunit in tracheal smooth muscle. In mice with the beta1-subunit-knockout allele, BK channel activity was significantly reduced in excised tracheal smooth muscle patches and spontaneous BK currents were reduced in whole tracheal smooth muscle cells. Knockout of the beta1-subunit resulted in an increase in resting Ca2+ levels and an increase in the sustained component of Ca2+ influx after cholinergic signaling. Tracheal constriction studies demonstrate that the level of constriction is the same with knockout of the beta1-subunit and BK channel block with paxillin, indicating that BK channels contribute little to airway relaxation in the absence of the beta1-subunit. Utilizing nifedipine, we found that the increased constriction caused by knockout of the beta1-subunit could be accounted for by an increased recruitment of L-type voltage-dependent Ca2+ channels. These results indicate that the beta1-subunit is required in airway smooth muscle for control of voltage-dependent Ca2+ influx during rest and after cholinergic signaling in BK channels.

The beta1 subunit enhances oxidative regulation of large-conductance calcium-activated K+ channels

Oxidative stress may alter the functions of many proteins including the Slo1 large conductance calcium-activated potassium channel (BK_Ca). Previous results demonstrated that in the virtual absence of Ca²⁺, the oxidant chloramine-T (Ch-T), without the involvement of cysteine oxidation, increases the open probability and slows the deactivation of BK_Ca channels formed by human Slo1 (hSlo1) α subunits alone. Because native BK_Ca channel complexes may include the auxiliary subunit β1, we investigated whether β1 influences the oxidative regulation of hSlo1. Oxidation by Ch-T with β1 present shifted the half-activation voltage much further in the hyperpolarizing direction (−75 mV) as compared with that with α alone (−30 mV). This shift was eliminated in the presence of high [Ca²⁺]_i, but the increase in open probability in the virtual absence of Ca²⁺ remained significant at physiologically relevant voltages. Furthermore, the slowing of channel deactivation after oxidation was even more dramatic in the presence of β1. Oxidation of cysteine and methionine residues within β1 was not involved in these potentiated effects because expression of mutant β1 subunits lacking cysteine or methionine residues produced results similar to those with wild-type β1. Unlike the results with α alone, oxidation by Ch-T caused a significant acceleration of channel activation only when β1 was present. The β1 M177 mutation disrupted normal channel activation and prevented the Ch-T–induced acceleration of activation. Overall, the functional effects of oxidation of the hSlo1 pore-forming α subunit are greatly amplified by the presence of β1, which leads to the additional increase in channel open probability and the slowing of deactivation. Furthermore, M177 within β1 is a critical structural determinant of channel activation and oxidative sensitivity. Together, the oxidized BK_Ca channel complex with β1 has a considerable chance of being open within the physiological voltage range even at low [Ca²⁺]_i.

Carbon monoxide activates KCa channels in newborn arteriole smooth muscle cells by increasing apparent Ca2+ sensitivity of alpha-subunits

Carbon monoxide (CO) is a gaseous vasodilator produced by many cell types, including endothelial and smooth muscle cells. The goal of the present study was to investigate signaling mechanisms responsible for CO activation of large-conductance Ca(2+)-activated K(+) (K(Ca)) channels in newborn porcine cerebral arteriole smooth muscle cells. In intact cells at 0 mV, CO (3 microM) or CO released from dimanganese decacarbonyl (10 microM), a novel light-activated CO donor, increased K(Ca) channel activity 4.9- or 3.5-fold, respectively. K(Ca) channel activation by CO was not blocked by 1-H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (25 microM), a soluble guanylyl cyclase inhibitor. In inside-out patches at 0 mV, CO shifted the Ca(2+) concentration-response curve for K(Ca) channels leftward and decreased the apparent dissociation constant for Ca(2+) from 31 to 24 microM. Western blotting data suggested that the low Ca(2+) sensitivity of newborn K(Ca) channels may be due to a reduced beta-subunit-to-alpha-subunit ratio. CO activation of K(Ca) channels was Ca(2+) dependent. CO increased open probability 3.7-fold with 10 microM free Ca(2+) at the cytosolic membrane surface but only 1.1-fold with 300 nM Ca(2+). CO left shifted the current-voltage relationship of cslo-alpha currents expressed in HEK-293 cells, increasing currents 2.2-fold at +50 mV. In summary, data suggest that in newborn arteriole smooth muscle cells, CO activates low-affinity K(Ca) channels via a direct effect on the alpha-subunit that increases apparent Ca(2+) sensitivity. The optimal tuning by CO of the micromolar Ca(2+) sensitivity of K(Ca) channels will lead to preferential activation by signaling modalities, such as Ca(2+) sparks, which elevate the subsarcolemmal Ca(2+) concentration within this range.

Sodium (2-sulfonatoethyl) methanethiosulfonate prevents S-nitroso-L-cysteine activation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeca

1. The modulation of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels by the nitric oxide (NO(.)) donor, S-nitroso-L-cysteine (NOCys) and three sulfhydryl-specific methanethiosulfonate (MTS) reagents, positively charged 2-aminoethyl MTS hydrobromide (MTSEA C(3)H(9)NO(2)S(2)HBr) and [2-(trimethylammonium) ethyl MTS bromide (MTSET C(6)H(16)NO(2)S(2)Br), and negatively charged sodium (2-sulfonatoethyl) MTS (MTSES C(3)H(7)O(5)S(3)Na) were compared in excised inside-out membrane patches of the guinea-pig taenia caeca. 2. In membrane patches bathed in a low Ca(2+) (15 nM) high K(+) physiological salt solution, 1-3 BK(Ca) channels opened with a low probability (N.P(o)) of 0.019+/-0.011 at 0 mV. N.P(o) readily increased with membrane depolarization, raised Ca(2+) concentration or upon the addition of NOCys (10 micro M for 2-5 min) such that 5-15 open BK(Ca) channels were evident. 3. MTSEA (2.5 mM) decreased, while MTSES (2.5 mM) increased N.P(o) (at 0 mV) and the number of open BK(Ca) channels at positive potentials. These changes in channel activity remained after a prolonged washout of these two MTS reagents. 4. MTSET (2.5 mM) increased N.P(o) (at 0 mV) and voltage-dependently decreased BK(Ca) current amplitudes in a manner readily reversed upon washout. 5. Pre-exposure of excised membrane patches to MTSES or N-ethylmaleimide (NEM 1 mM), a specific alkylating agent of cysteine sulfhydryls, but not MTSEA or MTSET, prevented the excitatory actions of NOCys (10 micro M). 6. It was concluded that NOCys-evoked increases in BK(Ca) channel activity occur via the S-nitrosylation of cysteine residues within basic regions of the channel alpha subunit that have an accessible water interface.

Variation in large-conductance, calcium-activated potassium channels from hair cells along the chicken basilar papilla

The mechanism for electrical tuning in non-mammalian hair cells rests within the widely diverse kinetics of functionally distinct, large-conductance potassium channels (BK), thought to result from alternative splicing of the pore-forming alpha subunit and variable co-expression with an accessory beta subunit. Inside-out patches from hair cells along the chicken basilar papilla revealed 'tonotopic' gradations in calcium sensitivity and deactivation kinetics. The resonant frequency for the hair cell from which the patch was taken was estimated from deactivation rates, and this frequency reasonably matched that predicted from the originating cell's tonotopic location. The rates of deactivation for native BK channels were much faster than rates reported for cloned chicken BK channels including both alpha and beta subunits. This result was surprising since patches were pulled from hair cells in the apical half of the papilla where beta subunits are most highly expressed. Heterogeneity in the properties of native chicken BK channels implies a high degree of molecular variation and hinders our ability to identify those molecular constituents.

Contribution of potential EF hand motifs to the calcium-dependent gating of a mouse brain large conductance, calcium-sensitive K(+) channel

1. The large conductance, calcium-sensitive K(+) channel (BK(Ca) channel) is a unique member of the K(+)-selective ion channel family in that activation is dependent upon both direct calcium binding and membrane depolarization. Calcium binding acts to dynamically shift voltage-dependent gating in a negative or left-ward direction, thereby adjusting channel opening to changes in cellular membrane potential. 2. We hypothesized that the intrinsic calcium-binding site within the BK(Ca) channel alpha subunit may contain an EF hand motif, the most common, naturally occurring calcium binding structure. Following identification of six potential sites, we introduced a single amino acid substitution (D/E to N/Q or A) at the equivalent of the -z position of a bona fide EF hand that would be predicted to lower calcium binding affinity at each of the six sites. 3. Using macroscopic current recordings of wild-type and mutant BK(Ca) channels in excised inside-out membrane patches from HEK 293 cells, we observed that a single point mutation in the C-terminus (Site 6, FLD(923)QD to N), adjacent to the 'calcium bowl' described by Salkoff and colleagues, shifted calcium-sensitive gating right-ward by 50--65 mV over the range of 2--12 microM free calcium, but had little effect on voltage-dependent gating in the absence of calcium. Combining this mutation at Site 6 with a similar mutation at Site 1 (PVD(81)EK to N) in the N-terminus produced a greater shift (70--90 mV) in calcium-sensitive gating over the same range of calcium. We calculated that these combined mutations decreased the apparent calcium binding affinity approximately 11-fold (129.5 microM vs. 11.3 microm) compared to the wild-type channel. 4. We further observed that a bacterially expressed protein encompassing Site 6 of the BK(Ca) channel C-terminus and bovine brain calmodulin were both able to directly bind (45)Ca(2+) following denaturation and polyacrylamide gel electrophoresis (e.g. SDS-PAGE). 5. Our results suggest that two regions within the mammalian BK(Ca) channel alpha subunit, with sequence similarities to an EF hand motif, functionally contribute to the calcium-sensitive gating of this channel.

Guanylyl cyclase stimulatory coupling to K(Ca) channels

We coexpressed the human large-conductance, calcium-activated K (K(Ca)) channel (alpha- and beta-subunits) and rat atrial natriuretic peptide (ANP) receptor genes in Xenopus oocytes to examine the mechanism of guanylyl cyclase stimulatory coupling to the channel. Exposure of oocytes to ANP stimulated whole cell K(Ca) currents by 21 +/- 3% (at 60 mV), without altering current kinetics. Similarly, spermine NONOate, a nitric oxide donor, increased K(Ca) currents (20 +/- 4% at 60 mV) in oocytes expressing the channel subunits alone. Stimulation of K(Ca) currents by ANP was inhibited in a concentration-dependent manner by a peptide inhibitor of cGMP-dependent protein kinase (PKG). Receptor/channel stimulatory coupling was not completely abolished by mutating the cAMP-dependent protein kinase phosphorylation site on the alpha-subunit (S869; Nars M, Dhulipals PD, Wang YX, and Kotlikoff MI. J Biol Chem 273: 14920-14924, 1998) or by mutating a neighboring consensus PKG site (S855), but mutation of both residues virtually abolished coupling. Spermine NONOate also failed to stimulate channels expressed from the double mutant cRNAs. These data indicate that nitric oxide donors stimulate K(Ca) channels through cGMP-dependent phosphorylation and that two serine residues (855 and 869) underlie this stimulatory coupling.

Mice with disrupted BK channel beta1 subunit gene feature abnormal Ca(2+) spark/STOC coupling and elevated blood pressure

Large-conductance potassium (BK) channels in vascular smooth muscle cells (VSMCs) sense both changes in membrane potential and in intracellular Ca(2+) concentration. BK channels may serve as negative feedback regulators of vascular tone by linking membrane depolarization and local increases in intracellular Ca(2+) concentration (Ca(2+) sparks) to repolarizing spontaneous transient outward K(+) currents (STOCs). BK channels are composed of channel-forming BKalpha and auxiliary BKbeta1 subunits, which confer to BK channels an increased sensitivity for changes in membrane potential and Ca(2+). To assess the in vivo functions of this ss subunit, mice with a disrupted BKbeta1 gene were generated. Cerebral artery VSMCs from BKbeta1 -/- mice generated Ca(2+) sparks of normal amplitude and frequency, but STOC frequencies were largely reduced at physiological membrane potentials. Our results indicate that BKbeta1 -/- mice have an abnormal Ca(2+) spark/STOC coupling that is shifted to more depolarized potentials. Thoracic aortic rings from BKbeta1 -/- mice responded to agonist and elevated KCl with a increased contractility. BKbeta1 -/- mice had higher systemic blood pressure than BKbeta1 +/+ mice but responded normally to alpha(1)-adrenergic vasoconstriction and nitric oxide-mediated vasodilation. We propose that the elevated blood pressure in BKbeta1 -/- mice serves to normalize Ca(2+) spark/STOC coupling for regulating myogenic tone. The full text of this article is available at http://www.circresaha.org.

beta subunits modulate alternatively spliced, large conductance, calcium-activated potassium channels of avian hair cells

Electrical tuning confers frequency selectivity onto sensory hair cells in the auditory periphery of frogs, turtles, and chicks. The resonant frequency is determined in large part by the number and kinetics of large conductance, calcium-activated potassium (BK) channels. BK channels in hair cells are encoded by the alternatively spliced slo gene and may include an accessory beta subunit. Here we examine the origins of kinetic variability among BK channels by heterologous expression of avian cochlear slo cDNAs. Four alternatively spliced forms of the slo-alpha gene from chick hair cells were co-expressed with accessory beta subunits (from quail cochlea) by transient transfection of human embryonic kidney 293 cells. Addition of the beta subunit increased steady-state calcium affinity, raised the Hill coefficient for calcium binding, and slowed channel deactivation rates, resulting in eight functionally distinct channels. For example, a naturally occurring splice variant containing three additional exons deactivated 20-fold more slowly when combined with beta. Deactivation kinetics were used to predict tuning frequencies and thus tonotopic location if hair cells were endowed with each of the expressed channels. All beta-containing channels were predicted to lie within the apical (low-frequency) 30% of the epithelium, consistent with previous in situ hybridization studies. Individual slo-alpha exons would be found anywhere within the apical 70%, depending on the presence of beta, and other alternative exons. Alternative splicing of the slo-alpha channel message provides intrinsic variability in gating kinetics that is expanded to a wider range of tuning by modulation with beta subunits.

Maxi-K(Ca), a Unique Member of the Voltage-Gated K Channel Superfamily

Large-conductance, voltage-, and Ca(2+)-sensitive K(+) (maxi-K(Ca)) channels regulate neuronal and smooth muscle excitability. Their pore-forming alpha-subunit shows similarities with voltage-gated channels and indeed can open in the practical absence of Ca(2+). The NH(2) terminus is unique, with a seventh transmembrane segment involved in beta-subunit modulation. The long COOH terminus is implied in Ca(2+) modulation.

Large conductance voltage- and calcium-dependent K+ channel, a distinct member of voltage-dependent ion channels with seven N-terminal transmembrane segments (S0-S6), an extracellular N terminus, and an intracellular (S9-S10) C terminus

Large conductance voltage- and Ca2+-dependent K+ (MaxiK) channels show sequence similarities to voltage-gated ion channels. They have a homologous S1-S6 region, but are unique at the N and C termini. At the C terminus, MaxiK channels have four additional hydrophobic regions (S7-S10) of unknown topology. At the N terminus, we have recently proposed a new model where MaxiK channels have an additional transmembrane region (S0) that confers beta subunit regulation. Using transient expression of epitope tagged MaxiK channels, in vitro translation, functional, and "in vivo" reconstitution assays, we now show that MaxiK channels have seven transmembrane segments (S0-S6) at the N terminus and a S1-S6 region that folds in a similar way as in voltage-gated ion channels. Further, our results indicate that hydrophobic segments S9-S10 in the C terminus are cytoplasmic and unequivocally demonstrate that S0 forms an additional transmembrane segment leading to an exoplasmic N terminus.