Search

The atrioventricular node (AVN) is a key component of the cardiac conduction system and takes over pacemaking of the ventricles if the sinoatrial node fails. IP 3 (inositol 1,4,5 tris phosphate) can modulate excitability of myocytes from other regions of the heart, but it is not known whether IP 3 receptor (IP 3 -R) activation modulates AVN cell pacemaking. Consequently, this study investigated effects of IP 3 on spontaneous action potentials (APs) from AVN cells isolated from rabbit hearts. Immunohistochemistry and confocal imaging demonstrated the presence of IP 3 -R2 in isolated AVN cells, with partial overlap with RyR2 ryanodine receptors seen in co-labelling experiments. In whole-cell recordings at physiological temperature, application of 10 µM membrane-permeant Bt 3 -(1,4,5)IP 3 -AM accelerated spontaneous AP rate and increased diastolic depolarization rate, without direct effects on I Ca,L , I Kr , I f or I NCX . By contrast, application via the patch pipette of 5 µM of the IP 3 -R inhibitor xestospongin C led to a slowing in spontaneous AP rate and prevented 10 µM Bt 3 -(1,4,5)IP 3 -AM application from increasing the AP rate. UV excitation of AVN cells loaded with caged-IP 3 led to an acceleration in AP rate, the magnitude of which increased with the extent of UV excitation. 2-APB slowed spontaneous AP rate, consistent with a role for constitutive IP 3 -R activity; however, it was also found to inhibit I Ca,L and I Kr , confounding its use for studying IP 3 -R. Under AP voltage clamp, UV excitation of AVN cells loaded with caged IP 3 activated an inward current during diastolic depolarization. Collectively, these results demonstrate that IP 3 can modulate AVN cell pacemaking rate.

The cardiac conduction system (CCS) is a network of specialized cardiomyocytes that coordinates electrical impulse generation and propagation for synchronized heart contractions. Although the components of the CCS, including the sinoatrial node, atrioventricular node, His bundle, bundle branches, and Purkinje fibers, were anatomically discovered more than 100 years ago, their molecular constituents and regulatory mechanisms remain incompletely understood. Here, we demonstrate the transcriptomic landscape of the postnatal mouse CCS at a single-cell resolution with spatial information. Integration of single-cell and spatial transcriptomics uncover region-specific markers and zonation patterns of expression. Network inference shows heterogeneous gene regulatory networks across the CCS. Notably, region-specific gene regulation is recapitulated in vitro using neonatal mouse atrial and ventricular myocytes overexpressing CCS-specific transcription factors, Tbx3 and/or Irx3. This finding is supported by ATAC-seq of different CCS regions, Tbx3 ChIP-seq, and Irx motifs. Overall, this study provides comprehensive molecular profiles of the postnatal CCS and elucidates gene regulatory mechanisms contributing to its heterogeneity., (© 2024. The Author(s).)

[Figure: see text].

Electrical activity in the heart exhibits 24-hour rhythmicity, and potentially fatal arrhythmias are more likely to occur at specific times of day. Here, we demonstrate that circadian clocks within the brain and heart set daily rhythms in sinoatrial (SA) and atrioventricular (AV) node activity, and impose a time-of-day dependent susceptibility to ventricular arrhythmia. Critically, the balance of circadian inputs from the autonomic nervous system and cardiomyocyte clock to the SA and AV nodes differ, and this renders the cardiac conduction system sensitive to decoupling during abrupt shifts in behavioural routine and sleep-wake timing. Our findings reveal a functional segregation of circadian control across the heart's conduction system and inherent susceptibility to arrhythmia.

It is well established that Pacemaker activity of the sino-atrial node (SAN) initiates the heartbeat. However, the atrioventricular node (AVN) can generate viable pacemaker activity in case of SAN failure, but we have limited knowledge of the ionic bases of AVN automaticity. We characterized pacemaker activity and ionic currents in automatic myocytes of the mouse AVN. Pacemaking of AVN cells (AVNCs) was lower than that of SAN pacemaker cells (SANCs), both in control conditions and upon perfusion of isoproterenol (ISO). Block of I(Na) by tetrodotoxin (TTX) or of I(Ca,L) by isradipine abolished AVNCs pacemaker activity. TTX-resistant (I(Nar)) and TTX-sensitive (I(Nas)) Na(+) currents were recorded in mouse AVNCs, as well as T-(I(Ca,T)) and L-type (I(Ca,L)) Ca(2+) currents I(Ca,L) density was lower than in SANCs (51%). The density of the hyperpolarization-activated current, (I(f)) and that of the fast component of the delayed rectifier current (I(Kr)) were, respectively, lower (52%) and higher (53%) in AVNCs than in SANCs. Pharmacological inhibition of I(f) by 3 µM ZD-7228 reduced pacemaker activity by 16%, suggesting a relevant role for I(f) in AVNCs automaticity. Some AVNCs expressed also moderate densities of the transient outward K(+) current (I(to)). In contrast, no detectable slow component of the delayed rectifier current (I(Ks)) could be recorded in AVNCs. The lower densities of I(f) and I(Ca,L), as well as higher expression of I(Kr) in AVNCs than in SANCs may contribute to the intrinsically slower AVNCs pacemaking than that of SANCs.

Recent evidence from intact hearts suggests that the function of cardiac nodal tissue may be particularly susceptible to acidosis. Little is currently known, however, about the effects of acidosis on the cellular electrophysiology of the atrioventricular node (AVN). This study was conducted, therefore, to determine the effect of acidosis on the spontaneous activity and membrane currents of myocytes isolated from the rabbit AVN, recorded at 35-37 degrees C using whole-cell patch-clamp. Reduction of extracellular pH (pH(e); from 7.4 to 6.8 or 6.3) produced pH-dependent slowing of spontaneous action potential rate and upstroke velocity, and reductions in maximum diastolic potential and action potential amplitude. Ionic current recordings under voltage-clamp indicated that acidosis (pH(e) 6.3) decreased L-type Ca current (I(Ca,L)), without significant changes in voltage-dependent activation or inactivation. Acidosis reduced the E-4031-sensitive, rapid delayed rectifier current (I(Kr)) tail amplitude at -40 mV following command pulses to between -30 and +50 mV, and accelerated tail-current deactivation. In contrast, the time-dependent hyperpolarisation-activated current, I(f), was unaffected by acidosis. Background current insensitive to E-4031 and nifedipine was reduced by acidosis. Measurement of intracellular pH (pH(i)) from undialysed cells using BCECF showed a reduction in mean pH(i) from 7.24 to 6.45 (n=17) when pH(e) was lowered from 7.4 to 6.3. We conclude that I(f) is unlikely to be involved in the response of the AVN to acidosis, whilst inhibition of I(Ca,L) and I(Kr) by acidosis are likely to play a significant role in effects on AVN cellular electrophysiology.

Spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) appears to play an important role in cardiac sinoatrial node pacemaking. However, comparatively little is known about the role of intracellular Ca(2+) in the atrioventricular node (AVN). Intracellular Ca(2+) was therefore monitored in cells isolated from the rabbit AVN, using fluo-3 in conjunction with confocal microscopy. These cells displayed spontaneous Ca(2+) transients and action potentials. Ca(2+) transients were normally preceded by a small, slow increase (ramp) of intracellular Ca(2+) which was sometimes, but not always, accompanied by Ca(2+) sparks. During the Ca(2+) transient, intracellular [Ca(2+)] increased initially at the cell periphery and propagated inhomogeneously to the cell centre. The rate of spontaneous activity was decreased by ryanodine (1muM) and increased by isoprenaline (500nM); these changes were accompanied by a decrease and increase, respectively, in the slope of the preceding Ca(2+) ramp, with no significant change in Ca(2+) spark characteristics. Rapidly reducing bathing [Na(+)] inhibited spontaneous activity. These findings provide the first information on Ca(2+) handling at the sub-cellular level and link cellular Ca(2+) cycling to the genesis of spontaneous activity in the AVN.