Small-conductance calcium-activated potassium (SK) channels contribute to action potential repolarization in human atria

Extracellular calcium ions support synaptic activity but also reduce excitability of central neurons. In the present study, the effect of calcium on excitability was explored in cultured hippocampal neurons. CaCl2 injected by pressure in the vicinity of a neuron that is bathed only in MgCl2 as the main divalent cation caused a depolarizing shift in action potential threshold and a reduction in excitability. This effect was not seen if the intracellular milieu consisted of Cs+ instead of K-gluconate as the main cation or when it contained ruthenium red, which blocks release of calcium from stores. The suppression of excitability by calcium was mimicked by caffeine, and calcium store antagonists cyclopiazonic acid or thapsigargin blocked this action. Neurons taken from synaptopodin-knockout mice show significantly reduced efficacy of calcium modulation of action potential threshold. Likewise, in Orai1 knockdown cells, calcium is less effective in modulating excitability of neurons. Activation of small-conductance K (SK) channels increased action potential threshold akin to that produced by calcium ions, whereas blockade of SK channels but not big K channels reduced the threshold for action potential discharge. These results indicate that calcium released from stores may suppress excitability of central neurons. NEW & NOTEWORTHY Extracellular calcium reduces excitability of cultured hippocampal neurons. This effect is mediated by calcium-gated potassium currents, possibly small-conductance K channels. Release of calcium from internal stores mimics the effect of extracellular calcium. It is proposed that calcium stores modulate excitability of central neurons.

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Small-conductance Ca2+-activated K+ channels modulate action potential-induced Ca2+ transients in hippocampal neurons

Journal of Neurophysiology ◽

10.1152/jn.00346.2012 ◽

2013 ◽

Vol 109 (6) ◽

pp. 1514-1524 ◽

Cited By ~ 9

Author(s):

Raffaella Tonini ◽

Teresa Ferraro ◽

Marisol Sampedro-Castañeda ◽

Anna Cavaccini ◽

Martin Stocker ◽

...

Keyword(s):

Action Potential ◽

Action Potentials ◽

Hippocampal Neurons ◽

Pyramidal Neurons ◽

Apical Dendrite ◽

Channel Blockers ◽

Sk Channels ◽

Hippocampal Pyramidal Neurons ◽

Voltage Gated ◽

Small Conductance

In hippocampal pyramidal neurons, voltage-gated Ca2+ channels open in response to action potentials. This results in elevations in the intracellular concentration of Ca2+ that are maximal in the proximal apical dendrites and decrease rapidly with distance from the soma. The control of these action potential-evoked Ca2+ elevations is critical for the regulation of hippocampal neuronal activity. As part of Ca2+ signaling microdomains, small-conductance Ca2+-activated K+ (SK) channels have been shown to modulate the amplitude and duration of intracellular Ca2+ signals by feedback regulation of synaptically activated Ca2+ sources in small distal dendrites and dendritic spines, thus affecting synaptic plasticity in the hippocampus. In this study, we investigated the effect of the activation of SK channels on Ca2+ transients specifically induced by action potentials in the proximal processes of hippocampal pyramidal neurons. Our results, obtained by using selective SK channel blockers and enhancers, show that SK channels act in a feedback loop, in which their activation by Ca2+ entering mainly through L-type voltage-gated Ca2+ channels leads to a reduction in the subsequent dendritic influx of Ca2+. This underscores a new role of SK channels in the proximal apical dendrite of hippocampal pyramidal neurons.

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Maternal Diabetes Increases Small Conductance Ca2+-Activated K+ (SK) Currents That Alter Action Potential Properties and Excitability of Cardiac Motoneurons in the Nucleus Ambiguus

Journal of Neurophysiology ◽

10.1152/jn.00671.2009 ◽

2010 ◽

Vol 104 (4) ◽

pp. 2125-2138 ◽

Cited By ~ 5

Author(s):

Min Lin ◽

Qing-Hui Chen ◽

Robert D. Wurster ◽

Jeff T. Hatcher ◽

Ye-Qi Liu ◽

...

Keyword(s):

Action Potential ◽

Firing Rate ◽

Maternal Diabetes ◽

Spike Frequency ◽

Nucleus Ambiguus ◽

Peak Amplitude ◽

Sk Channels ◽

Transient Currents ◽

Whole Cell ◽

Small Conductance

Parasympathetic cardiac motoneurons (PCMNs) in the nucleus ambiguus (NA) play a key role in regulating cardiac functions. In this study, we examined the effects of maternal diabetes on excitability, action potential (AP) properties, and small conductance Ca2+-activated K+ (SK) currents of PCMNs. Neonatal mice from diabetic (OVE26 female, NMDM) and normal (FVB female, control) mothers that had been mated with nondiabetic fathers (FVB male) were used. Tracer XRITC was injected into the pericardial sac at P7-9 to retrogradely label PCMNs. Two days later, XRITC-labeled PCMNs were identified in brain stem slices. The responses of spike frequency, AP repolarization (half-width) and afterhyperpolarization (AHP) of PCMNs to current injections were studied using whole cell current clamp. Outward and afterhyperpolarization currents ( IAHP) in response to voltage steps were measured using whole cell voltage clamp. In examining the effects of maternal diabetes on excitability and AP properties, we found that in NMDM spike frequency decreased, the half-width and AHP peak amplitude increased, and the peak amplitude of outward transient currents and IAHP increased compared with those measured in control. In examining the effects of maternal diabetes on SK channels, we found that after blockage of SK channels with a specific SK channel blocker apamin, maternal diabetes significantly increased apamin-sensitive outward transient currents and IAHP, and suppressed AHP amplitude in NMDM more than those in control. Further, apamin application increased the firing rate to current injections and completely abolished the difference of the firing rate between control and NMDM. We suggest that the augmented SK-mediated currents may contribute to the increased AHP amplitude and the attenuated excitability of PCMNs in NMDM.

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

Pflügers Archiv - European Journal of Physiology ◽

10.1007/s00424-021-02535-0 ◽

2021 ◽

Vol 473 (3) ◽

pp. 477-489 ◽

Cited By ~ 1

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.

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SK channel blockade prevents hypoxia-induced ventricular arrhythmias through inhibition of Ca2+/voltage uncoupling in hypertrophied hearts

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00777.2020 ◽

2021 ◽

Vol 320 (4) ◽

pp. H1456-H1469

Author(s):

Masayuki Takahashi ◽

Hisashi Yokoshiki ◽

Hirofumi Mitsuyama ◽

Masaya Watanabe ◽

Taro Temma ◽

...

Keyword(s):

Action Potential ◽

Action Potential Duration ◽

Ventricular Arrhythmias ◽

Channel Blockade ◽

Sk Channel ◽

Small Conductance

We demonstrated that hypoxia-induced ventricular arrhythmias were mainly initiated by Ca2+-loaded triggered activities in hypertrophied hearts. The blockades of small-conductance Ca2+-activated K+ channels, especially “apamin,” showed anti-arrhythmic effects by alleviation of not only action potential duration shortening but also Ca2+ handling abnormalities, most notably the “Ca2+/voltage uncoupling.”

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Maternal Diabetes Increases Small Conductance Ca2+‐activated K+ (SK) Currents Which Alters Action Potential Properties and Excitability of Cardiac Motoneurons in the Nucleus Ambiguus

The FASEB Journal ◽

10.1096/fasebj.24.1_supplement.625.13 ◽

2010 ◽

Vol 24 (S1) ◽

Author(s):

Min Lin ◽

Qing‐Hui Chen ◽

Lihua Li ◽

Robert D Wurster ◽

Zixi (Jack) Cheng

Keyword(s):

Action Potential ◽

Maternal Diabetes ◽

Nucleus Ambiguus ◽

Small Conductance

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Ceramide modulates HERG potassium channel gating by translocation into lipid rafts

AJP Cell Physiology ◽

10.1152/ajpcell.00462.2009 ◽

2010 ◽

Vol 299 (1) ◽

pp. C74-C86 ◽

Cited By ~ 19

Author(s):

Sindura B. Ganapathi ◽

Todd E. Fox ◽

Mark Kester ◽

Keith S. Elmslie

Keyword(s):

Potassium Channels ◽

Action Potential ◽

Lipid Rafts ◽

Negative Impact ◽

Cardiac Action Potential ◽

Cholesterol Depletion ◽

Cardiac Action ◽

Herg Channels ◽

The Impact ◽

Action Potential Repolarization

Human ether-à-go-go-related gene (HERG) potassium channels play an important role in cardiac action potential repolarization, and HERG dysfunction can cause cardiac arrhythmias. However, recent evidence suggests a role for HERG in the proliferation and progression of multiple types of cancers, making it an attractive target for cancer therapy. Ceramide is an important second messenger of the sphingolipid family, which due to its proapoptotic properties has shown promising results in animal models as an anticancer agent . Yet the acute effects of ceramide on HERG potassium channels are not known. In the present study we examined the effects of cell-permeable C6-ceramide on HERG potassium channels stably expressed in HEK-293 cells. C6-ceramide (10 μM) reversibly inhibited HERG channel current (IHERG) by 36 ± 5%. Kinetically, ceramide induced a significant hyperpolarizing shift in the current-voltage relationship (Δ V1/2 = −8 ± 0.5 mV) and increased the deactivation rate (43 ± 3% for τfast and 51 ± 3% for τslow). Mechanistically, ceramide recruited HERG channels within caveolin-enriched lipid rafts. Cholesterol depletion and repletion experiments and mathematical modeling studies confirmed that inhibition and gating effects are mediated by separate mechanisms. The ceramide-induced hyperpolarizing gating shift (raft mediated) could offset the impact of inhibition (raft independent) during cardiac action potential repolarization, so together they may nullify any negative impact on cardiac rhythm. Our results provide new insights into the effects of C6-ceramide on HERG channels and suggest that C6-ceramide can be a promising therapeutic for cancers that overexpress HERG.

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

Circulation ◽

10.1161/circ.138.suppl_1.17319 ◽

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.

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Guinea pig visceral C-fiber neurons are diverse with respect to the K+ currents involved in action-potential repolarization

Journal of Neurophysiology ◽

10.1152/jn.1994.71.2.561 ◽

1994 ◽

Vol 71 (2) ◽

pp. 561-574 ◽

Cited By ~ 16

Author(s):

E. P. Christian ◽

J. Togo ◽

K. E. Naper

Keyword(s):

Guinea Pig ◽

Action Potential ◽

Outward Current ◽

Dependent Manner ◽

Concentration Dependent Manner ◽

Whole Cell ◽

C Fiber ◽

K Current ◽

Action Potential Repolarization

1. Intracellular recordings were made from C-fiber neurons identified by antidromic conduction velocity in intact guinea pig nodose ganglia maintained in vitro, and whole-cell patch clamp recordings were made from dissociated guinea pig nodose neurons to investigate the contribution of various K+ conductances to action-potential repolarization. 2. The repolarizing phase of the intracellularly recorded action potential was prolonged in a concentration-dependent manner by charybdotoxin (Chtx; EC50 = 39 nM) or iberiatoxin (Ibtx; EC50 = 48 nM) in a subpopulation of 16/36 C-fiber neurons. In a subset of these experiments, removal of extracellular Ca2+ reversibly prolonged action-potential duration (APD) in the same 4/9 intracellularly recorded C-fiber neurons affected by Chtx (> or = 100 nM). These convergent results support that a Ca(2+)-activated K+ current (IC) contributes to action-potential repolarization in a restricted subpopulation of C-fiber neurons. 3. Tetraethylammonium (TEA; 1-10 mM) increased APD considerably further in the presence of 100-250 nM Chtx or Ibtx, or in nominally Ca(2+)-free superfusate in 14/14 intracellularly recorded C-fiber neurons. TEA affected APD similarly in subpopulations of neurons with and without IC, suggesting that a voltage-dependent K+ current (IK) contributes significantly to action-potential repolarization in most nodose C-fiber neurons. 4. Substitution of Mn2+ for Ca2+ reduced outward whole-cell currents elicited by voltage command steps positive to -30 mV (2-25 ms) in a subpopulation of 21/36 dissociated nodose neurons, supporting the heterogeneous expression of IC. The kinetics of outward tail current relaxations (tau s of 1.5-2 ms) measured at the return of 2-3 ms depolarizing steps to -40 mV were indistinguishable in neurons with and without IC, precluding a separation of the nodose IC and IK by a difference in deactivation rates. 5. Chtx (10-250 nM) reduced in a subpopulation of 3/8 C-fiber neurons the total outward current elicited by voltage steps depolarized to -30 mV in single microelectrode voltage-clamp recordings. TEA (5-10 mM) further reduced outward current in the presence of 100-250 nM Chtx in all eight experiments. The Chtx-sensitive current was taken to represent IC, and the TEA-sensitive current, the IK component contributing to action-potential repolarization. 6. Rapidly inactivating current (IA) was implicated in action-potential repolarization in a subpopulation of intracellularly recorded C-fiber neurons. In 4/7 neurons, incremented hyperpolarizing prepulses negative to -50 mV progressively shortened APD.(ABSTRACT TRUNCATED AT 400 WORDS)

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