Action Potential Conduction Velocity Research Articles

Arrhythmic effects of Epac-mediated ryanodine receptor activation in Langendorff-perfused murine hearts are associated with reduced conduction velocity.

SummaryRecent papers have attributed arrhythmic substrate in murine RyR2‐P2328S hearts to reduced action potential (AP) conduction velocities (CV), reflecting acute functional inhibition and/or reduced expression of sodium channels. We explored for acute effects of direct exchange protein directly activated by cAMP (Epac)‐mediated ryanodine receptor‐2 (RyR2) activation on arrhythmic substrate and CV. Monophasic action potential (MAP) recordings demonstrated that initial steady (8 Hz) extrinsic pacing elicited ventricular tachycardia (VT) in 0 of 18 Langendorff‐perfused wild‐type mouse ventricles before pharmacological intervention. The Epac activator 8‐CPT (8‐(4‐chlorophenylthio)‐2′‐O‐methyladenosine‐3′,5′‐cyclic monophosphate) (VT in 1 of 7 hearts), and the RyR2 blocker dantrolene, either alone (0 of 11) or with 8‐CPT (0 of 9) did not then increase VT incidence (P>.05). Both progressively increased pacing rates and programmed extrasystolic (S2) stimuli similarly produced no VT in untreated hearts (n=20 and n=9 respectively). 8‐CPT challenge then increased VT incidences (5 of 7 and 4 of 8 hearts respectively; P<.05). However, dantrolene, whether alone (0 of 10 and 1 of 13) or combined with 8‐CPT (0 of 10 and 0 of 13) did not increase VT incidence relative to those observed in untreated hearts (P>.05). 8‐CPT but not dantrolene, whether alone or combined with 8‐CPT, correspondingly increased AP latencies (1.14±0.04 (n=7), 1.04±0.03 (n=10), 1.09±0.05 (n=8) relative to respective control values). In contrast, AP durations, conditions for 2:1 conduction block and ventricular effective refractory periods remained unchanged throughout. We thus demonstrate for the first time that acute RyR2 activation reversibly induces VT in specific association with reduced CV.

Enhanced in vivo survival of Schwann cells by a synthetic oxygen carrier promotes sciatic nerve regeneration and functional recovery.

Local hypoxia in the early stages of peripheral nerve injury is a challenge for axonal regeneration. To address this issue, perfluorotributylamine (PFTBA)-based oxygen carrying fibrin hydrogel was prepared and injected into Schwann cell (SC)-seeded collagen-chitosan conduits to increase oxygen supply to SCs within the conduits. The conduit containing PFTBA-SC gel was then applied to bridge a 15-mm sciatic nerve defect in rats. It was observed that most of the GFP-labeled SCs initially seeded in the PFTBA hydrogel remained alive for approximately 28days after their in vivo implantation. The number of SCs was significantly higher in the PFTBA-SC scaffold than that in the SC scaffold without PFTBA. In addition, nerve regeneration and functional recovery were examined after nerve injury repair. We found that the PFTBA-SC scaffold was capable of promoting axonal regeneration and remyelination of the regenerated axons. Further studies showed the PFTBA-SC scaffold was able to accelerate the recovery of motor and sensory function of the regenerating nerves. Electrophysiological analysis showed area under the curve of compound muscle action potential and nerve conduction velocity were also improved, and gastrocnemius muscle atrophy was partially reversed by PFTBA-SC scaffold. Furthermore, microvessel density analysis showed PFTBA-SC composites were beneficial for microvascular growth, which provided sustained oxygen for regenerating nerve in the later stages of nerve regeneration. In conclusion, enhanced survival of SCs by PFTBA is capable of promoting sciatic nerve regeneration and functional recovery, which provides a new avenue for achieving better functional recovery in the treatment of peripheral nerve injuries. Copyright © 2016 John Wiley & Sons, Ltd.

Optical Mapping in Rat Models of Atrial Dilation

Atrial fibrillation (AF) is commonly associated with atrial dilatation caused by pressure or volume overload. Acute atrial stretch may create a myocardial substrate to promote AF via stretch-activated channels and mechano-electrical feedback mechanisms. However, enhanced AF vulnerability in chronic dilated atria occurs as a result of complex remodeling that includes changes of myocyte membrane currents, altered intracellular Ca2+ homeostasis and alterations of the extracellular matrix (e.g. interstitial fibrosis). Here, we dissect the role of acute stretch-related changes versus those occurring as part of the long-term remodeling process. Spontaneous hypertensive rats (SHR) with chronic atrial dilation were compared to normotensive Wistar-Kyoto rats (WKY) and to WKY rats with acute biatrial dilation (dil-WKY) obtained with atrial balloons (inserted into both atria and inflated with controlled pressure after baseline recording). Briefly, the hearts were perfused on a Langendorff's apparatus and then the ventricles were excised just below the atrio-ventricular junction. To maximize atrial perfusion and staining main coronary ventricular branches were cauterized. Atria were stained with a bolus injection of a red-shifted voltage-sensitive dye (di-4-ANBDQPQ). An ultrafast wide-field macroscope operating at 2 KHz (100 x 100 pixel) was developed to optically map action potential propagation of Langendorff's perfused-atria and to test AF vulnerability. AF was induced through burst pacing and occurred in 5.8% of WKY, 10.6% of dil-WKY and 17.4% of SHR atria. Action potential duration at 90% repolarization (APD90) was prolonged in SHR as compared to WKY and dil-WKY. APD90 variability between contiguous regions was increased in both dil-WKY and SHR compared to WKY, potentially contributing to the increased AF vulnerability. Next, we analyzed activation maps during AF episodes. Occasionally, they exhibited a stable reentry pattern characterized by similar origin and propagation on a beat-to-beat basis. Alternatevely, the origins of successive reentrant waves varied randomly and the arrhythmia was perpetuated by coexisting reentrant circuits, maintained through the continuous annihilation and creation of multiple wavelets. Voltage oscillations during AF were analyzed in time and frequency domains, and the spectrogram (Fourier transform) revealed coexisting reentrant circuits at different frequencies. Analysis of the dispersion of the action potential durations and conduction velocity maps will clarify the mechanism of AF in chronic and acute models of atrial dilation.

Superresolution imaging reveals activity-dependent plasticity of axon morphology linked to changes in action potential conduction velocity

Axons convey information to nearby and distant cells, and the time it takes for action potentials (APs) to reach their targets governs the timing of information transfer in neural circuits. In the unmyelinated axons of hippocampus, the conduction speed of APs depends crucially on axon diameters, which vary widely. However, it is not known whether axon diameters are dynamic and regulated by activity-dependent mechanisms. Using time-lapse superresolution microscopy in brain slices, we report that axons grow wider after high-frequency AP firing: synaptic boutons undergo a rapid enlargement, which is mostly transient, whereas axon shafts show a more delayed and progressive increase in diameter. Simulations of AP propagation incorporating these morphological dynamics predicted bidirectional effects on AP conduction speed. The predictions were confirmed by electrophysiological experiments, revealing a phase of slowed down AP conduction, which is linked to the transient enlargement of the synaptic boutons, followed by a sustained increase in conduction speed that accompanies the axon shaft widening induced by high-frequency AP firing. Taken together, our study outlines a morphological plasticity mechanism for dynamically fine-tuning AP conduction velocity, which potentially has wide implications for the temporal transfer of information in the brain.

Axonal Conduction Velocity Measurement

Action potential conduction velocity is the speed at which an action potential (AP) propagates along an axon. Measuring AP conduction velocity is instrumental in determining neuron health, function, and computational capability, as well as in determining short-term dynamics of neuronal communication and AP initiation (Ballo and Bucher, 2009; Bullock, 1951; Meeks and Mennerick, 2007; Rosenthal and Bezanilla, 2000; Städele and Stein, 2016; Swadlow and Waxman, 1976). Conduction velocity can be measured using extracellular recordings along the nerve through which the axon projects. Depending on the number of axons in the nerve, AP velocities of individual or many axons can be detected. This protocol outlines how to measure AP conduction velocity of (A) stimulated APs and (B) spontaneously generated APs by using two spatially distant extracellular electrodes. Although an invertebrate nervous system is used here, the principles of this technique are universal and can be easily adjusted to other nervous system preparations (including vertebrates).

Peripheral neuropathy in chronic obstructive pulmonary disease

IntroductionPeripheral neuropathy in chronic obstructive pulmonary disease (COPD) has received scanty attention. The purpose of this study was to evaluate objectively the functional changes in the peripheral nervous system in COPD by different electrophysiological parameters and to determine the frequencies of these changes in patients with COPD.AimAssessment of peripheral nerve conduction by evaluation of the motor and sensory nerve conduction (SNC) in COPD patients.Patients and methodsIn this case–control study, we recruited 25 COPD patients and matched 25 healthy controls. Motor and SNC studies for ulnar and median nerves were evaluated by means of electrophysiological nerve study. Motor nerve conduction velocity and sensory nerve conduction velocity (SNCV), distal latencies (DLs), and amplitude of compound motor action potential were recorded. Arterial blood gases including partial pressure of oxygen and carbon dioxide (PaO2and PaCO2), oxygen saturation (SaO2), and arterial pH were measured. Pulmonary function test was done and forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), and FEV1/FVC ratio were measured.ResultsThere was a significant difference between COPD patients and the control group in all spirometric and gasometric parameters recorded, except for the arterial pH. On studying motor nerve conduction through median and ulnar nerves, there was an increase in DL, decrease in motor nerve conduction velocity, and longer F-wave latency in the COPD group than in the control group in both nerves. SNC study of the median nerve revealed a decrease in SNCV and an increase in DL in the COPD group than in the control group. Median nerve motor neuropathy was proved in 28% of patients, ulnar nerve motor neuropathy was proved in 36% of patients, whereas sensory nerve study of median nerve proved that 68% of patients have sensory axonal neuropathy and 12% have demyelinating sensory neuropathy. Median nerve Distal Latency (DL) shows negative correlation with FEV1and FEV1/FVC ratio. SNCV of the median nerve was positively correlated to oxygen tension level.ConclusionThe incidence of neuropathy is high. The rate of axonal neuropathy was significantly higher than other types. Our study showed a significant positive correlation between the degree of hypoxemia and severity of neuropathy, whereas it showed negative correlation between spirometry parameters (FEV1and FEV1/FVC ratio) and median nerve DL.

Impact of height on sensory nerve conduction

Background: Change in height of the limb not only increases the length of nerve but also causes gradual tapering of nerve with decrease in fiber diameter and myelination. Aim and Objectives: To study the correlation of height with the nerve conduction study (NCS) variables, i. e., latency, duration, amplitude, and conduction velocity of sensory nerve action potential (SNAP) of the sensory nerves of the upper and lower limbs. Materials and Methods: This is cross-sectional study including 37 participants. After taking consent their age, sex, height, weight, and body mass index were noted. NCS test was carried out for median, ulnar, and sural sensory nerves. Pearson’s correlation coefficient was calculated. P < 0.05 (*) means difference is statistically significant while P < 0.01 (**) is highly significant difference. Results: Height showed a significant positive correlation with the SNAP latencies of all the sensory nerves: Median sensory (r = 0.704, P < 0.01), ulnar sensory (r = 0.350, P < 0.05), and sural (r = 0.392, P < 0.05). The SNAP duration and amplitude of ulnar and median sensory nerve were negatively correlated with height while that of sural sensory nerve showed insignificant positive correlation with height. However, a significant negative correlation was seen with the SNAP conduction velocity of median sensory (r = −0.740, P < 0.01) and sural sensory (r = −0.701, P < 0.01) nerve. For ulnar sensory nerve, correlation of height and conduction velocity (r = −0.220) was negative but not statistically significant. Overall, height had an inverse correlation with conduction velocity and positive correlation with latency of SNAP. Conclusion: Clinical recognition of this height effect is important as one should not label an individual with mild increase/decrease in peripheral nerve conduction velocity as abnormal which may be solely related to height.

Sarcopenia: Neurological Point of View.

Sarcopenia is an age-related geriatric syndrome which is characterized by the gradual loss of muscle mass, muscle strength, and muscle quality. There are a lot of neurologic insults on sarcopenia at various levels from the brain to the neuromuscular junctions (NMJs) to generate a volitional task. Dopaminergic downregulation, inadequate motor programming and motor coordination impairment lead to decline of supraspinal drive. Motor unit reorganization and inflammatory changes in motor neuron decrease conduction velocity and amplitude of compound muscle action potential. Furthermore, NMJ remodeling and age related neurophysiological alterations may contribute to neuromuscular impairment. Sarcopenia is an age-associated, lifelong process which links to multiple etiological factors. Although not all the causes are completely understood, we suggest that compromised nervous system function may be one of the important contributors to the sarcopenia.

Electrophysiologic effects of the IK1 inhibitor PA-6 are modulated by extracellular potassium in isolated guinea pig hearts.

The pentamidine analog PA‐6 was developed as a specific inward rectifier potassium current (IK 1) antagonist, because established inhibitors either lack specificity or have side effects that prohibit their use in vivo. We previously demonstrated that BaCl2, an established IK 1 inhibitor, could prolong action potential duration (APD) and increase cardiac conduction velocity (CV). However, few studies have addressed whether targeted IK 1 inhibition similarly affects ventricular electrophysiology. The aim of this study was to determine the effects of PA‐6 on cardiac repolarization and conduction in Langendorff‐perfused guinea pig hearts. PA‐6 (200 nm) or vehicle was perfused into ex‐vivo guinea pig hearts for 60 min. Hearts were optically mapped with di‐4‐ANEPPS to quantify CV and APD at 90% repolarization (APD 90). Ventricular APD 90 was significantly prolonged in hearts treated with PA‐6 (115 ± 2% of baseline; P < 0.05), but not vehicle (105 ± 2% of baseline). PA‐6 slightly, but significantly, increased transverse CV by 7%. PA‐6 significantly prolonged APD 90 during hypokalemia (2 mmol/L [K+]o), although to a lesser degree than observed at 4.56 mmol/L [K+]o. In contrast, the effect of PA‐6 on CV was more pronounced during hypokalemia, where transverse CV with PA‐6 (24 ± 2 cm/sec) was significantly faster than with vehicle (13 ± 3 cm/sec, P < 0.05). These results show that under normokalemic conditions, PA‐6 significantly prolonged APD 90, whereas its effect on CV was modest. During hypokalemia, PA‐6 prolonged APD 90 to a lesser degree, but profoundly increased CV. Thus, in intact guinea pig hearts, the electrophysiologic effects of the IK 1 inhibitor, PA‐6, are [K+]o‐dependent.

Nerve Conduction Study for Patients with Parkinson Disease Who Developed Persistent Sensory Symptom during Optimal Levodopa Treatment

Objective Sensory symptoms are common in patients with Parkinson disease (PD), although its exact prevalence and causes are not well known. We aimed to investigate whether peripheral pathology contribute to chronic sensory symptoms in patients with PD by nerve conduction test. Methods We recruited consecutively 34 patients with clinically probable PD (Hoehn-Yahr stage, 1–3) who developed persistent sensory symptoms in their lower extremities during optimal dopamine replacement therapy. Twenty-five patients who are above age of 70 or have other possible causes of neuropathy as determined by laboratory testing or medical record were excluded. Age-and gender-matched PD patients with same disease severity who have never developed sensory symptom during the same period were allocated to paired controls. Total 18 subjects participated in this study. Nerve conduction tests on peroneal motor nerve and sural sensory nerve were performed in all subjects. Amplitude and conduction velocity were compared between the paired subjects using Wilcoxon signed rank test. Results Subjective sensory symptoms of the 9 patients were variable: pain (4/9), chilling (3/9), burning (1/9), and tingling sense (1/9). There were no significant difference of mean values of compound muscle action potential/sensory nerve action potential amplitude and conduction velocity between patients and their paired controls. Conclusion This study supports the hypothesis that the origin of chronic sensory symptoms in PD patients is more central than peripheral. Key words: Parkinson disease; Sensory symptoms; Nerve conduction stduy

Beyond the cholinergic crisis

Introduction: Organophosphates (OP) are the most frequently involved pesticides in acute poisoning. In Sri Lanka it has been ranked as the sixth or seventh leading cause of hospital deaths for many years. Neurotoxic effects of acute OP have been hitherto under-explored. The aims of the studies were to assess the effects of acute OP poisoning on somatic, autonomic nerves, neuromuscular junction (NMJ), brain stem and cognitive function. Methods: Patients following self-ingestion of OP were recruited to cohort studies to evaluated the function of somatic, autonomic nerves, NMJ, brain stem and cognition. Motor and sensory nerve function was tested with nerve conduction studies. Cardiovascular reflexes based autonomic function tests and sympathetic skin response (SSR) was used to evaluate autonomic function. NMJ function was assessed with slow repetitive supramaximal stimulation of the median nerve of the dominant upper limb. Brain stem function and cognitive function were assessed with Brain Stem Evoked Response Audiometry (BERA) and Mini Mental State Examination (MMSE) respectively. The data of the patients were compared with age, gender and occupation matched controls. Results: There were 60-70 patients and equal number of controls in each study. Motor nerve conduction velocity, amplitude and area of compound muscle action potential on distal stimulation, sensory nerve conduction velocity and F-wave occurrence were significantly reduced. At one week the significant impairment in autonomic function were change of diastolic blood pressure 3 min after standing, heart rate variation during deep breathing (HR-DB), SSR-amplitude and post-void urine volume. All except HR-DB were reversed at six weeks. No significant impairment of NMJ function, BERA and MMSE were noted. Conclusions: Sub clinical somatic and autonomic nerve dysfunction was observed. There were no strong evidence of long term effects on NMJ, brain stem and higher function.

13 The effects of bio sugar influence on toad sciatic nerve-trunk action potential

Objectives MSD is a type of bio sugar made by an MS bio sugar research institute in South Korea. The bio sugar has a glycan structure with a functional group on the monosaccharide. Our objective was to observe the effects of different concentrations of MSD on toad sciatic nerve-trunk action potential, and whether MSD can improve the inhibitory effect on toad sciatic nerve-trunk action potential of hypertonic glucose solution. Methods Toad sciatic nerve trunks were randomly divided into three groups, namely a control group, an MSD group and a hypertonic glucose group. Following soaking with different concentrations of MSD, toad sciatic nerve-trunk action potential amplitudes and conduction velocities were determined using a BL-420 biological function experimental system. Results Compared with the control group, 1% MS inhibited toad sciatic nerve-trunk action potential amplitude and conduction velocity but the effects disappeared after 10 min, while 0.5% MSD decreased action potential amplitude and increased conduction velocity. Soaking in 10% hypertonic glucose with 0.5% MSD after processing permanently inhibited toad sciatic nerve-trunk action potential. MSD can inhibit toad sciatic nerve-trunk action potential amplitude and conduction velocity. Conclusions The activity of 0.5% MSD is better than that of 1% MSD. MSD can improve the inhibitory effects of 10% hypertonic glucose solution on toad sciatic nerve-trunk action potential. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Grant No. 31260207).

Optical magnetic detection of single-neuron action potentials using quantum defects in diamond

Magnetic fields from neuronal action potentials (APs) pass largely unperturbed through biological tissue, allowing magnetic measurements of AP dynamics to be performed extracellularly or even outside intact organisms. To date, however, magnetic techniques for sensing neuronal activity have either operated at the macroscale with coarse spatial and/or temporal resolution-e.g., magnetic resonance imaging methods and magnetoencephalography-or been restricted to biophysics studies of excised neurons probed with cryogenic or bulky detectors that do not provide single-neuron spatial resolution and are not scalable to functional networks or intact organisms. Here, we show that AP magnetic sensing can be realized with both single-neuron sensitivity and intact organism applicability using optically probed nitrogen-vacancy (NV) quantum defects in diamond, operated under ambient conditions and with the NV diamond sensor in close proximity (∼10 µm) to the biological sample. We demonstrate this method for excised single neurons from marine worm and squid, and then exterior to intact, optically opaque marine worms for extended periods and with no observed adverse effect on the animal. NV diamond magnetometry is noninvasive and label-free and does not cause photodamage. The method provides precise measurement of AP waveforms from individual neurons, as well as magnetic field correlates of the AP conduction velocity, and directly determines the AP propagation direction through the inherent sensitivity of NVs to the associated AP magnetic field vector.

Reciprocal Modulation of IK1–INa Extends Excitability in Cardiac Ventricular Cells

The inwardly rectifying potassium current (IK1) and the fast inward sodium current (INa) are reciprocally modulated in mammalian ventricular myocytes. An increase in the expression of channels responsible for one of these two currents results in a corresponding increase in expression of the other. These currents are critical in the propagation of action potentials (AP) during the normal functioning of the heart. This study identifies a physiological role for IK1–INa reciprocal modulation in ventricular fiber activation thresholds and conduction. Simulations of action potentials in single cells and propagating APs in cardiac fibers were carried out using an existing model of electrical activity in cardiac ventricular myocytes. The conductances, GK1, of the inwardly rectifying potassium current, and GNa, of the fast inward sodium current were modified independently and in tandem to simulate reciprocal modulation. In single cells, independent modulation of GK1 alone resulted in changes in activation thresholds that were qualitatively similar to those for reciprocal GK1–GNa modulation and unlike those due to independent modulation of GNa alone, indicating that GK1 determines the cellular activation threshold. On the other hand, the variations in conduction velocity in cardiac cell fibers were similar for independent GNa modulation and for tandem changes in GK1–GNa, suggesting that GNa is primarily responsible for setting tissue AP conduction velocity. Conduction velocity dependence on GK1–GNa is significantly affected by the intercellular gap junction conductance. While the effects on the passive fiber space constant due to changes in both GK1 and the intercellular gap junction conductance, Ggj, were in line with linear cable theory predictions, both conductances had surprisingly large effects on fiber activation thresholds. Independent modulation of GK1 rendered cardiac fibers inexcitable at higher levels of GK1 whereas tandem GK1–GNa changes allowed fibers to remain excitable at high GK1 values. Reciprocal modulation of the inwardly rectifying potassium current and the fast inward sodium current may have a functional role in allowing cardiac tissue to remain excitable when IK1 is upregulated.

Development of Dual Neurotrophins-Encapsulated Electrosupun Nanofibrous Scaffolds for Peripheral Nerve Regeneration.

Nerve growth factor (NGF) is widely used for repairing peripheral nerve injury because of its capability in dominating the survival, migration, proliferation, and differentiation of nerve cells. Monosialoganglioside (GM1), as another kind of nerve growth factor, works for regulating NGF function. In this study, GM1 and NGF were incorporated into the Poly(l-lactic acid-co-ε-caprolactone)/silk fibroin (PLCL/SF) nanofibers by the coaxial electrospinning. The fibers morphology and core–shell structure were characterized by SEM and TEM. The scaffolds demonstrated high tensile stress with good flexibility. In vitro cell viability studies indicated that the scaffolds incorporating both GM1 and NGF played a synergistic effect to enhance Schwann cells (SCs) proliferation and Pheochromocytoma (PC12) cells differentiation, in comparison to the scaffolds only incorporating NGF. Subsequently, the nanofibrous conduit scaffolds (NCSs) were evaluated in vivo in a rabbit sciatic nerve defect model. The NGF/GM1 incorporated NCSs group performed better nerve function recovery than single incorporated group, in consideration of the compound muscle action potential (CMAP) and nerve conduction velocity (NCV) results. Furthermore, hematoxylin and eosin (H&E) staining, toluidine blue (TB) staining, and transmission electron microscope (TEM) analysis displayed better nerve regeneration of NGF/GM1 incorporated NCSs both quantitatively and qualitatively. Therefore, the results indicated the dual neurotrophins-incorporated NCSs had potentials for the application in peripheral nerve repairing.

Endogenous GABA controls oligodendrocyte lineage cell number, myelination, and CNS internode length.

Adjusting the thickness and internodal length of the myelin sheath is a mechanism for tuning the conduction velocity of axons to match computational needs. Interactions between oligodendrocyte precursor cells (OPCs) and developing axons regulate the formation of myelin around axons. We now show, using organotypic cerebral cortex slices from mice expressing eGFP in Sox10‐positive oligodendrocytes, that endogenously released GABA, acting on GABAA receptors, greatly reduces the number of oligodendrocyte lineage cells. The decrease in oligodendrocyte number correlates with a reduction in the amount of myelination but also an increase in internode length, a parameter previously thought to be set by the axon diameter or to be a property intrinsic to oligodendrocytes. Importantly, while TTX block of neuronal activity had no effect on oligodendrocyte lineage cell number when applied alone, it was able to completely abolish the effect of blocking GABAA receptors, suggesting that control of myelination by endogenous GABA may require a permissive factor to be released from axons. In contrast, block of AMPA/KA receptors had no effect on oligodendrocyte lineage cell number or myelination. These results imply that, during development, GABA can act as a local environmental cue to control myelination and thus influence the conduction velocity of action potentials within the CNS. GLIA 2017;65:309–321

Regulation of gap junction conductance by calcineurin through Cx43 phosphorylation: implications for action potential conduction.

Cardiac arrhythmias are associated with raised intracellular [Ca2+] and slowed action potential conduction caused by reduced gap junction (GJ) electrical conductance (Gj). Ventricular GJs are composed of connexin proteins (Cx43), with Gj determined by Cx43 phosphorylation status. Connexin phosphorylation is an interplay between protein kinases and phosphatases but the precise pathways are unknown. We aimed to identify key Ca2+-dependent phosphorylation sites on Cx43 that regulate cardiac gap junction conductance and action potential conduction velocity. We investigated the role of the Ca2+-dependent phosphatase, calcineurin. Intracellular [Ca2+] was raised in guinea-pig myocardium by a low-Na solution or increased stimulation. Conduction velocity and Gj were measured in multicellular strips. Phosphorylation of Cx43 serine residues (S365 and S368) and of the intermediary regulator I1 at threonine35 was measured by Western blot. Measurements were made in the presence and absence of inhibitors to calcineurin, I1 or protein phosphatase-1 and phosphatase-2.Raised [Ca2 +]i decreased Gj, reduced Cx43 phosphorylation at S365 and increased it at S368; these changes were reversed by calcineurin inhibitors. Cx43-S368 phosphorylation was reversed by the protein kinase C inhibitor chelerythrine. Raised [Ca2+]i also decreased I1 phosphorylation, also prevented by calcineurin inhibitors, to increase activity of the Ca2+-independent phosphatase, PPI. The PP1 inhibitor, tautomycin, prevented Cx43-365 dephosphorylation, Cx43-S368 phosphorylation and Gj reduction in raised [Ca2+]i. PP2A had no role. Conduction velocity was reduced by raised [Ca2+]i and reversed by calcineurin inhibitors. Reduced action potential conduction and Gj in raised [Ca2+] are regulated by calcineurin-dependent Cx43-S365 phosphorylation, leading to Cx43-S368 dephosphorylation. The calcineurin action is indirect, via I1 dephosphorylation and subsequent activation of PP1.Electronic supplementary materialThe online version of this article (doi:10.1007/s00424-016-1885-7) contains supplementary material, which is available to authorized users.

Time Course of Changes in Neuromuscular Parameters During Sustained Isometric Muscle Actions

Smith, CM, Housh, TJ, Herda, TJ, Zuniga, JM, Camic, CL, Bergstrom, HC, Smith, DB, Weir, JP, Hill, EC, Cochrane, KC, Jenkins, NDM, Schmidt, RJ, and Johnson, GO. Time course of changes in neuromuscular parameters during sustained isometric muscle actions. J Strength Cond Res 30(10): 2697-2702, 2016-The objective of the present study was to identify the time course of changes in electromyographic (EMG) and mechanomyographic (MMG) time and frequency domain parameters during a sustained isometric muscle action of the leg extensors at 50% maximal voluntary isometric contraction. The EMG and MMG signals were measured from the vastus lateralis of 11 subjects to identify when motor unit activation strategies changed throughout the sustained isometric muscle action. The EMG amplitude (muscle activation) had a positive linear relationship (p = 0.018, r = 0.77) that began to increase at the initiation of the muscle action and continued until task failure. Electromyographic frequency (motor unit action potential conduction velocity) and MMG frequency (global motor unit firing rate) had negative quadratic relationships (p = 0.002, R = 0.99; p = 0.015, R = 0.94) that began to decrease at 30% of the time to exhaustion. The MMG amplitude (motor unit activation) had a cubic relationship (p = 0.001, R = 0.94) that increased from 10 to 30% of the time to exhaustion, then decreased from 40 to 70% of the time to exhaustion, and then markedly increased from 70% to task failure. The time course of changes in the neuromuscular parameters suggested that motor unit activation strategies changed at approximately 30 and 70% of the time to exhaustion during the sustained isometric muscle action. These findings indicate that the time course of changes in neuromuscular responses provide insight into the strategies used to delay the effects of fatigue and are valuable tools for quantifying changes in the fatiguing process during training programs or supplementation research.

Dynamic culture yields engineered myocardium with near-adult functional output.

Engineered cardiac tissues hold promise for cell therapy and drug development, but exhibit inadequate function and maturity. In this study, we sought to significantly improve the function and maturation of rat and human engineered cardiac tissues. We developed dynamic, free-floating culture conditions for engineering "cardiobundles", 3-dimensional cylindrical tissues made from neonatal rat cardiomyocytes or human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) embedded in fibrin-based hydrogel. Compared to static culture, 2-week dynamic culture of neonatal rat cardiobundles significantly increased expression of sarcomeric proteins, cardiomyocyte size (∼2.1-fold), contractile force (∼3.5-fold), and conduction velocity of action potentials (∼1.4-fold). The average contractile force per cross-sectional area (59.7mN/mm2) and conduction velocity (52.5cm/s) matched or approached those of adult rat myocardium, respectively. The inferior function of statically cultured cardiobundles was rescued by transfer to dynamic conditions, which was accompanied by an increase in mTORC1 activity and decline in AMPK phosphorylation and was blocked by rapamycin. Furthermore, dynamic culture effects did not stimulate ERK1/2 pathway and were insensitive to blockers of mechanosensitive channels, suggesting increased nutrient availability rather than mechanical stimulation as the upstream activator of mTORC1. Direct comparison with phenylephrine treatment confirmed that dynamic culture promoted physiological cardiomyocyte growth rather than pathological hypertrophy. Optimized dynamic culture conditions also augmented function of human cardiobundles made reproducibly from cardiomyocytes derived from multiple hPSC lines, resulting in significantly increased contraction force (∼2.5-fold) and conduction velocity (∼1.4-fold). The average specific force of 23.2mN/mm2 and conduction velocity of 25.8cm/s approached the functional metrics of adult human myocardium. In conclusion, we have developed a versatile methodology for engineering cardiac tissues with a near-adult functional output without the need for exogenous electrical or mechanical stimulation, and have identified mTOR signaling as an important mechanism for advancing tissue maturation and function invitro.

Interactions between Activation and Repolarization Restitution Properties in the Intact Human Heart: In-Vivo Whole-Heart Data and Mathematical Description.

BackgroundThe restitution of the action potential duration (APDR) and conduction velocity (CVR) are mechanisms whereby cardiac excitation and repolarization adapt to changes in heart rate. They modulate the vulnerability to dangerous arrhythmia, but the mechanistic link between restitution and arrhythmogenesis remains only partially understood.MethodsThis paper provides an experimental and theoretical study of repolarization and excitation restitution properties and their interactions in the intact human epicardium. The interdependence between excitation and repolarization dynamic is studied in 8 patients (14 restitution protocols, 1722 restitution curves) undergoing global epicardial mapping with multi-electrode socks before open heart surgery. A mathematical description of the contribution of both repolarization and conduction dynamics to the steepness of the APDR slope is proposed.ResultsThis study demonstrates that the APDR slope is a function of both activation and repolarization dynamics. At short cycle length, conduction delay significantly increases the APDR slope by interacting with the diastolic interval. As predicted by the proposed mathematical formulation, the APDR slope was more sensitive to activation time prolongation than to the simultaneous shortening of repolarization time. A steep APDR slope was frequently identified, with 61% of all cardiac sites exhibiting an APDR slope > 1, suggesting that a slope > 1 may not necessarily promote electrical instability in the human epicardium. APDR slope did not change for different activation or repolarization times, and it was not a function of local baseline APD. However, it was affected by the spatial organization of electrical excitation, suggesting that in tissue APDR is not a unique function of local electrophysiological properties. Spatial heterogeneity in both activation and repolarization restitution contributed to the increase in the modulated dispersion of repolarization, which for short cycle length was as high as 250 ms. Heterogeneity in conduction velocity restitution can translate into both activation and repolarization dispersion and increase cardiac instability. The proposed mathematical formulation shows an excellent agreement with the experimental data (correlation coefficient r = 0.94) and provides a useful tool for the understanding of the complex interactions between activation and repolarization restitution properties as well as between their measurements.