L-type but not T-type calcium current changes during postnatal development in rabbit sinoatrial node

Rad-GTPase contributes to heart rate via L-type calcium channel regulation

Sinoatrial node cardiomyocytes (SANcm) possess automatic, rhythmic electrical activity. SAN rate is influenced by autonomic nervous system input, including sympathetic nerve increases of heart rate (HR) via activation of β-adrenergic receptor signaling cascade (β-AR). L-type calcium channel (LTCC) activity contributes to membrane depolarization and is a central target of β-AR signaling. Recent studies revealed that the small G-protein Rad plays a central role in β-adrenergic receptor directed modulation of LTCC. These studies have identified a conserved mechanism in which β-AR stimulation results in PKA-dependent Rad phosphorylation: depletion of Rad from the LTCC complex, which is proposed to relieve the constitutive inhibition of CaV1.2 imposed by Rad association. Here, using a transgenic mouse model permitting conditional cardiomyocyte selective Rad ablation, we examine the contribution of Rad to the control of SANcm LTCC current (ICa,L) and sinus rhythm. Single cell analysis from a recent published database indicates that Rad is expressed in SANcm, and we show that SANcm ICa,L was significantly increased in dispersed SANcm following Rad silencing compared to those from CTRL hearts. Moreover, cRadKO SANcm ICa,L was not further increased with β-AR agonists. We also evaluated heart rhythm in vivo using radiotelemetered ECG recordings in ambulating mice. In vivo, intrinsic HR is significantly elevated in cRadKO. During the sleep phase cRadKO also show elevated HR, and during the active phase there is no significant difference. Rad-deletion had no significant effect on heart rate variability. These results are consistent with Rad governing LTCC function under relatively low sympathetic drive conditions to contribute to slower HR during the diurnal sleep phase HR. In the absence of Rad, the tonic modulated SANcm ICa,L promotes elevated sinus HR. Future novel therapeutics for bradycardia targeting Rad - LTCC can thus elevate HR while retaining βAR responsiveness.

Mechanistic Insights Into the Reduced Pacemaking Rate of the Rabbit Sinoatrial Node During Postnatal Development: A Simulation Study

Marked age- and development- related differences have been observed in morphology and characteristics of action potentials (AP) of neonatal and adult sinoatrial node (SAN) cells. These may be attributable to a different set of ion channel interactions between the different ages. However, the underlying mechanism(s) have yet to be elucidated. The objective of this study was to determine the mechanisms underlying different spontaneous APs and heart rate between neonatal and adult SAN cells of the rabbit heart by biophysical modeling approaches. A mathematical model of neonatal rabbit SAN cells was developed by modifying the current densities and/or kinetics of ion channels and transporters in an adult cell model based on available experimental data obtained from neonatal SAN cells. The single cell models were then incorporated into a multi-cellular, two-dimensional model of the intact SAN-atrium to investigate the functional impact of altered ion channels during maturation on pacemaking electrical activities and their conduction at the tissue level. Effects of the neurotransmitter acetylcholine on the pacemaking activities in neonatal cells were also investigated and compared to those in the adult. Our results showed: (1) the differences in ion channel properties between neonatal and adult SAN cells are able to account for differences in their APs and the heart rate, providing mechanistic insight into understanding the reduced pacemaking rate of the rabbit sinoatrial node during postnatal development; (2) in the 2D model of the intact SAN-atria, it was shown that cellular changes during postnatal development impaired pacemaking activity through increasing the activation time and reducing the conduction velocity across the SAN; (3) the neonatal SAN model, with its faster beating rates, showed a greater sensitivity to parasympathetic modulation in response to acetylcholine than did the adult model. These results provide novel insights into the understanding of the cellular mechanisms underlying the differences in the cardiac pacemaking activities of the neonatal and adult SAN.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

Postnatal Increases in Axonal Conduction Velocity of an Identified Drosophila Interneuron Require Fast Sodium, L-Type Calcium and Shaker Potassium Channels

During early postnatal life, speed up of signal propagation through many central and peripheral neurons has been associated with an increase in axon diameter or/and myelination. Especially in unmyelinated axons postnatal adjustments of axonal membrane conductances is potentially a third mechanism but solid evidence is lacking. Here, we show that axonal action potential (AP) conduction velocity in the Drosophila giant fiber (GF) interneuron, which is required for fast long-distance signal conduction through the escape circuit, is increased by 80% during the first day of adult life. Genetic manipulations indicate that this postnatal increase in AP conduction velocity in the unmyelinated GF axon is likely owed to adjustments of ion channel expression or properties rather than axon diameter increases. Specifically, targeted RNAi knock-down of either Para fast voltage-gated sodium, Shaker potassium (Kv1 homologue), or surprisingly, L-type like calcium channels counteracts postnatal increases in GF axonal conduction velocity. By contrast, the calcium-dependent potassium channel Slowpoke (BK) is not essential for postnatal speeding, although it also significantly increases conduction velocity. Therefore, we identified multiple ion channels that function to support fast axonal AP conduction velocity, but only a subset of these are regulated during early postnatal life to maximize conduction velocity. Despite its large diameter (∼7 µm) and postnatal regulation of multiple ionic conductances, mature GF axonal conduction velocity is still 20-60 times slower than that of vertebrate Aβ sensory axons and α motoneurons, thus unraveling the limits of long-range information transfer speed through invertebrate circuits.

Neuronal Intrinsic Physiology Changes During Development of a Learned Behavior

Juvenile male zebra finches learn their songs over distinct auditory and sensorimotor stages, the former requiring exposure to an adult tutor song pattern. The cortical premotor nucleus HVC (acronym is name) plays a necessary role in both learning stages, as well as the production of adult song. Consistent with neural network models where synaptic plasticity mediates developmental forms of learning, exposure to tutor song drives changes in the turnover, density, and morphology of HVC synapses during vocal development. A network's output, however, is also influenced by the intrinsic properties (e.g., ion channels) of the component neurons, which could change over development. Here, we use patch clamp recordings to show cell-type-specific changes in the intrinsic physiology of HVC projection neurons as a function of vocal development. Developmental changes in HVC neurons that project to the basal ganglia include an increased voltage sag response to hyperpolarizing currents and an increased rebound depolarization following hyperpolarization. Developmental changes in HVC neurons that project to vocal-motor cortex include a decreased resting membrane potential and an increased spike amplitude. HVC interneurons, however, show a relatively stable range of intrinsic features across vocal development. We used mathematical models to deduce possible changes in ionic currents that underlie the physiological changes and to show that the magnitude of the observed changes could alter HVC circuit function. The results demonstrate developmental plasticity in the intrinsic physiology of HVC projection neurons and suggest that intrinsic plasticity may have a role in the process of song learning.

Functional role of voltage gated Ca(2+) channels in heart automaticity

Pacemaker activity of automatic cardiac myocytes controls the heartbeat in everyday life. Cardiac automaticity is under the control of several neurotransmitters and hormones and is constantly regulated by the autonomic nervous system to match the physiological needs of the organism. Several classes of ion channels and proteins involved in intracellular Ca(2+) dynamics contribute to pacemaker activity. The functional role of voltage-gated calcium channels (VGCCs) in heart automaticity and impulse conduction has been matter of debate for 30 years. However, growing evidence shows that VGCCs are important regulators of the pacemaker mechanisms and play also a major role in atrio-ventricular impulse conduction. Incidentally, studies performed in genetically modified mice lacking L-type Cav1.3 (Cav1.3(-/-)) or T-type Cav3.1 (Cav3.1(-/-)) channels show that genetic inactivation of these channels strongly impacts pacemaking. In cardiac pacemaker cells, VGCCs activate at negative voltages at the beginning of the diastolic depolarization and importantly contribute to this phase by supplying inward current. Loss-of-function of these channels also impairs atrio-ventricular conduction. Furthermore, inactivation of Cav1.3 channels promotes also atrial fibrillation and flutter in knockout mice suggesting that these channels can play a role in stabilizing atrial rhythm. Genomic analysis demonstrated that Cav1.3 and Cav3.1 channels are widely expressed in pacemaker tissue of mice, rabbits and humans. Importantly, human diseases of pacemaker activity such as congenital bradycardia and heart block have been attributed to loss-of-function of Cav1.3 and Cav3.1 channels. In this article, we will review the current knowledge on the role of VGCCs in the generation and regulation of heart rate and rhythm. We will discuss also how loss of Ca(2+) entry through VGCCs could influence intracellular Ca(2+) handling and promote atrial arrhythmias.

Electrical excitability of the heart in a Chondrostei fish, the Siberian sturgeon (Acipenser baerii)

Sturgeon (family Acipenseridae) are regarded as living fossils due to their ancient origin and exceptionally slow evolution. To extend our knowledge of fish cardiac excitability to a Chondrostei fish, we examined electrophysiological phenotype of the Siberian sturgeon ( Acipenser baerii) heart with recordings of epicardial ECG, intracellular action potentials (APs), and sarcolemmal ion currents. Epicardial ECG of A. baerii had the typical waveform of the vertebrate ECG with Q-T interval (average duration of ventricular AP) of 650 ± 30 ms and an intrinsic heart rate of 45.5 ± 5 beats min−1 at 20°C. Similar to other fish species, atrial AP was shorter in duration (402 ± 33 ms) than ventricular AP (585 ± 40) ( P < 0.05) at 20°C. Densities of atrial and ventricular Na+ currents were similar (−47.6 ± 4.5 and −53.2 ± 5.1 pA/pF, respectively) and close to the typical values of teleost hearts. Two major K+ currents, the inward rectifier K+ current ( IK1), and the delayed rectifier K+ current ( IKr) were found under basal conditions in sturgeon cardiomyocytes. The atrial IKr (3.3 ± 0.2 pA/pF) was about twice as large as the ventricular IKr (1.3 ± 0.4 pA/pF) ( P < 0.05) conforming to the typical pattern of teleost cardiac IKr. Divergent from other fishes, the ventricular IK1 was remarkably small (−2.5 ± 0.07 pA/pF) and not different from that of the atrial myocytes (−1.9 ± 0.06 pA/pF) ( P > 0.05). Two ligand-gated K+ currents were also found: ACh-activated inward rectifier ( IKACh) was present only in atrial cells, while ATP-sensitive K+ current ( IKATP) was activated by a mitochondrial blocker, CCCP, in both atrial and ventricular cells. The most striking difference to other fishes appeared in Ca2+ currents ( ICa). In atrial myocytes, ICa was predominated by nickel-sensitive and nifedipine-resistant T-type ICa, while ventricular myocytes had mainly nifedipine-sensitive and nickel-resistant L-type ICa. ICaT/ ICaL ratio of the sturgeon atrial myocytes (2.42) is the highest value ever measured for a vertebrate species. In ventricular myocytes, ICaT/ ICaL ratio was 0.09. With the exception of the large atrial ICaT and small ventricular IK1, electrical excitability of A. baerii heart is similar to that of teleost hearts.

The mechanism of increased postnatal heart rate and sinoatrial node pacemaker activity in mice

Heart rate (HR) of mammalian species changes postnatally, i.e., HR of large animals including humans decreases, while HR in small animals such as mice and rats increases. To clarify cellular mechanisms underlying the postnatal HR changes, we performed in vivo HR measurement and electrophysiological analysis on sinoatrial node (SAN) cells in mice. The in vivo HR was ~320 beats min(-1) (bpm) immediately after birth, and increased with age to ~690 bpm at postnatal day 14. Under blockage of autonomic nervous systems, HR remained constant until postnatal day 5 and then increased day by day. The spontaneous beating rate of SAN preparation showed a similar postnatal change. The density of the L-type Ca(2+) current (LCC) was smaller in neonatal SAN cells than in adult cells, accompanied by a positive shift of voltage-dependent activation. Thus, the postnatal increase in HR is caused by both the increased sympathetic influence and the intrinsic activity of SAN cells. The different conductance and kinetics of LCC may be involved in the postnatal increase in pacemaker activity.

Gender-related differences in a process of the age-dependent alterations of the elements in monkey sino-atrial node

Gender differences in the trace elements of monkey sino-atrial (SA) node were investigated in a process of age-dependent alterations. Sixty hearts from Japanese and rhesus monkeys (30 male and 30 female) used were aged ranging from 1-day- to 30-year-old. The elements were analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-AES). Advancing age decreased all the trace elements. Ca, P, S and Mg significantly decreased. The correlation coefficients of Ca and P were -0.178±0.081 (p<0.05) and -0.088±0.022 (p<0.05) in male (n=30), and -0.095±0.026 (p<0.05) and -0.069±0.017 (p<0.05) in female (n=30), respectively. The age-dependent coefficients for Fe/Ca, Zn/Ca, Fe/P, Fe/S, Zn/S, Fe/Mg and Zn/Mg were exhibited markedly in male, but all was less in female. In gender-related differences, only a ratio of P/Ca (p<0.05) was significantly observed with ageing. The trace elements such as Cu, Se and Sn were less detected in the SA nodes. These results indicate that the age-dependent changes in the ratios of elements are appeared more rapidly in male monkey SA node, and the gender difference is observed in ratio of P/Ca. The different attenuations may be involved with the age- and gender-related SA nodal functions.

Age-dependent changes in Na current magnitude and TTX-sensitivity in the canine sinoatrial node

In rabbit, sodium current (I(Na)) contributes to newborn sinoatrial node (SAN) automaticity but is absent in adult SAN, where heart rate is slower. In contrast, heart rate is high and I(Na) is functional in adult mouse SAN. Given the slower heart rates of large mammals, we asked if I(Na) is functionally active in SAN of newborn or adult canine heart. SAN cells were isolated from newborn (6-10 days), young (40-43 days) and adult mongrels. I(Na) was observed in >80% of cells from each age. However, current density was markedly greater in newborn, decreasing with age. At all ages, I(Na) was sensitive to nanomolar tetrodotoxin (TTX); 100 nmol/L inhibited I(Na) by 46.7%, 59.9% and 90.7% in newborn, young and adult cells, respectively. While high TTX sensitivity suggested the presence of non-cardiac isoforms, steady-state inactivation was relatively negative (midpoints -89.7+/-0.7 mV, -95.1+/-1.2 mV and -93.4+/-1.9 mV from newborn to adult). Consequently, I(Na) should be unavailable at physiological potentials under normal conditions, and 100 nmol/L TTX did not change cycle length or action potential parameters of spontaneous adult SAN cells. However, computer modeling predicts the large newborn I(Na) protects against excess rate slowing from strong vagal stimulation. The results show that canine SAN cells have TTX-sensitive I(Na) which decreases with post-natal age. The current does not contribute to normal automaticity in isolated adult cells but can be recruited to sustain excitability if nodal cells are hyperpolarized. This is particularly relevant in newborn, where I(Na) is large and parasympathetic/sympathetic balance favors vagal tone.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Simulated ischemia enhances L-type calcium current in pacemaker cells isolated from the rabbit sinoatrial node

Ischemic-like conditions (a glucose-free, pH 6.6 Tyrode solution bubbled with 100% N2) enhance L-type Ca current ( ICa,L) in single pacemaker cells (PCs) isolated from the rabbit sinoatrial node (SAN). In contrast, studies of ventricular myocytes have shown that acidic extracellular pH, as employed in our “ischemic” Tyrode, reduces ICa,L. Therefore, our goal was to explain why ICa,Lis increased by “ischemia” in SAN PCs. The major findings were the following: 1) blockade of Ca-induced Ca release with ryanodine, exposure of PCs to BAPTA-AM, or replacement of extracellular Ca2+with Ba2+failed to prevent the ischemia-induced enhancement of ICa,L; 2) inhibition of protein kinase A with H-89, or calcium/calmodulin-dependent protein kinase II with KN-93, reduced ICa,Lbut did not prevent its augmentation by ischemia; 3) ischemic Tyrode or pH 6.6 Tyrode shifted the steady-state inactivation curve in the positive direction, thereby reducing inactivation; 4) ischemic Tyrode increased the maximum conductance but did not affect the activation curve; 5) in rabbit atrial myocytes isolated and studied with exactly the same techniques used for SAN PCs, ischemic Tyrode reduced the maximum conductance and shifted the activation curve in the positive direction; pH 6.6 Tyrode also shifted the steady-state inactivation curve in the positive direction. We conclude that the acidic pH of ischemic Tyrode enhances ICa,Lin SAN PCs, because it increases the maximum conductance and reduces inactivation. Furthermore, the opposite results obtained with rabbit atrial myocytes cannot be explained by differences in cell isolation or patch-clamp techniques.

Electrophysiology and pacemaker function of the developing sinoatrial node

The sinoatrial node performs its task as a cardiac impulse generator throughout the life of the organism, but this important function is not a constant. Rather, there are significant developmental changes in the expression and function of ion channels and other cellular elements, which lead to a postnatal slowing of heart rate and may be crucial to the reliable functioning of the node during maturation. In this review, we provide an overview of current knowledge regarding these changes, with the main focus placed on maturation of the ion channel expression profile. Studies on Na(+) and pacemaker currents have shown that their contribution to automaticity is greater in the newborn than in the adult, but this age-dependent decrease is at least partially opposed by an increased contribution of L-type Ca(2+) current. Whereas information regarding age-dependent changes in other transmembrane currents within the sinoatrial node are lacking, there are data on other relevant parameters. These include an increase in the nodal content of fibroblasts and in the area of nonexpression of connexin43, considered a molecular marker of nodal tissue. Although much remains to be done before a comprehensive view of the developmental biology of the node is available, important evidence in support of a molecular interpretation of developmental slowing of the intrinsic sinoatrial rate is beginning to emerge.

Computational evaluation of the roles of Na+ current, iNa, and cell death in cardiac pacemaking and driving

Voltage-dependent sodium (Na+) channels are heterogeneously distributed through the pacemaker of the heart, the sinoatrial node (SA node). The measured sodium channel current ( iNa) density is higher in the periphery but low or zero in the center of the SA node. The functional roles of iNa in initiation and conduction of cardiac pacemaker activity remain uncertain. We evaluated the functional roles of iNa by computer modeling. A gradient model of the intact SA node and atrium of the rabbit heart was developed that incorporates both heterogeneities of the SA node electrophysiology and histological structure. Our computations show that a large iNa in the periphery helps the SA node to drive the atrial muscle. Removal iNa from the SA node slows down the pacemaking rate and increases the sinoatrial node-atrium conduction time. In some cases, reduction of the SA node iNa results in impairment of impulse initiation and conduction that leads to the SA node-atrium conduction exit block. Decrease in active SA node cell population has similar effects. Combined actions of reduced cell population and removal of iNa from the SA node have greater impacts on weakening the ability of the SA node to pace and drive the atrium.

Specific pattern of ionic channel gene expression associated with pacemaker activity in the mouse heart

Even though sequencing of the mammalian genome has led to the discovery of a large number of ionic channel genes, identification of the molecular determinants of cellular electrical properties in different regions of the heart has been rarely obtained. We developed a high-throughput approach capable of simultaneously assessing the expression pattern of ionic channel repertoires from different regions of the mouse heart. By using large-scale real-time RT-PCR, we have profiled 71 channels and related genes in the sinoatrial node (SAN), atrioventricular node (AVN), the atria (A) and ventricles (V). Hearts from 30 adult male C57BL/6 mice were microdissected and RNA was isolated from six pools of five mice each. TaqMan data were analysed using the threshold cycle (C(t)) relative quantification method. Cross-contamination of each region was checked with expression of the atrial and ventricular myosin light chains. Two-way hierarchical clustering analysis of the 71 genes successfully classified the six pools from the four distinct regions. In comparison with the A, the SAN and AVN were characterized by higher expression of Nav beta 1, Nav beta 3, Cav1.3, Cav3.1 and Cav alpha 2 delta 2, and lower expression of Kv4.2, Cx40, Cx43 and Kir3.1. In addition, the SAN was characterized by higher expression of HCN1 and HCN4, and lower expression of RYR2, Kir6.2, Cav beta 2 and Cav gamma 4. The AVN was characterized by higher expression of Nav1.1, Nav1.7, Kv1.6, Kvbeta1, MinK and Cav gamma 7. Other gene expression profiles discriminate between the ventricular and the atrial myocardium. The present study provides the first genome-scale regional ionic channel expression profile in the mouse heart.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.