scholarly journals Small-Conductance Calcium-Activated Potassium Channels Control Excitability and Firing Dynamics in Gonadotropin-Releasing Hormone (GnRH) Neurons

Endocrinology ◽  
2008 ◽  
Vol 149 (7) ◽  
pp. 3598-3604 ◽  
Author(s):  
Xinhuai Liu ◽  
Allan E. Herbison
Keyword(s):  
Fluorescent Protein ◽  
Current Injection ◽  
Sk Channels ◽  
Cellular Mechanisms ◽  
Burst Frequency ◽  
Single Action ◽  
Sk Channel ◽  
Gnrh Neurons ◽  
Green Fluorescent ◽  
Small Conductance

The cellular mechanisms determining the firing patterns of GnRH neurons are presently under intense investigation. In this study, we used GnRH-green fluorescent protein transgenic mice and perforated-patch electrophysiology to examine the role of small conductance calcium-activated potassium (SK) channels in determining the electrical excitability and burst-firing characteristics of adult GnRH neurons. After establishing an appropriate protocol for examining the afterhyperpolarization potential (AHP) currents in GnRH neurons, the highly selective SK channel blocker apamin was used to demonstrate that all GnRH neurons express functional SK channels (35.7 ± 2.7 pA, mean decay time constant = 2167 msec, apamin IC50 = 9.6 nm) and that this channel underlies approximately 90% of the AHP in these cells. Current-clamp experiments showed that apamin-sensitive SK channels were tonically active in the majority (74%) of GnRH neurons, with apamin (100 nm) administration resulting in a mean 6.9 ± 0.5 mV membrane depolarization. Apamin also elevated the firing rate of GnRH neurons, including increased burst frequency and duration in spontaneously bursting cells as well as the ability of GnRH neurons to fire action potentials in response to current injection. In GnRH neurons activated by current injection, apamin significantly enhanced the amplitude of the afterdepolarization potential after a single action potential and eliminated spike frequency adaptation. Together, these studies show that apamin-sensitive SK channels play a key role in restraining GnRH neuron excitability. Through direct modulation of the AHP and indirect actions on the afterdepolarization potential, the SK channel exerts a powerful tonic influence upon the firing dynamics of GnRH neurons.

scholarly journals Cardiac small-conductance calcium-activated potassium channels in health and disease

2021 ◽  
Vol 473 (3) ◽  
pp. 477-489 ◽  
Author(s):  
Xiao-Dong Zhang ◽  
Phung N. Thai ◽  
Deborah K. Lieu ◽  
Nipavan Chiamvimonvat
Keyword(s):  
Ventricular Myocytes ◽  
Pulmonary Veins ◽  
Differential Sensitivity ◽  
Sk Channels ◽  
Sk Channel ◽  
Ventricular Cells ◽  
Intracellular Ca ◽  
Life Threatening ◽  
Health And Disease ◽  
Small Conductance

AbstractSmall-conductance Ca2+-activated K+ (SK, KCa2) channels are encoded by KCNN genes, including KCNN1, 2, and 3. The channels play critical roles in the regulation of cardiac excitability and are gated solely by beat-to-beat changes in intracellular Ca2+. The family of SK channels consists of three members with differential sensitivity to apamin. All three isoforms are expressed in human hearts. Studies over the past two decades have provided evidence to substantiate the pivotal roles of SK channels, not only in healthy heart but also with diseases including atrial fibrillation (AF), ventricular arrhythmia, and heart failure (HF). SK channels are prominently expressed in atrial myocytes and pacemaking cells, compared to ventricular cells. However, the channels are significantly upregulated in ventricular myocytes in HF and pulmonary veins in AF models. Interests in cardiac SK channels are further fueled by recent studies suggesting the possible roles of SK channels in human AF. Therefore, SK channel may represent a novel therapeutic target for atrial arrhythmias. Furthermore, SK channel function is significantly altered by human calmodulin (CaM) mutations, linked to life-threatening arrhythmia syndromes. The current review will summarize recent progress in our understanding of cardiac SK channels and the roles of SK channels in the heart in health and disease.

Abstract 17319: Distinct Effects of Human Calmodulinopathy on the Function of Small Conductance Ca 2+ -Activated K + (SK) Channels

Circulation ◽  
2018 ◽  
Vol 138 (Suppl_1) ◽  
Author(s):  
Hannah A Ledford ◽  
Seojin Park ◽  
Duncan Muir ◽  
Wen Smith ◽  
Ryan L Woltz ◽  
...  
Keyword(s):  
Ion Channels ◽  
Intracellular Signaling ◽  
Critical Role ◽  
Dominant Negative ◽  
Polymorphic Ventricular Tachycardia ◽  
Dominant Negative Effect ◽  
Sk Channels ◽  
Channel Function ◽  
Sk Channel ◽  
Small Conductance

Background: Calmodulin (CaM) plays a critical role in intracellular signaling and regulation of Ca 2+ -dependent ion channels. Mutations in CALM1, CALM2, and CALM3 have recently been linked to cardiac arrhythmias, such as Long QT Syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia (CPVT), and familial idiopathic ventricular fibrillation (IVF). Small-conductance Ca 2+ - activated K + channels (SK) are voltage-independent channels that are regulated solely from beat-to-beat changes in intracellular calcium. CaM regulates the function of multiple ion channels, including SK channels, although the effect of CaM mutations on these channels is not yet understood. We hypothesize that human CaM mutations linked to sudden cardiac death disrupt SK channel function by distinct mechanisms. Methods and Results: We tested the effects of LQTS (CaM D96V , CaM D130G ), CPVT (CaM N54I , CaM N98S ), and IVF (CaM F90L ) CaM mutants compared to CaM WT on SK channel function. Using whole-cell voltage-clamp recordings, we found that CaM D96V and CaM D130G mutants significantly inhibited apamin-sensitive currents. Similarly, action potential studies in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) also revealed significant knockdown of apamin-sensitive currents. Immunofluorescent confocal microscopy confirmed that this effect was not due to changes in SK channel trafficking. Rather, co-immunoprecipitation studies showed a significant decrease in the association of these CaM mutants with the SK channel. Rosetta molecular modeling was used to identify a conformational change in CaM F90L structure compared to that of CaM WT . Conclusions: We found that CaM D96V and CaM D130G mutants significantly reduced apamin-sensitive currents, through a dominant negative effect on SK channel function. Consistent with our hypothesis, CaM F90L resulted in the least inhibitory effects. The data suggests that specific mutations with phenylalanine to leucine (CaM F90L ) may disrupt the interaction between apo-CaM with CaMBD on the SK2 channel.

scholarly journals Suppression of Basal Spontaneous Gonadotropin-Releasing Hormone Neuronal Activity during Lactation: Role of Inhibitory Effects of Neuropeptide Y

Endocrinology ◽  
2008 ◽  
Vol 150 (1) ◽  
pp. 333-340 ◽  
Author(s):  
Jing Xu ◽  
Melissa A. Kirigiti ◽  
Michael A. Cowley ◽  
Kevin L. Grove ◽  
M. Susan Smith
Keyword(s):  
Neuropeptide Y ◽  
Neuronal Activity ◽  
Fluorescent Protein ◽  
Resting Membrane Potential ◽  
Inhibitory Effects ◽  
Gnrh Neurons ◽  
Cell Current ◽  
Green Fluorescent ◽  
Hypothalamic Slices

Increased neuropeptide Y (NPY) activity drives the chronic hyperphagia of lactation and may contribute to the suppression of GnRH activity. The majority of GnRH neurons are contacted by NPY fibers, and GnRH cells express NPY Y5 receptor (Y5R). Therefore, NPY provides a neurocircuitry for information about food intake/energy balance to be directly transmitted to GnRH neurons. To investigate the effects of lactation on GnRH neuronal activity, hypothalamic slices were prepared from green fluorescent protein-GnRH transgenic rats. Extracellular loose-patch recordings determined basal GnRH neuronal activity from slices of ovariectomized control and lactating rats. Compared with controls, hypothalamic slices from lactating rats had double the number of quiescent GnRH neurons (14.51 ± 2.86 vs. 7.04 ± 2.84%) and significantly lower firing rates of active GnRH neurons (0.25 ± 0.02 vs. 0.37 ± 0.03 Hz). To study the NPY-postsynaptic Y5R system, whole-cell current-clamp recordings were performed in hypothalamic slices from control rats to examine NPY/Y5R antagonist effects on GnRH neuronal resting membrane potential. Under tetrodotoxin treatment, NPY hyperpolarized GnRH neurons from −56.7 ± 1.94 to −62.1 ± 1.83 mV; NPY’s effects were blocked by Y5R antagonist. To determine whether increased endogenous NPY tone contributes to GnRH neuronal suppression during lactation, hypothalamic slices were treated with Y5R antagonist. A significantly greater percentage of GnRH cells were activated in slices from lactating rats (52%) compared with controls (28%). These results suggest that: 1) basal GnRH neuronal activity is suppressed during lactation; 2) NPY can hyperpolarize GnRH neurons via postsynaptic Y5R; and 3) increased inhibitory NPY tone during lactation is a component of the mechanisms responsible for suppression of GnRH neuronal activity. Neuropeptide Y (NPY) directly hyperpolarizes GnRH neurons via postsynaptic NPY Y5 receptor. Increased inhibitory NPY tone during lactation is an important component of the suppression of GnRH neuronal activity.

scholarly journals EF hands at the N-lobe of calmodulin are required for both SK channel gating and stable SK–calmodulin interaction

2009 ◽  
Vol 134 (4) ◽  
pp. 281-293 ◽  
Author(s):  
Weiyan Li ◽  
David B. Halling ◽  
Amelia W. Hall ◽  
Richard W. Aldrich
Keyword(s):  
Modular Design ◽  
Binding Capacity ◽  
Current Model ◽  
Sk Channels ◽  
Channel Activity ◽  
Wild Type ◽  
Sk Channel ◽  
Ef Hand ◽  
Ef Hands ◽  
Small Conductance

Small conductance calcium-activated potassium (SK) channels respond to intracellular Ca2+ via constitutively associated calmodulin (CaM). Previous studies have proposed a modular design for the interaction between CaM and SK channels. The C-lobe and the linker of CaM are thought to regulate the constitutive binding, whereas the N-lobe binds Ca2+ and gates SK channels. However, we found that coexpression of mutant CaM (E/Q) where the N-lobe has only one functional EF hand leads to rapid rundown of SK channel activity, which can be recovered with exogenously applied wild-type (WT), but not mutant, CaM. Our results suggest that the mutation at the N-lobe EF hand disrupts the stable interaction between CaM and SK channel subunits, such that mutant CaM dissociates from the channel complex when the inside of the membrane is exposed to CaM-free solution. The disruption of the stable interaction does not directly result from the loss of Ca2+-binding capacity because SK channels and WT CaM can stably interact in the absence of Ca2+. These findings question a previous conclusion that CaM where the N-lobe has only one functional EF hand can stably support the gating of SK channels. They cannot be explained by the current model of modular interaction between CaM and SK channels, and they imply a role for N-lobe EF hand residues in binding to the channel subunits. Additionally, we found that a potent enhancer for SK channels, 3-oxime-6,7-dichloro-1H-indole-2,3-dione (NS309), enables the recovery of channel activity with CaM (E/Q), suggesting that NS309 stabilizes the interaction between CaM and SK channels. CaM (E/Q) can regulate Ca2+-dependent gating of SK channels in the presence of NS309, but with a lower apparent Ca2+ affinity than WT CaM.

Pharmacological Modulation of the Gating Properties of Small Conductance Ca2+-Activated K+ Channels Alters the Firing Pattern of Dopamine Neurons In Vivo

2010 ◽  
Vol 104 (3) ◽  
pp. 1726-1735 ◽  
Author(s):  
Kjartan F. Herrik ◽  
Palle Christophersen ◽  
Paul D. Shepard
Keyword(s):  
Firing Rate ◽  
Dopamine Neurons ◽  
Substantia Nigra Pars Compacta ◽  
Pacemaker Activity ◽  
Sk Channels ◽  
Pharmacological Modulation ◽  
Sk Channel ◽  
Discharge Patterns ◽  
Small Conductance

Dopamine (DA) neurons are autonomous pacemakers that occasionally fire bursts of action potentials, discharge patterns thought to reflect tonic and phasic DA signaling, respectively. Pacemaker activity depends on the concerted and cyclic interplay between intrinsic ion channels with small conductance Ca2+-activated K+ (SK) channels playing an important role. Bursting activity is synaptically initiated but neither the transmitters nor the specific ion conductances involved have been definitively identified. Physiological and pharmacological regulation of SK channel Ca2+ sensitivity has recently been demonstrated and could represent a powerful means of modulating the expression of tonic/phasic signaling in DA neurons in vivo. To test this premise, we characterized the effects of intravenous administration of the novel positive and negative SK channel modulators NS309 and NS8593, respectively, on the spontaneous activity of substantia nigra pars compacta DA neurons in anesthetized C57BL/6 mice. NS309, dose-dependently decreased DA cell firing rate, increased the proportion of regular firing cells, and eventually stopped spontaneous firing. By contrast, systemic administration of the negative SK channel modulator NS8593 increased firing rate and shifted the pattern toward increased irregularity/bursting; an effect similar to local application of the pore blocking peptide apamin. The altered firing patterns resulting from inhibiting SK currents persisted independently of changes in firing rates induced by administration of DA autoreceptor agonists/antagonists. We conclude that pharmacological modulation of SK channel Ca2+-sensitivity represents a powerful mechanism for switching DA neuron firing activity between tonic and phasic signaling modalities in vivo.

Myometrial expression of small conductance Ca2+-activated K+ channels depresses phasic uterine contraction

2007 ◽  
Vol 292 (2) ◽  
pp. C832-C840 ◽  
Author(s):  
Amber Brown ◽  
Trudy Cornwell ◽  
Iryna Korniyenko ◽  
Viktoriya Solodushko ◽  
Chris T. Bond ◽  
...  
Keyword(s):  
Uterine Contraction ◽  
Feedback Regulation ◽  
Cyclopiazonic Acid ◽  
Sk Channels ◽  
Wild Type ◽  
Uterine Contractility ◽  
Sk Channel ◽  
Channel Expression ◽  
Spontaneous Contractions ◽  
Small Conductance

Mechanisms regulating uterine contractility are poorly understood. We hypothesized that a specific isoform of small conductance Ca2+-activated K+ (SK) channel, SK3, promotes feedback regulation of myometrial Ca2+ and hence relaxation of the uterus. To determine the specific functional impact of SK3 channels, we assessed isometric contractions of uterine strips from genetically altered mice (SK3T/T), in which SK3 is overexpressed and can be suppressed by oral administration of doxycycline (SK3T/T+Dox). We found SK3 protein in mouse myometrium, and this expression was substantially higher in SK3T/T mice and lower in SK3T/T+Dox mice compared with wild-type (WT) controls. Sustained contractions elicited by 60 mM KCl were not different among SK3T/T, SK3T/T+Dox, and WT mice. However, the rate of onset and magnitude of spontaneously occurring phasic contractions was muted significantly in isolated uterine strips from SK3T/T mice compared with those from WT mice. These spontaneous contractions were augmented greatly by blockade of SK channels with apamin or by suppression of SK3 expression. Phasic but not tonic contraction in response to oxytocin was depressed in uterine strips from SK3T/T mice, whereas suppression of SK3 channel expression or treatment with apamin promoted the predominance of large coordinated phasic events over tone. Spontaneous contractions and the phasic component of oxytocin contractions were blocked by nifedipine but not by cyclopiazonic acid. Our findings suggest that SK3 channels play an important role in regulating uterine function by limiting influx through L-type Ca2+ channels and disrupting the development of concerted phasic contractile events.

scholarly journals Dose-Dependent Switch in Response of Gonadotropin-Releasing Hormone (GnRH) Neurons to GnRH Mediated through the Type I GnRH Receptor

Endocrinology ◽  
2004 ◽  
Vol 145 (2) ◽  
pp. 728-735 ◽  
Author(s):  
Chun Xu ◽  
Xu-Zhi Xu ◽  
Craig S. Nunemaker ◽  
Suzanne M. Moenter
Keyword(s):  
Firing Rate ◽  
Fluorescent Protein ◽  
Brain Slices ◽  
Dose Dependence ◽  
Gnrh Receptor ◽  
Neuron Activity ◽  
Type I ◽  
Pulsatile Release ◽  
Gnrh Neurons ◽  
Green Fluorescent

Abstract Pulsatile release of GnRH provides central control of reproduction. GnRH neuron activity is likely synchronized to produce hormone pulses, but the mechanisms are largely unknown. One candidate for communication among these neurons is GnRH itself. Cultured embryonic and immortalized GnRH neurons express GnRH receptor type I (GnRHR-1), but expression has not been shown in adult GnRH neurons. Using mice that express green fluorescent protein (GFP) in GnRH neurons, we tested whether adult GnRH neurons express GnRHR-1. GFP-positive (n = 42) and -negative neurons (n = 22) were harvested from brain slices, and single-cell RT-PCR was performed with cell contents. Fifty-two percent of the GnRH neurons tested expressed GnRHR-1, but only 9% of non-GnRH hypothalamic neurons expressed GnRHR-1; no false harvest controls (n = 13) were positive. GnRHR-1 expression within GnRH neurons suggested a physiological ultrashort loop feedback role for GnRH. Thus, we examined the effect of GnRH on the firing rate of GnRH neurons. Low-dose GnRH (20 nm) significantly decreased firing rate in 12 of 22 neurons (by 42 ± 4%, P < 0.05), whereas higher doses increased firing rate (200 nm, five of 10 neurons, 72 ± 26%; 2000 nm, nine of 13 neurons, 53 ± 8%). Interestingly, the fraction of GnRH neurons responding was similar to the fraction in which GnRHR-1 was detected. Together, these data demonstrate that a subpopulation of GnRH neurons express GnRHR-1 and respond to GnRH with altered firing. The dose dependence suggests that this autocrine control of GnRH neurons may be not only a mechanism for generating and modulating pulsatile release, but it may also be involved in the switch between pulse and surge modes of release.

scholarly journals Arrhythmia development during inhibition of small-conductance calcium-activated potassium channels in acute myocardial infarction in a porcine model

EP Europace ◽  
2019 ◽  
Vol 21 (10) ◽  
pp. 1584-1593 ◽  
Author(s):  
Anniek F Lubberding ◽  
Stefan M Sattler ◽  
Morten Grunnet ◽  
Ulrik S Sørensen ◽  
Jacob Tfelt-Hansen ◽  
...  
Keyword(s):  
Myocardial Infarction ◽  
Acute Myocardial Infarction ◽  
Porcine Model ◽  
Sk Channels ◽  
Sk Channel ◽  
Channel Inhibition ◽  
Ventricular Extrasystoles ◽  
Calcium Activated Potassium Channels ◽  
Qrs Duration ◽  
Small Conductance

Abstract Aims  Acute myocardial infarction (AMI) is associated with intracellular Ca2+ build-up. In healthy ventricles, small conductance Ca2+-activated K+ (SK) channels are present but do not participate in repolarization. However, SK current is increased in chronic myocardial infarction and heart failure, and recently, SK channel inhibition was demonstrated to reduce arrhythmias in AMI rats. Hence, we hypothesized that SK channel inhibitors (NS8593 and AP14145) could reduce arrhythmia development during AMI in a porcine model. Methods and results  Twenty-seven pigs were randomized 1:1:1 to control, NS8593, or AP14145. Haemodynamic and electrophysiological parameters [electrocardiogram (ECG) and monophasic action potentials (MAP)] were continuously recorded. A balloon was placed in the mid-left anterior descending artery, blinded to treatment. Infusion lasted from 10 min before occlusion until 30 min after. Occlusion was maintained for 1 h, followed by 2 h of reperfusion. Upon occlusion, cardiac output dropped similarly in all groups, while blood pressure remained stable. Heart rate decreased in the NS8593 and AP14145 groups. QRS duration increased upon occlusion in all groups but more prominently in AP14145-treated pigs. Inhibition of SK channels did not affect QT interval. Infarct MAP duration shortened comparably in all groups. Ventricular fibrillation developed in 4/9 control-, 4/9 AP14145-, and 2/9 NS8593-treated pigs. Ventricular tachycardia was rarely observed in either group, whereas ventricular extrasystoles occurred comparably in all groups. Conclusion  Inhibition of SK channels was neither beneficial nor detrimental to ventricular arrhythmia development in the setting of AMI in this porcine model.

Properties and expression of Ca2+-activated K+ channels in H9c2 cells derived from rat ventricle

1999 ◽  
Vol 276 (5) ◽  
pp. H1559-H1566 ◽  
Author(s):  
Wei Wang ◽  
Makino Watanabe ◽  
Takeshi Nakamura ◽  
Yoshihisa Kudo ◽  
Rikuo Ochi
Keyword(s):  
Time Course ◽  
External Solution ◽  
Pcr Analysis ◽  
Sk Channels ◽  
Sk Channel ◽  
Current Voltage ◽  
Rat Ventricle ◽  
Current Voltage Relation ◽  
Small Conductance

H9c2 is a clonal myogenic cell line derived from embryonic rat ventricle that can serve as a surrogate for cardiac or skeletal muscle in vitro. Using whole cell clamp with H9c2 myotubes, we observed that depolarizing pulses activated slow outward K+ currents and then slow tail currents. The K+ currents were abolished in a Ca2+-free external solution, indicating that they were Ca2+-activated K+ currents. They were blocked by apamin, a small-conductance Ca2+-activated K+ (SK) channel antagonist (IC50 = 6.2 nM), and by d-tubocurarine (IC50 = 49.4 μM). Activation of SK channels exhibited a bell-shaped voltage dependence that paralleled the current-voltage relation for L-type Ca2+ currents ( I Ca,L). I Ca,L exhibited a slow time course similar to skeletal I Ca,L, were unaffected by apamin, and were only slightly depressed by d-tubocurarine. RT-PCR analysis of the mRNAs revealed that rSK3, but not rSK1 or rSK2, was expressed in H9c2 myotubes but not in myoblasts. These results suggest that rSK3 channels are expressed in H9c2 myotubes and are primarily activated by I Ca,L directly or indirectly via Ca2+-induced Ca2+ release from sarcoplasmic reticulum.

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