Beta-1 adrenergic receptors modulate pacemaker activity of mouse sino-atrial myocytes through L-type Cav1.3 channels

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Doctoral program in Biology and Biotechnologies

b1- and b2- adrenergic receptors (ARs) are co-expressed in different regions of the heart. The b2/ b1 expression ratio is higher in the sino-atrial node (SAN) than in atria and ventricles, but the specific contribution of either type of receptor to modulation of pacemaker activity is still not well established. Specific stimulation of b2-ARs in rabbit SAN myocytes is associated with a positive shift of the pacemaker "funny" current (If) activation curve. However, previous studies showed that L-type Cav1.3 channels play an important role in the generation of cardiac pacemaker activity by contributing to the diastolic depolarization (DD) in SAN myocytes. Since Cav1.3 channels are positively regulated by b-ARs activation2, we investigated which is the main b-ARs isoform that modulates Cav1.3-mediated ICaL andthe pacemaker activity of SAN myocytes. To address this point, we recorded spontaneous activity and Cav1.3-mediated ICaL from mouse SAN myocytes. We found that the positive chronotropic effect of the non-selective b-AR agonist isoproterenol (ISO, 0.1mM) was decreased by the b1-ARs antagonist CGP-20712 (0.3mM) and the b2-ARs antagonist ICI-118,551 (1mM) by -18% and -9%, respectively. Perfusion of CGP-20712 strongly reduced the positive chronotropic effect induced by ISO. Finally, we recorded Cav1.3-mediated L-type currents in presence of the b1-ARs antagonist. CGP-20712 reduced the basal Cav1.3-mediated ICaL. Furthermore, the increase in Cav1.3-mediated ICaL by isoproterenol was abolished during b1-ARs inhibition by CGP-20712. In conclusion, these preliminary data show that b1- and b2-ARs differently modulate the spontaneous activity of mouse SAN myocytes.  In addition, b1-ARs play a predominant role in the  adrenergic regulation of L-type Cav1.3 channels to increase pacemaker activity.  Future studies will be performed to clarify the role of b2-ARs antagonist on Cav1.3-mediated ICaL and the functional relationships between b-ARs and Cav1.3 channels.

The increase of extracellular Ca2+ from physiological concentrations to hypercalcemia impairs sino-atrial automaticity

Funding Acknowledgements

Type of funding sources: Foundation. Main funding source(s): ESC FRM Lefoulon Delalande

Aims

To investigate whether extracellular hypercalcemia alters the conduction through L-type Ca2+ channels (LTCCs), impairing the pacemaker activity of the heart.

Introduction

In the sino-atrial node (SAN), membrane currents and the dynamics of intracellular Ca2+ ([Ca2+]i) generate the pacemaker activity of the heart. SAN dysfunctions (SNDs) harm heart automaticity and have been associated with abnormal dynamics of [Ca2+]i. The LTCCs, Cav1.2 and Cav1.3 carry the main Ca2+ influx of SAN cells, which is necessary to sustain [Ca2+]i dynamics. Modified extracellular Ca2+ ([Ca2+]o) could alter Ca2+ influx through these channels. For example, cancer and hyperparathyroidism can raise [Ca2+]o, causing an extracellular hypercalcemia that could alter [Ca2+]i dynamics and impair SAN activity and heart automaticity.

Methods and results

To test this hypothesis, we measured contractions, [Ca2+]i release and L-type Ca2+ current (ICa,L) in spontaneous cells of the murine SAN. Then, we recorded rate and propagation of the spontaneous action potentials (APs) generated by the SAN tissue ex-vivo.

In spontaneously beating SAN cells, we observed that the modification of [Ca2+]o affected [Ca2+]i and cell contractility through changes of ICa,L. In particular, the increase of [Ca2+]o dysregulated pacemaker activity, likely through excessive Ca2+ influx mediated by Cav1.2.

[Ca2+]o increase to hypercalcemia induced arrhythmia also in the intact SAN tissues, activating ectopic leading regions of pacemaking and impairing conduction towards the atria.

Conclusions

Hypercalcemia causes excessive Cav1.2-mediated Ca2+ influx, which alters [Ca2+]I leading to pacemaker impairment. Modulation of LTCC may reduce pacemaker dysfunctions, preventing SND progression.

Cholinergic regulation of cardiac pacemaker activity by L-type Cav1.3 channels

Funding Acknowledgements

Type of funding sources: Foundation. Main funding source(s): Fondation Recherche Médicale

Introduction

The cholinergic regulation of heart rate (HR) is mediated by acetylcholine (ACh)-dependent activation of M2-receptors (M2R). Activated M2R promote release of the βγ-subunit of G-proteins to directly gate GIRK1/4 channels (underlying the cardiac IKACh current), while αi-subunits inhibit adenylate cyclase (AC) activity. AC inhibition reduces the intracellular concentration of cAMP, decreasing the activity of ion channels involved in pacemaking, including "funny" f-(HCN4) and L-type Cav1.3 calcium channels.

Purpose

To determine the role of L-type Cav1.3 channels in cholinergic regulation of heart rate.

Methods

We recorded the frequency of activation and position of pacemaker leading site in ex vivo sinus nodes and the HR of isolated Langendorff perfused hearts of mice at baseline or during ACh perfusion.  We used control wild type (WT) mice, and five genetically modified mouse models: Cav1.3 knockout (KO, ablated Cav1.3-mediated L-type current), GIRK4KO (ablated IKACh current), HCN4-CNBD (selective deletion of cAMP-dependent regulation of HCN4), GIRK4KO/HCN4-CNBD and GIRK4KO/Cav1.3KO. We performed in vivo telemetric recordings of heart rate (HR) in WT and GIRK4KO/Cav1.3KO animals.

Results

Data from optical mapping experiments showed that, under basal conditions, perfusion of 3 μM ACh significantly reduced the frequency of action potentials in WT (44%), HCN4-CNBD (38%), Cav1.3KO (65%) and GIRK4KO (8%) isolated mouse sinus node tissues. ACh application did not significantly affect the frequency of action potentials recorded in tissue from GIRK4KO/HCN4-CNBD and GIRK4KO/Cav1.3KO animals. Furthermore, in all sinus nodes tested, regardless of genotype, ACh shifted the pacemaker leading site from its normal position by at least 0.7 mm.

Upon stimulation of the β-adrenergic pathway by Isoproterenol, to reproduce conditions of accentuated antagonism, 3µM ACh reduced HR in isolated hearts from WT (43.8%), HCN4-CNBD (38.7%), Cav1.3KO (25,4%), GIRK4KO (16.9%) and GIRK4KO/HCN4-CNBD (16.4%) mice. No significant HR reduction was recorded in hearts from GIRK4KO/Cav1.3KO animals.

In vivo data indicate that HR reduction induced by combined injection of Hexamethonium ( a Nicotinic acetylcholine receptor blocker) with Carbamoylcholine (CCH, M2 receptor agonist) or with 2-Chloro-N6-Cyclopentyladenosine (CCPA, A1 receptor agonist) is higher in WT than in GIRK4KO/Cav1.3KO animals (68% vs 48% CCH, and 79% vs 62% CCPA, respectively).

Conclusion

Our data indicate that L-type Cav1.3 channels are involved in cholinergic regulation of heart rate in mice. In addition, when the intracellular concentration of cAMP is elevated (i.e. under conditions of accentuated antagonism), cholinergic regulation of sinus node pacemaking is reliant on Cav1.3 and KACh channels.

Autonomous initiation of pacemaking by L-type Cav1.3 calcium channels in mouse sino-atrial myocytes

Funding Acknowledgements

Type of funding sources: Other. Main funding source(s): Ecole doctorale

Background

Cardiac pacemaking relies on the spontaneous electrical activity in the right atrium of sino-atrial myocytes (SANCs). Automaticity in SANCs results from a robust interplay of membrane ion channels activity and intracellular calcium dynamics. However, only a fraction of isolated SANCs exhibit rhythmic firing, whereas most SANCs show irregular (dysrhythmic) firing or remain dormant.

Purpose

To study the capability of L-type Cav1.3 calcium channels to initiate automaticity in dormant SANCs under β-adrenergic stimulation, we used a knock-in mouse strain in which the sensitivity of Cav1.2 α1 subunits to dihydropyridines (DHP) was inactivated (Cav1.2DHP-/-).

Methods

We performed current and voltage-clamp recordings on isolated SANCs under isoprenaline (ISO, 100 nM) and in the absence or presence of the DHP blocker Nifedipine (Nife, 3 µM).

Results

Nife significantly reduced the spontaneous firing under ISO perfusion in all rhythmic SANCs (ISO: 447 ± 12, ISO + Nife: 233 ± 25 bpm) and 60% of dysrhythmic SANCs (ISO: 386 ± 12, ISO + Nife: 188 ± 47 bpm) whereas it completely stopped it in the remaining 40% (295 ± 29 bpm to 0). On 25 dormant SANCs, 50% started firing after ISO perfusion (0 to 320 ± 46 bpm). Strikingly, in 75% of them, Nife totally blocked this ISO-induced firing. Interestingly, these cells exhibited a significantly slower rate and a slower slope of the diastolic depolarization under ISO perfusion compared to the remaining 25% dormant SANCs in which Nife only reduced the ISO-induced firing. Moreover, dormant SANCs showed a statistically significant increase in action potential (AP) threshold under ISO compared to dysrhythmic and rhythmic SANCs (dormant: -30.1 ± 2.5, dysrhythmic: -43.3 ± 2.3, rhythmic: -41.2 ± 2.1 mV). No significant difference was observed in the other AP parameters between dormant, dysrhythmic and rhythmic SANCs under ISO.

Conclusion

 Our results seem to point at a difference of expression in ionic channels (Cav1.3, HCN4) within isolated SANCs. Preliminary results on If density support this hypothesis with a lesser density in dormant SANCs compared to dysrhythmic SANCs. These results also tend to indicate that Cav1.3 channels can generate pacemaker activity autonomously, at least in a particular subpopulation of SANCs.

Cholinergic regulation of cardiac pacemaker activity by l-type cav1.3 channels

Introduction

The cholinergic regulation of heart rate (HR) is mediated by acetylcholine (ACh)-dependent activation of M2-receptors (M2R). Activated M2R promote release of the βγ-subunit of G-proteins to directly gate GIRK1/4 channels (underlying the cardiac IKACh current), while αi-subunits inhibit adenylate cyclase (AC) activity. AC inhibition reduces the intracellular concentration of cAMP, decreasing the activity of ion channels involved in pacemaking, including “funny” f-(HCN4) and L-type Cav1.3 calcium channels.

Purpose

To determine the importance of L-type Cav1.3 channels in the cholinergic regulation of heart rate.

Methods

We recorded the frequency and the position of the pacemaker leading site in ex vivo sinus nodes and the HR of isolated Langendorff perfused hearts of mice in control or during ACh perfusion. We used control wild type (WT) mice, and five genetically modified mouse models: Cav1.3 knockout (KO, ablated Cav1.3-mediated L-type current), GIRK4KO (ablated IKACh current), HCN4-CNBD (selective deletion of cAMP-dependent regulation of HCN4), GIRK4KO/HCN4-CNBD and GIRK4KO/Cav1.3KO.

Results

Data from optical mapping experiments showed that, under basal conditions, perfusion of 3 μM ACh significantly reduced the frequency of action potentials in WT (44%), HCN4-CNBD (38%), Cav1.3KO (65%) and GIRK4KO (8%) isolated mouse sinus node tissues. ACh application did not significantly affect the frequency of action potentials recorded in tissue from GIRK4KO/HCN4-CNBD and GIRK4KO/Cav1.3KO animals. Furthermore, in all the sinus node tissues tested, regardless of the genotypes, ACh shifted the pacemaker leading site from its normal position by at least 0.7 mm.

Upon stimulation of the β-adrenergic pathway by Isoproterenol, to reproduce conditions of accentuated antagonism, 3μM ACh reduced HR in isolated hearts from WT (43.8%), HCN4-CNBD (38.7%), Cav1.3KO (25,4%), GIRK4KO (16.9%) and GIRK4KO/HCN4-CNBD (16.4%) mice. No significant HR reduction was recorded in hearts from GIRK4KO/Cav1.3KO animals.

Conclusion

Our data indicate that L-type Cav1.3 channels are involved in cholinergic regulation of heart rate in mice. In addition, when the intracellular concentration of cAMP is elevated (i.e. under conditions of accentuated antagonism), the cholinergic regulation of sinus node pacemaking is predominantly ensured by Cav1.3 and KACh channels.

Funding Acknowledgement

Type of funding source: Foundation. Main funding source(s): “Fondation pour la recherche medicale” FRM

1800 MHz radiofrequency (mobile phones, different Global System for Mobile communication modulations) does not affect apoptosis and heat shock protein 70 level in peripheral blood mononuclear cells from young and old donors

PURPOSE

To study if prolonged in vitro exposure to 1800MHz radiofrequency (RF) could exert an effect on human peripheral blood mononuclear cells (PBMC) from young and elderly donors by affecting apoptosis, mitochondrial membrane potential and heat shock protein (HSP) 70 levels.

MATERIALS AND METHODS

Endpoints were analysed in the presence or absence of the apoptosis-inducing agent 2-deoxy-D-ribose. Three different signal modulations typical of the Global System for Mobile communication (GSM) system were applied. The modulations are widely used in mobile telephony (GSM Basic, discontinuous transmission [DTX] and Talk) at specific absorption rates of 1.4 and 2.0 W kg(-1).

RESULTS

In all conditions and for all endpoints tested, there was no significant difference between RF- and sham-exposed cells.

CONCLUSION

1800MHz RF could not induce apoptosis by itself or affect the apoptotic phenomenon when induced by an apoptotic agent. Moreover, RF did not modify the mitochondrial functionality and the expression of HSP 70.

Effects of exogenous electromagnetic fields on a simplified ion channel model

In this paper, we calculate the effect of an exogenous perturbation (an electromagnetic field [EMF] oscillating in the range of microwave frequencies in the range of 1 GHz) on the flux of two ion species through a cylindrical ion channel, implementing a continuous model, the Poisson-Smoluchowski system of equations, to study the dynamics of charged particle density inside the channel. The method was validated through comparison with Brownian dynamics simulations, supposed to be more accurate but computationally more demanding, obtaining a very good agreement. No EMF effects were observed for low field intensities below the level for thermal effects, as the highly viscous regime and the simplicity of the channel do not exhibit resonance phenomena. For high intensities of the external field (>10(5) V/m), we observed slightly different behavior of ion concentration oscillations and ion currents as a function of EMF orientation with respect to the channel axis.

Various effects on transposition activity and survival of Escherichia coli cells due to different ELF-MF signals

Previous assays with weak sinusoidal magnetic fields (SMF) have shown that bacteria that had been exposed to a 50 Hz magnetic field (0.1-1 mT) gave colonies with significantly lower transposition activity as compared to sham-exposed bacteria. These experiments have now been extended by using a pulsed-square wave magnetic field (PMF) and, unexpectedly, it was found that bacteria exposed to PMF showed a higher transposition activity compared to the controls. The increase of the transposition activity was positively correlated with the intensity of the magnetic fields (linear dose-effect relation). This phenomenon was not affected by any bacterial cell proliferation, since no significant difference was observed in number and size of PMF-exposed and sham-exposed colonies. In addition, the cell viability of E. coli was significantly higher than that of the controls when exposed to SMF, and lower than that of the controls when exposed to PMF. Under our experimental conditions it was shown that exposure to PMF stimulates the transposition activity and reduces cell viability of bacteria, whereas exposure to SMF reduces the transposition mobility and enhances cell viability. These results suggest that the biological effects of magnetic fields may critically depend on the physical characteristics of the magnetic signal, in particular the wave shape.

Extremely low frequency magnetic fields affect transposition activity in Escherichia coli

The aim of this study was to verify whether extremely low frequency (ELF) magnetic fields (MF) could affect transposition activity like some environmental stress factors such as heat shock or UV irradiation. Using an Escherichia coli Lac Z(-) strain transformed with a plasmid containing a Tn 10 derivative element expressing beta-galactosidase only after transposition, it was possible to determine the events of transposition evaluating the rate at which the colonies developed dark coloured papillae (Lac Z(+)). We found that those bacteria that had been exposed for a long time (58 h) to a 50 Hz low intensity MF (0.1-1 mT) gave colonies with significantly lower transposition activity compared to sham-exposed bacteria. Such reduction in transposition activity was positively correlated to the intensity of the MF, in a dose-effect manner. This phenomenon was not affected by bacterial cell proliferation, since no significant differences were observed in number, diameter and perimeter between sham-exposed and MF-exposed colonies.

A four coil exposure system (tetracoil) producing a highly uniform magnetic field

We propose a magnetic field exposure system (tetracoil) for in vitro and in vivo experiments, composed of two couples of circular coils satisfying a spherical constraint, whose characteristics are chosen in order to maximize the uniformity region of the magnetic field. Analytical calculations and computer simulations show that our system, as compared to the other most largely used magnetic field exposure systems, represents an optimal compromise in terms of field uniformity, accessibility for biological experiments, and ratio between overall dimension and uniformity region.

[Results of specific neonatal screening for congenital dysplasia of the hip at the Vicenza Hospital. A prospective study of subjects at risk]

The aim of the study was to evaluate the incidence of pathological neonatal hips in the community served by the central Vicenza local health authority and to establish a protocol to minimize demands on available equipment, staff and the family. From May 1992 to May 1993 all neonates at San Bortolo Hospital in Vicenza were subjected to specific clinical examination of the hip by staff experienced in neonatal care. Neonates then underwent ultrasound examination--catalogued according to Graf--if they presented risk factors (dynamic ultrasound test was omitted). The orthopaedic examination was carried out in all cases. The total number of neonates involved was 1939 (994 m., 945 f.). Of these, 142 (7.3%) underwent ultrasonography (60 m., 82 f.). Family history and breech delivery were the most frequent anamnestic risk factors justifying ultrasound examination while among objective risk factors the most frequent being a clicking sound. Considering the clinical and ultrasound findings the resulting overall incidence of pathological hips is 0.25%. So far, there have been no late cases of c.d.h. A screening protocol such as ours cannot realistically aim to identify all pathological hips, however the great majority can be diagnosed at this early stage. Resources comparable to those used for our study are available to many other local health authorities. Higher diagnostic standard depend closely on local health policy.

Sodium cromoglycate and carbachol-induced bronchoprovocation in asthmatic children

Ten children with mild bronchial asthma underwent bronchial challenge with carbachol on three consecutive days; using a double-blind technique, they were given inhaled saline placebo on the first day and placebo or sodium cromoglycate (SCG) on the second and third days 15 min prior to the bronchial provocation test with carbachol. We did not find any significant difference between the baseline and placebo values, but we did obtain a statistically significant difference when comparing baseline with post-SCG and placebo with post-SCG values. These results demonstrate a protective effect of SCG on carbachol-induced bronchospasm in asthmatic children.

Influence of aeroallergens on bronchial reactivity in children sensitized to grass pollens

Nine children, sensitized to grass pollens, with seasonal rhinitis and mild asthma underwent bronchial challenge with carbachol and EIA test repeatedly either during a pre-seasonal or in seasonal period. We failed to find any significant difference between pre-seasonal and seasonal values in both tests. We believe that atmospheric conditions are an important determinant of our results. It is our hypothesis that a threshold concentration in atmospheric pollen may exist which must be reached before an allergic reaction is evident.

[Biofeedback in asthmatic children]

This paper reports the results of the treatment of continuous bronchial asthma in children, 7-14 years old, by means of biofeedback and the counter-conditioning. We observed the remission of the symptomatology in all the cases, with a statistical significance.