Sebastian Maier

Ca2+/calmodulin-dependent protein kinase II regulates cardiac Na+ channels.

In heart failure (HF), Ca(2+)/calmodulin kinase II (CaMKII) expression is increased. Altered Na(+) channel gating is linked to and may promote ventricular tachyarrhythmias (VTs) in HF. Calmodulin regulates Na(+) channel gating, in part perhaps via CaMKII. We investigated effects of adenovirus-mediated (acute) and Tg (chronic) overexpression of cytosolic CaMKIIdelta(C) on Na(+) current (I(Na)) in rabbit and mouse ventricular myocytes, respectively (in whole-cell patch clamp). Both acute and chronic CaMKIIdelta(C) overexpression shifted voltage dependence of Na(+) channel availability by -6 mV (P < 0.05), and the shift was Ca(2+) dependent. CaMKII also enhanced intermediate inactivation and slowed recovery from inactivation (prevented by CaMKII inhibitors autocamtide 2-related inhibitory peptide [AIP] or KN93). CaMKIIdelta(C) markedly increased persistent (late) inward I(Na) and intracellular Na(+) concentration (as measured by the Na(+) indicator sodium-binding benzofuran isophthalate [SBFI]), which was prevented by CaMKII inhibition in the case of acute CaMKIIdelta(C) overexpression. CaMKII coimmunoprecipitates with and phosphorylates Na(+) channels. In vivo, transgenic CaMKIIdelta(C) overexpression prolonged QRS duration and repolarization (QT intervals), decreased effective refractory periods, and increased the propensity to develop VT. We conclude that CaMKII associates with and phosphorylates cardiac Na(+) channels. This alters I(Na) gating to reduce availability at high heart rate, while enhancing late I(Na) (which could prolong action potential duration). In mice, enhanced CaMKIIdelta(C) activity predisposed to VT. Thus, CaMKII-dependent regulation of Na(+) channel function may contribute to arrhythmogenesis in HF.

Distinct Subcellular Localization of Different Sodium Channel α and β Subunits in Single Ventricular Myocytes From Mouse Heart

Background—Voltage-gated sodium channels composed of pore-forming &agr; and auxiliary &bgr; subunits are responsible for the rising phase of the action potential in cardiac muscle, but their localizations have not yet been clearly defined. Methods and Results—Immunocytochemical studies show that the principal cardiac &agr; subunit isoform Nav1.5 and the &bgr;2 subunit are preferentially localized in intercalated disks, identified by immunostaining of connexin 43, the major protein of cardiac gap junctions. The brain &agr; subunit isoforms Nav1.1, Nav1.3, and Nav1.6 are preferentially localized with &bgr;1 and &bgr;3 subunits in the transverse tubules, identified by immunostaining of &agr;-actinin, a cardiac z-line protein. The &bgr;1 subunit is also present in a small fraction of intercalated disks. The recently cloned &bgr;4 subunit, which closely resembles &bgr;2 in amino acid sequence, is also expressed in ventricular myocytes and is localized in intercalated disks as are &bgr;2 and Nav1.5. Conclusions—Our results suggest that the primary sodium channels present in ventricular myocytes are composed of Nav1.5 plus &bgr;2 and/or &bgr;4 subunits in intercalated disks and Nav1.1, Nav1.3, and Nav1.6 plus &bgr;1 and/or &bgr;3 subunits in the transverse tubules.

Quantum-dot spin–photon entanglement via frequency downconversion to telecom wavelength

Long-distance quantum teleportation and quantum repeater technologies require entanglement between a single matter quantum bit (qubit) and a telecommunications (telecom)-wavelength photonic qubit. Electron spins in III–V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation and photon emission, but entanglement between a single quantum-dot spin qubit and a flying (propagating) photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to near-infrared wavelengths, where silica fibre losses are so high that long-distance quantum communication protocols become difficult to implement. Here we demonstrate entanglement between an InAs quantum-dot electron spin qubit and a photonic qubit, by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres. The use of sub-10-picosecond pulses at a wavelength of 2.2 micrometres in the frequency downconversion process provides the necessary quantum erasure to eliminate which-path information in the photon energy. Together with previously demonstrated indistinguishable single-photon emission at high repetition rates, the present technique advances the III–V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication.

On-demand single photons with high extraction efficiency and near-unity indistinguishability from a resonantly driven quantum dot in a micropillar

Scalable photonic quantum technologies require on-demand single-photon sources with simultaneously high levels of purity, indistinguishability, and efficiency. These key features, however, have only been demonstrated separately in previous experiments. Here, by s-shell pulsed resonant excitation of a Purcell-enhanced quantum dot-micropillar system, we deterministically generate resonance fluorescence single photons which, at π pulse excitation, have an extraction efficiency of 66%, single-photon purity of 99.1%, and photon indistinguishability of 98.5%. Such a single-photon source for the first time combines the features of high efficiency and near-perfect levels of purity and indistinguishabilty, and thus opens the way to multiphoton experiments with semiconductor quantum dots.

An unexpected requirement for brain-type sodium channels for control of heart rate in the mouse sinoatrial node

Voltage-gated Na+ channels are composed of pore-forming α and auxiliary β subunits. The majority of Na+ channels in the heart contain tetrodotoxin (TTX)-insensitive Nav1.5 α subunits, but TTX-sensitive brain-type Na+ channel α subunits are present and functionally important in the transverse tubules of ventricular myocytes. Sinoatrial (SA) nodal cells were identified in cardiac tissue sections by staining for connexin 43 (which is expressed in atrial tissue but not in SA node), and Na+ channel localization was analyzed by immunocytochemical staining with subtype-specific antibodies and confocal microscopy. Brain-type TTX-sensitive Nav1.1 and Nav1.3 α subunits and all four β subunits were present in mouse SA node, but Nav1.5 α subunits were not. Nav1.1 α subunits were also present in rat SA node. Isolated mouse hearts were retrogradely perfused in a Langendorff preparation, and electrocardiograms were recorded. Spontaneous heart rate and cycle length were constant, and heart rate variability was small under control conditions. In contrast, in the presence of 100 nM TTX to block TTX-sensitive Na+ channels specifically, we observed a significant reduction in spontaneous heart rate and markedly greater heart rate variability, similar to sick-sinus syndrome in man. We hypothesize that brain-type Na+ channels are required because their more positive voltage dependence of inactivation allows them to function at the depolarized membrane potential of SA nodal cells. Our results demonstrate an important contribution of TTX-sensitive brain-type Na+ channels to SA nodal automaticity in mouse heart and suggest that they may also contribute to SA nodal function and dysfunction in human heart.

Requirement of neuronal‐ and cardiac‐type sodium channels for murine sinoatrial node pacemaking

The majority of Na+ channels in the heart are composed of the tetrodotoxin (TTX)‐resistant (KD, 2–6 μm) Nav1.5 isoform; however, recently it has been shown that TTX‐sensitive (KD, 1–10 nm) neuronal Na+ channel isoforms (Nav1.1, Nav1.3 and Nav1.6) are also present and functionally important in the myocytes of the ventricles and the sinoatrial (SA) node. In the present study, in mouse SA node pacemaker cells, we investigated Na+ currents under physiological conditions and the expression of cardiac and neuronal Na+ channel isoforms. We identified two distinct Na+ current components, TTX resistant and TTX sensitive. At 37°C, TTX‐resistant iNa and TTX‐sensitive iNa started to activate at ∼−70 and ∼−60 mV, and peaked at −30 and −10 mV, with a current density of 22 ± 3 and 18 ± 1 pA pF−1, respectively. TTX‐sensitive iNa inactivated at more positive potentials as compared to TTX‐resistant iNa. Using action potential clamp, TTX‐sensitive iNa was observed to activate late during the pacemaker potential. Using immunocytochemistry and confocal microscopy, different distributions of the TTX‐resistant cardiac isoform, Nav1.5, and the TTX‐sensitive neuronal isoform, Nav1.1, were observed: Nav1.5 was absent from the centre of the SA node, but present in the periphery of the SA node, whereas Nav1.1 was present throughout the SA node. Nanomolar concentrations (10 or 100 nm) of TTX, which block TTX‐sensitive iNa, slowed pacemaking in both intact SA node preparations and isolated SA node cells without a significant effect on SA node conduction. In contrast, micromolar concentrations (1–30 μm) of TTX, which block TTX‐resistant iNa as well as TTX‐sensitive iNa, slowed both pacemaking and SA node conduction. It is concluded that two Na+ channel isoforms are important for the functioning of the SA node: neuronal (putative Nav1.1) and cardiac Nav1.5 isoforms are involved in pacemaking, although the cardiac Nav1.5 isoform alone is involved in the propagation of the action potential from the SA node to the surrounding atrial muscle.

Calcium/Calmodulin-Dependent Protein Kinase II Contributes to Cardiac Arrhythmogenesis in Heart Failure

Background —Transgenic CaMKIIδC (TG) mice have heart failure and isoproterenol (ISO)-inducible arrhythmias. We hypothesized that CaMKII contributes to arrhythmias and to underlying cellular events and that inhibition of CaMKII reduces cardiac arrhythmogenesis in vitro and in vivo. Methods and Results —Under baseline conditions, isolated cardiac myocytes from TG mice revealed an increased incidence of early afterdepolarizations (ADs) as compared to wild-type (WT) myocytes (P<0.05). CaMKII-inhibition (AIP) completely abolished these ADs in TG cells ( P <0.05). Elevating intracellular Ca stores using ISO (10-8 M) induced a larger amount of delayed ADs and spontaneous action potentials in TG as compared to WT ( P <0.05). This seems to be due to an increased sarcoplasmic reticulum (SR) Ca leak since diastolic [Ca] rose clearly upon ISO in TG but not in WT cells (+20±5% vs. +3±4% at 10-6 M ISO, P <0.05). In parallel, SR Ca leak assessed by spontaneous SR Ca release events showed an increased Ca spark frequency (3.9±0.5 vs. 2.0±0.4 sparks per 100 μm-1*s-1, P <0.05). However, CaMKII-inhibition (either pharmacologically using KN-93 or genetically using an isoform-specific CaMKIIδ-knockout mouse model) significantly reduced SR Ca spark frequency although this rather increased SR Ca content. In parallel, ISO increased the incidence of early (54% vs. 4%, P <0.05) and late (86% vs. 43%, P <0.05) non-stimulated events (NSEs) in TG vs. WT myocytes but CaMKII-inhibition (KN-93 and KO) reduced these proarrhythmogenic events ( P <0.05). In addition, CaMKII-inhibition in TG mice (KN-93) clearly reduced ISO-induced arrhythmias in vivo ( P <0.05). Conclusions —We conclude that CaMKII contributes to cardiac arrhythmogenesis in TG CaMKIIδC mice having heart failure and suggest the increased SR Ca leak as an important mechanism. Moreover, CaMKII-inhibition reduces cardiac arrhythmias in vitro and in vivo and may therefore indicate a potential role for future antiarrhythmic therapies warranting further studies.Background—Transgenic (TG) Ca/calmodulin-dependent protein kinase II (CaMKII)&dgr;C mice have heart failure and isoproterenol (ISO)-inducible arrhythmias. We hypothesized that CaMKII contributes to arrhythmias and underlying cellular events and that inhibition of CaMKII reduces cardiac arrhythmogenesis in vitro and in vivo. Methods and Results—Under baseline conditions, isolated cardiac myocytes from TG mice showed an increased incidence of early afterdepolarizations compared with wild-type myocytes (P<0.05). CaMKII inhibition (AIP) completely abolished these afterdepolarizations in TG cells (P<0.05). Increasing intracellular Ca stores using ISO (10−8 M) induced a larger amount of delayed afterdepolarizations and spontaneous action potentials in TG compared with wild-type cells (P<0.05). This seems to be due to an increased sarcoplasmic reticulum (SR) Ca leak because diastolic [Ca]i rose clearly on ISO in TG but not in wild-type cells (+20±5% versus +3±4% at 10−6 M ISO, P<0.05). In parallel, SR Ca leak assessed by spontaneous SR Ca release events showed an increased Ca spark frequency (3.9±0.5 versus 2.0±0.4 sparks per 100 &mgr;m−1·s−1, P<0.05). However, CaMKII inhibition (either pharmacologically using KN-93 or genetically using an isoform-specific CaMKII&dgr;-knockout mouse model) significantly reduced SR Ca spark frequency, although this rather increased SR Ca content. In parallel, ISO increased the incidence of early (54% versus 4%, P<0.05) and late (86% versus 43%, P<0.05) nonstimulated events in TG versus wild-type myocytes, but CaMKII inhibition (KN-93 and KO) reduced these proarrhythmogenic events (P<0.05). In addition, CaMKII inhibition in TG mice (KN-93) clearly reduced ISO-induced arrhythmias in vivo (P<0.05). Conclusions—We conclude that CaMKII contributes to cardiac arrhythmogenesis in TG CaMKII&dgr;C mice having heart failure and suggest the increased SR Ca leak as an important mechanism. Moreover, CaMKII inhibition reduces cardiac arrhythmias in vitro and in vivo and may therefore indicate a potential role for future antiarrhythmic therapies warranting further studies.

Detection of atrial high-rate events by continuous home monitoring: clinical significance in the heart failure-cardiac resynchronization therapy population.

Aims Uncertainty exists over the importance of device-detected short-duration atrial arrhythmias. Continuous atrial diagnostics, through home monitoring (HM) technology (BIOTRONIK, Berlin, Germany), provides a unique opportunity to assess frequency and quantity of atrial fibrillation (AF) episodes defined as atrial high-rate events (AHRE). Methods and results Prospective data from 560 heart failure (HF) patients (age 67 ± 10 years, median ejection fraction 27%) patients with a cardiac resynchronization therapy (CRT) device capable of HM from two multi-centre studies were analysed. Atrial high-rate events burden was defined as the duration of mode switch in a 24-h period with atrial rates of >180 beats for at least 1% or total of 14 min per day. The primary endpoint was incidence of a thromboembolic (TE) event. Secondary endpoints were cardiovascular death, hospitalization because of AF, or worsening HF. Over a median 370-day follow-up AHRE occurred in 40% of patients with 11 (2%) patients developing TE complications and mortality rate of 4.3% (24 deaths, 16 with cardiovascular aetiology). Compared with patients without detected AHRE, patients with detected AHRE>3.8 h over a day were nine times more likely to develop TE complications (P= 0.006). The majority of patients (73%) did not show a temporal association with the detected atrial episode and their adverse event, with a mean interval of 46.7 ± 71.9 days (range 0–194) before the TE complication. Conclusion In a high-risk cohort of HF patients, device-detected atrial arrhythmias are associated with an increased incidence of TE events. A cut-off point of 3.8 h over 24 h was associated with significant increase in the event rate. Routine assessment of AHRE should be considered with other data when assessing stroke risk and considering anti-coagulation initiation and should also prompt the optimization of cardioprotective HF therapy in CRT patients.

Utility and Limitations of the Traditional Diagnostic Approach to Hyponatremia: A Diagnostic Study

BACKGROUND The differential diagnosis of hyponatremia is often challenging because of its association with multiple underlying pathophysiological mechanisms, diseases, and treatment options. Several algorithms are available to guide the diagnostic approach to hyponatremia, but their diagnostic and clinical utility has never been evaluated. We aimed to assess in detail the diagnostic utility as well as the limitations of the existing approaches to hyponatremia. METHODS Each of the 121 consecutive subjects presenting with hyponatremia (serum sodium <130 mmoL/L) underwent 3 different and independent diagnostic and therapeutic approaches: inexperienced doctor applying an established Algorithm, intensive care senior physicians acting as Senior Physician, and senior endocrinologist serving as Reference Standard. RESULTS The overall diagnostic agreement between Algorithm and Reference Standard was 71% (respective Cohens kappa and delta values were 0.64 and 0.70), the overall diagnostic agreement between Senior Physician and Reference Standard was 32% (0.20 and 0.19, respectively). Regarding the therapeutic consequences, the diagnostic accuracy of the Algorithm was 86% (0.70 and 0.72, respectively) and of the Senior Physician was 48% (0.01 and 0.04, respectively). In retrospect, by disregarding the patients extracellular fluid volume and assessing the effective arterial blood volume by determination of the fractional urate excretion, the Algorithm improved its diagnostic accuracy to 95%. CONCLUSION Although the Algorithm performed reasonably well, several shortcomings became apparent, rendering it difficult to apply the Algorithm without reservation. Whether some modifications may enhance its diagnostic accuracy and simplify the management of hyponatremia needs to be determined.

Highly indistinguishable on-demand resonance fluorescence photons from a deterministic quantum dot micropillar device with 74% extraction efficiency

The implementation and engineering of bright and coherent solid state quantum light sources is key for the realization of both on chip and remote quantum networks. Despite tremendous efforts for more than 15 years, the combination of these two key prerequisites in a single, potentially scalable device is a major challenge. Here, we report on the observation of bright single photon emission generated via pulsed, resonance fluorescence conditions from a single quantum dot (QD) deterministically centered in a micropillar cavity device via cryogenic optical lithography. The brightness of the QD fluorescence is greatly enhanced on resonance with the fundamental mode of the pillar, leading to an overall device efficiency of η = (74 ± 4) % for a single photon emission as pure as g(2)(0) = 0.0092 ± 0.0004. The combination of large Purcell enhancement and resonant pumping conditions allows us to observe a two-photon wave packet overlap up to ν = (88 ± 3) %.