Background— Spontaneous Ca ²⁺ release from the sarcoplasmic reticulum (SR) can generate afterdepolarizations, and these have the potential to initiate arrhythmias. Therefore, an association may exist between spontaneous SR Ca ²⁺ release and initiation of atrial fibrillation (AF), but this has not yet been reported. Methods and Results— Spontaneous Ca ²⁺ release from the SR, manifested as Ca ²⁺ sparks and Ca ²⁺ waves, was recorded with confocal microscopy in atrial myocytes isolated from patients with and those without AF. In addition, the spontaneous inward current associated with Ca ²⁺ waves was measured with the use of the perforated patch-clamp technique. The Ca ²⁺ spark frequency was higher in 8 patients with AF than in 16 patients without (6.0±1.2 versus 2.8±0.8 sparks/mm per second, P <0.05). Similarly, the spontaneous Ca ²⁺ wave frequency was greater in patients with AF (2.8±0.5 versus 1.1±0.3 waves/mm per second, P <0.01). The spontaneous inward current frequency was also higher in 10 patients with AF than in 13 patients without this arrhythmia (0.101±0.028 versus 0.031±0.007 per second, P <0.05, at a clamped potential of −80 mV). In contrast, both the Ca ²⁺ released from the SR and the Na ⁺ -Ca ²⁺ exchange rate induced by a rapid caffeine application were comparable in patients with and without AF. Conclusions— The observed increase in spontaneous Ca ²⁺ release in patients with AF probably is due to an upregulation of the SR Ca ²⁺ release channel activity, which may contribute to the development of AF.

Background—: Pitx2 is a homeobox transcription factor that plays a pivotal role in early left/right determination during embryonic development. Pitx2 loss-of-function mouse mutants display early embryonic lethality with severe cardiac malformations, demonstrating the importance of Pitx2 during cardiogenesis. Recently, independent genome-wide association studies have provided new evidence for a putative role of PITX2 in the adult heart. These studies have independently reported several risk variants close to the PITX2 locus on chromosome 4q25 that are strongly associated with atrial fibrillation in humans. Methods and Results—: We show for the first time that PITX2C expression is significantly decreased in human patients with sustained atrial fibrillation, thus providing a molecular link between PITX2 loss of function and atrial fibrillation. In addition, morphological, molecular, and electrophysiological characterization of chamber-specific Pitx2 conditional mouse mutants reveals that atrial but not ventricular chamber-specific deletion of Pitx2 results in differences in the action potential amplitude and resting membrane potential in the adult heart as well as ECG characteristics of atrioventricular block. Lack of Pitx2 in atrial myocardium impairs sodium channel and potassium channel expression, mediated in part by miRNA misexpression. Conclusions—: This study thus identifies Pitx2 as an upstream transcriptional regulator of atrial electric function, the insufficiency of which results in cellular and molecular changes leading to atrial electric and structural remodeling linked to arrhythmogenesis.

Aims: Recent trial data demonstrate beneficial effects of active rhythm management in patients with atrial fibrillation (AF) and support the concept that a low arrhythmia burden is associated with a low risk of AF-related complications. The aim of this document is to summarize the key outcomes of the 9th AFNET/EHRA Consensus Conference of the Atrial Fibrillation NETwork (AFNET) and the European Heart Rhythm Association (EHRA). Methods and results: Eighty-three international experts met in Münster for 2 days in September 2023. Key findings are as follows: (i) Active rhythm management should be part of the default initial treatment for all suitable patients with AF. (ii) Patients with device-detected AF have a low burden of AF and a low risk of stroke. Anticoagulation prevents some strokes and also increases major but non-lethal bleeding. (iii) More research is needed to improve stroke risk prediction in patients with AF, especially in those with a low AF burden. Biomolecules, genetics, and imaging can support this. (iv) The presence of AF should trigger systematic workup and comprehensive treatment of concomitant cardiovascular conditions. (v) Machine learning algorithms have been used to improve detection or likely development of AF. Cooperation between clinicians and data scientists is needed to leverage the potential of data science applications for patients with AF. Conclusions: Patients with AF and a low arrhythmia burden have a lower risk of stroke and other cardiovascular events than those with a high arrhythmia burden. Combining active rhythm control, anticoagulation, rate control, and therapy of concomitant cardiovascular conditions can improve the lives of patients with AF.

Clinical and basic studies shed light on the mechanism, diagnosis, and treatment of cardiac ryanodine receptor Ca ²⁺ release deficiency syndrome.

Passive Ca binding was measured with a Ca-selective minielectrode in suspensions of permeabilized rabbit ventricular myocytes equilibrated with 5 microM thapsigargin and 30 microM ruthenium red to prevent sarcoplasmic reticulum (SR) or mitochondrial Ca uptake. Passive Ca binding was obtained by titration of the myocytes with Ca and subtraction of Ca binding in a blank titration without myocytes. Passive Ca binding could be described by a Michaelis binding curve with two sites: K1 = 0.42 microM n1 = 1.27 nmol/mg cell protein and K2 = 79 microM, n2 = 4.13 nmol/mg cell protein. The passive Ca buffering over the physiological Ca concentration was approximately twice the value expected from the values compiled by Fabiato [A. Fabiato. Am. J. Physiol. 245 (Cell Physiol. 14): C1-C14, 1983]. The maximal SR Ca uptake in the presence of 30 microM ruthenium red was fit by an uptake curve with a maximal uptake of 5.16 nmol/mg cell protein and a K 1/2 of 1.0 microM. In the presence of 5 microM thapsigargin and no ruthenium red, a significant Ca uptake attributed to mitochondria was measured between 10 and 100 microM free Ca. Rapid changes in free Ca concentration ([Ca]) measured with a Ca electrode were slower than simultaneous measurements of free [Ca] with indo-1 in permeabilized myocytes. However, oxalate, which buffers Ca and maximizes SR Ca uptake, increased the uptake rate and eliminated the difference in free [Ca] measured with Ca electrode and indo-1. This suggests that spatial gradients of [Ca] exist in permeabilized myocytes without Ca buffering. The new estimates of the buffering of intracellular Ca in cardiac myocytes should be valuable in developing quantitative insights into cardiac Ca regulation.