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Giommi A, Gurgel ARB, Smith GL, Workman AJ. Does the small conductance Ca 2+-activated K + current I SK flow under physiological conditions in rabbit and human atrial isolated cardiomyocytes? J Mol Cell Cardiol 2023; 183:70-80. [PMID: 37704101 DOI: 10.1016/j.yjmcc.2023.09.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/26/2023] [Revised: 08/16/2023] [Accepted: 09/02/2023] [Indexed: 09/15/2023]
Abstract
BACKGROUND The small conductance Ca2+-activated K+ current (ISK) is a potential therapeutic target for treating atrial fibrillation. AIM To clarify, in rabbit and human atrial cardiomyocytes, the intracellular [Ca2+]-sensitivity of ISK, and its contribution to action potential (AP) repolarisation, under physiological conditions. METHODS Whole-cell-patch clamp, fluorescence microscopy: to record ion currents, APs and [Ca2+]i; 35-37°C. RESULTS In rabbit atrial myocytes, 0.5 mM Ba2+ (positive control) significantly decreased whole-cell current, from -12.8 to -4.9 pA/pF (P < 0.05, n = 17 cells, 8 rabbits). By contrast, the ISK blocker apamin (100 nM) had no effect on whole-cell current, at any set [Ca2+]i (∼100-450 nM). The ISK blocker ICAGEN (1 μM: ≥2 x IC50) also had no effect on current over this [Ca2+]i range. In human atrial myocytes, neither 1 μM ICAGEN (at [Ca2+]i ∼ 100-450 nM), nor 100 nM apamin ([Ca2+]i ∼ 250 nM) affected whole-cell current (5-10 cells, 3-5 patients/group). APs were significantly prolonged (at APD30 and APD70) by 2 mM 4-aminopyridine (positive control) in rabbit atrial myocytes, but 1 μM ICAGEN had no effect on APDs, versus either pre-ICAGEN or time-matched controls. High concentration (10 μM) ICAGEN (potentially ISK-non-selective) moderately increased APD70 and APD90, by 5 and 26 ms, respectively. In human atrial myocytes, 1 μM ICAGEN had no effect on APD30-90, whether stimulated at 1, 2 or 3 Hz (6-9 cells, 2-4 patients/rate). CONCLUSION ISK does not flow in human or rabbit atrial cardiomyocytes with [Ca2+]i set within the global average diastolic-systolic range, nor during APs stimulated at physiological or supra-physiological (≤3 Hz) rates.
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Affiliation(s)
- Alessandro Giommi
- School of Cardiovascular and Metabolic Health, University of Glasgow, Glasgow, UK
| | - Aline R B Gurgel
- School of Cardiovascular and Metabolic Health, University of Glasgow, Glasgow, UK
| | - Godfrey L Smith
- School of Cardiovascular and Metabolic Health, University of Glasgow, Glasgow, UK
| | - Antony J Workman
- School of Cardiovascular and Metabolic Health, University of Glasgow, Glasgow, UK.
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2
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van Herck IGM, Seutin V, Bentzen BH, Marrion NV, Edwards AG. Gating kinetics and pharmacological properties of small-conductance Ca 2+-activated potassium channels. Biophys J 2023; 122:1143-1157. [PMID: 36760125 PMCID: PMC10111258 DOI: 10.1016/j.bpj.2023.02.004] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2022] [Revised: 11/20/2022] [Accepted: 02/06/2023] [Indexed: 02/11/2023] Open
Abstract
Small-conductance (SK) calcium-activated potassium channels are a promising treatment target in atrial fibrillation. However, the functional properties that differentiate SK inhibitors remain poorly understood. The objective of this study was to determine how two unrelated SK channel inhibitors, apamin and AP14145, impact SK channel function in excised inside-out single-channel recordings. Surprisingly, both apamin and AP14145 exert much of their inhibition by inducing a class of very-long-lived channel closures (apamin: τc,vl = 11.8 ± 7.1 s, and AP14145: τc,vl = 10.3 ± 7.2 s), which were never observed under control conditions. Both inhibitors also induced changes to the three closed and two open durations typical of normal SK channel gating. AP14145 shifted the open duration distribution to favor longer open durations, whereas apamin did not alter open-state kinetics. AP14145 also prolonged the two shortest channel closed durations (AP14145: τc,s = 3.50 ± 0.81 ms, and τc,i = 32.0 ± 6.76 ms versus control: τc,s = 1.59 ± 0.19 ms, and τc,i = 13.5 ± 1.17 ms), thus slowing overall gating kinetics within bursts of channel activity. In contrast, apamin accelerated intraburst gating kinetics by shortening the two shortest closed durations (τc,s = 0.75 ± 0.10 ms and τc,i = 5.08 ± 0.49 ms) and inducing periods of flickery activity. Finally, AP14145 introduced a unique form of inhibition by decreasing unitary current amplitude. SK channels exhibited two clearly distinguishable amplitudes (control: Ahigh = 0.76 ± 0.03 pA, and Alow = 0.54 ± 0.03 pA). AP14145 both reduced the fraction of patches exhibiting the higher amplitude (AP14145: 4/9 patches versus control: 16/16 patches) and reduced the mean low amplitude (0.38 ± 0.03 pA). Here, we have demonstrated that both inhibitors introduce very long channel closures but that each also exhibits unique effects on other components of SK gating kinetics and unitary current. The combination of these effects is likely to be critical for understanding the functional differences of each inhibitor in the context of cyclical Ca2+-dependent channel activation in vivo.
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Affiliation(s)
- Ilsbeth G M van Herck
- Computational Physiology Department, Simula Research Laboratory, Oslo, Norway; Institute of Informatics, University of Oslo, Oslo, Norway
| | - Vincent Seutin
- Neurophysiology Unit, GIGA Neurosciences, University of Liège, Liège, Belgium
| | - Bo H Bentzen
- Acesion Pharma, Copenhagen, Denmark; Biomedical Institute, University of Copenhagen, Copenhagen, Denmark
| | - Neil V Marrion
- School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, UK
| | - Andrew G Edwards
- Computational Physiology Department, Simula Research Laboratory, Oslo, Norway; Department of Pharmacology, University of California, Davis, California.
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3
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Butler AS, Hancox JC, Marrion NV. Preferential formation of human heteromeric SK2:SK3 channels limits homomeric SK channel assembly and function. J Biol Chem 2022; 299:102783. [PMID: 36502918 PMCID: PMC9841042 DOI: 10.1016/j.jbc.2022.102783] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2022] [Revised: 11/29/2022] [Accepted: 12/04/2022] [Indexed: 12/13/2022] Open
Abstract
Three isoforms of small conductance, calcium-activated potassium (SK) channel subunits have been identified (SK1-3) that exhibit a broad and overlapping tissue distribution. SK channels have been implicated in several disease states including hypertension and atrial fibrillation, but therapeutic targeting of SK channels is hampered by a lack of subtype-selective inhibitors. This is further complicated by studies showing that SK1 and SK2 preferentially form heteromeric channels during co-expression, likely limiting the function of homomeric channels in vivo. Here, we utilized a simplified expression system to investigate functional current produced when human (h) SK2 and hSK3 subunits are co-expressed. When expressed alone, hSK3 subunits were more clearly expressed on the cell surface than hSK2 subunits. hSK3 surface expression was reduced by co-transfection with hSK2. Whole-cell recording showed homomeric hSK3 currents were larger than homomeric hSK2 currents or heteromeric hSK2:hSK3 currents. The smaller amplitude of hSK2:hSK3-mediated current when compared with homomeric hSK3-mediated current suggests hSK2 subunits regulate surface expression of heteromers. Co-expression of hSK2 and hSK3 subunits produced a current that arose from a single population of heteromeric channels as exhibited by an intermediate sensitivity to the inhibitors apamin and UCL1684. Co-expression of the apamin-sensitive hSK2 subunit and a mutant, apamin-insensitive hSK3 subunit [hSK3(H485N)], produced an apamin-sensitive current. Concentration-inhibition relationships were best fit by a monophasic Hill equation, confirming preferential formation of heteromers. These data show that co-expressed hSK2 and hSK3 preferentially form heteromeric channels and suggest that the hSK2 subunit acts as a chaperone, limiting membrane expression of hSK2:hSK3 heteromeric channels.
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Affiliation(s)
- Andrew S Butler
- School of Physiology, Pharmacology and Neuroscience, Medical Sciences Building, University of Bristol, University Walk, Bristol, United Kingdom
| | - Jules C Hancox
- School of Physiology, Pharmacology and Neuroscience, Medical Sciences Building, University of Bristol, University Walk, Bristol, United Kingdom.
| | - Neil V Marrion
- School of Physiology, Pharmacology and Neuroscience, Medical Sciences Building, University of Bristol, University Walk, Bristol, United Kingdom.
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4
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Assessing Drug-Induced Mitochondrial Toxicity in Cardiomyocytes: Implications for Preclinical Cardiac Safety Evaluation. Pharmaceutics 2022; 14:pharmaceutics14071313. [PMID: 35890211 PMCID: PMC9319223 DOI: 10.3390/pharmaceutics14071313] [Citation(s) in RCA: 11] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2022] [Revised: 06/09/2022] [Accepted: 06/14/2022] [Indexed: 02/07/2023] Open
Abstract
Drug-induced cardiotoxicity not only leads to the attrition of drugs during development, but also contributes to the high morbidity and mortality rates of cardiovascular diseases. Comprehensive testing for proarrhythmic risks of drugs has been applied in preclinical cardiac safety assessment for over 15 years. However, other mechanisms of cardiac toxicity have not received such attention. Of them, mitochondrial impairment is a common form of cardiotoxicity and is known to account for over half of cardiovascular adverse-event-related black box warnings imposed by the U.S. Food and Drug Administration. Although it has been studied in great depth, mitochondrial toxicity assessment has not yet been incorporated into routine safety tests for cardiotoxicity at the preclinical stage. This review discusses the main characteristics of mitochondria in cardiomyocytes, drug-induced mitochondrial toxicities, and high-throughput screening strategies for cardiomyocytes, as well as their proposed integration into preclinical safety pharmacology. We emphasize the advantages of using adult human primary cardiomyocytes for the evaluation of mitochondrial morphology and function, and the need for a novel cardiac safety testing platform integrating mitochondrial toxicity and proarrhythmic risk assessments in cardiac safety evaluation.
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Verkerk AO, Marchal GA, Zegers JG, Kawasaki M, Driessen AHG, Remme CA, de Groot JR, Wilders R. Patch-Clamp Recordings of Action Potentials From Human Atrial Myocytes: Optimization Through Dynamic Clamp. Front Pharmacol 2021; 12:649414. [PMID: 33912059 PMCID: PMC8072333 DOI: 10.3389/fphar.2021.649414] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/04/2021] [Accepted: 02/18/2021] [Indexed: 11/29/2022] Open
Abstract
Introduction: Atrial fibrillation (AF) is the most common cardiac arrhythmia. Consequently, novel therapies are being developed. Ultimately, the impact of compounds on the action potential (AP) needs to be tested in freshly isolated human atrial myocytes. However, the frequent depolarized state of these cells upon isolation seriously hampers reliable AP recordings. Purpose: We assessed whether AP recordings from single human atrial myocytes could be improved by providing these cells with a proper inward rectifier K+ current (IK1), and consequently with a regular, non-depolarized resting membrane potential (RMP), through “dynamic clamp”. Methods: Single myocytes were enzymatically isolated from left atrial appendage tissue obtained from patients with paroxysmal AF undergoing minimally invasive surgical ablation. APs were elicited at 1 Hz and measured using perforated patch-clamp methodology, injecting a synthetic IK1 to generate a regular RMP. The injected IK1 had strong or moderate rectification. For comparison, a regular RMP was forced through injection of a constant outward current. A wide variety of ion channel blockers was tested to assess their modulatory effects on AP characteristics. Results: Without any current injection, RMPs ranged from −9.6 to −86.2 mV in 58 cells. In depolarized cells (RMP positive to −60 mV), RMP could be set at −80 mV using IK1 or constant current injection and APs could be evoked upon stimulation. AP duration differed significantly between current injection methods (p < 0.05) and was shortest with constant current injection and longest with injection of IK1 with strong rectification. With moderate rectification, AP duration at 90% repolarization (APD90) was similar to myocytes with regular non-depolarized RMP, suggesting that a synthetic IK1 with moderate rectification is the most appropriate for human atrial myocytes. Importantly, APs evoked using each injection method were still sensitive to all drugs tested (lidocaine, nifedipine, E-4031, low dose 4-aminopyridine, barium, and apamin), suggesting that the major ionic currents of the atrial cells remained functional. However, certain drug effects were quantitatively dependent on the current injection approach used. Conclusion: Injection of a synthetic IK1 with moderate rectification facilitates detailed AP measurements in human atrial myocytes. Therefore, dynamic clamp represents a promising tool for testing novel antiarrhythmic drugs.
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Affiliation(s)
- Arie O Verkerk
- Department of Medical Biology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands.,Department of Experimental Cardiology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
| | - Gerard A Marchal
- Department of Experimental Cardiology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
| | - Jan G Zegers
- Department of Medical Biology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
| | - Makiri Kawasaki
- Department of Experimental Cardiology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
| | - Antoine H G Driessen
- Department of Cardiothoracic Surgery, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
| | - Carol Ann Remme
- Department of Experimental Cardiology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
| | - Joris R de Groot
- Department of Cardiology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
| | - Ronald Wilders
- Department of Medical Biology, Amsterdam UMC, University of Amsterdam, Amsterdam, Netherlands
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Darkow E, Nguyen TT, Stolina M, Kari FA, Schmidt C, Wiedmann F, Baczkó I, Kohl P, Rajamani S, Ravens U, Peyronnet R. Small Conductance Ca 2 +-Activated K + (SK) Channel mRNA Expression in Human Atrial and Ventricular Tissue: Comparison Between Donor, Atrial Fibrillation and Heart Failure Tissue. Front Physiol 2021; 12:650964. [PMID: 33868017 PMCID: PMC8047327 DOI: 10.3389/fphys.2021.650964] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2021] [Accepted: 03/01/2021] [Indexed: 12/25/2022] Open
Abstract
In search of more efficacious and safe pharmacological treatments for atrial fibrillation (AF), atria-selective antiarrhythmic agents have been promoted that target ion channels principally expressed in the atria. This concept allows one to engage antiarrhythmic effects in atria, but spares the ventricles from potentially proarrhythmic side effects. It has been suggested that cardiac small conductance Ca2+-activated K+ (SK) channels may represent an atria-selective target in mammals including humans. However, there are conflicting data concerning the expression of SK channels in different stages of AF, and recent findings suggest that SK channels are upregulated in ventricular myocardium when patients develop heart failure. To address this issue, RNA-sequencing was performed to compare expression levels of three SK channels (KCNN1, KCNN2, and KCNN3) in human atrial and ventricular tissue samples from transplant donor hearts (no cardiac disease), and patients with cardiac disease in sinus rhythm or with AF. In addition, for control purposes expression levels of several genes known to be either chamber-selective or differentially expressed in AF and heart failure were determined. In atria, as compared to ventricle from transplant donor hearts, we confirmed higher expression of KCNN1 and KCNA5, and lower expression of KCNJ2, whereas KCNN2 and KCNN3 were statistically not differentially expressed. Overall expression of KCNN1 was low compared to KCNN2 and KCNN3. Comparing atrial tissue from patients with AF to sinus rhythm samples we saw downregulation of KCNN2 in AF, as previously reported. When comparing ventricular tissue from heart failure patients to non-diseased samples, we found significantly increased ventricular expression of KCNN3 in heart failure, as previously published. The other channels showed no significant difference in expression in either disease. Our results add weight to the view that SK channels are not likely to be an atria-selective target, especially in failing human hearts, and modulators of these channels may prove to have less utility in treating AF than hoped. Whether targeting SK1 holds potential remains to be elucidated.
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Affiliation(s)
- Elisa Darkow
- Institute for Experimental Cardiovascular Medicine, University Heart Center Freiburg-Bad Krozingen, Freiburg im Breisgau, Germany.,Medical Center and Faculty of Medicine, University of Freiburg, Freiburg im Breisgau, Germany.,Spemann Graduate School of Biology and Medicine (SGBM), University of Freiburg, Freiburg im Breisgau, Germany.,Faculty of Biology, University of Freiburg, Freiburg im Breisgau, Germany
| | - Thong T Nguyen
- Genome Analysis Unit, Amgen Research, Amgen Inc., South San Francisco, CA, United States
| | - Marina Stolina
- Department of Cardiometabolic Disorders, Amgen Research, Amgen Inc., Thousand Oaks, CA, United States
| | - Fabian A Kari
- Medical Center and Faculty of Medicine, University of Freiburg, Freiburg im Breisgau, Germany.,Department of Cardiovascular Surgery, University Heart Center Freiburg-Bad Krozingen, Freiburg im Breisgau, Germany
| | - Constanze Schmidt
- Department of Cardiology, University Hospital Heidelberg, Heidelberg, Germany.,DZHK (German Center for Cardiovascular Research) Partner Site Heidelberg/Mannheim, Heidelberg University, Heidelberg, Germany
| | - Felix Wiedmann
- Department of Cardiology, University Hospital Heidelberg, Heidelberg, Germany.,DZHK (German Center for Cardiovascular Research) Partner Site Heidelberg/Mannheim, Heidelberg University, Heidelberg, Germany
| | - István Baczkó
- Department of Pharmacology and Pharmacotherapy, University of Szeged, Szeged, Hungary
| | - Peter Kohl
- Institute for Experimental Cardiovascular Medicine, University Heart Center Freiburg-Bad Krozingen, Freiburg im Breisgau, Germany.,Medical Center and Faculty of Medicine, University of Freiburg, Freiburg im Breisgau, Germany.,CIBSS Centre for Integrative Biological Signalling Studies, University of Freiburg, Freiburg im Breisgau, Germany
| | - Sridharan Rajamani
- Translational Safety and Bioanalytical Sciences, Amgen Research, Amgen Inc., South San Francisco, CA, United States
| | - Ursula Ravens
- Institute for Experimental Cardiovascular Medicine, University Heart Center Freiburg-Bad Krozingen, Freiburg im Breisgau, Germany.,Medical Center and Faculty of Medicine, University of Freiburg, Freiburg im Breisgau, Germany
| | - Rémi Peyronnet
- Institute for Experimental Cardiovascular Medicine, University Heart Center Freiburg-Bad Krozingen, Freiburg im Breisgau, Germany.,Medical Center and Faculty of Medicine, University of Freiburg, Freiburg im Breisgau, Germany
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Qi MM, Qian LL, Wang RX. Modulation of SK Channels: Insight Into Therapeutics of Atrial Fibrillation. Heart Lung Circ 2021; 30:1130-1139. [PMID: 33642173 DOI: 10.1016/j.hlc.2021.01.009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2020] [Revised: 01/12/2021] [Accepted: 01/31/2021] [Indexed: 11/19/2022]
Abstract
Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia in the world. Although much technological progress in the treatment of AF has been made, there is an urgent need for better treatment of AF due to its high rates of morbidity and mortality. The anti-arrhythmic drugs currently approved for marketing have significant limitations and side effects such as life-threatening ventricular arrhythmias and hypotension. The small conductance Ca2+-activated K+ channels (SK channels) are dependent on intracellular Ca2+ concentrations, which tightly integrate with membrane potential. Given the predominant expression in the atria of many species, including humans, they are now emerging as a therapeutic target for treating AF. This review aimed to illustrate the characteristics and function of SK channels. Moreover, it discussed the regulation of SK channels and their potential as a therapeutic target of AF.
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Affiliation(s)
- Miao-Miao Qi
- Department of Cardiology, Wuxi People's Hospital Affiliated to Nanjing Medical University, Wuxi, Jiangsu, China
| | - Ling-Ling Qian
- Department of Cardiology, Wuxi People's Hospital Affiliated to Nanjing Medical University, Wuxi, Jiangsu, China
| | - Ru-Xing Wang
- Department of Cardiology, Wuxi People's Hospital Affiliated to Nanjing Medical University, Wuxi, Jiangsu, China.
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Impact of I SK Voltage and Ca 2+/Mg 2+-Dependent Rectification on Cardiac Repolarization. Biophys J 2020; 119:690-704. [PMID: 32668235 DOI: 10.1016/j.bpj.2020.06.022] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2019] [Revised: 06/02/2020] [Accepted: 06/10/2020] [Indexed: 10/24/2022] Open
Abstract
Cardiac small conductance Ca2+-activated K+ (SK) channels are activated solely by Ca2+, but the SK current (ISK) is inwardly rectified. However, the impact of inward rectification in shaping action potentials (APs) in ventricular cardiomyocytes under β-adrenergic stimulation or in disease states remains undefined. Two processes underlie this inward rectification: an intrinsic rectification caused by an electrostatic energy barrier from positively charged amino acids at the inner pore and a voltage-dependent Ca2+/Mg2+ block. Thus, Ca2+ has a biphasic effect on ISK, activating at low [Ca2+] yet inhibiting ISK at high [Ca2+]. We examined the effect of ISK rectification on APs in rat cardiomyocytes by simultaneously recording whole-cell apamin-sensitive currents and Ca2+ transients during an AP waveform and developed a computer model of SK channels with rectification features. The typical profile of ISK during AP clamp included an initial peak (mean 1.6 pA/pF) followed by decay to the point that submembrane [Ca2+] reached ∼10 μM. During the rest of the AP stimulus, ISK either plateaued or gradually increased as the cell repolarized and submembrane [Ca2+] decreased further. We used a six-state gating model combined with intrinsic and Ca2+/Mg2+-dependent rectification to simulate ISK and investigated the relative contributions of each type of rectification to AP shape. This SK channel model replicates key features of ISK recording during AP clamp showing that intrinsic rectification limits ISK at high Vm during the early and plateau phase of APs. Furthermore, the initial rise of Ca2+ transients activates, but higher [Ca2+] blocks SK channels, yielding a transient outward-like ISK trajectory. During the decay phase of Ca2+, the Ca2+-dependent block is released, causing ISK to rise again and contribute to repolarization. Therefore, ISK is an important repolarizing current, and the rectification characteristics of an SK channel determine its impact on early, plateau, and repolarization phases of APs.
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