Small Conductance Research Articles

Gating small conductance calcium-activated potassium channels in the thalamic reticular nucleus.

Small conductance calcium-activated potassium (SK) channels are well-known regulators of neuronal excitability. In the thalamic hub, SK2 channels act as pacemakers of thalamic reticular neurons, which play a key role in the thalamocortical circuit. Several disease-linked genes are highly enriched in these neurons, including genes known to be associated with schizophrenia and attentional disorders, which could affect neuronal firing. The present study assessed the effect of pharmacological modulation of SK channels in the firing pattern and intrinsic properties of thalamic reticular neurons by performing whole cell patch clamp recordings in brain slices. Two SK positive allosteric modulators and one negative allosteric modulator were used: CyPPA, NS309, and NS8593, respectively. By acting on the burst afterhyperpolarization (AHP), negative modulation of SK channels resulted in increased action potential (AP) firing, increased burst duration, and decreased intervals between bursts. Conversely, both CyPPA and NS309 increased the afterburst AHP, prolonging the interburst interval, which additionally resulted in reduced AP firing in the case of NS309. Alterations in SK channel activity would be expected to alter functioning of thalamocortical circuits. Targeting SK channels could be promising in treating disorders involving thalamic reticular dysfunction such as psychiatric and neurodevelopmental disorders.

Physical mechanism on ladderphane copolymers with electron localization and phonon delocalization for capacitive energy storage

In this article, the properties of trapezoidal polymer PSBNP-co-PTNI for capacitive energy storage are investigated theoretically, stimulated by the recent experimental reports [Chen et al., Nature 615, 62 (2023)]. The flatband structures of PSBNP-co-PTNI copolymer reveal that very large effective mass of electron results in Anderson electron localization and then small electric conductance. The TNI doping significantly decreases optical absorption in an ultraviolet region, which can significantly suppress photocurrent. Raman spectrum demonstrates that phonon Debye temperature can be significantly decreased, due to the U-process of phonon scattering by TNI doping, and then results in the phonon delocalization, and then the large thermal conductivity and thermal capacity. The simultaneous regulations on decreasing electric conductivity and on increasing thermal conductivity can significantly increase the ability of capacitive energy storage. The unit-length dependent thermal electric current manifests that the thermal electric current can be significantly decreased by slightly increase in length of copolymer. Overall, our research contributes to the understanding of the impact of TNI doping on the energy storage properties of the polymer and provides a valuable reference for future research focused on polymer energy storage.

Origin, evolution and diversification of plant mechanosensitive channel of small conductance-like (MSL) proteins

Mechanosensitive (MS) ion channels provide efficient molecular mechanism for transducing mechanical forces into intracellular ion fluxes in all kingdoms of life. The mechanosensitive channel of small conductance (MscS) was one of the best-studied MS channels and its homologs (MSL, MscS-like) were widely distributed in cell-walled organisms. However, the origin, evolution and expansion of MSL proteins in plants are still not clear. Here, we identified more than 2100 MSL proteins from 176 plants and conducted a broad-scale phylogenetic analysis. The phylogenetic tree showed that plant MSL proteins were divided into three groups (I, II and III) prior to the emergence of chlorophytae algae, consistent with their specific subcellular localization. MSL proteins were distributed unevenly into each of plant species, and four parallel expansion was identified in angiosperms. In Brassicaceae, most MSL duplicates were derived by whole-genome duplication (WGD)/segmental duplications. Finally, a hypothetical evolutionary model of MSL proteins in plants was proposed based on phylogeny. Our studies illustrate the evolutionary history of the MSL proteins and provide a guide for future functional diversity analyses of these proteins in plants.

154 Inhibition of the intermediate (KCa3.1) and small (KCa2.3 and KCa2.2) conductance, Ca2+-activated K+ channels by CFTR correctors

154 Inhibition of the intermediate (KCa3.1) and small (KCa2.3 and KCa2.2) conductance, Ca2+-activated K+ channels by CFTR correctors

Does the small conductance Ca2+-activated K+ current ISK flow under physiological conditions in rabbit and human atrial isolated cardiomyocytes?

BackgroundThe small conductance Ca2+-activated K+ current (ISK) is a potential therapeutic target for treating atrial fibrillation. AimTo clarify, in rabbit and human atrial cardiomyocytes, the intracellular [Ca2+]-sensitivity of ISK, and its contribution to action potential (AP) repolarisation, under physiological conditions. MethodsWhole-cell-patch clamp, fluorescence microscopy: to record ion currents, APs and [Ca2+]i; 35–37°C. ResultsIn 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). ConclusionISK 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.

Adaptive spike threshold dynamics associated with sparse spiking of hilar mossy cells are captured by a simple model.

Hilar mossy cells (hMCs) in the dentate gyrus (DG) receive inputs from DG granule cells (GCs), CA3 pyramidal cells and inhibitory interneurons, and provide feedback input to GCs. Behavioural and in vivo recording experiments implicate hMCs in pattern separation, navigation and spatial learning. Our experiments link hMC intrinsic excitability to their synaptically evoked in vivo spiking outputs. We performed electrophysiological recordings from DG neurons and found that hMCs displayed an adaptative spike threshold that increased both in proportion to the intensity of injected currents, and in response to spiking itself, returning to baseline over a long time scale, thereby instantaneously limiting their firing rate responses. The hMC activity is additionally limited by a prominent medium after-hyperpolarizing potential (AHP) generated by small conductance K+ channels. We hypothesize that these intrinsic hMC properties are responsible for their low in vivo firing rates. Our findings extend previous studies that compare hMCs, CA3 pyramidal cells and hilar inhibitory cells and provide novel quantitative data that contrast the intrinsic properties of these cell types. We developed a phenomenological exponential integrate-and-fire model that closely reproduces the hMC adaptive threshold nonlinearities with respect to their threshold dependence on input current intensity, evoked spike latency and long-lasting spike-induced increase in spike threshold. Our robust and computationally efficient model is amenable to incorporation into large network models of the DG that will deepen our understanding of the neural bases of pattern separation, spatial navigation and learning. KEY POINTS: Previous studies have shown that hilar mossy cells (hMCs) are implicated in pattern separation and the formation of spatial memory, but how their intrinsic properties relate to their in vivo spiking patterns is still unknown. Here we show that the hMCs display electrophysiological properties that distinguish them from the other hilar cell types including a highly adaptive spike threshold that decays slowly. The spike-dependent increase in threshold combined with an after-hyperpolarizing potential mediated by a slow K+ conductance is hypothesized to be responsible for the low-firing rate of the hMC observed in vivo. The hMC's features are well captured by a modified stochastic exponential integrate-and-fire model that has the unique feature of a threshold intrinsically dependant on both the stimulus intensity and the spiking history. This computational model will allow future work to study how the hMCs can contribute to spatial memory formation and navigation.

KV7 but not dual small and intermediate KCa channel openers inhibit the activation of colonic afferents by noxious stimuli.

In numerous subtypes of central and peripheral neurons, small and intermediate conductance Ca2+-activated K+ (SK and IK, respectively) channels are important regulators of neuronal excitability. Transcripts encoding SK channel subunits, as well as the closely related IK subunit, are coexpressed in the soma of colonic afferent neurons with receptors for the algogenic mediators ATP and bradykinin, P2X3 and B2, highlighting the potential utility of these channels as drug targets for the treatment of abdominal pain in gastrointestinal diseases such as irritable bowel syndrome. Despite this, pretreatment with the dual SK/IK channel opener SKA-31 had no effect on the colonic afferent response to ATP, bradykinin, or noxious ramp distention of the colon. Inhibition of SK or IK channels with apamin or TRAM-34, respectively, yielded no change in spontaneous baseline afferent activity, indicating these channels are not tonically active. In contrast to its lack of effect in electrophysiological experiments, comparable concentrations of SKA-31 abolished ongoing peristaltic activity in the colon ex vivo. Treatment with the KV7 channel opener retigabine blunted the colonic afferent response to all applied stimuli. Our data therefore highlight the potential utility of KV7, but not SK/IK, channel openers as analgesic agents for the treatment of abdominal pain.NEW & NOTEWORTHY Despite marked coexpression of small (Kcnn1, Kcnn2) and intermediate (Kcnn4) conductance calcium-activated potassium channel transcripts with P2X3 (P2rx3) or bradykinin B2 (Bdkrb2) receptors in colonic sensory neurons, pharmacological activation of these channels had no effect on the colonic afferent response to ATP, bradykinin or luminal distension of the colon. This is in contrast to the robust inhibitory effect of the KV7 channel opener, retigabine.

Transient Receptor Potential channels (TRP) in GtoPdb v.2023.2

Transient Receptor Potential channels (TRP) in GtoPdb v.2023.2

TMEM63 proteins function as monomeric high-threshold mechanosensitive ion channels

OSCA/TMEM63s form mechanically activated (MA) ion channels in plants and animals, respectively. OSCAs and related TMEM16s and transmembrane channel-like (TMC) proteins form homodimers with two pores. Here, we uncover an unanticipated monomeric configuration of TMEM63 proteins. Structures of TMEM63A and TMEM63B (referred to as TMEM63s) revealed a single highly restricted pore. Functional analyses demonstrated that TMEM63s are bona fide mechanosensitive ion channels, characterized by small conductance and high thresholds. TMEM63s possess evolutionary variations in the intracellular linker IL2, which mediates dimerization in OSCAs. Replacement of OSCA1.2 IL2 with TMEM63A IL2 or mutations to key variable residues resulted in monomeric OSCA1.2 and MA currents with significantly higher thresholds. Structural analyses revealed substantial conformational differences in the mechano-sensing domain IL2 and gating helix TM6 between TMEM63s and OSCA1.2. Our studies reveal that mechanosensitivity in OSCA/TMEM63 channels is affected by oligomerization and suggest gating mechanisms that may be shared by OSCA/TMEM63, TMEM16, and TMC channels.

A chlorzoxazone–folic acid combination improves cognitive affective decline in SCA2-58Q mice

Spinocerebellar ataxia type 2 (SCA2) is a polyglutamine disorder caused by a pathological expansion of CAG repeats in ATXN2 gene. SCA2 is accompanied by cerebellar degeneration and progressive motor decline. Cerebellar Purkinje cells (PCs) seem to be primarily affected in this disorder. The majority of the ataxia research is focused on the motor decline observed in ataxic patients and animal models of the disease. However, recent evidence from patients and ataxic mice suggests that SCA2 can also share the symptoms of the cerebellar cognitive affective syndrome. We previously reported that SCA2-58Q PC-specific transgenic mice exhibit anxiolytic behavior, decline in spatial memory, and a depressive-like state. Here we studied the effect of the activation of the small conductance calcium-activated potassium channels (SK channels) by chlorzoxazone (CHZ) combined with the folic acid (FA) on the PC firing and also motor, cognitive and affective symptoms in SCA2-58Q mice. We realized that CHZ-FA combination improved motor and cognitive decline as well as ameliorated mood alterations in SCA2-58Q mice without affecting the firing rate of their cerebellar PCs. Our results support the idea of the combination therapy for both ataxia and non-motor symptoms in ataxic mice without affecting the firing frequency of PCs.

Differential effect of gold nanoparticles on cerebrovascular function and biomechanical properties.

Human stroke serum (HSS) has been shown to impair cerebrovascular function, likely by factors released into the circulation after ischemia. 20 nm gold nanoparticles (GNPs) have demonstrated anti-inflammatory properties, with evidence that they decrease pathologic markers of ischemic severity. Whether GNPs affect cerebrovascular function, and potentially protect against the damaging effects of HSS on the cerebral circulation remains unclear. HSS obtained 24 h poststroke was perfused through the lumen of isolated and pressurized third-order posterior cerebral arteries (PCAs) from male Wistar rats with and without GNPs (~2 × 109 GNP/ml), or GNPs in vehicle, in an arteriograph chamber (n = 8/group). All vessels were myogenically reactive ≥60 mmHg intravascular pressure; however, vessels containing GNPs had significantly less myogenic tone. GNPs increased vasoreactivity to small and intermediate conductance calcium activated potassium channel activation via NS309; however, reduced vasoconstriction to nitric oxide synthase inhibition. Hydraulic conductivity and transvascular filtration, were decreased by GNPs, suggesting a protective effect on the blood-brain barrier. The stress-strain curves of PCAs exposed to GNPs were shifted leftward, indicating increased vessel stiffness. This study provides the first evidence that GNPs affect the structure and function of the cerebrovasculature, which may be important for their development and use in biomedical applications.

Microcirculation in Hypertension: A Therapeutic Target to Prevent Cardiovascular Disease?

Arterial hypertension is a common condition worldwide and an important risk factor for cardio- and cerebrovascular events, renal diseases, as well as microvascular eye diseases. Established hypertension leads to the chronic vasoconstriction of small arteries as well as to a decreased lumen diameter and the thickening of the arterial media or wall with a consequent increased media-to-lumen ratio (MLR) or wall-to-lumen ratio (WLR). This process, defined as vascular remodeling, was firstly demonstrated in small resistance arteries isolated from subcutaneous biopsies and measured by micromyography, and this is still considered the gold-standard method for the assessment of structural alterations in small resistance arteries; however, microvascular remodeling seems to represent a generalized phenomenon. An increased MLR may impair the organ flow reserve, playing a crucial role in the maintenance and, probably, also in the progressive worsening of hypertensive disease, as well as in the development of hypertension-mediated organ damage and related cardiovascular events, thus possessing a relevant prognostic relevance. New non-invasive techniques, such as scanning laser Doppler flowmetry or adaptive optics, are presently under development, focusing mainly on the evaluation of WLR in retinal arterioles; recently, also retinal microvascular WLR was demonstrated to have a prognostic impact in terms of cardio- and cerebrovascular events. A rarefaction of the capillary network has also been reported in hypertension, which may contribute to flow reduction in and impairment of oxygen delivery to different tissues. These microvascular alterations seem to represent an early step in hypertension-mediated organ damage since they might contribute to microvascular angina, stroke, and renal dysfunction. In addition, they can be markers useful in monitoring the beneficial effects of antihypertensive treatment. Additionally, conductance arteries may be affected by a remodeling process in hypertension, and an interrelationship is present in the structural changes in small and large conductance arteries. The review addresses the possible relations between structural microvascular alterations and hypertension-mediated organ damage, and their potential improvement with antihypertensive treatment.

Modulation of SK Channels via Calcium Buffering Tunes Intrinsic Excitability of Parvalbumin Interneurons in Neuropathic Pain: A Computational and Experimental Investigation.

Parvalbumin-expressing interneurons (PVINs) play a crucial role within the dorsal horn of the spinal cord by preventing touch inputs from activating pain circuits. In both male and female mice, nerve injury decreases PVINs' output via mechanisms that are not fully understood. In this study, we show that PVINs from nerve-injured male mice change their firing pattern from tonic to adaptive. To examine the ionic mechanisms responsible for this decreased output, we used a reparametrized Hodgkin-Huxley type model of PVINs, which predicted (1) the firing pattern transition is because of an increased contribution of small conductance calcium-activated potassium (SK) channels, enabled by (2) impairment in intracellular calcium buffering systems. Analyzing the dynamics of the Hodgkin-Huxley type model further demonstrated that a generalized Hopf bifurcation differentiates the two types of state transitions observed in the transient firing of PVINs. Importantly, this predicted mechanism holds true when we embed the PVIN model within the neuronal circuit model of the spinal dorsal horn. To experimentally validate this hypothesized mechanism, we used pharmacological modulators of SK channels and demonstrated that (1) tonic firing PVINs from naive male mice become adaptive when exposed to an SK channel activator, and (2) adapting PVINs from nerve-injured male mice return to tonic firing on SK channel blockade. Our work provides important insights into the cellular mechanism underlying the decreased output of PVINs in the spinal dorsal horn after nerve injury and highlights potential pharmacological targets for new and effective treatment approaches to neuropathic pain.SIGNIFICANCE STATEMENT Parvalbumin-expressing interneurons (PVINs) exert crucial inhibitory control over Aβ fiber-mediated nociceptive pathways at the spinal dorsal horn. The loss of their inhibitory tone leads to neuropathic symptoms, such as mechanical allodynia, via mechanisms that are not fully understood. This study identifies the reduced intrinsic excitability of PVINs as a potential cause for their decreased inhibitory output in nerve-injured condition. Combining computational and experimental approaches, we predict a calcium-dependent mechanism that modulates PVINs' electrical activity following nerve injury: a depletion of cytosolic calcium buffer allows for the rapid accumulation of intracellular calcium through the active membranes, which in turn potentiates SK channels and impedes spike generation. Our results therefore pinpoint SK channels as potential therapeutic targets for treating neuropathic symptoms.

Cl− induces endothelium-dependent mesenteric arteriolar vasorelaxation through the NKCC1/TRPV4/NCX axis

AimsAlthough absorbed NaCl increases intestinal blood flow to facilitate absorption and transportation, it is unclear if it can directly mediate mesenteric arterial relaxation. We aimed to investigate and test our hypothesis that Cl− induces mesenteric arterial vasorelaxation via endothelium-dependent hyperpolarization (EDH). Main methodsWe used wire myograph to study NaCl-induced vasorelaxation of mesenteric arteries isolated from mice. Cl−, Ca2+ and K+ imaging was performed in human vascular endothelial cells pre-treated with pharmacological agents. Key findingsThe Cl− concentration-dependently induced vasorelaxation of mesenteric arteries likely through EDH. The Cl−-induced vasorelaxation was attenuated in TRPV4 KO mice and inhibited by selective blockers of Na+-K+-2Cl− cotransporter 1 (NKCC1) (bumetanide, 10 μM), transient receptor potential vanilloid 4 (TRPV4) (RN-1734, 40 μM), and small conductance Ca2+-activated K+ channels (SKCa) (apamin, 3 μM)/ intermediate conductance Ca2+-activated K+ channels (IKCa) (TRAM-34, 10 μM) and myoendothelial gap junction (18α-glycyrrhetinic acid, 10 μM), but enhanced by a selective activator of IKCa/SKCa (SKA-31, 0.3 μM). Cl− decreased intracellular K+ concentrations in endothelial cells, which was reversed by apamin (200 nM) plus TRAM-34 (500 nM). Extracellular Cl− raised intracellular Cl− concentrations in endothelial cells, which was attenuated by bumetanide (10 μM). Finally, Cl− induced a transient Ca2+ signaling via TRPV4 in endothelial cells, which became sustained when the Ca2+ exit mode of Na+-Ca2+ exchanger (NCX) was blocked. SignificanceCl− induces a pure EDH-mediated vasorelaxation of mesenteric arteries through activation of endothelial NKCC1/TRPV4/NCX axis. We have provided a novel insight into the role of Cl−-induced vasorelaxation via EDH mechanism.

Insights into the mechanisms governing P01 scorpion toxin effect against U87 glioblastoma cells oncogenesis.

The emerging concept of small conductance Ca2+-activated potassium channels (SKCa) as pharmacological target for cancer treatment has significantly increased in recent years. In this study, we isolated the P01 toxin from Androctonus australis (Aa) scorpion venom and investigated its effect on biological properties of glioblastoma U87, breast MDA-MB231 and colon adenocarcinoma LS174 cancer cell lines. Our results showed that P01 was active only on U87 glioblastoma cells. It inhibited their proliferation, adhesion and migration with IC50 values in the micromolar range. We have also shown that P01 reduced the amplitude of the currents recorded in HEK293 cells expressing SK2 channels with an IC50 value of 3pM, while it had no effect on those expressing SK3 channels. The investigation of the SKCa channels expression pattern showed that SK2 transcripts were expressed differently in the three cancer cell lines. Particularly, we highlighted the presence of SK2 isoforms in U87 cells, which could explain and rely on the specific activity of P01 on this cell line. These experimental data highlighted the usefulness of scorpion peptides to decipher the role of SKCa channels in the tumorigenesis process, and develop potential therapeutic molecules targeting glioblastoma with high selectivity.

Gate controllable band alignment transition in 2D black-arsenic/WSe2 heterostructure

Controlling the manner of band alignment of heterostructures increases design freedom with novel physical properties, enables the design of new functional devices, and improves device performance, but the lattice matching limits the diversity of traditional heterostructures. Van der Waals heterostructures (vdWHs) fabricated by rationally mechanical restacking different two-dimensional (2D) layered materials or sequential synthesis can overcome this limitation. However, it is difficult to achieve full control over the band alignment for a specific vdWHs by means of an applied vertical electric field. Here, we take advantage of the band structure alignment properties of narrow-bandgap black-arsenic (b-As) and large-bandgap WSe2 to realize b-As/WSe2 vdWHs with type-I band alignment. The band alignment can be tuned from type I to type II by gate electric field, which greatly improves the photoresponsivity over 103. An ultra-fast photoresponse of about 570 ns is obtained, which is much better than that of vdWHs with the same structure. The b-As/WSe2 vdWHs also can achieve high-performance rectifier phototransistor with an ultra-high rectification ratio exceeding 106, a small conductance slope of about 86 mV/dec, and a low curvature coefficient of about 46 V−1. Our work paves the way for the exploitation of b-As heterojunction for ultra-fast and low-power optoelectronic applications.

Dissipation during the Gating Cycle of the Bacterial Mechanosensitive Ion Channel Approaches the Landauer Limit.

The Landauer principle sets a thermodynamic bound of kBT ln 2 on the energetic cost of erasing each bit of information. It holds for any memory device, regardless of its physical implementation. It was recently shown that carefully built artificial devices can attain this bound. In contrast, biological computation-like processes, e.g., DNA replication, transcription and translation use an order of magnitude more than their Landauer minimum. Here, we show that reaching the Landauer bound is nevertheless possible with biological devices. This is achieved using a mechanosensitive channel of small conductance (MscS) from E. coli as a memory bit. MscS is a fast-acting osmolyte release valve adjusting turgor pressure inside the cell. Our patch-clamp experiments and data analysis demonstrate that under a slow switching regime, the heat dissipation in the course of tension-driven gating transitions in MscS closely approaches its Landauer limit. We discuss the biological implications of this physical trait.

Does a Blatter Radical Retain Its Open-Shell Character When Incorporated into Gold–Molecule–Gold Junctions?

One of the key issues for the use of organic radicals in high-performance molecular electronic, spintronic and thermoelectric devices is the retention, at room temperature, of their open-shell character when they are in contact with the metal electrodes of a junction. By means of first-principles quantum transport calculations, we have investigated the stability and the electronic transport properties of single-molecule junctions incorporating a Blatter radical (BR) with thiomethyl linker groups. Our calculations suggest that the BR can unequivocally retain its open-shell nature even when bound to gold electrodes through undercoordinated gold adatoms. Such a state cannot be destroyed by the surrounding solvent molecules and the applied bias. The calculated low-bias conductance values of junctions with BR, an oxidized BR derivative, and a closed-shell stilbene molecule are in quantitative agreement with the available experimental measurements. The small conductance of the BR can be associated with the localization of its π-type singly occupied and singly unoccupied molecular orbitals (SOMO and SUMO), leading to an asymmetrically weak electronic coupling to the two gold electrodes, even though these two orbitals lie very close to the Fermi energy. Furthermore, the nonmonotonic modulation of the junction conductance controlled by a charge gate can be used in experiments to further verify the presence of the unpaired electron. This proof may be realized in practice with an electrostatic solid-state or an electrochemical gate. Our findings deepen the current understanding of the radical–metal interfacial properties and facilitate the design of future radical-based multifunctional molecular devices.

PO-02-147 THE ANTIARRHYTHMIC EFFECTS OF MIRABEGRON IN ISOLATED RABBIT HEARTS

β-3 adrenoceptor (AR) activation opposes the effects of β-1 and β-2 AR activation and rescues the effects of excessive catecholamines. BRL37344 (beta3-AR agonist) is antiarrhythmic in canine models. Mirabegron is a β-3 agonist approved by the FDA for the treatment of overactive bladder. A previous study showed that isoproterenol activates the small conductance Ca-activated K current more in female than in male ventricles. To test the hypothesis that mirabegron reduces the proarrhythmic effects of isoproterenol more effectively in female than in male ventricles. We performed optical mapping studies in 6 male and 6 female Langendorff-perfused rabbit hearts first at baseline, then sequentially after isoproterenol (ISO, 100 nM), mirabegron (1000 nM), apamin (100 nM) administration and washout in study 1. In study 2 (5 males and 5 females), we studied at baseline, then after adding mirabegron (250 nM, 500 nM, 750 nM, and 1000 nM), followed by isoproterenol and washout. All hearts underwent 10 attempts of ventricular fibrillation (VF) induction with a ventricular burst (20 Hz for 10 s) pacing at each stage of the experiment. The number of phase singularities (PSs) was determined per episode of VF (PSs/VF). In study 1, mirabegron reduced the VF inducibility (number of VF after 10 attempts) during isoproterenol infusion by an average of -2.75 (p = 0.026). When VF was induced, the PSs/VF increased by 2.4±2.1 in males and 6.6±3.3 in females (p=0.03, Figure A). Mirabegron significantly reduced the PSs/VF in both sexes and eliminated the sex differences. Apamin given after mirabegron in the presence of isoproterenol did not further change PSs/VF (p=0.63) (Figure B). The conduction velocity (CV), estimated by the inverse of the total activation time (1/s), was 16±2.7 for all hearts at baseline and 35±10 after ISO, respectively (p=0.0006). ISO did not increase CV differentially by sex (p=0.93). Mirabegron reduced the CV by an average of 15.6 (from 34±11.4 to 18±3.9) in females, which was significantly (p=0.043) more than 5.1 (from 35±8.7 to 31±9.3) in males. Figure C shows a summary of study 2. Mirabegron significantly reduced PSs/VF at 750 nM in female (p=0.015 vs. baseline) and at 1000 nM in male (p=0.033 vs. baseline) hearts. The addition of ISO did not increase PSs/VF. During ISO infusion, mirabegron reduces the CV and the PSs/VF more in females than males during VF. These data suggest that the β3 agonist is a sex-specific antiarrhythmic agent.

PO-02-241 CARDIAC SYMPATHETIC REMODELING AND SK CHANNELS UPREGULATION, THE POSSIBLE MECHANISMS OF NOISE EXPOSURE ASSOCIATED ATRIAL ARRHYTHMOGENESIS

Environmental noise exposure may increase sympathetic tone and lead to atrial fibrillation (AF) and mortality. The small conductance Ca2+- activated K+ (SK) channels play important roles in the repolarization of atria. The regulation of SK channels might be involved in modulating cardiac electrophysiology via the autonomic nervous system (ANS). Apamin blocks SK channels specifically and is used for evaluating the roles of SK channels regulation related to noise exposure. To determine how ANS regulates SK channels under noise exposure. We used a noise exposure (NOE) mice model with broad-band noise 20-20k Hz for 28 days to induce atrial myopathy. A total of 12 NOE and 9 control mice were used in the study. Dual optical mapping was performed via Langendorff perfused intact hearts to measure the baseline and post-apamin atrial action potential duration (APD), calcium transient duration (CaTD) and conduction velocity (CV) in the NOE and control mice. The expression of SK channel proteins (SK1-3), tyrosine hydroxylase (TH) and growth-associated protein 43 (GAP43) was detected by immunohistochemistry. The degree of myocardial fibrosis in the atria was detected by Masson's trichrome staining. Optical mapping study showed no APD difference (26.26±3.69 vs 27.76±2.36 ms, p=0.69) but prolonged CaTD (52.67±1.91 vs 49.65±3.4 ms, p=0.03) in NOE compared with control mice atria at baseline. The ratio of ΔAPD (the difference in the APD after and before apamin) and baseline was higher in NOE than in control mice (0.221±0.231 vs 0.011±0.52, p=0.015), meaning that noise exposure contributed to the upregulation of SK channels. Atrial CV was higher in NOE than in control mice (44.9±4 vs 39.2±3.5 cm/s, p=0.019); after apamin administration, it was significantly lower in NOE than in control mice (30.9±4.8 vs 34.2±3.5 cm/s, p=0.045). IHC study showed SK1-3, TH, GAP43 and trichrome staining were significantly higher in NOE than in control atria (18445.3±2527.9 vs 11228.4±2276.9 μm2/ mm2, 16063.6±1974 vs 9760.17±2576.8 μm2/mm2, 9304.3±633.1 vs 5994.7±1131.8 μm2/ mm2, 1019.9±189.8 vs 622.6±252.3 μm2/ mm2, 733.5±449.1 vs 117.4±53.4 μm2/ mm2, 7449.9±4274.7 vs 492.6±353.9 μm2/ mm2, all p≤0.01). Additionally, AF can be induced in 17% (2/12) of NOE but never in control mice (p=0.14). Noise exposure induces cardiac ANS remodeling, fibrosis and SK currents upregulation. It partially explains the mechanism of noise-induced atrial arrhythmia.