Peak Sodium Research Articles

Effects of phosphodiesterase-1 inhibitor on pulmonary vein electrophysiology and arrhythmogenesis.

Phosphodiesterase (PDE) isoform inhibitors have mechanical and electrical effects on the heart. Inhibition of PDE-1 enzymes is a novel strategy for treating heart failure. However, the electrophysiological effects of PDE-1 inhibition on the heart remain unclear. This study explored the effects of PDE-1 inhibition using ITI-214 on electrical activity in the pulmonary vein (PV), the most common trigger of atrial fibrillation, and investigated the underlying ionic mechanisms. Conventional microelectrodes or whole-cell patch clamps were employed to study the effects of ITI-214 (0.1-10μM) on PV electrical activity, mechanical responses and ionic currents in isolated rabbit PV tissue specimens and isolated single PV cardiomyocytes. ITI-214 at 1μM and 10μM (but not 0.1μM) significantly reduced PV spontaneous beating rate (10±2% and 10±3%, respectively) and PV diastolic tension (11±3% and 17±3%, respectively). ITI-24 (1μM) significantly reduced late sodium current (INa-Late ), L-type calcium current (ICa-L ) and the reverse mode of the sodium-calcium exchanger (NCX), but it did not affect peak sodium currents. ITI-214 reduces PV spontaneous activity and PV diastolic tension by reducing INa-Late , ICa-L and NCX current. Considering its therapeutic potential in heart failure, targeting PDE-1 inhibition may provide a novel strategy for managing atrial arrhythmogenesis.

Acute Transient Central Diabetes Insipidus: A Rare Case of DI Secondary to Vasopressin Withdrawal

Introduction: Transient Central Diabetes Insipidus (tCDI) induced by vasopressin withdrawal is a rare condition which is possibly under recognized. It occurs in very sick, often critically ill patients and usually complicates an already complex clinical picture, so early recognition and treatment are critical to reduce morbidity.Case Description: A 47-y/o male with a PMH of Coffin Lowry syndrome, atrial fibrillation, and GERD presented with abdominal pain and flu like symptoms. His home medications were metoprolol, loratadine and colestipol. Work up revealed bowel perforation for which he was taken to the OR for repair. Intraoperatively he developed septic shock requiring pressor support with norepinephrine and vasopressin. He was weaned off norepinephrine on post-op day (POD)1, and vasopressin on POD 2. Approximately three hours after withdrawal of vasopressin support his urine output increased dramatically up to a peak of 350 cc/hour with a recorded 24hr urine volume of >5L. Concurrently, his serum sodium was found to have increased from 147 mmol/L to 173 mmol/L (n 135-145) over the course of 13 hours. Clinically, he became increasingly lethargic with abnormal eye movements. His sodium did not improve with fluid management with D5W. His other laboratory values included a urine osmolality of 141 mOsm/kg, urine sodium of 60 mmol/L and a peak serum sodium of 177mmol/L. He was administered 1mcg desmopressin and his D5W rate was increased. His urine output dropped gradually to ~150cc/hr, his serum sodium level started to trend down to 168 mmol/L and his urine osmolality increased to 439 mOsm/kg five hours after desmopressin administration, with improvement in mental status. The patient received a total of two doses of desmopressin and continued support with IV fluids. His sodium eventually normalized, and his polyuria did not return.Discussion: This patient’s clinical picture is consistent with tCDI secondary to discontinuation of vasopressin. Transient Central diabetes insipidus due to vasopressin withdrawal is a phenomenon that is not well understood, but there is a strong male preponderance, and it tends to occur more commonly in patients with underlying neurological conditions, as did our patient. Current proposed mechanisms include decreased production and release of exogenous ADH due to negative feedback, down regulation of the V2 receptors, and hypoperfusion to the posterior pituitary. This condition deserves more investigation to better understand the incidence, risk factors and pathophysiological mechanisms.

Central Diabetes Insipidus Due to Acute Myeloid Leukemia

Abstract Central diabetes insipidus (CDI) is a condition characterized by decreased secretion of antidiuretic hormone (ADH) and is commonly seen with pathology involving the hypothalamic/pituitary area. Common etiologies include congenital disorders, neurosurgery or trauma, infiltrative disorders, primary or secondary cancers, and idiopathic causes. CDI associated with acute myeloid leukemia (AML) is extremely rare, with about 100 published case reports, and typically occurs in patients with chromosome 3 or 7 abnormalities. A 49-year-old woman was admitted to the inpatient oncology service for chemotherapy with gemtuzumab with a goal of inducing remission of her AML prior to allogeneic hematopoietic cell transplantation. She had been diagnosed with AML 7 months prior to this admission and was considered high risk for poor clinical outcome due to cytogenetics demonstrating deletion of chromosome 7 and inversion of chromosome 3. She was noted to have increasing serum sodium and endocrine was consulted for evaluation of hypernatremia. Patient had a peak sodium of 154 meq/L which improved with free water replacement. She reported excessive thirst with associated polyuria and nocturia. Laboratory tests demonstrated inappropriately dilute urine for the degree of serum osmolality, consistent with a low ADH tone. A water deprivation test was performed which resulted in plasma and urine osmolalities of 311 mosm/kg and 103 mosm/kg, respectively. After the administration of desmopressin the Uosm increased to 345 mosm/kg at 2 hours and 484 mosm/kg at 5 hours, confirming a diagnosis of Central DI. Evaluation of other pituitary axes showed normal TSH and FT4 with a normal cortisol response on a cosyntropin stimulation test. She had low-normal LH and FSH plus a mildly elevated prolactin level (38.7 ng/mL) which were attributed to stress. An MRI of the brain done to rule out any hypothalamic/pituitary metastasis or hypophysitis was normal. AML associated with Central DI occurs rarely, is associated with a poor prognosis, and the pathogenesis is unclear. The most commonly reported cytogenetic aberrations in patients with AML and CDI are monosomy 7 and 3q alterations. Our patient had monosomy 7 and chromosome 3 inversion. Proposed pathogenic mechanisms include leukemic infiltration of the pituitary, overexpression of ectopic virus integration (EVI-1) site interfering with hypothalamic neuroendocrine secretion, and abnormal thrombopoiesis interfering with ADH levels in blood. This case illustrates a lesser-known cause for central diabetes insipidus. While other more common etiologies of CDI should still be considered first, recognition of this association can provide clarity to the origin of DI in select patients.

Inhibitory effects of aloperine on voltage-gated Na+ channels in rat ventricular myocytes.

Aloperine (ALO), a quinolizidine alkaloid extracted from Sophora alopecuroides L., modulates hypertension, ventricular remodeling, and myocardial ischemia. However, few studies have evaluated the effects of ALO on other cardiovascular parameters. Accordingly, in this study, we used a rat model of aconitine-induced ventricular arrhythmia to assess the effects of ALO. Notably, ALO pretreatment delayed the onset of ventricular premature and ventricular tachycardia and reduced the incidence of fatal ventricular fibrillation. Moreover, whole-cell patch-clamp assays in rats' ventricular myocyte showed that ALO (3, 10, and 30 μM) significantly reduced the peak sodium current density of voltage-gated Na+ channel currents (INa) in a concentration-dependent manner. The gating kinetics characteristics showed that the steady-state activation and recovery curve were shifted in positive direction along the voltage axis, respectively, and the steady-state inactivation curve was shifted in negative direction along the voltage axis, i.e., which was similar to the inhibitory effects of amiodarone. These results indicated that ALO had anti-arrhythmic effects, partly attributed to INa inhibition. ALO may act as a class I sodium channel anti-arrhythmia agent.

Copeptin is increased by nausea and vomiting during hypertonic saline infusion in healthy individuals.

Measurement of hypertonic saline-stimulated copeptin has recently been described for the differentiation of polyuria-polydipsia syndrome. This study aims to determine the copeptin response to intravenous 3% hypertonic saline, including evaluation of adverse effects, in a local cohort of healthy adults >18years in Australia. Prospective clinical study. Twenty healthy volunteers (10 males and 10 females) were recruited. Participants underwent infusion of 3% hypertonic saline via a previously described standardized protocol, until the plasma sodium was ≥150mmol/L, with measurement of plasma copeptin. Mean peak sodium was 152mmol/L±SD 1.4 with osmolality 315mmol/kg±SD 3.9. Median volume of hypertonic saline infused to reach target sodium≥150mmol/L was 1536mL (IQR 1362, 1992). Mean rate of plasma sodium rise was 5.9mmol/L/hour±SD 1.5. Hypertonic saline-stimulated copeptin had non-parametrical distribution with median of 33.8pmol/L (IQR 27.6, 63.6). Overall median symptom burden was 6/10 (range 3/10-9/10). Copeptin was significantly higher for those who experienced nausea and/or vomiting (n=13) (median 39.0pmol/L; IQR 32.5, 90), compared to those participants who did not experience either (median 20.0pmol/L; IQR 13.0, 31.0) (P=0.003). There were no serious adverse events. Hypertonic saline-stimulated copeptin measurements were similar in our population compared with previously reported reference intervals in healthy volunteers. There is a wide range of stimulated copeptin measurements in the healthy population. Nausea and vomiting are common adverse effects which enhance the copeptin response.

Electronic Expression of Background and Delayed Rectifier Currents via Dynamic Clamp Produces Physiological Action Potentials in Stem Cell Derived Glutamatergic Neurons

Human stem-cell derived glutamatergic neurons (iPSC-GNs) are a model system for studying neurological disorders, such as epilepsy. However, progress for disease modelling in neuronal iPSC-GNs is hindered by the low expression of K+ currents. This results in depolarized resting membrane potentials (RMPs) which leads to the inability to evoke physiologically shaped action potentials (APs). Our goal was to overcome this limitation by developing a neuronal dynamic clamp system to electronically “express” K+ currents in real-time. iPSC-GNs (iCell GlutaNeurons from Fujifilm Cellular Dynamics, WI) were cultured according to the manufacturer's instructions and used 7-30 days after plating. Recordings were performed using the whole-cell ruptured patch clamp configuration. Dynamic clamp was implemented via the Cybercyte dynamic clamp system (Cytocybernetics, NY). iPSC-GNs had a peak sodium current at −20 mV (−198 ±21 pA/pF, n=32). The sodium current was blocked with tetrodotoxin. APs were triggered with 0.5-1.0 nA pulses for 0.3-1.5 ms at 0.5 Hz. Without electronic expression of a virtual K+ current, the RMP was depolarized (‑40 ± 2.1 mV, n=35). The electronic expression of an outwardly rectifying K+ leak current, modeled after Goldman-Hodgkin-Katz equation, allowed for the stabilization of the RMP to hyperpolarized potentials (∼55-60mV) and enabled the recording of evoked, stable and physiologically shaped APs. We also electronically expressed an A-type current which also caused hyperpolarization of the membrane potential and a physiologically shaped AP. The data shows that electronic addition of background and delayed rectifier K+ currents in hiPSC neurons allows for the unmasking of physiological AP properties that can be used for modelling neurological diseases.

Expression defect of the rare variant/Brugada mutation R1512W depends upon the SCN5A splice variant background and can be rescued by mexiletine and the common polymorphism H558R

ABSTRACT Background : Mutations in SCN5A that decrease Na current underlie arrhythmia syndromes such as the Brugada syndrome (BrS). SCN5A in humans has two splice variants, one lacking a glutamine at position 1077 (Q1077del) and one containing Q1077. We investigated the effect of splice variant background on loss-of-function and rescue for R1512W, a mutation reported to cause BrS. Methods and results : We made the mutation in both variants and expressed them in HEK-293 cells for voltage-clamp study. After 24 hours of transfection, the current expression level of R1512W was reduced by ~50% in both Q1077del and Q1077 compared to the wild-type (WT) channel, respectively. The activation and inactivation midpoint were not different between WT and mutant channels in both splice variant backgrounds. However, slower time constants of recovery and enhanced intermediate inactivation were observed for R1512W/Q1077 compared with WT-Q1077, while the recovery and intermediate inactivation parameters of R1512W/Q1077del were similar to WT-Q1077del. Furthermore, both mexiletine and the common polymorphism H558R restored peak sodium current (I Na) amplitude of the mutant channel by increasing the cell surface expression of SCN5A. Conclusion : These findings provide further evidence that the splice variant affects the molecular phenotype with implications for the clinical phenotype, and they provide insight into the expression defect mechanisms and potential treatment in BrS.

A novel proline-rich M-superfamily conotoxin that can simultaneously affect sodium, potassium and calcium currents.

BackgroundConotoxins have become a research hotspot in the neuropharmacology field for their high activity and specificity in targeting ion channels and neurotransmitter receptors. There have been reports of a conotoxin acting on two ion channels, but rare reports of a conotoxin acting on three ion channels. MethodsVr3a, a proline-rich M-superfamily conotoxin from a worm-hunting Conus varius, was obtained by solid-phase synthesis and identified by mass spectrometry. The effects of synthesized Vr3a on sodium, potassium and calcium currents were tested on rat DRG cells by patch clamp experiments. The further effects of Vr3a on human Cav1.2 and Cav2.2 currents were tested on HEK293 cells.Results About 10 μM Vr3a has no effects on the peak sodium currents, but can induce a ~10 mV shift in a polarizing direction in the current-voltage relationship. In addition, 10 μM Vr3a can increase 19.61 ± 5.12% of the peak potassium currents and do not induce a shift in the current-voltage relationship. An amount of 10 μM Vr3a can inhibit 31.26% ± 4.53% of the peak calcium currents and do not induce a shift in the current-voltage relationship. The IC50 value of Vr3a on calcium channel currents in rat DRG neurons is 19.28 ± 4.32 μM. Moreover, 10 μM Vr3a can inhibit 15.32% ± 5.41% of the human Cav1.2 currents and 12.86% ± 4.93% of the human Cav2.2 currents.ConclusionsVr3a can simultaneously affect sodium, potassium and calcium currents. This novel triple-target conotoxin Vr3a expands understanding of conotoxin functions.

CaMKII delta interaction with neuronal sodium channel Nav1.8 contributes to arrhythmogenic triggers in failing human and mouse cardiomyocytes

Abstract In heart failure, enhanced persistent current through neuronal sodium channel NaV1.8 (INaL) may induce influx of Na+ into cardiomyocytes. This may cause Ca2+ influx via the Na+/Ca2+ exchanger leading to increased proarrhythmogenic diastolic sarcoplasmic reticulum (SR) Ca2+ leak. This Ca2+ may activate Ca2+/calmodulin-dependent protein kinase IIδ (CaMKIIδ) which can induce INaL augmentation by phosphorylating NaV1.5 channels leading to a vicious cycle between INaL and CaMKIIδ. Here, we examined whether CaMKIIδ associates with NaV1.8 in human and mouse cardiomyocytes thereby regulating its function. Interaction and co-localisation of CaMKIIδ and NaV1.8 were confirmed by co-immunoprecipitation and immunocytochemistry. Whole-cell patch clamp showed a potent reduction of INaL after addition of novel specific Nav1.8 blockers, either A-803467 (30 nmol/L) or PF-01247324 (1 μmol/L) in failing mouse cardiomyocytes overexpressing CaMKIIδc (CaMKIIδc+/T: −109.4±10.6 vs A-803467: −56.9±11.7 and PF-01247324:−-69.9±8.6 A*ms*F-1). In failing human cardiomyocytes inhibition of either NaV1.8 or CaMKIIδ using AIP (1 μmol/L) or AIP and PF-01247324 together led to a significant and comparable decrease of INaL (control: −93.7±7.1 vs PF-01247324: −56.8±6.6; AIP: −44.2±6.6; AIP+PF-01247324: −39.8±5.4 A*ms*F-1). Furthermore, to confirm whether observed alterations in INaL after inhibition of NaV1.8 are not due to an overall reduction in peak sodium current (INa) we measured INa properties in mouse cardiomyocytes. Importantly, we observed no difference neither in the peak nor in inactivation between wild type (WT), WT with PF-01247324 and in mice lacking NaV1.8. Using confocal microscopy we investigated whether inhibition of the NaV1.8-mediated INaL could attenuate the increase of proarrhythmogenic SR Ca2+ spark frequency (CaSpF) caused by overexpression of CaMKIIδ in mice. We observed a significant reduction of CaSpF in both NaV1.8 inhibitor groups (PF-01247324: 0.51±0.08 and A-803467: 0.57±0.08 μm–1 s–1) compared to control (1.00±0.13 μm–1 s–1). Incubation of human failing cardiomyocytes with either AIP (0.35±0.06 μm–1 s–1) or PF-01247324 (0.44±0.11 μm–1 s–1), or blocking CaMKIIδ and NaV1.8 together (0.30±0.08 μm–1 s–1) resulted in significant decrease of CaSpF compared to control (0.89±0.13 μm–1 s–1). In conclusion, we show for the first time subcellular localisation of the neuronal sodium channel NaV1.8 and its interaction with CaMKIIδ in both human and mouse ventricular cardiomyocytes. Moreover, pharmacological inhibition of NaV1.8 caused a reduction of the augmented INaL and spontaneous diastolic SR-Ca2+ release in both failing human and mouse cardiomyocytes. NaV1.8 and CaMKIIδ interaction seem to play a relevant role for the generation of arrhythmogenic triggers (INaL & spontaneous diastolic SR-Ca2+ release) in both human and mouse cardiomyocytes from failing hearts. Funding Acknowledgement Type of funding source: None

The Impact of Hypernatremia in Preterm Infants on Neurodevelopmental Outcome at 18 Months of Corrected Age.

The study objective was to assess the correlation between hypernatremia during the first week of life and neurodevelopmental outcomes at 18 months of corrected age in premature infants. A retrospective observational study of preterm infants born at less than 32 weeks of gestation who had a neurodevelopmental assessment with the Bayley scales of infant and toddler development III at 18 ± 6 months of corrected age. Serum sodium levels from birth through 7 days of life were collected. The study cohort was divided into two groups: infants with a peak serum sodium of >145 mmol/L (hypernatremia group) and infants with a peak serum sodium level of <145 mmol/L (no hypernatremia group). Prenatal, intrapartum, and postnatal hospital course and neurodevelopmental data at 18 ± 6 months were collected. Logistic regression analysis was used to assess the correlation between neonatal hypernatremia and neurodevelopment with adjustment for selected population characteristics. Eighty-eight preterm infants with complete neurodevelopmental outcome data at 18 ± 6 months of corrected gestational age were included in the study. Thirty-five neonates were in the hypernatremia group and 53 were in the no hypernatremia group. Maternal and neonatal characteristics were similar between the two groups except that the hypernatremia group had a significantly lower average birth weight and gestational age. Comparison of the mean neurodevelopmental scores between the two groups showed that patients in the hypernatremia group as compared with those in the no hypernatremia group had significantly lower neurodevelopmental scaled scores in the fine motor domain (p = 0.01). This difference remained significant (p = 0.03, odds ratio [OR] = 0.8, 95% confidence interval [CI]: 0.6-0.97) when adjusted for birth weight and gestational age. Preterm infants born at less than 32 weeks of gestation with hypernatremia in the first week of life have lower fine motor scores at 18 months of corrected age. · Hypernatremia is a common electrolyte disturbance in preterm neonates.. · Hypernatremia may be associated with long-term neurodevelopmental outcomes in preterm infants.. · Hypernatremia is a potentially modifiable risk factor..

GS-967 and Eleclazine Block Sodium Channels in Human Induced Pluripotent Stem Cell–Derived Cardiomyocytes

GS-967 and eleclazine (GS-6615) are novel sodium channel inhibitors exhibiting antiarrhythmic effects in various in vitro and in vivo models. The antiarrhythmic mechanism has been attributed to preferential suppression of late sodium current (I NaL). Here, we took advantage of a high throughput automated electrophysiology platform (SyncroPatch 768PE) to investigate the molecular pharmacology of GS-967 and eleclazine on peak sodium current (I NaP) recorded from human induced pluripotent stem cell-derived cardiomyocytes. We compared the effects of GS-967 and eleclazine with the antiarrhythmic drug lidocaine, the prototype I NaL inhibitor ranolazine, and the slow inactivation enhancing drug lacosamide. In human induced pluripotent stem cell-derived cardiomyocytes, GS-967 and eleclazine caused a reduction of I NaP in a frequency-dependent manner consistent with use-dependent block (UDB). GS-967 and eleclazine had similar efficacy but evoked more potent UDB of I NaP (IC50 = 0.07 and 0.6 µM, respectively) than ranolazine (7.8 µM), lidocaine (133.5 µM), and lacosamide (158.5 µM). In addition, GS-967 and eleclazine exerted more potent effects on slow inactivation and recovery from inactivation compared with the other sodium channel blocking drugs we tested. The greater UDB potency of GS-967 and eleclazine was attributed to the higher association rates and moderate unbinding rate of these two compounds with sodium channels. We propose that substantial UDB contributes to the observed antiarrhythmic efficacy of GS-967 and eleclazine. SIGNIFICANCE STATEMENT: We investigated the molecular pharmacology of GS-967 and eleclazine on sodium channels in human induced pluripotent stem cell-derived cardiomyocytes using a high throughput automated electrophysiology platform. Sodium channel inhibition by GS-967 and eleclazine has unique effects, including accelerating the onset of slow inactivation and impairing recovery from inactivation. These effects combined with rapid binding and moderate unbinding kinetics explain potent use-dependent block, which we propose contributes to their observed antiarrhythmic efficacy.

Abstract 346: Cardiac Expression of Fetal and Adult SCN5A Isoforms in Sudden Unexplained Infant Death (SUID)

Sodium Voltage-Gated Channel Alpha Subunit 5 (SNC5A) plays a vital role in cardiac depolarization and has been implicated in Long QT Syndrome and sudden cardiac death. SCN5A encodes fetal and adult isoforms, differing by splicing of exon 6A or 6B, respectively. Fetal SCN5A decreases peak sodium current, leading to slower channel activation and inactivation and providing a potential substrate for arrhythmia. Little is known about developmental expression of these isoforms. We investigated developmental expression of SCN5A isoforms in infants, toddlers, and adults, including expression ratios in SUID cases and controls (0-12 months). We extracted RNA from heart tissue collected at autopsy (fetus-24 months) with 80 SUID cases and 27 controls, and 5 adult donor hearts. RNA Integrity Numbers ranged from 4.8-9.5. Relative SCN5A isoform expression was quantified by qRT-PCR. Non-parametric t-test and ANOVA were used; P&lt;0.05 was considered statistically significant. Our results show a significant stepwise increase in adult/fetal isoform expression with age. Relative expression of adult/fetal SCN5A increased significantly from fetal-4 months to 5-24 months (P&lt;0.0001), and again in adulthood (P&lt;0.0001). There was no significant difference between case and control groups at 0-4 or 0-12 months. Expression of adult/fetal SCN5A increases significantly from birth to 24 months of age and into adulthood. Relative adult/fetal SCN5A expression is lowest during the neonatal period through 4 months, corresponding with the peak incidence of SUID. Further investigation of protein expression of SCN5A isoforms may shed light onto the age-dependent incidence of sudden death.

Telethonin variants found in Brugada syndrome, J-wave pattern ECG, and ARVC reduce peak Nav 1.5 currents in HEK-293 cells.

Telethonin (TCAP) is a Z-disk protein that maintains cytoskeletal integrity and various signaling pathways in cardiomyocytes. TCAP is shown to modulate α-subunit of the human cardiac sodium channel (hNav 1.5) by direct interactions. Several TCAP variants are found in cardiomyopathies. We sought to investigate whether TCAP variants are associated with arrhythmia syndromes. Mutational analyses for TCAP were performed in 303 Japanese patients with Brugada syndrome, arrhythmogenic right ventricular cardiomyopathy, and J-wave pattern ECG. Using patch-clamp techniques, electrophysiological characteristics of hNav 1.5 were studied in HEK-293 cells stably expressing hNav 1.5 and transiently transfected with wild-type (WT) or variant TCAP. We identified two TCAP variants, c.145G>A:p.E49K and c.458G>A:p.R153H, in four individuals. p.E49K was found in two patients with ARVC or BrS. p.R153H was found in two patients with BrS or J-wave pattern ECG. No patient had variant hNav 1.5. Patch-clamp experiments demonstrated that peak sodium currents were significantly reduced in cells expressing p.R153H and p.E49K compared with WT-TCAP (66%, p.R153H; 72%, p.E49K). Voltage dependency of peak IV curve was rightward-shifted by 5mV in cells expressing p.E49K compared with WT-TCAP. Voltage dependency of activation was not leftward-shifted by p.R153H, while voltage dependency of steady-state inactivation was leftward-shifted by p.E49K. We found two TCAP variants in the patients with BrS, J-wave pattern ECG, and ARVC that can cause loss-of-function of the hNav 1.5 in heterologous expression systems. Our observation suggests that these variants might impair INa and be associated with the patients' electrophysiological phenotypes. Further studies linking our experimental data to clinical phenotypes are warranted.

E1784K, the most common Brugada syndrome and long-QT syndrome type 3 mutant, disrupts sodium channel inactivation through two separate mechanisms.

Inheritable and de novo variants in the cardiac voltage-gated sodium channel, Nav1.5, are responsible for both long-QT syndrome type 3 (LQT3) and Brugada syndrome type 1 (BrS1). Interestingly, a subset of Nav1.5 variants can cause both LQT3 and BrS1. Many of these variants are found in channel structures that form the channel fast inactivation machinery, altering the rate, voltage dependence, and completeness of the fast inactivation process. We used a series of mutants at position 1784 to show that the most common inheritable Nav1.5 variant, E1784K, alters fast inactivation through two separable mechanisms: (1) a charge-dependent interaction that increases the noninactivating current characteristic of E1784K; and (2) a hyperpolarized voltage dependence and accelerated rate of fast inactivation that decreases the peak sodium current. Using a homology model built on the NavPaS structure, we find that the charge-dependent interaction is between E1784 and K1493 in the DIII-DIV linker of the channel, five residues downstream of the putative inactivation gate. This interaction can be disrupted by a positive charge at position 1784 and rescued with the K1493E/E1784K double mutant that abolishes the noninactivating current. However, the double mutant does not restore either the voltage dependence or rates of fast inactivation. Conversely, a mutant at the bottom of DIVS4, K1641D, causes a hyperpolarizing shift in the voltage dependence of fast inactivation and accelerates the rate of fast inactivation without causing an increase in noninactivating current. These findings provide novel mechanistic insights into how the most common inheritable arrhythmogenic mixed syndrome variant, E1784K, simultaneously decreases transient sodium currents and increases noninactivating currents, leading to both BrS1 and LQT3.

Neurotoxicity of Tityus bahiensis (brown scorpion) venom in sympathetic vas deferens preparations and neuronal cells

Systemic scorpion envenomation is characterized by massive neurotransmitter release from peripheral nerves mediated primarily by scorpion venoms neurotoxins. Tityus bahiensis is one of the medically most important species in Brazil, but its venom pharmacology, especially regarding to peripheral nervous system, is poorly understood. Here, we evaluated the T. bahiensis venom activity on autonomic (sympathetic) neurotransmission by using a variety of approaches, including vas deferens twitch-tension recordings, electrophysiological measurements (resting membrane potentials, spontaneous excitatory junctional potentials and whole-cell patch-clamp), calcium imaging and histomorphological analysis. Low concentrations of venom (≤ 3 μg/mL) facilitated the electrically stimulated vas deferens contractions without affecting postsynaptic receptors or damaging the smooth muscle cells. Transient TTX-sensitive sustained contractions and resting membrane depolarization were mediated mainly by massive spontaneous ATP release. High venom concentrations (≥ 10 μg/mL) blocked the muscle contractions and induced membrane depolarization. In neuronal cells (ND7-23wt), the venom increased the peak sodium current, modified the current-voltage relationship by left-shifting the Nav-channel activation curve, thereby facilitating the opening of these channels. The venom also caused a time-dependent increase in neuronal calcium influx. These results indicate that the sympathetic hyperstimulation observed in systemic envenomation is presynaptically driven, probably through the interaction of α- and β-toxins with neuronal sodium channels.

Chloroquine or Hydroxychloroquine for COVID-19: Is Cardiotoxicity a Concern?

Chloroquine or Hydroxychloroquine for COVID-19: Is Cardiotoxicity a Concern?

SAT-235 Young Male with Persistent Central Diabetes Insipidus After a Car Accident

BACKGROUND: Central diabetes insipidus (CDI) occurs in 20% of cases of traumatic brain injury (TBI). Most cases of post-TBI CDI resolve within 2–5 days. Only 6% of long-term survivors of TBI have evidence of persistent CDI.1 We report a patient with persistent CDI after TBI. Clinical Case: A 27 year old male was referred for polyuria. Three months prior he was in a rollover motor vehicle accident. At that time, CT head revealed frontal and occipital contusions with scattered subarachnoid, subdural, and intraventricular blood. There was no evidence of skull fracture, mass-effect, or midline shift. On hospital day 4, his urine output increased to > 3L/day, with serum sodium of 146 mEq/L (n 135–145) and urine specific gravity of 1.015 (n 1.005–1.030). Repeat head CT revealed bilateral subdural hematomas causing mild mass effect. During the rest of his 2-month hospitalization, he continued to have polyuria with specific gravity as low as 1.005, and occasional hypernatremia, with a peak serum sodium of 149 mEq/L. Serum sodium on discharge was 144 mEq/L. On presentation to our clinic, his family and caretakers reported polydipsia, polyuria and nocturia. He had no history of diabetes mellitus or lithium use. On exam he was tachycardic but normotensive with no signs of dehydration. Neurologic exam was normal except for distractibility and impaired long- and short-term memory. After 3 hours of water deprivation, laboratory testing revealed serum sodium 150 mEq/L, serum osmolality 307 mOsmol/kg (n 270–295), urine osmolality 119 mOsmol/kg (n 300–900), and ADH 3 pmol/L (n </= 14); consistent with CDI. Oral desmopressin led to resolution of polydipsia and polyuria. Evaluation of anterior pituitary function was normal. Six months post TBI, CT head revealed increased left frontal subdural hematoma with effacement of the right lateral ventricle and 1cm left-to-right midline shift. A burr hole procedure was performed but CDI persisted. Conclusion: Animal studies have shown that neurohypophyseal apoptosis occurs by inducing intracranial hypertension lasting 12 hours or more.2 Persistent DI may herald rising intracranial pressure (ICP), as reflected by our patient’s case.1 Clinicians should be aware of the reciprocal association between increased ICP and persistent CDI following TBI.

SUN-297 Development of a Local Reference Range for Hypertonic Saline-Stimulated Copeptin

Differentiating between primary polydipsia and central diabetes insipidus (DI) can be challenging. The water deprivation test has traditionally been used to diagnosis DI, however has poor diagnostic accuracy (1). Direct measurement of anti-diuretic hormone (ADH) is limited clinically. Copeptin is the C-terminal glycoprotein moiety of ADH prohormone, and correlates well with plasma ADH. Unlike ADH, copeptin is easy to measure (2).Hypertonic saline stimulated copeptin measurements have recently been described for the diagnosis of central DI. A copeptin cut-off of >4.9 pmol/L has a diagnostic accuracy of 96.5% for distinguishing primary polydipsia from central DI (3). A copeptin assay has recently been established in our laboratory. Validation of hypertonic saline-stimulated copeptin concentrations in our local population is needed before this test can be used with confidence in patients presenting to our institution with polyuria-polydipsia syndrome. The aim of this study was to develop a local reference range for hypertonic saline-stimulated copeptin in healthy volunteers.Twenty healthy volunteers (10 male and 10 female) were recruited. Subjects underwent a hypertonic saline test, as previously described (3). Hypertonic saline (3%) was administered as an initial 250 mL bolus followed by 0.15 mL/kg/minute until a target serum sodium of ≥150 mmol/L was reached. At this time, blood was drawn for copeptin.Twelve healthy volunteers (7 females; 5 males) have undergone the study to date. Median age was 28 years (range 26-50); median body weight 75.7 kg (range 57.9 -94.5); median baseline plasma sodium 138 mmol/L (range 136 - 140) and median serum osmolality 289.5 (range 281-297). Median peak sodium was 152 mmol/L (range 150-154) with osmolality 314.5 mmol/kg (range 306-320). Median volume of hypertonic saline infused was 1583 mL (1230-2177) and median hypertonic saline stimulated copeptin was 29.2 pmol/L (9.6-167.4). Overall symptom burden was 5/10 (range 3/10-9/10). There were no serious adverse events.Development of a local reference range for hypertonic saline stimulated copeptin measurements will assist in interpretation of the test in our local population of patients presenting with polyuria-polydipsia syndrome.

Heritable arrhythmia syndromes associated with abnormal cardiac sodium channel function: ionic and non-ionic mechanisms.

The cardiac sodium channel NaV1.5, encoded by the SCN5A gene, is responsible for the fast upstroke of the action potential. Mutations in SCN5A may cause sodium channel dysfunction by decreasing peak sodium current, which slows conduction and facilitates reentry-based arrhythmias, and by enhancing late sodium current, which prolongs the action potential and sets the stage for early afterdepolarization and arrhythmias. Yet, some NaV1.5-related disorders, in particular structural abnormalities, cannot be directly or solely explained on the basis of defective NaV1.5 expression or biophysics. An emerging concept that may explain the large disease spectrum associated with SCN5A mutations centres around the multifunctionality of the NaV1.5 complex. In this alternative view, alterations in NaV1.5 affect processes that are independent of its canonical ion-conducting role. We here propose a novel classification of NaV1.5 (dys)function, categorized into (i) direct ionic effects of sodium influx through NaV1.5 on membrane potential and consequent action potential generation, (ii) indirect ionic effects of sodium influx on intracellular homeostasis and signalling, and (iii) non-ionic effects of NaV1.5, independent of sodium influx, through interactions with macromolecular complexes within the different microdomains of the cardiomyocyte. These indirect ionic and non-ionic processes may, acting alone or in concert, contribute significantly to arrhythmogenesis. Hence, further exploration of these multifunctional effects of NaV1.5 is essential for the development of novel preventive and therapeutic strategies.

Inhibitory effect of gallic acid on voltage-gated Na+ channels in rat cardiomyocytes.

Gallic acid (GA) has a protective effect on the cardiovascular system. To study its cardiac electrophysiological effects, voltage-gated Na+ channel currents (INa ) were recorded in rat cardiomyocytes using whole-cell patch clamp techniques. Moreover, the effects of GA on aconitine-induced arrhythmias were assessed using electrocardiograms in vivo. We found that the current-voltage characteristic curve (I-V curve) of INa significantly shifted in the presence of 1, 3, and 10μmol/L of GA. The peak sodium current density (INa -Peak) was reduced from -84.02±5.68 pA/pF to -65.78±3.96 pA/pF with 1μmol/L, -54.45±5.18 pA/pF with 3μmol/L, and -44.20±4.35 pA/pF with 10μmol/L, respectively. GA shifted the steady-state activation curve of INa and recovery curve to the right and the steady-state inactivation curve to the left. The observed inhibitory effect was comparable to that of amiodarone. GA pre-treatment significantly prolonged the onset of fatal ventricular fibrillation. Our results indicated that GA inhibited INa in rat ventricular myocytes and aconitine-induced arrhythmias in vivo. These results suggest the potential of GA for development as a novel anti-arrhythmic therapeutic.