Intrinsic Pacemaker Activity Research Articles

BS-514-02 TRANSIENT BIOLOGICAL PACING BASED ON AAV6-HCN2/SKM1 GENE TRANSFER IN PIGS WITH COMPLETE HEART BLOCK

In an effort to develop hardware-free pacemakers, we and others have engaged into the development of gene therapy-based biological pacemakers by means of ion channel overexpression. Here we report our progress with regard to adeno associated virus (AAV) vectors-mediated delivery of HCN2/SkM1 to generate long-term biological pacing. To study the long-term efficacy of biological pacing based on AAV-mediated gene transfer of HCN2/SkM1. We tested in vitro AAV-mediated overexpression of HCN2 and SkM1 on transduced neonatal rat ventricular myocytes (NRVMs). In vivo gene transfer was further validated after injecting 1E11 GC of AAV6-cTNT-HCN2 or AAV6-CMV-SkM1 into the left ventricle free wall of adult mice. Functional biological pacemaker studies were performed in pigs with radiofrequency ablation-induced complete AV block (CAVB). Four weeks after the ablation, we proceeded with three groups: non-injected, saline, and AAV6-HCN2/SkM1 (5E12 GC per vector). All animals were then observed for another four weeks to evaluate in vivo biological pacemaker performance. AAV6-HCN2 and AAV6-SkM1 showed both robust expression in vitro (NRVMs) and in vivo (mice) (Figure 1B and 1C). We thus proceeded with in vivo testing in CAVB pigs. Within the first two weeks of gene delivery, maximal beating rate increased in the HCN2/SkM1-transduced animals (Figure 2B) and mean heart rate modestly trended up after one week after injection, although this trend was not sustained (Figure 2A). This increase in beating rates was accompanied by an abrupt reduction of electronically paced beats in HCN2/SkM1 animals showing dependency on backup pacing of less than 50% - one week after of transgene delivery (Figure 2C). Finally, we evaluated corrected ventricular recovery times after overdrive suppression pacing which indicated enhanced intrinsic pacemaker activity in HCN2/SkM1-injected animals (Figure 2D). AAV-mediated gene transfer results in robust long-term overexpression of HCN2 and SKM1 in mouse myocardium. AAV-HCN2/SkM1 gene transfer in CAVB pigs resulted in transient biological pacemaker activity, potentially stemming from insufficient in vivo transduction and/or immunological interference.View Large Image Figure ViewerDownload Hi-res image Download (PPT)

Intrinsic pacemaker activity and propulsive forces provided by the myosalpinx are necessary for egg and embryo transport in the oviduct.

Oviducts or fallopian tubes are essential for normal fertilization and development of the early embryo; however, their functions are still poorly understood. Oviducts facilitate several physiological processes including (i) transport of the ovum from the ovary to the site of fertilization [1], (ii) aiding the transport of spermatozoa to the site of fertilization [2], (iii) providing a suitable environment for ova and protection from mechanical damage [3], (iv) the maintenance of viability of ova [3], and (v) timely transport of the early embryo to the uterus for implantation [4]. Pathophysiological conditions that disrupt the normal functions of the oviduct often lead to serious health problems, including infertility, ectopic pregnancy, failed early embryonic development, and subsequent loss of embryos. Thus, the oviduct is essential for normal female fertility. In a recent study by Bianchi et al. [5], the authors...

Cardiac Pacemaker Activity and Aging.

A progressive decline in maximum heart rate (mHR) is a fundamental aspect of aging in humans and other mammals. This decrease in mHR is independent of gender, fitness, and lifestyle, affecting in equal measure women and men, athletes and couch potatoes, spinach eaters and fast food enthusiasts. Importantly, the decline in mHR is the major determinant of the age-dependent decline in aerobic capacity that ultimately limits functional independence for many older individuals. The gradual reduction in mHR with age reflects a slowing of the intrinsic pacemaker activity of the sinoatrial node of the heart, which results from electrical remodeling of individual pacemaker cells along with structural remodeling and a blunted β-adrenergic response. In this review, we summarize current evidence about the tissue, cellular, and molecular mechanisms that underlie the reduction in pacemaker activity with age and highlight key areas for future work.

P1605Increased irregularity and spectral complexity of the intrinsic pacemaker beat-to-beat variability correlates with increased metabolic syndrome components

Abstract Background Metabolic syndrome (MetS) is becoming one of the future potential leading risk factors for heart and cardiovascular disease. MetS relates to a condition associated with at least three metabolic risk factors raising risk for health diseases concomitantly such as diabetes, stroke, hypertension, obesity and dyslipidemia. This can lead to chest pain, heart attack, heart damage and overall higher prevalence of cardiovascular disease, atrial fibrillation and sudden cardiac death. One of the underlying mechanisms of the progressive remodeling in presence of MetS components could be altered automaticity, which would reflect modifications of sinus node activity. These phenomena can be evaluated analyzing the components of heart rate variability (HRV). Purpose Our aim was to examine the modifications of sinus node variability in an isolated heart model of diet-induced obesity and MetS. Methods Male NZW rabbits were randomly assigned to high-fat (HF, n=8), control (HF-C, n=7), high-fat, high-sucrose (HFHS, n=9), and control (HFHS-C, n=9) groups, fed with their respective diets during 18/28 weeks. After euthanasia their hearts were isolated in a Langendorff system. We recorded 10–15 minutes of spontaneous activity. Short RR time series were analyzed, and standard HRV parameters were determined with special interest in the time-course of spectral, time-frequency and non-linear components. One-way ANOVA, Kruskal-Wallis test and bivariate correlations were used for statistical analysis (p<0.05). Results We did find an increase in the complexity and irregularity of intrinsic pacemaker activity as shown by modifications of entropy (ApEn: p=0.011 vs HF, p=0.002 vs HF-C, p=0.019 vs HFHS-C) and the complexity index (CI: p=0.006 vs HF, p=0.047 vs HF-C, p=0.027 vs HFHS-C) in HFHS animals (Figure). Higher dispersion on RR differences distributions was observed in the HFHS group. Time-frequency spectral heterogeneity increased in HFHS group (p=0.002 vs HF, p=0.050 vs HF-X, p=0.027 vs HFHS-C) even though no differences were found in standard time and frequency-domain analyses. High-band and low-band spectral concentration ratios showed decreased organization in HFHS when compared to HF (p=0.002) and controls HFHS-C (p=0.027) and HF-C (p=0.050). Interestingly, animal weight and glucose intolerance were highly correlated with the modifications of intrinsic pacemaker variability. Modifications of intrinsic IHRV. Conclusions Modifications of intrinsic HRV seemed to be reliant on the number of components of MetS present, given that only HFHS group showed significant changes towards an increased complexity and irregularity of intrinsic pacemaker variability. Acknowledgement/Funding This work was partially supported by: GV2015-062, CB16/11/00486.

Reduced Basal Heart Rate Variability in a Calcineurin Aβ Deficient Mouse

PurposeHeart rate and heart rhythm, quantified by heart rate variability (HRV) in vivo, are both determined by intrinsic pacemaker activity of sinoatrial node (SAN) pacemaker cells (SANC), and input to SAN from the autonomic nervous system (ANS). The spontaneous action potential (AP) firing rate and rhythm of SANC are governed, at least in part, by phosphorylation of proteins that regulate their intracellular calcium cycling and ion channel functions. Calcineurin Aβ (CnAβ), the catalytic subunit of a calcium‐activated serine/threonine phosphatase, influences SANC function protein phosphorylation. Increased calcium cycling and ion channel phosphorylation leads to an increase in SANC AP firing rate and reduced AP firing interval variability. CnAβ likely impacts SANC AP firing rate and AP interval variability, making CnAβ a prime candidate of in vivo HR and HRV regulation. Thus, we hypothesized the role of CnAβ in CnAβ−/− mice, in which cardiac Cn activity is decreased by 80%, would manifest an increased in vivo HR and reduced HRV.MethodsElectrocardiograms (EKG) were measured (via telemetry) in awake, unrestrained, 3‐month‐old male CnAβ−/− mice (n = 9) and wildtype littermates (n = 9). Four 1.5‐hour basal recordings were obtained at least 24 hours apart. A segment of at least 2048 beats with minimal noise were selected for each EKG time series recording and analyzed for different types of HRV parameters (LabChart). HRV parameters in the time domain quantify variance of unordered beat‐to‐beat intervals, frequency domain parameters measure how variance of ordered beat‐to‐beat intervals embedded within an EKG time series distribute as a function of frequency, and non‐linear domain parameters detect variation in fractals buried within the heart rhythm. Statistical analyses were performed using a linear mixed effect model (R) accounting for the four repeated measures.ResultsC.F. table. Mean basal HR was substantially higher in CnAβ−/− than in WT, and time, non‐linear, and most frequency domain HRV descriptors were markedly reduced in CnAβ−/− than WT.ConclusionBecause CnAβ regulates intrinsic SANC function to enable increased calcium cycling and ion channel phosphorylation, our results suggest intrinsic SANC function is a factor that contributes to HR and HRV in vivo. Whether the altered HR and HRV in the CnAβ−/− heart are driven solely by CnAβ dependent mechanisms intrinsic to SANC or, in addition, reflect changes in ANS input to the heart require further studies.Support or Funding InformationFunded by the National Institute of Health/National Institute on Aging Intramural Research Program. Average^ Basal HRV Values in CnAβ−/− vs WT Mice Parameter WT CnAβ−/− Mean HR (bpm) 460 ± 10.8 507 ± 9.6* TIME DOMAIN SD NN (ms) 9.87 ±0.8 5.39 ± 0.4** Range NN (ms) 47.77 ± 3.7 26.68 ± 2.4** Coefficient of Variance (%) 7.22 ± 0.5 4.34 ± 0.3** FREQUENCY DOMAIN Total Power (Power Spectral Density) 284.93 ± 23.5 184.21 ± 19.0* High Frequency (PSD) 70.28 ± 6.2 66.42 ± 9.5 Low Frequency (PSD) 62.58 ± 5.2 40.09 ± 4.7* Very Low Frequency (PSD) 152.07 ± 19.8 77.70 ± 10.5* NON‐LINEAR DOMAIN Beta Slope −2.15 ± 0.1 −2.84 ± 0.1** SDl(ms) 5.72 ± 0.4 2.80 ± 0.7* SD2 (ms) 12.57 ± 0.9 7.00 ± 0.6** Multiscale Entropy 1.57 ± 0.1 0.93 ± 0.1** Values are mean ± SE p<0.05 p<0.01 Average of 4 basal recordings over separate days This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Modifications of short-term intrinsic pacemaker variability in diet-induced metabolic syndrome: a study on isolated rabbit heart.

Metabolic syndrome (MetS) describes a condition associated with multiple diseases concomitantly such as diabetes, hypertension, obesity, and dyslipidemia. It has been linked with higher prevalence of cardiovascular disease, atrial fibrillation, and sudden cardiac death. One of the underlying mechanisms could be altered automaticity, which would reflect modifications of sinus node activity. These phenomena can be evaluated analyzing the components of heart rate variability (HRV). Our aim was to examine the modifications of sinus node variability in an isolated heart model of diet-induced obesity and MetS. Male NZW rabbits were randomly assigned to high-fat (HF, n = 8), control (HF-C, n = 7), high-fat, high-sucrose (HFHS, n = 9), and control (HFHS-C, n = 9) groups, fed with their respective diets during 18/28weeks. After euthanasia, their hearts were isolated in a Langendorff system. We recorded 10-15min of spontaneous activity. Short RR time series were analyzed, and standard HRV parameters were determined. One-way ANOVA, Kruskal-Wallis test, and bivariate correlation were used for statistical analysis (p < 0.05). We did find an increase in the complexity and irregularity of intrinsic pacemaker activity as shown by modifications of approximate entropy, sample entropy, minimum multiscale entropy, and complexity index in HFHS animals. Even though no differences were found in standard time and frequency-domain analyses, spectral heterogeneity increased in HFHS group. Animal weight and glucose intolerance were highly correlated with the modifications of intrinsic pacemaker variability. Finally, modifications of intrinsic HRV seemed to be reliant on the number of components of MetS present, given that only HFHS group showed significant changes towards an increased complexity and irregularity of intrinsic pacemaker variability.

Spontaneous Electrical Activity and Rhythmicity in Gastrointestinal Smooth Muscles.

The gastrointestinal (GI) tract has multifold tasks of ingesting, processing, and assimilating nutrients and disposing of wastes at appropriate times. These tasks are facilitated by several stereotypical motor patterns that build upon the intrinsic rhythmicity of the smooth muscles that generate phasic contractions in many regions of the gut. Phasic contractions result from a cyclical depolarization/repolarization cycle, known as electrical slow waves, which result from intrinsic pacemaker activity. Interstitial cells of Cajal (ICC) are electrically coupled to smooth muscle cells (SMCs) and generate and propagate pacemaker activity and slow waves. The mechanism of slow waves is dependent upon specialized conductances expressed by pacemaker ICC. The primary conductances responsible for slow waves in mice are Ano1, Ca2+-activated Cl- channels (CaCCs), and CaV3.2, T-type, voltage-dependent Ca2+ channels. Release of Ca2+ from intracellular stores in ICC appears to be the initiator of pacemaker depolarizations, activation of T-type current provides voltage-dependent Ca2+ entry into ICC, as slow waves propagate through ICC networks, and Ca2+-induced Ca2+ release and activation of Ano1 in ICC amplifies slow wave depolarizations. Slow waves conduct to coupled SMCs, and depolarization elicited by these events enhances the open-probability of L-type voltage-dependent Ca2+ channels, promotes Ca2+ entry, and initiates contraction. Phasic contractions timed by the occurrence of slow waves provide the basis for motility patterns such as gastric peristalsis and segmentation. This chapter discusses the properties of ICC and proposed mechanism of electrical rhythmicity in GI muscles.

Sub-cellular Electrical Heterogeneity Revealed by Loose Patch Recording Reflects Differential Localization of Sarcolemmal Ion Channels in Intact Rat Hearts.

The cardiac action potential (AP) is commonly recoded as an integral signal from isolated myocytes or ensembles of myocytes (with intracellular microelectrodes and extracellular macroelectrodes, respectively). These signals, however, do not provide a direct measure of activity of ion channels and transporters located in two major compartments of a cardiac myocyte: surface sarcolemma and the T-tubule system, which differentially contribute to impulse propagation and excitation-contraction (EC) coupling. In the present study we investigated electrical properties of myocytes within perfused intact rat heart employing loose patch recording with narrow-tip (2 μm diameter) extracellular electrodes. Using this approach, we demonstrated two distinct types of electric signals with distinct waveforms (single peak and multi-peak AP; AP1 and AP2, respectively) during intrinsic pacemaker activity. These two types of waveforms depend on the position of the electrode tip on the myocyte surface. Such heterogeneity of electrical signals was lost when electrodes of larger pipette diameter were used (5 or 10 μm), which indicates that the electric signal was assessed from a region of <5 μm. Importantly, both pharmacological and mathematical simulation based on transverse (T)-tubular distribution suggested that while the AP1 and the initial peak of AP2 are predominantly attributable to the fast, inward Na+ current in myocyte's surface sarcolemma, the late components of AP2 are likely representative of currents associated with L-type Ca2+ channel and Na+/Ca2+ exchanger (NCX) currents which are predominantly located in T-tubules. Thus, loose patch recording with narrow-tip pipette provides a valuable tool for studying cardiac electric activity on the subcellular level in the intact heart.

Cyclic AMP reverses the effects of aging on pacemaker activity and If in sinoatrial node myocytes.

Aerobic capacity decreases with age, in part because of an age-dependent decline in maximum heart rate (mHR) and a reduction in the intrinsic pacemaker activity of the sinoatrial node of the heart. Isolated sinoatrial node myocytes (SAMs) from aged mice have slower spontaneous action potential (AP) firing rates and a hyperpolarizing shift in the voltage dependence of activation of the "funny current," If Cyclic AMP (cAMP)is a critical modulator of bothAP firing rate and Ifin SAMs.Here,we testthe ability of endogenous and exogenous cAMP to overcome age-dependent changes in acutely isolated murine SAMs. We found that maximal stimulation of endogenous cAMP with 3-isobutyl-1-methylxanthine (IBMX) and forskolin significantly increased AP firing rate and depolarized the voltage dependence of activation of If in SAMs from both young and aged mice. However, these changes were insufficient to overcome the deficits in aged SAMs, and significant age-dependent differences in AP firing rate and If persisted in the presence of IBMX and forskolin. In contrast, the effects of aging on SAMs were completely abolished by a high concentration of exogenous cAMP, which restored AP firing rate and If activation to youthful levels in cells from aged animals. Interestingly, the age-dependent differences in AP firing rates and If were similar in whole-cell and perforated-patch recordings, and the hyperpolarizing shift in If persisted in excised inside-out patches, suggesting a limited role for cAMP in causing these changes. Collectively, the data indicate that aging does not impose an absolute limit on pacemaker activity and that it does not act by simply reducing the concentration of freely diffusible cAMP in SAMs.

Presympathetic neuron dysfunction--time to reconsider increased intrinsic activity as the cause of neurogenic hypertension.

Presympathetic neuron dysfunction--time to reconsider increased intrinsic activity as the cause of neurogenic hypertension.

Intrinsic properties of rostral ventrolateral medulla presympathetic and bulbospinal respiratory neurons of juvenile rats are not affected by chronic intermittent hypoxia.

The presympathetic neurons in the rostral ventrolateral medulla (RVLM) are considered to be the source of the sympathetic activity, and there is experimental evidence that these cells present intrinsic autodepolarization. There is also evidence that an important respiratory neuronal population located in the RVLM/Bötzinger complex (BötC) corresponds to augmenting expiratory neurons (aug-E), which send projections to the phrenic nucleus in the spinal cord. However, the pacemaker activity of presympathetic neurons and the intrinsic properties of aug-E neurons had not been evaluated in brainstem slices of juvenile rats (postnatal day35). Chronic intermittent hypoxia (CIH) is a sympathetic-mediated hypertension model, which seems to produce an associated increase in the activity of aug-E neurons. In this study, we evaluated the effects of CIH on the intrinsic properties of RVLM/BötC presympathetic and phrenic nucleus-projecting neurons (aug-E) in brainstem slices of juvenile rats (postnatal day35). We observed that all presympathetic neurons presented spontaneous action potential firing (n=18), which was not abolished by ionotropic receptor antagonism. In addition, exposure to 10days of CIH produced no changes in their intrinsic passive properties, firing pattern or excitability. Most aug-E neurons presented spontaneous firing in control conditions (13 of 15 neurons), and this characteristic was preserved after blocking fast synaptic transmission (12 of 15 neurons), clearly demonstrating their intrinsic pacemaker activity. Chronic intermittent hypoxia also produced no changes in intrinsic passive properties, frequency and pattern of discharge or excitability of the aug-E neurons. The present study shows that: (i)it is possible to record the electrophysiological properties of RVLM/BötC presympathetic and aug-E neurons in brainstem slices from juvenile rats; (ii)these neurons present characteristics of intrinsic pacemakers; and (iii)their intrinsic properties were not altered by chronic intermittent hypoxia.

The cardiac sodium–calcium exchanger NCX1 is a key player in the initiation and maintenance of a stable heart rhythm

The complex molecular mechanisms underlying spontaneous cardiac pacemaking are not fully understood. Recent findings point to a co-ordinated interplay between intracellular Ca(2+) cycling and plasma membrane-localized cation transport determining the origin and periodicity of pacemaker potentials. The sodium-calcium exchanger (NCX1) is a key sarcolemmal protein for the maintenance of calcium homeostasis in the heart. Here, we investigated the contribution of NCX1 to cardiac pacemaking. We used an inducible and sinoatrial node-specific Cre transgene to create micelacking NCX1 selectively in cells of the cardiac pacemaking and conduction system (cpNCX1KO). RT-PCR and immunolabeling experiments confirmed the precise tissue-specific and temporally controlled deletion. Ablation of NCX1 resulted in a progressive slowing of heart rate accompanied by severe arrhythmias. Isolated sinoatrial tissue strips displayed a significantly decreased and irregular contraction rate underpinning a disturbed intrinsic pacemaker activity. Mutant animals displayed a gradual increase in the heart-to-body weight ratio and developed ventricular dilatation; however, their ventricular contractile performance was not significantly affected. Pacemaker cells from cpNCX1KO showed no NCX1 activity in response to caffeine-induced Ca(2+) release, determined by Ca(2+) imaging. Regular spontaneous Ca(2+) discharges were frequently seen in control, but only sporadically in knockout (KO) cells. The majority of NCX1KO cells displayed an irregular and a significantly reduced frequency of spontaneous Ca(2+) signals. Furthermore, Ca(2+) transients measured during electrical field stimulation were of smaller magnitude and decelerated kinetics in KO cells. Our results establish NCX1 as a critical target for the proper function of cardiac pacemaking.

DDTA mode pacing in the treatment of complete conduction block from the right atrium to the ventricles and interatrial electrical dissociation after surgical procedures

This case report describes a 61-year-old man with a complete conduction block from the right atrium to both the ventricles and interatrial electrical dissociation that developed after removal of a left atrial myxoma and a maze surgery. A pacemaker was implanted with biatrial pacing using dual-chamber DDD pacing with triggered atrial pacing (DDTA) mode to detect the right atrial potential and to pace the left atrium while simultaneously pacing the ventricles. This approach was useful for restoring the native atrioventricular conduction, based on the intrinsic pacemaker activity in the right atrium.

A defined heteromeric KV1 channel stabilizes the intrinsic pacemaking and regulates the output of deep cerebellar nuclear neurons to thalamic targets

The output of the cerebellum to the motor axis of the central nervous system is orchestrated mainly by synaptic inputs and intrinsic pacemaker activity of deep cerebellar nuclear (DCN) projection neurons. Herein, we demonstrate that the soma of these cells is enriched with K(V)1 channels produced by mandatory multi-merization of K(V)1.1, 1.2 α and KV β2 subunits. Being constitutively active, the K(+) current (IK(V)1) mediated by these channels stabilizes the rate and regulates the temporal precision of self-sustained firing of these neurons. Placed strategically, IK(V)1 provides a powerful counter-balance to prolonged depolarizing inputs, attenuates the rebound excitation, and dampens the membrane potential bi-stability. Somatic location with low activation threshold render IK(V)1 instrumental in voltage-dependent de-coupling of the axon initial segment from the cell body of projection neurons, impeding invasion of back-propagating action potentials into the somato-dendritic compartment. The latter is also demonstrated to secure the dominance of clock-like somatic pacemaking in driving the regenerative firing activity of these neurons, to encode time variant inputs with high fidelity. Through the use of multi-compartmental modelling and retro-axonal labelling, the physiological significance of the described functions for processing and communication of information from the lateral DCN to thalamic relay nuclei is established.

17 Changes in the Role of the SR in SAN Function Across the Lifespan May Be responsible for Changing Pacemaker Stability and Response

BackgroundIntracellular calcium regulation is important for regulating sinoatrial node (SAN) activity. Ventricular expression of the sarcoendoplasmic recticulum (SR) Ca-ATPase pump (SERCA) has been shown to decline with age; a change...

Regulation of gastrointestinal motility—insights from smooth muscle biology

Gastrointestinal motility results from coordinated contractions of the tunica muscularis, the muscular layers of the alimentary canal. Throughout most of the gastrointestinal tract, smooth muscles are organized into two layers of circularly or longitudinally oriented muscle bundles. Smooth muscle cells form electrical and mechanical junctions between cells that facilitate coordination of contractions. Excitation-contraction coupling occurs by Ca(2+) entry via ion channels in the plasma membrane, leading to a rise in intracellular Ca(2+). Ca(2+) binding to calmodulin activates myosin light chain kinase; subsequent phosphorylation of myosin initiates cross-bridge cycling. Myosin phosphatase dephosphorylates myosin to relax muscles, and a process known as Ca(2+) sensitization regulates the activity of the phosphatase. Gastrointestinal smooth muscles are 'autonomous' and generate spontaneous electrical activity (slow waves) that does not depend upon input from nerves. Intrinsic pacemaker activity comes from interstitial cells of Cajal, which are electrically coupled to smooth muscle cells. Patterns of contractile activity in gastrointestinal muscles are determined by inputs from enteric motor neurons that innervate smooth muscle cells and interstitial cells. Here we provide an overview of the cells and mechanisms that generate smooth muscle contractile behaviour and gastrointestinal motility.

Functional characterization of ether-à-go-go-related gene potassium channels in midbrain dopamine neurons - implications for a role in depolarization block.

Bursting activity by midbrain dopamine neurons reflects the complex interplay between their intrinsic pacemaker activity and synaptic inputs. Although the precise mechanism responsible for the generation and modulation of bursting in vivo has yet to be established, several ion channels have been implicated in the process. Previous studies with nonselective blockers suggested that ether-à-go-go-related gene (ERG) K(+) channels are functionally significant. Here, electrophysiology with selective chemical and peptide ERG channel blockers (E-4031 and rBeKm-1) and computational methods were used to define the contribution made by ERG channels to the firing properties of midbrain dopamine neurons in vivo and in vitro. Selective ERG channel blockade increased the frequency of spontaneous activity as well as the response to depolarizing current pulses without altering spike frequency adaptation. ERG channel block also accelerated entry into depolarization inactivation during bursts elicited by virtual NMDA receptors generated with the dynamic clamp, and significantly prolonged the duration of the sustained depolarization inactivation that followed pharmacologically evoked bursts. In vivo, somatic ERG blockade was associated with an increase in bursting activity attributed to a reduction in doublet firing. Taken together, these results show that dopamine neuron ERG K(+) channels play a prominent role in limiting excitability and in minimizing depolarization inactivation. As the therapeutic actions of antipsychotic drugs are associated with depolarization inactivation of dopamine neurons and blockade of cardiac ERG channels is a prominent side effect of these drugs, ERG channels in the central nervous system may represent a novel target for antipsychotic drug development.

A New Concept for Pacemaker Activity in the Human Stomach (J Physiol 2011:589[Pt 24];6105-6118)

Rhee et al1 has reported a study entitled Analysis of pacemaker activity in the human stomach in the December issue of Journal of Physiology 2011. Extracellular electrical recording techniques using animal models have contributed to establish important concepts of human gastric physiology.2,3 The electrical quiescence in the gastric fundus, 3 cycles per minute (cpm) pacemaker activity in corpus and antrum, and proximal-to-distal slow wave frequency gradient tended to accept as standard concepts. However, several findings which do not concur with those classic concepts have been presented in Rhee's study. Rhee et al1 experimented using human gastric muscles obtained from patients undergoing gastric resection for gastric cancer and investigated the slow wave pacemaker activity recorded with intracellular microelectrodes, the contractions recorded by isometric force techniques and the distribution of interstitial cells of Cajal (ICC). Interestingly, different from the recent studies, slow waves were routinely recorded from gastric fundic muscles. And similar waveforms as slow waves in more distal regions were presented with couple to phasic contractions. Also gastric slow wave frequency was significantly greater than 3 cpm in all regions of the stomach. Additionally antral slow wave frequency often exceeded the highest frequency of pacemaker activity in the corpus. These observations could not be explained as chronotropic mechanisms such as muscarinic and prostaglandin receptor binding, stretch, extracelluar Ca2+ and temperature. After all, muscles from all regions through the thickness of the muscularis demonstrated intrinsic pacemaker activity, and this corresponded with the widespread distribution in ICC.

Analysis of pacemaker activity in the human stomach

Extracellular electrical recording and studies using animal models have helped establish important concepts of human gastric physiology. Accepted standards include electrical quiescence in the fundus, 3 cycles per minute (cpm) pacemaker activity in corpus and antrum, and a proximal-to-distal slow wave frequency gradient. We investigated slow wave pacemaker activity, contractions and distribution of interstitial cells of Cajal (ICC) in human gastric muscles. Muscles were obtained from patients undergoing gastric resection for cancer, and the anatomical locations of each specimen were mapped by the operating surgeon to 16 standardized regions of the stomach. Electrical slow waves were recorded with intracellular microelectrodes and contractions were recorded by isometric force techniques. Slow waves were routinely recorded from gastric fundus muscles. These events had similar waveforms as slow waves in more distal regions and were coupled to phasic contractions. Gastric slow wave frequency was significantly greater than 3 cpm in all regions of the stomach. Antral slow wave frequency often exceeded the highest frequency of pacemaker activity in the corpus. Chronotropic mechanisms such as muscarinic and prostaglandin receptor binding, stretch, extracelluar Ca(2+) and temperature were unable to explain the observed slow wave frequency that exceeded accepted normal levels. Muscles from all regions through the thickness of the muscularis demonstrated intrinsic pacemaker activity, and this corresponded with the widespread distribution in ICC we mapped throughout the tunica muscularis. Our findings suggest that extracellular electrical recording has underestimated human slow wave frequency and mechanisms of human gastric function may differ from standard laboratory animal models.

Ivabradine

Ivabradine