Clamp Data Research Articles

Reverse-engineered models reveal differential membrane properties of autonomic and cutaneous unmyelinated fibers.

Unmyelinated C-fibers constitute the vast majority of axons in peripheral nerves and play key roles in homeostasis and signaling pain. However, little is known about their ion channel expression, which controls their firing properties. Also, because of their small diameters (~ 1 μm), it has not been possible to characterize their membrane properties using voltage clamp. We developed a novel library of isoform-specific ion channel models to serve as the basis functions of our C-fiber models. We then developed a particle swarm optimization (PSO) framework that used the isoform-specific ion channel models to reverse engineer C-fiber membrane properties from measured autonomic and cutaneous C-fiber conduction responses. Our C-fiber models reproduced experimental conduction velocity, chronaxie, action potential duration, intracellular threshold, and paired pulse recovery cycle. The models also matched experimental activity-dependent slowing, a property not included in model optimization. We found that simple conduction responses, characterizing the action potential, were controlled by similar membrane properties in both the autonomic and cutaneous C-fiber models, but complicated conduction response, characterizing the afterpotenials, were controlled by differential membrane properties. The unmyelinated C-fiber models constitute important tools to study autonomic signaling, assess the mechanisms of pain, and design bioelectronic devices. Additionally, the novel reverse engineering approach can be applied to generate models of other neurons where voltage clamp data are not available.

Label-free, non-contact determination of resting membrane potential using dielectrophoresis

Measurement of cellular resting membrane potential (RMP) is important in understanding ion channels and their role in regulation of cell function across a wide range of cell types. However, methods available for the measurement of RMP (including patch clamp, microelectrodes, and potential-sensitive fluorophores) are expensive, slow, open to operator bias, and often result in cell destruction. We present non-contact, label-free membrane potential estimation which uses dielectrophoresis to determine the cytoplasm conductivity slope as a function of medium conductivity. By comparing this to patch clamp data available in the literature, we have demonstratet the accuracy of this approach using seven different cell types, including primary suspension cells (red blood cells, platelets), cultured suspension cells (THP-1), primary adherent cells (chondrocytes, human umbilical mesenchymal stem cells), and adherent (HeLa) and suspension (Jurkat) cancer cell lines. Analysis of the effect of ion channel inhibitors suggests the effects of pharmaceutical agents (TEA on HeLa; DMSO and neuraminidase on red blood cells) can also be measured. Comparison with published values of membrane potential suggest that the differences between our estimates and values recorded by patch clamp are accurate to within published margins of error. The method is low-cost, non-destructive, operator-independent and label-free, and has previously been shown to allow cells to be recovered after measurement.

Functional characterization of native Piezo1 as calcium and magnesium influx pathway in human myeloid leukemia cells.

Piezo1 is a Ca2+-permeable mechanically activated ion channel that is involved in various physiological processes and cellular responses to mechanical stimuli. The study of biophysical characteristics of Piezo1 is important for understanding the mechanisms of its function and regulation. Stretch activation, a routine approach that is applied to stimulate Piezo1 activity in the plasma membrane, has a number of significant limitations that complicate precise single-channel analysis. Here, we aimed to determine pore properties of native Piezo1, specifically to examine permeation for physiologically relevant signaling divalent ions (calcium and magnesium) in human myeloid leukemia K562 cells using Piezo1-specific chemical agonist, Yoda1. Using a combination of low-noise single-current patch-clamp recordings of Piezo1 activity in response to Yoda1, we have determined single-channel characteristics of native Piezo1 under various ionic conditions. Whole-cell assay allowed us to directly measure Piezo1 single currents carried by Ca2+ or Mg2+ ions in the absence of other permeable cations in the extracellular solutions; unitary conductance values estimated at various concentrations of Mg2+ revealed strong saturation effect. Patch clamp data complemented with fluorescent imaging clearly evidenced Ca2+ and Mg2+ entry via native Piezo1 channel in human leukemia K562 cells. Mg2+ influx via Piezo1 was detected under quasi-physiological conditions, thus showing that Piezo1 channels could potentially provide the physiological relevant pathway for Mg2+ ion transport and contribute to the regulation of Mg2+-dependent intracellular signaling.

Single-cell ionic current phenotyping elucidates non-canonical features and predictive potential of cardiomyocytes during automated drug experiments.

All new drugs must go through preclinical screening tests to determine their proarrhythmic potential. While these assays effectively filter out dangerous drugs, they are too conservative, often misclassifying safe compounds as proarrhythmic. In this study, we attempt to address this shortcoming with a novel, medium-throughput drug-screening approach: we use an automated patch-clamp system to acquire optimized voltage clamp (VC) and action potential (AP) data from human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) at several drug concentrations (baseline, 3×, 10× and 20× the effective free plasma concentrations). With our novel method, we show correlations between INa block and upstroke slowing after treatment with flecainide or quinine. Additionally, after quinine treatment, we identify significant reductions in current during voltage steps designed to isolate If and IKs. However, we do not detect any IKr block by either drug, and upon further investigation, do not see any IKr present in the iPSC-CMs when prepared for automated patch experiments (i.e. in suspension) - this is in contrast to similar experiments we have conducted with these cells using the manual patch setup. In this study, we: (1) present a proof-of-concept demonstration of a single-cell medium-throughput drug study, and (2) characterize the non-canonical electrophysiology of iPSC-CMs when prepared for experiments in a medium-throughput setting. KEY POINTS: Human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) offer potential as an in vitro model to study the proarrhythmic potential of drugs, but insights from these cells are often limited by the low throughput of manual patch-clamp. In this study, we use a medium-throughput automated patch-clamp system to acquire action potential (AP) and complex voltage clamp (VC) data from single iPSC-CMs at multiple drug concentrations. A correlation between AP upstroke and INa transients was identified and drug-induced changes in ionic currents found. We also characterize the substantially altered physiology of iPSC-CMs when patched in an automated system, suggesting the need to investigate differences between manual and automated patch experiments.

NEURAL INFLAMMATION CONTRIBUTES TO VOLTAGE-GATED POTASSIUM CHANNELS DYSFUNCTION IN LUMBAR DORSAL ROOT GANGLIA IN CHRONIC HEART FAILURE RATS

Objective: Skeletal muscle afferent sensitization causes excessive sympatho-excitation and exercise intolerance during physical activity in the chronic heart failure (CHF) state. Our recent study reported that downregulated voltage-gated potassium (Kv) channels in lumbar dorsal root ganglia (DRGs) contribute to muscle afferent sensitization in CHF rats. However, the cellular mechanisms underlying downregulated Kv channels in CHF remain unknown. We hypothesized that chronic macrophage activation contributes to Kv channels dysfunction in the lumbar DRGs in CHF. Design and method: Myocardial infarction (MI) was produced by left coronary artery ligation in rats. Molecular experiments were carried out to assess macrophage activation, cytokines and Kv channel expression in L4-L6 DRGs post MI. In vitro co-culture of lipopolysaccharide (LPS)-pretreated RAW264.7 (activated M1 macrophage) with 50B11 DRG neurons were performed. Patch clamp experiments were performed to assess Kv channel activity in acutely isolated muscle afferent DRG neurons. Chronic oral administration of minocycline (Mino, 20 mg/kg/day) was used to suppress macrophage activation in L4-L6 DRGs. Results: Compared to sham rats, male MI rats exhibited a robust increase in the number of Iba1-positive macrophages and the protein expression of Interferon regulatory factor 8 (IRF8, a pro-inflammatory macrophage marker) in L4-L6 DRGs at 8 weeks post MI, which was largely prevented by Mino (Figure 1). SEM showed macrophage infiltration into lumbar DRGs 8 weeks post MI. PCR data suggested that the mRNA expressions of pro-inflammatory cytokines including IL1beta and IL6 in lumbar DRGs were significantly elevated in male rats at 8-weeks post MI compared to sham rats. Protein expression of Kv channels including Kv1.4, 3.4, 4.2 and 4.3, was significantly downregulated in male rat L4-L6 DRGs at 8 weeks post MI. This was largely attenuated by Mino. Patch clamp data demonstrated decreased Ito current in isolated male rat muscle afferent DRG neurons 8 weeks post MI, also attenuated by Mino. Moreover, LPS-pretreated BV2 cells or RAW264.7 (LPS was washed out 4 hours after exposure, co-cultured with 50B11 DRG neurons significantly downregulated several Kv isoforms in DRG neurons. Conclusions: Macrophage activation contributes to Kv channels dysfunction in lumbar DRGs in CHF male rats.

Mir155hg Accelerates Hippocampal Neuron Injury in Convulsive Status Epilepticus by Inhibiting Microglial Phagocytosis.

Convulsive status epilepticus (CSE) is a common critical neurological condition that can lead to irreversible hippocampal neuron damage and cognitive dysfunction. Multiple studies have demonstrated the critical roles that long non-coding RNA Mir155hg plays in a variety of diseases. However, less is known about the function and mechanism of Mir155hg in CSE. Here we investigate and elucidate the mechanism underlying the contribution of Mir155hg to CSE-induced hippocampal neuron injury. By applying high-throughput sequencing, we examined the expression of differentially expressed genes in normal and CSE rats. Subsequent RT-qPCR enabled us to measure the level of Mir155hg in rat hippocampal tissue. Targeted knockdown of Mir155hg was achieved by the AAV9 virus. Additionally, we utilized HE and Tunel staining to evaluate neuronal injury. Immunofluorescence (IF), Golgi staining, and brain path clamping were also used to detect the synaptic plasticity of hippocampal neurons. Finally, through IF staining and Sholl analysis, we assessed the degree of microglial phagocytic function. It was found that the expression of Mir155hg was elevated in CSE rats. HE and Tunel staining results showed that Mir155hg knockdown suppressed the hippocampal neuron loss and apoptosis followed CSE. IF, Golgi staining and brain path clamp data found that Mir155hg knockdown enhanced neuronal synaptic plasticity. The results from IF staining and Sholl analysis showed that Mir155hg knockdown enhanced microglial phagocytosis. Our findings suggest that Mir155hg promotes CSE-induced hippocampal neuron injury by inhibiting microglial phagocytosis.

Single-cell ionic current phenotyping explains stem cell-derived cardiomyocyte action potential morphology.

Human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) are a promising tool to study arrhythmia-related factors, but the variability of action potential (AP) recordings from these cells limits their use as an in vitro model. In this study, we use recently published brief (10 s), dynamic voltage-clamp (VC) data to provide mechanistic insights into the ionic currents contributing to AP heterogeneity; we call this approach rapid ionic current phenotyping (RICP). Features of this VC data were correlated to AP recordings from the same cells, and we used computational models to generate mechanistic insights into cellular heterogeneity. This analysis uncovered several interesting links between AP morphology and ionic current density: both L-type calcium and sodium currents contribute to upstroke velocity, rapid delayed rectifier K+ current is the main determinant of the maximal diastolic potential, and an outward current in the activation range of slow delayed rectifier K+ is the main determinant of AP duration. Our analysis also identified an outward current in several cells at 6 mV that is not reproduced by iPSC-CM mathematical models but contributes to determining AP duration. RICP can be used to explain how cell-to-cell variability in ionic currents gives rise to AP heterogeneity. Because of its brief duration (10 s) and ease of data interpretation, we recommend the use of RICP for single-cell patch-clamp experiments that include the acquisition of APs.NEW & NOTEWORTHY We present rapid ionic current phenotyping (RICP), a current quantification approach based on an optimized voltage-clamp protocol. The method captures a rich snapshot of the ionic current dynamics, providing quantitative information about multiple currents (e.g., ICa,L, IKr) in the same cell. The protocol helped to identify key ionic determinants of cellular action potential heterogeneity in iPSC-CMs. This included unexpected results, such as the critical role of IKr in establishing the maximum diastolic potential.

Inhibitory effect of daidzein on the calcium-activated chloride channel TMEM16A and its anti-lung adenocarcinoma activity

TMEM16A is highly expressed in a variety of tumor cells and is involved in the growth and metastasis of malignancies. It has been established that down-regulation of TMEM16A expression or functional activity can inhibit tumor cells growth. However, there is a lack of targeted inhibitors with high efficiency and low toxicity. Here, we identified a novel inhibitor daidzein from dozens of natural product molecules. Whole-cell patch clamp data indicated that daidzein inhibits TMEM16A channel in a dose-dependent manner, with IC50 of 1.39 ± 0.59 μM. Western blot result showed that daidzein can also reduce the expression of TMEM16A protein in LA795 cells. These results indicated that the inhibitory effects of daidzein exert on TMEM16A in two ways, both inhibiting TMEM16A current and decreasing its protein expression. In addition, the putative binding sites of daidzein on TMEM16A are G608, G628, and K839 through molecular docking. Moreover, daidzein concentration-dependently reduced cell viability and cell migration, causing G1/S cell cycle arrest in vitro. It was also confirmed that daidzein can effectively inhibit the growth of LA795 lung adenocarcinoma cells implanted nude mice in vivo. In conclusion, daidzein can be used as a lead compound for the development of therapeutic drugs for lung adenocarcinoma.

The Action Potential Clamp Technique as a Tool for Risk Stratification of Sinus Bradycardia Due to Loss-of-Function Mutations in HCN4: An In Silico Exploration Based on In Vitro and In Vivo Data.

These days, in vitro functional analysis of gene variants is becoming increasingly important for risk stratification of cardiac ion channelopathies. So far, such risk stratification has been applied to SCN5A, KCNQ1, and KCNH2 gene variants associated with Brugada syndrome and long QT syndrome types 1 and 2, respectively, but risk stratification of HCN4 gene variants related to sick sinus syndrome has not yet been performed. HCN4 is the gene responsible for the hyperpolarization-activated 'funny' current If, which is an important modulator of the spontaneous diastolic depolarization underlying the sinus node pacemaker activity. In the present study, we carried out a risk classification assay on those loss-of-function mutations in HCN4 for which in vivo as well as in vitro data have been published. We used the in vitro data to compute the charge carried by If (Qf) during the diastolic depolarization phase of a prerecorded human sinus node action potential waveform and assessed the extent to which this Qf predicts (1) the beating rate of the comprehensive Fabbri-Severi model of a human sinus node cell with mutation-induced changes in If and (2) the heart rate observed in patients carrying the associated mutation in HCN4. The beating rate of the model cell showed a very strong correlation with Qf from the simulated action potential clamp experiments (R2 = 0.95 under vagal tone). The clinically observed minimum or resting heart rates showed a strong correlation with Qf (R2 = 0.73 and R2 = 0.71, respectively). While a translational perspective remains to be seen, we conclude that action potential clamp on transfected cells, without the need for further voltage clamp experiments and data analysis to determine individual biophysical parameters of If, is a promising tool for risk stratification of sinus bradycardia due to loss-of-function mutations in HCN4. In combination with an If blocker, this tool may also prove useful when applied to human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) obtained from mutation carriers and non-carriers.

Independent euglycaemic hyperinsulinaemic clamp studies validate clinically applicable formulae to estimate insulin sensitivity in people with type 1 diabetes

Background and aimLow insulin sensitivity (IS) increases Type 1 diabetes (T1D) complication risk and can be estimated by simple formulae developed from complex euglycemic hyperinsulinaemic clamp studies. We aimed to validate these formulae using independent clamp data. MethodsClamps were performed in 104 T1D adults. Measured glucose disposal rate (GDR) was correlated with eGDR and eLog10 M/I calculated by five IS formulae. ResultsCorrelations ranged between 0.23–0.40. Two IS formulae (by the authors), using age, sex, HDL-C, HbA1c, pulse pressure, BMI, and waist-hip-ratio had the highest correlation with measured GDR and the best performance in detecting low IS.

Modeling the kinetics of heteromeric potassium channels.

Mechanistic mathematical modeling has long been used as a tool for answering questions in cellular physiology. To mathematically describe cellular processes such as cell excitability, volume regulation, neurotransmitter release, and hormone secretion requires accurate descriptions of ion channel kinetics. One class of ion channels currently lacking a physiological model framework is the class of channels built with multiple different potassium protein subunits called heteromeric voltage gated potassium channels. Here we present a novel mathematical model for heteromeric potassium channels that captures both the number and type of protein subunits present in each channel. Key model assumptions are validated by showing our model is the reduction of a Markov model and through observations about voltage clamp data. We then show our model's success in replicating kinetic properties of concatemeric channels with different numbers of K v 1.1 and K v 1.2 subunits. Finally, through comparisons with multiple expression experiments across multiple voltage gated potassium families, we use the model to make predictions about the importance and effect of genetic mutations in heteromeric channel formation.

Electrophysiological remodeling in tachycardia-induced cardiomyopathy

Abstract Background Tachycardia-induced cardiomyopathy (TCM) is a reversible and likely underrecognized form of heart failure. Thus, a better understanding of the TCM-pathophysiology is warranted as the underlying early mechanisms that mediate the progression of TCM remain unclear. Purpose This study aimed to identify the cellular mechanisms of TCM. Methods and results Human induced pluripotent stem cell cardiomyocytes (iPSC-CM) were utilized as a translational human-based model. We performed chronic tachycardic (120 bpm) or normofrequent (control, 60bpm) cell culture pacing to study cellular changes during TCM progression. Already after 24h of tachycardic stimulation of iPSC-CM, we detected a decrease in Ca transient amplitude compared to control (Fura-2, n=49/44 cells/9 differentiations). Diastolic Ca levels and cytosolic Ca elimination were not affected after 24h of tachycardia (n=49/44/9). We detected no difference in sarcoplasmic reticulum (SR) Ca load (assessed via caffeine application) or SERCA activity (Ksys-Kcaff) after 24h of tachycardia (n=13/15/5). However, demonstrating the progress of TCM, 7d of tachycardia resulted in progressive decline of Ca transient amplitude together with an impaired Ca elimination, while diastolic Ca concentration was unchanged (n=73/66/8). These changes may underlie the reduced systolic force and impaired relaxation in TCM. We could explain these results by a significantly reduced SR Ca load and a diminished SERCA activity after 7d tachycardia (n=13/7 vs. 13/4). Using confocal microscopy (Fluo-4) we detected no difference in SR Ca spark frequency after 24h of tachycardia (n=82/66/8), while 7d of tachycardia caused an increase of Ca spark frequency (n=76/79/7), which is a typical hallmark of maladaptive remodeling in HF and likely underlie the reduced SR Ca load. Voltage clamp data of late Na current (INaL) showed no difference in INaL after 24h of stimulation (n=17/6 vs. 19/7), whereas INaL was increased after 7d of tachycardia (n=26/7 vs. 19/6). Accordingly, whole-cell current clamp experiments revealed a prolongation of the action potential after 7d of tachycardia compared to control (n=21/6 vs. 19/5), while no difference of action potential duration could be detected after 24h (n=37/31/8). Resting membrane potential and action potential amplitude were not changed. Finally, we investigated tachycardia-mediated effects on explanted human failing hearts. 8h of tachycardic stimulation (120 bpm) of human failing ventricular trabeculae already compromised systolic force, and diastolic tension and relaxation time were markedly increased compared to control (60bpm, n=8/6 trabeculae /7/6 human hearts). Conclusion This study demonstrates that persistent tachycardia adversely alters cardiomyocyte excitation-contraction coupling via electrophysiological cellular remodeling. Our translational investigation in human myocardium may help to understand the pathophysiology of an underrated but prevalent disease. Funding Acknowledgement Type of funding sources: Foundation. Main funding source(s): Else Kröner-Fresenius-Stiftung (EKFS)Deutsche Gesellschaft für Innere Medizin

Use of high throughput ion channel profiling and statistical modeling to predict off-target arrhythmia risk - One pharma's experience and perspective

IntroductionThe use of high throughput patch clamp profiling to determine mixed ion channel-mediated arrhythmia risk was assessed using profiling data generated using proprietary internal and clinical reference compounds. We define the reproducibility of the platform and highlight inherent platform issues. The data generated was used to develop predictive models for cardiac arrhythmia risk, specifically Torsades de Pointes (TdP). MethodsA retrospective analysis was performed using profiling data generated over a 3-year period, including patch clamp data from hERG, Cav1.2, and Nav1.5 (peak/late), together with hERG binding. ResultsAssay reproducibility was robust over the 3-year period examined. High throughput hERG patch IC50 values correlated well with GLP-hERG data (Pearson = 0.87). A disconnect between hERG binding and patch was observed for ∼10% compounds and trended with passive cellular permeability. hERG and Cav1.2 potency did not correlate for proprietary compounds, with more potent hERG compounds showing selectivity versus Cav1.2. For clinical compounds where hERG and Cav1.2 activity was more balanced, an analysis of TdP risk versus hERG/Cav1.2 ratio demonstrated low TdP probability when the hERG/Cav1.2 potency ratios were < 1. Modeling of clinical compound data revealed a lack of impact of the Nav1.5 (late) current in predicting TdP. Moreover, models using hERG binding data (ROC AUC = 0.876) showed an improved ability to predict TdP risk versus hERG patch clamp (ROC AUC = 0.787). DiscussionThe data highlight the value of high throughput patch clamp data in the prediction of TdP risk, as well as some potential limitations with this approach.

684-P: Tracking Type 2 Diabetes Progression Using CGM Systems

Disposition index (DI) is an index quantifying the quality of glucose control, usable to track disease progression. It is usually calculated as the product of beta-cell function and insulin sensitivity, assessed, in hospitalized setting, either from clamp data, surrogate indices or model-based methods, like the Oral Minimal Model (OMM) . The increasing use of continuous glucose monitoring (CGM) systems, even in individuals with type 2 diabetes (T2D) , opens the door to the use of such devices to monitor glucose control in outpatients. Here, we propose a method to assess DI from CGM data, validated it against OMM and proved its usability to track disease progression. We studied 100 early stage (ES) and 100 advanced stage (AS) subjects with T2D of the Padova T2D simulator, matched for age (54 [42,75] y) and different BMI (33 [29,37] vs. 31 [28,34] kg/m2; p=0.016) , which underwent a mixed meal challenge (75g of carbohydrates) while wearing CGM. Plasma glucose, insulin and C-peptide concentrations were measured for 6 hours after the meal and used for the estimation of the DI with the OMM (DIMM) , while CGM sensor data were used to calculate DI with the sensor-based method (DISB) . DISB well correlated with DIMM (ρES = 0.81 and ρAS = 0.77; p&amp;lt;0.001) and both were significantly lower in AD than ES (p&amp;lt;0.001, Fig. 1) . We conclude that DISB is usable to assess DI in subjects with different stage of T2D wearing CGM and thus, potentially, to track disease progression in noninvasive way. Disclosure M.Schiavon: None. C.Dalla man: Research Support; Becton, Dickinson and Company, Sanofi-Aventis Deutschland GmbH. Funding MIUR (Italian Minister for Education) under the initiative "Departments of Excellence" (Law 232/2016)

Abstract 11153: Massively Parallel, High-Throughput Approach for Identifying Defective KCNH2 (hERG) Variants

Introduction: Individuals with congenital long QT syndrome (LQTS) are at high risk for the life-threatening arrhythmia torsades de pointes . Loss-of-function KCNH2 (aka hERG ) variants that reduce repolarizing current, I Kr , are responsible for &gt;30% of LQTS cases with ~6% of sudden death in the young and ~1% in infants. Most KCNH2 variants are observed in some individual(s) making it difficult to obtain clinical evidence to assign pathogenicity; &gt;80% of KCNH2 missense variants in ClinVar are listed as variants of uncertain significance. To help adjudicate variants lacking in compelling clinical data, we demonstrate a method to characterize KCNH2 variant dysfunctions. Methods: We combined deep mutational scanning with a HA tagged KCNH2 construct to enable massively parallel characterization of channel trafficking of missense variants in KCNH2 . We have applied this method to the first 160 residues of KCNH2 ; 3,123 missense variants in total, a known hotspot for disease causing variants. For a subset of 458 non-redundant single nucleotide missense variants, we compared trafficking data, and in silico classifiers, with patch clamp data collected on the SyncroPatch platform. Results: Trafficking data is better than in silico features at predicting peak tail I Kr values, with an area under the receiver operating characteristic curve of &gt; 0.9 for trafficking data versus 0.8 for in silico feature. The Spearman correlation coefficients were 0.73 [0.67-0.78] compared to 0.51 [0.44-0.58] for trafficking and in silico predictions respectively. Our assay identified substitution sensitive regions, surrounded by more permissive residues. We also found substitution intolerant sites involved in networked hydrogen-bonding. Conclusion: A massively parallel trafficking assay can recapitulate most KCNH2 defects more effectively than in silico methods. Extended application of our method will enable the assessment of all missense variants in KCNH2 and other cardiac ion channels.

TRPV3 and TRPV4 as candidate proteins for intestinal ammonium absorption.

Absorption of ammonia from the gut has consequences that range from encephalitis in hepatic disease to global climate change induced by nitrogenous excretions from livestock. Since patch clamp data show that certain members of the transient receptor potential (TRP) family are permeable to NH4+ , participation in ammonium efflux was investigated. Digesta, mucosa and muscular samples from stomach, duodenum, jejunum, ileum, caecum and colon of pigs were analysed via colourimetry, qPCR, Western blot, immunohistochemistry and Ussing chambers. qPCR data show high duodenal expression of TRPV6. TRPM6 was highest in jejunum and colon, with expression of TRPM7 ubiquitous. TRPM8 and TRPV1 were below detection. TRPV2 was highest in the jejunum but almost non-detectable in the colon. TRPV4 was ubiquitously expressed by mucosal and muscular layers. TRPV3 mRNA was only found in the mucosa of the caecum and colon, organs in which NH4+ was highest (>7mmol·L-1 ). Immunohistochemically, an apical expression of TRPV3 and TRPV4 could be detected in all tissues, with effects of 2-APB and GSK106790A supporting functional expression. In symmetrical NaCl Ringer, removal of mucosal Ca2+ and Mg2+ increased colonic short circuit current (Isc ) and conductance (Gt ) by 0.18±0.06 µeq·cm-2 ·h-1 and 4.70±0.85 mS·cm-2 (P<.05, N/n=4/17). Application of mucosal NH4 Cl led to dose-dependent and divalent-sensitive increases in Gt and Isc , with effects highest in the caecum and colon. We propose that TRP channels contribute to the intestinal transport of ammonium, with TRPV3 and TRPV4 promising candidate proteins. Pharmacological regulation may be possible.

625-P: Validation of Formulae to Estimate Insulin Sensitivity in Type 1 Diabetes

Introduction: The “gold standard” measure of insulin sensitivity (IS), a euglycemic hyperinsulinemic clamp, is costly, time- and labour-intensive. Several formulae, developed using clamp data estimate insulin sensitivity, expressed as estimated glucose disposal rate (eGDR) or insulin sensitivity index (eM/I or elog10M/I). Due to clamp complexity and cost these formulae often lacks independent validation.Aim: To validate several formulae estimating IS using independent euglycaemic hyperinsulinemic clamp data.Methods: Euglycemic, hyperinsulinemic clamps were performed in 108 T1D adults (age (mean±SD) 34±7 yrs, T1D duration 10±4 yrs, HbA1c 7.7±1.5%; 33 with microvascular complications). Measured GDR (last 30min of the clamp) ranged 0.5 - 9.6 mg/kg/min (median (LQ, UQ) 4.45 (3.04, 6.5 mg/kg/min) were compared with eGDR and eLog10M/I calculated using simple formulae by (1) Williams, (2) Zheng, (3) Dabelea and (4) and (5) (Australia (AU)) authors, all derived from routine clinical chemistry and demographics. eGDR formula (Duca) was not assessed as it includes adiponectin levels.Results: Results are in Table 1.Conclusion: Authors (AU) formulae to estimate IS, including age, sex, HDL-C, BMI, HbA1c, pulse pressure and WHR had highest correlation with measured GDR and better performance (AUROC) in detecting low IS than other formulae.View largeDownload slideView largeDownload slide DisclosureA. S. Januszewski: None. P. Niedzwiecki: None. N. Sachithanandan: None. G. M. Ward: None. C. Karschimkus: None. D. N. O’neal: Advisory Panel; Self; Abbott Diabetes, Medtronic, Sanofi, Research Support; Self; Dexcom, Inc., GlySens Incorporated, Medtronic, Pacific Diabetes Technologies. D. Zozulinska-ziolkiewicz: None. A. Uruska: None. A. Jenkins: Advisory Panel; Self; Abbott Diabetes, Sanofi-Aventis, Other Relationship; Self; Amgen Inc., Research Support; Self; Abbott, International Diabetes Federation, JDRF, Medtronic, Mylan N. V.

Physiological Roles of the Rapidly Activated Delayed Rectifier K+ Current in Adult Mouse Heart Primary Pacemaker Activity.

Robust, spontaneous pacemaker activity originating in the sinoatrial node (SAN) of the heart is essential for cardiovascular function. Anatomical, electrophysiological, and molecular methods as well as mathematical modeling approaches have quite thoroughly characterized the transmembrane fluxes of Na+, K+ and Ca2+ that produce SAN action potentials (AP) and ‘pacemaker depolarizations’ in a number of different in vitro adult mammalian heart preparations. Possible ionic mechanisms that are responsible for SAN primary pacemaker activity are described in terms of: (i) a Ca2+-regulated mechanism based on a requirement for phasic release of Ca2+ from intracellular stores and activation of an inward current-mediated by Na+/Ca2+ exchange; (ii) time- and voltage-dependent activation of Na+ or Ca2+ currents, as well as a cyclic nucleotide-activated current, If; and/or (iii) a combination of (i) and (ii). Electrophysiological studies of single spontaneously active SAN myocytes in both adult mouse and rabbit hearts consistently reveal significant expression of a rapidly activating time- and voltage-dependent K+ current, often denoted IKr, that is selectively expressed in the leading or primary pacemaker region of the adult mouse SAN. The main goal of the present study was to examine by combined experimental and simulation approaches the functional or physiological roles of this K+ current in the pacemaker activity. Our patch clamp data of mouse SAN myocytes on the effects of a pharmacological blocker, E4031, revealed that a rapidly activating K+ current is essential for action potential (AP) repolarization, and its deactivation during the pacemaker potential contributes a small but significant component to the pacemaker depolarization. Mathematical simulations using a murine SAN AP model confirm that well known biophysical properties of a delayed rectifier K+ current can contribute to its role in generating spontaneous myogenic activity.

Excitability and propagation of the electrical impulse in Venus flytrap; a comparative electrophysiological study of unipolar electrograms with myocardial tissue

Mammalian heart cells and cells of leaves of Dionaea muscipula share the ability to generate propagated action potentials, because the excitable cells are electrically coupled. In the heart the propagated action potential causes synchronized contraction of the heart muscle after automatic generation of the impulse in the sinus node. In Dionaea propagation results in closure of the trap after activation of trigger hairs by an insect. The electrical activity can be recorded in the extracellular space as an extracellular electrogram, resulting from transmembrane currents. Although the underlying physiological mechanism that causes the electrogram is similar for heart and Dionaea cells, the contribution of the various ions to the transmembrane current is different. We recorded extracellular electrograms from Dionaea leaves and compared the recorded signals with those known from the heart. The morphology of the electrograms differed considerably. In comparison to activation in mammalian myocardium, electrograms of Dionaea are more temporally and spatially variable. Whereas electrograms in healthy myocardium recorded at some distance from the site of activation reveal a simple biphasic pattern, Dionaea activation showed positive, negative or biphasic deflections. Comparison of patch clamp data from plant cells and cardiomyocytes suggests a role of temperature and ion concentrations in extracellular space for the diversity of morphologies of the Dionaea electrograms.

Robust Hidden Markov Model Selection by Bayesian Network-Based Algorithms Embracing Various Problems of Experimental Data

We present results of a Bayesian network analysis method for posterior estimation of the rates of a linear stochastic network for ion channel data. The Bayesian network is an analytical stochastic approximation of the underlying master equation, enabling a precise modeling of stochastic processes that are traditionally modeled by deterministic rate equations. The full Bayesian treatment of macroscopic patch clamp (PC) and related fluorescence data has superior model selection and inference properties over traditional maximum likelihood techniques and deterministic rate equations.