Effect of targeted deletions of beta1- and beta2-adrenergic-receptor subtypes on heart rate variability

The ric-8b protein (resistance to inhibitors of cholinesterase 8b) is key to preserving contractile function in the adult heart

Resistance to inhibitors of cholinesterases (ric-8 proteins) are involved in modulating G-protein function, but little is known of their potential physiological importance in the heart. In the present study, we assessed the role of resistance to inhibitors of cholinesterase 8b (Ric-8b) in determining cardiac contractile function. We developed a murine model in which it was possible to conditionally delete ric-8b in cardiac tissue in the adult animal after the addition of tamoxifen. Deletion of ric-8b led to severely reduced contractility as measured using echocardiography days after administration of tamoxifen. Histological analysis of the ventricular tissue showed highly variable myocyte size, prominent fibrosis, and an increase in cellular apoptosis. RNA sequencing revealed transcriptional remodeling in response to cardiac ric-8b deletion involving the extracellular matrix and inflammation. Phosphoproteomic analysis revealed substantial downregulation of phosphopeptides related to myosin light chain 2. At the cellular level, the deletion of ric-8b led to loss of activation of the L-type calcium channel through the β-adrenergic pathways. Using fluorescence resonance energy transfer-based assays, we showed ric-8b protein selectively interacts with the stimulatory G-protein, Gαs. We explored if deletion of Gnas (the gene encoding Gαs) in cardiac tissue using a similar approach in the mouse led to an equivalent phenotype. The conditional deletion of the Gαs gene in the ventricle led to comparable effects on contractile function and cardiac histology. We conclude that ric-8b is essential to preserve cardiac contractile function likely through an interaction with the stimulatory G-protein and downstream phosphorylation of myosin light chain 2.

Effect of stress on the chronotropic and inotropic responses to β-adrenergic agonists in isolated atria of KOβ2 mice

Altered sensitivity to the chronotropic and inotropic effects of catecholamines and reduction in β₁/β₂-adrenoceptor (β₁/β₂-AR) ratio were reported in failing and in senescent human heart, as well as in isolated atria and ventricle of rats submitted to stress. This was due to downregulation of β₁-AR with or without up-regulation of β₂-AR.

AIMS

To investigate the stress-induced behavior of β₁-AR in the heart of mice expressing a non-functional β₂-AR subtype. The guiding hypothesis is that the absence of β₂-AR signaling will not affect the behavior of β₁-AR during stress and that those are independent processes.

MATERIALS AND METHODS

The chronotropic and inotropic responses to β-AR agonists in isolated atria of stressed mice expressing a non-functional β₂-AR were analyzed. The mRNA and protein expressions of β₁- and β₂-AR were also determined.

KEY FINDINGS

No deaths were observed in mice under stress protocol. Atria of stressed mice displayed reduced sensitivity to isoprenaline compared to the controls, an effect that was abolished by the β₂- and β₁-AR antagonists 50 nM ICI118,551 and 300 nM CGP20712A, respectively. Sensitivity and maximum response to the β-agonists dobutamine and salbutamol were not altered by stress or ICI118,551. The responses to dobutamine and salbutamol were prevented by CGP20712A. The expression of β₁-AR was reduced at protein levels.

SIGNIFICANCE

Collectively, our data provide evidence that the cardiac β₂-AR is not essential for survival in a stressful situation and that the stress-induced reduction of β₁-AR expression was independent of the β₂-AR presence.

Evolution of the N-Terminal Regulation of Cardiac Troponin I for Heart Function of Tetrapods: Lungfish Presents an Example of the Emergence of Novel Submolecular Structure to Lead the Capacity of Adaptation

Troponin-based Ca2+ regulation of striated muscle contraction emerged approximately 700 million years ago with largely conserved functions during evolution. Troponin I (TnI) is the inhibitory subunit of troponin and has evolved into three muscle type-specific isoforms in vertebrates. Cardiac TnI is specifically expressed in the adult heart and has a unique N-terminal extension implicating a specific value during natural selection. The N-terminal extension of cardiac TnI in higher vertebrates contains β-adrenergic-regulated protein kinase A (PKA) phosphorylation sites as a mechanism to enhance cardiac muscle relaxation and facilitate ventricular filling. Phylogenic studies showed that the N-terminal extension of cardiac TnI first emerged in the genomes of early tetrapods as well as primordial lobe-finned fishes such as the coelacanth whereas it is absent in ray-finned fish. This apparently rapid evolution of β-adrenergic regulation of cardiac function suggests a high selection value for the heart of vertebrate animals on land to work under higher metabolic demands. Sequencing and PKA phosphorylation data showed that lungfish cardiac TnI has evolved with an amphibian-like N-terminal extension with prototype PKA phosphorylation sites while its overall structure remained fish like. The data demonstrate that the submolecular structure of TnI may evolve ahead of the whole protein for cardiac muscle contractility to adapt to new environmental conditions. Understanding the evolution of the β-adrenergic regulation of TnI and cardiac adaptation to the increased energetic demands of life on land adds knowledge for the treatment of human heart diseases and failure.

Activation of the Thiazide-Sensitive Sodium-Chloride Cotransporter by Beta3-Adrenoreceptor in the Distal Convoluted Tubule

We previously showed that the beta-3 adrenergic receptor (BAR3) is expressed in most segments of the nephron where its agonism promotes a potent antidiuretic effect. We localized BAR3 in distal convoluted tubule (DCT) cells expressing the thiazide-sensitive sodium-chloride cotransporter (NCC). Aim of this study is to investigate the possible functional role of BAR3 on NCC modulation in DCT cells. Here, we found that, in mice, the knockout of BAR3 was paralleled by a significant attenuation of NCC phosphorylation, paralleled by reduced expression and activation of STE-20/SPS1-related proline-alanine-rich kinase (SPAK) and WNKs the main kinases involved in NCC activation. Conversely, in BAR1/2 knockout mice, we found reduced NCC abundance with no changes in the phosphorylation state of NCC. Moreover, selective BAR3 agonism promotes both SPAK and NCC activation in wild-type mouse kidney slices. In conclusion, our findings suggest a novel role for BAR3 in the regulation of NCC in DCT.

The role of resistance to inhibitors of cholinesterase 8b in the control of heart rate

We have assessed the role of ric-b8 in the control of heart rate after the gene was implicated in a recent genome-wide association study of resting heart rate. We developed a novel murine model in which it was possible to conditionally delete ric-8b in the sinoatrial (SA) node after the addition of tamoxifen. Despite this, we were unable to obtain homozygotes and thus studied heterozygotes. Haploinsufficiency of ric-8b in the sinoatrial node induced by the addition of tamoxifen in adult animals leads to mice with a reduced heart rate. However, other electrocardiographic intervals (e.g., PR and QRS) were normal, and there was no apparent arrhythmia such as heart block. The positive chronotropic response to isoprenaline was abrogated, whereas the response to carbachol was unchanged. The pacemaker current I_f (funny current) has an important role in regulating heart rate, and its function is modulated by both isoprenaline and carbachol. Using a heterologous system expressing HCN4, we show that ric-8b can modulate the HCN4 current. Overexpression of ric-8b led to larger HCN4 currents, whereas silencing ric-8b led to smaller currents. Ric-8b modulates heart rate responses in vivo likely via its actions on the stimulatory G-protein.

Nardilysin controls cardiac sympathetic innervation patterning through regulation of p75 neurotrophin receptor

Cardiac sympathetic innervation is critically involved in the regulation of circulatory dynamics. However, the molecular mechanism for the innervation patterning has remained elusive. Here, we demonstrate that nardilysin (NRDC, Nrdc), an enhancer of ectodomain shedding, regulates cardiac sympathetic innervation. Nardilysin-deficient (Nrdc^-/- ) mice show hypoplastic hearts, hypotension, bradycardia, and abnormal sympathetic innervation patterning. While the innervation of left ventricle (LV) of wild-type mice is denser in the subepicardium than in the subendocardium, Nrdc^-/- LV lacks such a polarity and is uniformly and more abundantly innervated. At the molecular level, the full-length form of p75 neurotrophin receptor (p75^NTR , Ngfr) is increased in Nrdc^-/- LV due to the reduced ectodomain shedding of p75^NTR . Importantly, the reduction of p75^NTR rescued the abnormal innervation phenotype of Nrdc^-/- mice. Moreover, sympathetic neuron-specific, but not cardiomyocyte-specific deletion of Nrdc recapitulated the abnormal innervation patterning of Nrdc^-/- mice. In conclusion, neuronal nardilysin critically regulates cardiac sympathetic innervation and circulatory dynamics via modulation of p75^NTR .

Downregulation of the β1 adrenergic receptor in the myocardium results in insensitivity to metoprolol and reduces blood pressure in spontaneously hypertensive rats

The β1‑adrenergic receptor (AR) is the primary β‑AR subtype in the heart and is the target of metoprolol (Met), which is commonly used to treat angina and hypertension. Previous studies have revealed a positive correlation between the methylation levels of the adrenoreceptor β1 gene (Adrb1) promoter in the myocardium with the antihypertensive activity of Met in spontaneously hypertensive rats (SHR), which affects β1‑AR expression in H9C2 cells. The aim of the present study was to investigate the effects of myocardial β1‑AR downregulation using short‑hairpin RNA (shRNA) against Adrb1 on the antihypertensive activity of Met in SHR. Recombinant adeno‑associated virus type 9 (rAAV9) vectors carrying Adrb1 shRNA (rAAV9‑Adrb1) or a negative control sequence (rAAV9‑NC) were generated and used to infect rat hearts via the pericardial cavity. The results of reverse transcription‑quantitative polymerase chain reaction, immunohistochemistry and western blotting analyses demonstrated that cardiac β1‑AR expression in the rAAV9‑Adrb1 group was significantly downregulated when compared with the rAAV9‑NC group (P<0.001, P<0.001 and P=0.032, respectively). In addition, a greater reduction in systolic blood pressure (SBP) was observed in the rAAV9‑NC group compared with the rAAV9‑Adrb1 group following Met treatment (P=0.035). Furthermore, downregulation of myocardial β1‑AR was associated with a significant decrease in SBP (P<0.001). In conclusion, these data suggest that suppression of β1‑AR expression in the myocardium reduces SBP and sensitivity to Met in SHR.

β3 adrenergic receptor in the kidney may be a new player in sympathetic regulation of renal function

To date, the study of the sympathetic regulation of renal function has been restricted to the important contribution of β1- and β2-adrenergic receptors (ARs). Here we investigate the expression and the possible physiologic role of β3-adrenergic receptor (β3-AR) in mouse kidney. The β3-AR is expressed in most of the nephron segments that also express the type 2 vasopressin receptor (AVPR2), including the thick ascending limb and the cortical and outer medullary collecting duct. Ex vivo experiments in mouse kidney tubules showed that β3-AR stimulation with the selective agonist BRL37344 increased intracellular cAMP levels and promoted 2 key processes in the urine concentrating mechanism. These are accumulation of the water channel aquaporin 2 at the apical plasma membrane in the collecting duct and activation of the Na-K-2Cl symporter in the thick ascending limb. Both effects were prevented by the β3-AR antagonist L748,337 or by the protein kinase A inhibitor H89. Interestingly, genetic inactivation of β3-AR in mice was associated with significantly increased urine excretion of water, sodium, potassium, and chloride. Stimulation of β3-AR significantly reduced urine excretion of water and the same electrolytes. Moreover, BRL37344 promoted a potent antidiuretic effect in AVPR2-null mice. Thus, our findings are of potential physiologic importance as they uncover the antidiuretic effect of β3-AR stimulation in the kidney. Hence, β3-AR agonism might be useful to bypass AVPR2-inactivating mutations.

Comprehensive multilevel in vivo and in vitro analysis of heart rate fluctuations in mice by ECG telemetry and electrophysiology

The normal heartbeat slightly fluctuates around a mean value; this phenomenon is called physiological heart rate variability (HRV). It is well known that altered HRV is a risk factor for sudden cardiac death. The availability of genetic mouse models makes it possible to experimentally dissect the mechanism of pathological changes in HRV and its relation to sudden cardiac death. Here we provide a protocol that allows for a comprehensive multilevel analysis of heart rate (HR) fluctuations. The protocol comprises a set of techniques that include in vivo telemetry and in vitro electrophysiology of intact sinoatrial network preparations or isolated single sinoatrial node (SAN) cells. In vitro preparations can be completed within a few hours, with data acquisition within 1 d. In vivo telemetric ECG requires 1 h for surgery and several weeks for data acquisition and analysis. This protocol is of interest to researchers investigating cardiovascular physiology and the pathophysiology of sudden cardiac death.

The New Age of Sudomotor Function Testing: A Sensitive and Specific Biomarker for Diagnosis, Estimation of Severity, Monitoring Progression, and Regression in Response to Intervention

Sudorimetry technology has evolved dramatically, as a rapid, non-invasive, robust, and accurate biomarker for small fibers that can easily be integrated into clinical practice. Though skin biopsy with quantitation of intraepidermal nerve fiber density is still currently recognized as the gold standard, sudorimetry may yield diagnostic information not only on autonomic dysfunction but also enhance the assessment of the small somatosensory nerves, disease detection, progression, and response to therapy. Sudorimetry can be assessed using Sudoscan™, which measures electrochemical skin conductance (ESC) of hands and feet. It is based on different electrochemical principles (reverse iontophoresis and chronoamperometry) to measure sudomotor function than prior technologies, affording it a much more practical and precise performance profile for routine clinical use with potential as a research tool. Small nerve fiber dysfunction has been found to occur early in metabolic syndrome and diabetes and may also be the only neurological manifestation in small fiber neuropathies, beneath the detection limits of traditional nerve function tests. Test results are robust, accomplished within minutes, require little technical training and no calculations, since established norms have been provided for the effects of age, gender, and ethnicity. Sudomotor testing has been greatly under-utilized in the past, restricted to specialized centers capable of handling the technically demanding and expensive technology. Yet, evaluation of autonomic and somatic nerve function has been shown to be one of the best estimates of cardiovascular risk. Evaluation of sweating has the appeal of quantifiable non-invasive determination of the integrity of the peripheral autonomic nervous system, and can now be accomplished rapidly at point of care clinics with the determination of ESC, allowing intervention for morbid complications prior to permanent structural nerve damage. We review here sudomotor function testing technology, the research evidence accumulated supporting the clinical utility of measuring ESC, the medical applications of sudorimetry now available to physicians with this device, and clinical vignettes illustrating its use in the clinical decision-making process.

αB-Crystallin R120G variant causes cardiac arrhythmias and alterations in the expression of Ca(2+) -handling proteins and endoplasmic reticulum stress in mice

Mutations of αB-crystallin (CryαB), a small heat shock protein abundantly expressed in cardiac and skeletal muscles, are known to cause desmin-related myopathies. The CryαB R120G allele has been linked to a familial desminopathy and, in transgenic mice, causes a sudden death at about 28 weeks of age. To investigate the mechanisms of the sudden cardiac arrest of CryαB R120G transgenic mice, we prepared protein samples from left ventricular tissues of two different age groups (10 and 28 weeks) and examined Ca(2+) -handling proteins. Expression of sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) 2, phospholamban, ryanodine receptor 2 and calsequestrin 2 was significantly decreased in 28- versus 10-week-old CryαB R120G transgenic mice. In addition, low heart rate variability, including heart rate, total power and low frequency, was observed and continuous electrocardiogram monitoring revealed cardiac arrhythmias, such as ventricular tachycardia, atrioventricular block and atrial flutter, in 28-week-old CryαB R120G transgenic mice. In contrast, expression of endoplasmic reticulum (ER) degradation enhancing α-mannosidase-like protein, inositol requirement 1 and X-box binding protein 1 were increased significantly in 28- versus 10-week-old CryαBR120G transgenic mice, suggesting that the CryαBR120G transgenic mice exhibit increased ER stress compared with wild-type mice. Together, the data suggest that the CryαB R120G dominant variant induces ER stress and impairs Ca(2+) regulation, leading to ageing-related cardiac dysfunction, arrhythmias and decreased autonomic tone with shortened lifespan.

Adrenergic regulation of the rapid component of delayed rectifier K+ currents in guinea pig cardiomyocytes

BACKGROUND

Guinea pig ventricular cardiomyocytes display the rapid component of the delayed rectifier potassium current (Ikr) that contributes to ventricular repolarization and promotes stress-induced arrhythmias. Adrenergic stimulation favors ventricular arrhythmogenesis but its effects on Ikr are poorly understood.

METHODS

Adrenergic modulation of Ikr was studied in isolated guinea pig ventricular cardiomyocytes using whole-cell patch clamping.

RESULTS

We found that the Ikr amplitude was reduced to 0.66±0.02 and 0.62±0.03 in response to 0.1 µM phenylephrine (PE), an α1AR agonist, and 10 µM isoproterenol (ISO), a βAR agonist, respectively. The effect of PE can be blocked by the selective α1A-adrenoceptor antagonist 5-methylurapidil, but not by the α1B-adrenoceptor antagonist chloroethylclonidine or α1D-adrenoceptor antagonist BMY7378. Additionally, the effect of ISO can be blocked by the β1-selective AR antagonist CGP-20712A, but not by the β2-selective AR antagonist ICI-118551. Although PE and ISO was continuously added to cells, ISO did not decrease the current to a greater extent when cells were first given PE. In addition, PE's effect on Ikr was suppressed by β1AR stimulation.

CONCLUSIONS

Ikr can by regulated by both the α1 and β ARs system, and that in addition to direct regulation by each receptor system, crosstalk may exist between the two systems.

Clenbuterol plus granulocyte colony-stimulating factor regulates stem/progenitor cell mobilization and exerts beneficial effect by increasing neovascularization in rats with heart failure

BACKGROUND

Treatment of beta2-adrenergic receptor agonists with myeloid cytokines, such as granulocyte colony-stimulating factor (G-CSF) has been reported to enhance stem/progenitor cell mobilization and proliferation in ischemic myocardium. However, whether the combination therapy of G-CSF and clenbuterol (Clen) contributes to improved left ventricular (LV) function remains uncertain. We investigated whether this combination therapy induced bone marrow-derived stem/progenitor cell mobilization, neovascularization, and altered LV function after acute myocardial infarction (MI).

METHODS AND RESULTS

Following MI, rats were treated with single Clen, high-dose Clen, and G-CSF + Clen. We evaluated LV function and remodeling with the use of echocardiography in addition to hemodynamics 3 weeks after MI. Treatment with G-CSF + Clen increased (P < .05), compared with no treatment, LV ejection fraction 46 ± 3% vs 34 ± 2%, LV dP/dt 5,789 ± 394 mm Hg vs 4,503 ± 283 mm Hg, and the percentage of circulating CD34+ cells, appearing to correlate with improvements in LV function.

CONCLUSIONS

Combination therapy improved LV function 3 weeks after MI, suggesting that G-CSF + Clen might augment stem/progenitor cell migration, contributing to tissue healing. These data raise the possibility that enhancing endogenous bone marrow-derived stem/progenitor cell mobilization may be a new treatment for ischemic heart failure after MI.

Novel, gel-free proteomics approach identifies RNF5 and JAMP as modulators of GPCR stability

The maturation and folding of G protein-coupled receptors are governed by mechanisms that remain poorly understood. In an effort to characterize these biological events, we optimized a novel, gel-free proteomic approach to identify partners of the β2-adrenergic receptor (β2AR). In addition to a number of known interacting proteins such as heterotrimeric G protein subunits, this allowed us to identify proteins involved in endoplasmic reticulum (ER) QC of the receptor. Among β2AR-associated proteins is Ring finger protein 5 (RNF5), an E3 ubiquitin ligase anchored to the outer membrane of the ER. Coimmunoprecipitation assays confirmed, in a cellular context, the interaction between RNF5 and the β2AR as well as the prostaglandin D2 receptor (DP). Confocal microscopy revealed that DP colocalized with RNF5 at the ER. Coexpression of RNF5 with either receptor increased levels of their expression, whereas small interfering RNA-mediated knockdown of endogenous RNF5 promoted the opposite. RNF5 did not modulate the ubiquitination state of β2AR or DP. Instead, RNF5 ubiquitinated JNK-associated membrane protein (JAMP), a protein that recruits the proteasome to the ER membrane and that is negatively regulated by RNF5-mediated ubiquitination. JAMP coimmunoprecipitated with both β2AR and DP and decreased total receptor protein levels through proteasomal degradation. Expression of DP, a receptor largely retained in the ER, promoted proteasome recruitment by JAMP. Degradation of both receptors via JAMP was increased when RNF5 was depleted. Our data suggest that RNF5 regulates the turnover of specific G protein-coupled receptors by ubiquitinating JAMP and preventing proteasome recruitment.

The in vivo regulation of heart rate in the murine sinoatrial node by stimulatory and inhibitory heterotrimeric G proteins

Reciprocal physiological modulation of heart rate is controlled by the sympathetic and parasympathetic systems acting on the sinoatrial (SA) node. However, there is little direct in vivo work examining the role of stimulatory and inhibitory G protein signaling in the SA node. Thus, we designed a study to examine the role of the stimulatory (Gαs) and inhibitory G protein (Gαi2) in in vivo heart rate regulation in the SA node in the mouse. We studied mice with conditional deletion of Gαs and Gαi2 in the conduction system using cre-loxP technology. We crossed mice in which cre recombinase expression was driven by a tamoxifen-inducible conduction system-specific construct with “Gαs floxed” and “Gαi2 floxed” mice. We studied the heart rate responses of adult mice compared with littermate controls by using radiotelemetry before and after administration of tamoxifen. The mice with conditional deletion of Gαs and Gαi2 had a loss of diurnal variation and were bradycardic or tachycardic, respectively, in the daytime. In mice with conditional deletion of Gαs, there was a selective loss of low-frequency power, while with deletion of Gαi2, there was a loss of high-frequency power in power spectral analysis of heart rate variability. There was no evidence of pathological arrhythmia. Pharmacological modulation of heart rate by isoprenaline was impaired in the Gαs mice, but a muscarinic agonist was still able to slow the heart rate in Gαi2 mice. We conclude that Gαs- and Gαi2-mediated signaling in the sinoatrial node is important in the reciprocal regulation of heart rate through the autonomic nervous system.

Hormonal regulation of β-myosin heavy chain expression in the mouse left ventricle

We investigated the influence of sex hormones on the expression of α- and β-cardiac myosin heavy chain isoforms (α-MHC and β-MHC) in C57bl/6 mice of both sexes under physiological and pathological conditions. In the left ventricles (LVs) of fertile female mice, β-MHC expression was tenfold higher compared with the age-matched males, whereas no difference was found in α-MHC expression. These differences disappeared after ovariectomy or in immature mice. We also found a sex-related difference in expression of β-adrenoceptors (β1-AR), as mRNA levels of this gene were 40% lower in fertile females compared with males of the same age but did not differ in prepubertal or ovariectomized animals. Interestingly, the deletion of both β1- and β2-ARs abolished sex difference of β-MHC expression, as mRNA levels in the LVs of knockout males were increased and reached values comparable to those of knockout females. Moreover, the β1-AR antagonist metoprolol induced about a threefold increase in β-MHC expression in adult male mice. The capability of gender to regulate β-MHC expression was also evaluated in the presence of hemodynamic overload. Thoracic aortic coarctation (TAC) produced cardiac hypertrophy along with a 12-fold increase in β-MHC and a 50% decrease in β1-AR expression in males but not in females, thus abolishing the gender difference observed in sham animals for such genes. By contrast, TAC did not change β2-AR expression. In conclusion, our results show that the expression of β-MHC and β1-AR in the LVs undergo gender-related and correlated changes under both physiological and pathological conditions and suggest a role of β1-AR-mediated signaling.

Total beta-adrenoceptor knockout slows conduction and reduces inducible arrhythmias in the mouse heart

INTRODUCTION

Beta-adrenoceptors (β-AR) play an important role in the neurohumoral regulation of cardiac function. Three β-AR subtypes (β(1), β(2), β(3)) have been described so far. Total deficiency of these adrenoceptors (TKO) results in cardiac hypotrophy and negative inotropy. TKO represents a unique mouse model mimicking total unselective medical β-blocker therapy in men. Electrophysiological characteristics of TKO have not yet been investigated in an animal model.

METHODS

In vivo electrophysiological studies using right heart catheterisation were performed in 10 TKO mice and 10 129SV wild type control mice (WT) at the age of 15 weeks. Standard surface ECG, intracardiac and electrophysiological parameters, and arrhythmia inducibility were analyzed.

RESULTS

The surface ECG of TKO mice revealed a reduced heart rate (359.2±20.9 bpm vs. 461.1±33.3 bpm; p<0.001), prolonged P wave (17.5±3.0 ms vs. 15.1±1.2 ms; p = 0.019) and PQ time (40.8±2.4 ms vs. 37.3±3.0 ms; p = 0.013) compared to WT. Intracardiac ECG showed a significantly prolonged infra-Hisian conductance (HV-interval: 12.9±1.4 ms vs. 6.8±1.0 ms; p<0.001). Functional testing showed prolonged atrial and ventricular refractory periods in TKO (40.5±15.5 ms vs. 21.3±5.8 ms; p = 0.004; and 41.0±9.7 ms vs. 28.3±6.6 ms; p = 0.004, respectively). In TKO both the probability of induction of atrial fibrillation (12% vs. 24%; p<0.001) and of ventricular tachycardias (0% vs. 26%; p<0.001) were significantly reduced.

CONCLUSION

TKO results in significant prolongations of cardiac conduction times and refractory periods. This was accompanied by a highly significant reduction of atrial and ventricular arrhythmias. Our finding confirms the importance of β-AR in arrhythmogenesis and the potential role of unspecific beta-receptor-blockade as therapeutic target.

Decrease in heart adrenoceptor gene expression and receptor number as compensatory tool for preserved heart function and biological rhythm in M(2) KO animals

Muscarinic receptors (MR) are main cardioinhibitory receptors. We investigated the changes in gene expression, receptor number, echocardiography, muscarinic/adrenergic agonist/antagonist changes in heart rate (HR) and HR biorhythm in M(2) KO mice (mice lacking the main cardioinhibitory receptors) in the left ventricle (LV) and right ventricle (RV). We hypothesize that the disruption of M(2) MR, key players in parasympathetic bradycardia, would change the number of receptors with antagonistic effects on the heart (β(1)- and β(2)-adrenoceptors, BAR), while the function of the heart would be changed only marginally. We have found changes in LV, but not in RV: decrease in M(3) MR, β(1)- and β(2)-adrenoceptor gene expressions that were accompanied by a decrease in MR and BAR receptor binding. No changes were found both in LV systolic and diastolic function as assessed by echocardiography (e.g., similar LV end-systolic and end-diastolic diameter, fractional shortening, mitral flow characteristics, and maximal velocity in LV outflow tract). We have found only marginal changes in specific HR biorhythm parameters. The effects of isoprenaline and propranolol on HR were similar in WT and KO (but with lesser extent). Atropine was not able to increase HR in KO animals. Carbachol decreased the HR in WT but increased HR in KO, suggesting the presence of cardiostimulatory MR. Therefore, we can conclude that although the main cardioinhibitory receptors are not present in the heart, the function is not much affected. As possible mechanisms of almost normal cardiac function, the decreases of both β(1)- and β(2)-adrenoceptor gene expression and receptor binding should be considered.

Autonomic nervous system modulation affects the inflammatory immune response in mice with acute Chagas disease

The aim of the present study was to evaluate the effects of changes to the autonomic nervous system in mice during the acute phase of Chagas disease, which is an infection caused by the parasite Trypanosoma cruzi. The following types of mice were inoculated with T. cruzi (CHG): wild-type (WT) and vesicular acetylcholine transporter knockdown (KDVAChT) C57BL/6j mice; wild-type non-treated (NT) FVB mice; FVB mice treated with pyridostigmine bromide (PYR) or salbutamol (SALB); and β(2)-adrenergic receptor knockout (KOβ2) FVB mice. During infection and at 18-21 days after infection (acute phase), the survival curves, parasitaemia, electrocardiograms, heart rate variability, autonomic tonus and histopathology of the animals were evaluated. Negative control groups were matched for age, genetic background and treatment. The KDVAChT-CHG mice exhibited a significant shift in the electrocardiographic, autonomic and histopathological profiles towards a greater inflammatory immune response that was associated with a reduction in blood and tissue parasitism. In contrast, the CHG-PYR mice manifested reduced myocardial inflammation and lower blood and tissue parasitism. Similar results were observed in CHG-SALB animals. Unexpectedly, the KOβ2-CHG mice exhibited less myocardial inflammation and higher blood and tissue parasitism, which were associated with reduced mortality. These findings could have been due to the increase in vagal tone observed in the KOβ2 mice, which rendered them more similar to the CHG-PYR animals. In conclusion, our results indicate a marked immunomodulatory role for the parasympathetic and sympathetic autonomic nervous systems, which inhibit both the inflammatory immune response and parasite clearance during the acute phase of experimental Chagas heart disease in mice.

Sarcalumenin plays a critical role in age-related cardiac dysfunction due to decreases in SERCA2a expression and activity

Impaired Ca(2+) reuptake into the sarcoplasmic reticulum (SR) underlies a primary pathogenesis of heart failure in the aging heart. Sarcalumenin (SAR), a Ca(2+)-binding glycoprotein located in the longitudinal SR, regulates Ca(2+) reuptake by interacting with SR Ca(2+)-ATPase (SERCA). Here we found that the expression levels of both SAR and SERCA2 proteins were significantly downregulated in senescent wild-type mice (18-month old) and that downregulation of SAR protein preceded downregulation of SERCA2 protein. The downregulation of SERCA2 protein was greater in senescent SARKO mice than in age-matched senescent wild-type mice, which was at least in part due to progressive degradation of SERCA2 protein in SARKO mice. Senescent SARKO mice exhibited typical findings of heart failure such as increased sympathetic activity, impaired exercise tolerance, and upregulation of biomarkers of cardiac stress. Consequently, cardiac function was progressively decreased in senescent SARKO. We also found that the expression levels of endoplasmic reticulum (ER) stress-related genes such as x-box binding protein 1 (XBP1) were significantly increased in senescent SARKO mice, indicating that senescent SARKO mice exhibited ER stress. Thus we uncovered the important role of SAR in maintaining Ca(2+) transport activity of SERCA2a and cardiac function in the senescent population.

Frequency ranges of heart rate variability related to autonomic nerve activity in the mouse

Mice have gained more and more attention in recent years and been widely used in transgenic experiments. Although the number of researches on the heart rate variability (HRV) of mice has been gradually increasing, a consensus on the frequency ranges of autonomic modulation has not been established. Therefore, the main purpose of this study was to find a HRV "prototype" for conscious mice in the state of being motionless and breathing regularly (called "genuinely resting"), and to determine the frequency ranges corresponding to the autonomic modulation. Further, whether these frequencies will change when the mice move freely was studied to evaluate the feasibility of the HRV spectrum as an index of the autonomic modulation of mice. The recording sites were specially arranged to simultaneously obtain the electrocardiography and electromyography data to be provided for the use of HRV analysis and motion monitoring, respectively. The states of being motionless and breathing regularly as judged from the electromyography results were selected as a genuine resting state of a conscious mouse. The frequencies related to autonomic modulation of HRV were determined by comparing the spectrum changes before and after blockades of the autonomic tone by different pharmaceutical agents in both the genuine resting state and freely moving states. Our results showed that the HRV of mice is not suitable for indexing sympathetic modulation; however, it is possible to use the spectral power in the frequency range between 0.1 and 1 Hz as an index of parasympathetic modulation.

Modeling the association between HR variability and illness in elite swimmers

PURPOSE

To determine whether HR variability (HRV), an indirect measure of autonomic control, is associated with upper respiratory tract and pulmonary infections, muscular affections, and all-type pathologies in elite swimmers.

METHODS

For this study, 7 elite international and 11 national swimmers were observed weekly for 2 yr. The indexes of cardiac autonomic regulation in supine and orthostatic position were assessed as explanatory variables by time domain (SD1, SD2) and spectral analyses (high frequency [HF] = 0.15-0.40 Hz, low frequency [LF] = 0.04-0.15 Hz, and HF/LF ratio) of HRV. Logistic mixed models described the relationship between the explanatory variables and the risk of upper respiratory tract and pulmonary infections, muscular affections, and all-type pathologies.

RESULTS

The risk of all-type pathologies was higher for national swimmers and in winter (P < 0.01). An increase in the parasympathetic indexes (HF, SD1) in the supine position assessed 1 wk earlier was linked to a higher risk of upper respiratory tract and pulmonary infections (P < 0.05) and to a higher risk of muscular affections (increase in HF, P < 0.05). Multivariate analyses showed (1) a higher all-type pathologies risk in winter and for an increase in the total power of HRV associated with a decline SD1 in supine position, (2) a higher all-type pathologies risk in winter associated with a decline in HF assessed 1 wk earlier in orthostatic position, and (3) a higher risk of muscular affections in winter associated with a decrease SD1 and an increase LF in orthostatic position.

CONCLUSIONS

Swimmers' health maintenance requires particular attention when autonomic balance shows a sudden increase in parasympathetic indices in the supine position assessed 1 wk earlier evolving toward sympathetic predominance in supine and orthostatic positions.

In vivo and in vitro assessment of cardiac beta-adrenergic receptors in larval zebrafish (Danio rerio)

β-Adrenergic receptors (βARs) are crucial for maintaining the rate and force of cardiac muscle contraction in vertebrates. Zebrafish (Danio rerio) have one β1AR gene and two β2AR genes (β2aAR and β2bAR). We examined the roles of these receptors in larval zebrafish in vivo by assessing the impact of translational gene knockdown on cardiac function. Zebrafish larvae lacking β1AR expression by morpholino knockdown displayed lower heart rates than control fish, whereas larvae deficient in both β2aAR and β2bAR expression exhibited significantly higher heart rates than controls. These results suggested a potential inhibitory role for one or both β2AR genes. By using cultured HEK293 cells transfected with zebrafish βARs, we demonstrated that stimulation with adrenaline or procaterol (a β2AR agonist) resulted in an increase in intracellular cAMP levels in cells expressing any of the three zebrafish βARs. In comparison with its human βAR counterpart, zebrafish β2aAR expressed in HEK293 cells appeared to exhibit a unique binding affinity profile for adrenergic ligands. Specifically, zebrafish β2aAR had a high binding affinity for phenylephrine, a classical α-adrenergic receptor agonist. The zebrafish receptors also had distinct ligand binding affinities for adrenergic agonists when compared with human βARs in culture, with zebrafish β2aAR being distinct from human β2AR and zebrafish β2bAR. Overall, this study provides insight into the function and evolution of both fish and mammalian β-adrenergic receptors.

TGF-β1-mediated fibrosis and ion channel remodeling are key mechanisms in producing the sinus node dysfunction associated with SCN5A deficiency and aging

BACKGROUND

Mutations in the cardiac Na(+) channel gene (SCN5A) can adversely affect electric function in the heart, but effects can be age dependent. We explored the interacting effects of Scn5a disruption and aging on the pathogenesis of sinus node dysfunction in a heterozygous Scn5a knockout (Scn5a(+/-)) mouse model.

METHODS AND RESULTS

We compared functional, histological, and molecular features in young (3 to 4 month) and old (1 year) wild type and Scn5a(+/-) mice. Both Scn5a disruption and aging were associated with decreased heart rate variability, reduced sinoatrial node automaticity, and slowed sinoatrial conduction. They also led to increased collagen and fibroblast levels and upregulated transforming growth factor-β(1) (TGF-β(1)) and vimentin transcripts, providing measures of fibrosis and reduced Nav1.5 expression. All these effects were most noticeable in old Scn5a(+/-) mice. Na(+) channel inhibition by Nav1.5-E3 antibody directly increased TGF-β(1) production in both cultured human cardiac myocytes and fibroblasts. Finally, aging was associated with downregulation of a wide range of ion channel and related transcripts and, again, was greatest in old Scn5a(+/-) mice. The quantitative results from these studies permitted computer simulations that successfully replicated the observed sinoatrial node phenotypes shown by the different experimental groups.

CONCLUSIONS

These results implicate a tissue degeneration triggered by Nav1.5 deficiency manifesting as a TGF-β(1)-mediated fibrosis accompanied by electric remodeling in the sinus node dysfunction associated with Scn5a disruption or aging. The latter effects interact to produce the most severe phenotype in old Scn5a(+/-) mice. In demonstrating this, our findings suggest a novel regulatory role for Nav1.5 in cellular biological processes in addition to its electrophysiologic function.

Particulate matter inhalation exacerbates cardiopulmonary injury in a rat model of isoproterenol-induced cardiomyopathy

Ambient particulate matter (PM) exposure is linked to cardiovascular events and death, especially among individuals with heart disease. A model of toxic cardiomyopathy was developed in Spontaneously Hypertensive Heart Failure (SHHF) rats to explore potential mechanisms. Rats were infused with isoproterenol (ISO; 2.5 mg/kg/day subcutaneous [sc]), a beta-adrenergic agonist, for 28 days and subsequently exposed to PM by inhalation. ISO induced tachycardia and hypotension throughout treatment followed by postinfusion decrements in heart rate, contractility, and blood pressures (systolic, diastolic, pulse), and fibrotic cardiomyopathy. Changes in heart rate and heart rate variability (HRV) 17 days after ISO cessation indicated parasympathetic dominance with concomitantly altered ventilation. Rats were subsequently exposed to filtered air or Harvard Particle 12 (HP12) (12 mg/m(3))--a metal-rich oil combustion-derived PM--at 18 and 19 days (4 h/day) after ISO infusion via nose-only inhalation to determine if cardio-impaired rats were more responsive to the effects of PM exposure. Inhalation of PM among ISO-pretreated rats significantly increased pulmonary lactate dehydrogenase, serum high-density lipoprotein (HDL) cholesterol, and heart-to-body mass ratio. PM exposure increased the number of ISO-pretreated rats that experienced bradyarrhythmic events, which occurred concomitantly with acute alterations of HRV. PM, however, did not significantly affect mean HRV in the ISO- or saline-pretreated groups. In summary, subchronic ISO treatment elicited some pathophysiologic and histopathological features of heart failure, including cardiomyopathy. The enhanced sensitivity to PM exposure in SHHF rats with ISO-accelerated cardiomyopathy suggests that this model may be useful for elucidating the mechanisms by which PM exposure exacerbates heart disease.

Effects of cyclooxygenase-2 gene inactivation on cardiac autonomic and left ventricular function in experimental diabetes

Glucose-mediated oxidative stress and the upregulation of cyclooxygenase (COX)-2 pathway activity have been implicated in the pathogenesis of several vascular complications of diabetes including diabetic neuropathy. However, in nondiabetic subjects, the cardiovascular safety of selective COX-2 inhibition is controversial. The aim of this study was to explore the links between hyperglycemia, oxidative stress, activation of the COX-2 pathway, cardiac sympathetic integrity, and the development of left ventricular (LV) dysfunction in experimental diabetes. R wave-to-R wave interval (R-R interval) and parameters of LV function measured by echocardiography using 1% isoflurane, LV sympathetic nerve fiber density, LV collagen content, and markers of myocardial oxidative stress, inflammation, and PG content were assessed after 6 mo in control and diabetic COX-2-deficient (COX-2(-/-)) and littermate, wild-type (COX-2(+/+)) mice. There were no differences in blood glucose, LV echocardiographic measures, collagen content, sympathetic nerve fiber density, and markers of oxidative stress and inflammation between nondiabetic (ND) COX-2(+/+) and COX-2(-/-) mice at baseline and thereafter. After 6 mo, diabetic COX-2(+/+) mice developed significant deteriorations in the R-R interval and signs of LV dysfunction. These were associated with a loss of LV sympathetic nerve fiber density, increased LV collagen content, and a significant increase in myocardial oxidative stress and inflammation compared with those of ND mice. Diabetic COX-2(-/-) mice were protected against all these biochemical, structural, and functional deficits. These data suggest that in experimental diabetes, selective COX-2 inactivation confers protection against sympathetic denervation and LV dysfunction by reducing intramyocardial oxidative stress, inflammation, and myocardial fibrosis.

The role of inhibitory heterotrimeric G proteins in the control of in vivo heart rate dynamics

Multiple isoforms of inhibitory Galpha-subunits (Galphai1,2,3, as well as Galphao) are present within the heart, and their role in modulating pacemaker function remains unresolved. Do inhibitory Galpha-subunits selectively modulate parasympathetic heart rate responses? Published findings using a variety of experimental approaches have implicated roles for Galphai2, Galphai3, and Galphao in parasympathetic signal transduction. We have compared in vivo different groups of mice with global genetic deletion of Gialpha1/Galphai3, Galphai2, and Galphao against littermate controls using implanted ECG telemetry. Significant resting tachycardia was observed in Galphai2(-/-) and Galphao(-/-) mice compared with control and Galphai1(-/-)/Galphai3(-/-) mice (P < 0.05). Loss of diurnal heart rate variation was seen exclusively in Galphao(-/-) mice. Using heart rate variability (HRV) analysis, compared with littermate controls (4.02 ms2 +/- 1.17; n = 6, Galphai2(-/-)) mice have a selective attenuation of high-frequency (HF) power (0.73 ms2 +/- 0.31; n = 5, P < 0.05). Galphai1(-/-)/Galphai3(-/-) and Galphao(-/-) cohorts have nonsignificant changes in HF power. Galphao(-/-) mice have a different basal HRV signature. The observed HRV phenotype in Galphai2(-/-) mice was qualitatively similar to atropine (1 mg/kg)-treated controls [and mice treated with the GIRK channel blocker tertiapinQ (0.05 mg/kg)]. Maximal cardioinhibitory response to the M(2)-receptor agonist carbachol (0.5 mg/kg) compared with basal heart rate was attenuated in Galphai2(-/-) mice (0.08 +/- 0.04; n = 6) compared to control (0.27 +/- 0.04; n = 7 P < 0.05). Our data suggest a selective defect of parasympathetic heart rate modulation in mice with Galphai2 deletion. Mice with Galphao deletion also have a defect in short-term heart rate dynamics, but this is qualitatively different to the effects of atropine, tertiapinQ, and Galphai2 deletion. In contrast, Galphai1 and Galphai3 do not appear to be essential for parasympathetic responses in vivo.

Vagal tone dominates autonomic control of mouse heart rate at thermoneutrality

It is generally accepted that cardiac sympathetic tone dominates the control of heart rate (HR) in mice. However, we have recently challenged this notion given that HR in the mouse is responsive to ambient temperature (Ta) and that the housing Tais typically 21–23°C, well below the thermoneutral zone (∼30°C) of this species. To specifically test the hypothesis that cardiac sympathetic tone is the primary mediator of HR control in the mouse, we first examined the metabolic and cardiovascular responses to rapid changes in Tato demonstrate the sensitivity of the mouse cardiovascular system to Ta. We then determined HR in 1) mice deficient in cardiac sympathetic tone (“β-less” mice), 2) mice deficient in cardiac vagal tone [muscarinic M2receptor ( M2R−/−) mice], and 3) littermate controls. At a Taof 30°C, the HR of β-less mice was identical to that of wild-type mice (351 ± 11 and 363 ± 10 beats/min, respectively). However, the HR of M2R−/−mice was significantly greater (416 ± 7 beats/min), demonstrating that vagal tone predominates over HR control at this Ta. When these mice were calorically restricted to 70% of normal intake, HR fell equally in wild-type, β-less, and M2R−/−mice (ΔHR = 73 ± 9, 76 ± 3, and 73 ± 7 beats/min, respectively), suggesting that the fall in intrinsic HR governs bradycardia of calorically restricted mice. Only when the Tawas relatively cool, at 23°C, did β-less mice exhibit a HR (442 ± 14 beats/min) that was different from that of littermate controls (604 ± 10 beats/min) and M2R−/−mice (602 ± 5 beats/min). These experiments conclusively demonstrate that in the absence of cold stress, regulation of vagal tone and modulation of intrinsic rate are important determinants of HR control in the mouse.

Persistence of circadian variation in arterial blood pressure in beta1/beta2-adrenergic receptor-deficient mice

The beta-adrenergic pathway has been considered one important effector of circadian variation in arterial pressure. Experiments were performed in beta1/beta2-adrenergic receptor-deficient mice (beta1/beta2ADR-/-) to assess whether this pathway is required for circadian variation in mean arterial pressure (MAP) and to determine the impact of its loss on the response to changes in dietary salt. Twenty-four-hour recordings of MAP, heart rate (HR), and locomotor activity were made in conscious 16- to 17-wk-old mice [wild-type, (WT), n = 7; beta1/beta2ADR-/-, n = 10] by telemetry. Both WT and beta1/beta2ADR-/- mice demonstrated robust circadian variation in MAP and HR, although 24-h mean MAP was 10% lower (102.02 +/- 1.81 vs. 92.11 +/- 2.62 mmHg) in beta1/beta2ADR-/- than WT, HR was 16% lower and day-night differences reduced. Both WT and beta1/beta2ADR-/- mice adapted to changed salt intake without changed MAP. However, the beta1/beta2ADR-/- mice demonstrated a striking reduction in locomotor activity in light and dark phases of the day. In WT mice, MAP was markedly affected by locomotor activity, resulting in bimodal distributions in both light and dark. When MAP was analyzed using only intervals without locomotor activity, bimodality and circadian differences were reduced, and there was no significant difference between the two genotypes. The results indicate that there is no direct effect or role for the beta-adrenergic system in circadian variation of arterial pressure in mice, aside from the indirect consequences of altered locomotor activity. Our results also confirm that locomotor activity contributes strongly to circadian variation in blood pressure in mice.

Genome-wide linkage scan for submaximal exercise heart rate in the HERITAGE family study

The purpose of this study was to identify regions of the human genome linked to submaximal exercise heart rates in the sedentary state and in response to a standardized 20-wk endurance training program in blacks and whites of the HERITAGE Family Study. A total of 701 polymorphic markers covering the 22 autosomes were used in the genome-wide linkage scan, with 328 sibling pairs from 99 white nuclear families and 102 pairs from 115 black family units. Steady-state heart rates were measured at the relative intensity of 60% maximal oxygen uptake (HR60) and at the absolute intensity of 50 W (HR50). Baseline phenotypes were adjusted for age, sex, and baseline body mass index (BMI) and training responses (posttraining minus baseline, Δ) were adjusted for age, sex, baseline BMI, and baseline value of the phenotype. Two analytic strategies were used, a multipoint variance components and a regression-based multipoint linkage analysis. In whites, promising linkages (LOD > 1.75) were identified on 18q21-q22 for baseline HR50 (LOD = 2.64; P = 0.0002) and ΔHR60 (LOD = 2.10; P = 0.0009) and on chromosome 2q33.3 for ΔHR50 (LOD = 2.13; P = 0.0009). In blacks, evidence of promising linkage for baseline HR50 was detected with several markers within the chromosomal region 10q24-q25.3 (peak LOD = 2.43, P = 0.0004 with D10S597). The most promising regions for fine mapping in the HERITAGE Family Study were found on 2q33 for HR50 training response in whites, on 10q25-26 for baseline HR60 in blacks, and on 18q21–22 for both baseline HR50 and ΔHR60 in whites.

Adrenergic and calcium modulation of the heart in stress: from molecular biology to function

There is strong evidence about the importance of catecholamines and calcium signaling in heart function. Also, interaction of these two systems is well documented. Catecholamines signal through adrenergic receptors, and further activate calcium transport either from the extracellular space, or from the intracellular calcium stores. This review summarizes current knowledge on catecholamine production in the heart, with special focus on the final enzyme in the catecholamine synthesizing pathway, phenylethanolamine N-methyltransferase (PNMT), in different cell types in the heart. Further, signaling through different types of adrenergic receptors in physiological conditions and after exposure to different stressors is discussed. Also, part of this review considers activation of an intracellular calcium transport system via inositol 1,4,5-trisphosphate receptor and to possible functional consequences in control and stress conditions.

Sema3a maintains normal heart rhythm through sympathetic innervation patterning

Sympathetic innervation is critical for effective cardiac function. However, the developmental and regulatory mechanisms determining the density and patterning of cardiac sympathetic innervation remain unclear, as does the role of this innervation in arrhythmogenesis. Here we show that a neural chemorepellent, Sema3a, establishes cardiac sympathetic innervation patterning. Sema3a is abundantly expressed in the trabecular layer in early-stage embryos but is restricted to Purkinje fibers after birth, forming an epicardial-to-endocardial transmural sympathetic innervation patterning. Sema3a(-/-) mice lacked a cardiac sympathetic innervation gradient and exhibited stellate ganglia malformation, which led to marked sinus bradycardia due to sympathetic dysfunction. Cardiac-specific overexpression of Sema3a in transgenic mice (SemaTG) was associated with reduced sympathetic innervation and attenuation of the epicardial-to-endocardial innervation gradient. SemaTG mice demonstrated sudden death and susceptibility to ventricular tachycardia, due to catecholamine supersensitivity and prolongation of the action potential duration. We conclude that appropriate cardiac Sema3a expression is needed for sympathetic innervation patterning and is critical for heart rate control.

Mapping the contribution of beta3-containing GABAA receptors to volatile and intravenous general anesthetic actions

Background

Agents belonging to diverse chemical classes are used clinically as general anesthetics. The molecular targets mediating their actions are however still only poorly defined. Both chemical diversity and substantial differences in the clinical actions of general anesthetics suggest that general anesthetic agents may have distinct pharmacological targets. It was demonstrated previously that the immobilizing action of etomidate and propofol is completely, and the immobilizing action of isoflurane partly mediated, by β3-containing GABA_A receptors. This was determined by using the β3(N265M) mice, which carry a point mutation known to decrease the actions of general anesthetics at recombinant GABA_A receptors. In this communication, we analyzed the contribution of β3-containing GABA_A receptors to the pharmacological actions of isoflurane, etomidate and propofol by means of β3(N265M) mice.

Results

Isoflurane decreased core body temperature and heart rate to a smaller degree in β3(N265M) mice than in wild type mice, indicating a minor but significant role of β3-containing GABA_A receptors in these actions. Prolonged time intervals in the ECG and increased heart rate variability were indistinguishable between genotypes, suggesting no involvement of β3-containing GABA_A receptors. The anterograde amnesic action of propofol was indistinguishable in β3(N265M) and wild type mice, suggesting that it is independent of β3-containing GABA_A receptors. The increase of heart rate variability and prolongation of ECG intervals by etomidate and propofol were also less pronounced in β3(N265M) mice than in wild type mice, pointing to a limited involvement of β3-containing GABA_A receptors in these actions. The lack of etomidate- and propofol-induced immobilization in β3(N265M) mice was also observed in congenic 129X1/SvJ and C57BL/6J backgrounds, indicating that this phenotype is stable across different backgrounds.

Conclusion

Our results provide evidence for a defined role of β3-containing GABA_A receptors in mediating some, but not all, of the actions of general anesthetics, and confirm the multisite model of general anesthetic action. This pharmacological separation of anesthetic endpoints also suggests that subtype-selective substances with an improved side-effect profile may be developed.

Involvement of voltage-dependent Ca(2+) channel beta(3) subunit in the autonomic control of heart rate variability

Noradrenaline release from sympathetic nerve terminals is dependent on Ca(2+) entry through neuronal voltage-gated N-type Ca(2+) channels. The accessory beta(3) subunits of Ca(2+) channels (Ca(V)beta(3)) are preferentially associated with the alpha(1B) subunit to form N-type Ca(2+) channels, and are therefore expected to play a functional role in the stimulation-evoked release of noradrenaline. In this study, we employed Ca(V)beta(3)-null, Ca(V)beta(3)-overexpressing (Ca(V)beta(3)-Tg), and wild-type (WT) mice to investigate the possible roles of Ca(V)beta(3) in the sympathetic regulation of heart rate in vivo. Telemetry was used to monitor the ECG and both time and frequency domain analyses were carried out to evaluate heart rate variability. In the frequency domain analysis, power spectral density of the RR interval series was computed using the fast Fourier transform algorithm. The resting heart rate was increased in Ca(V)beta(3)-Tg mice compared with both Ca(V)beta(3)-null and WT mice. Mice overexpressing Ca(V)beta(3) displayed decreased heart rate variability, which was measured by the time domain analysis of the standard deviation of RR intervals. In the frequency domain analysis, Ca(V)beta(3)-Tg mice showed decreased spectral powers compared with WT and Ca(V)beta(3)-null mice. Pharmacological blockade of beta-adrenergic receptors with metoprolol decreased the heart rate in all genotypes, but the extent of the decrease was most obvious in Ca(V)beta(3)-Tg mice. On the other hand, the spectral powers were decreased in response to parasympathetic blockade (atropine) in WT and Ca(V)beta(3)-Tg mice. These results indicate the functional roles of Ca(V)beta(3) in regulating sympathetic nerve signaling.