Calcium signalling silencing in atrial fibrillation.

Subcellular calcium signalling silencing is a novel and distinct cellular and molecular adaptive response to rapid cardiac activation. Calcium signalling silencing develops during short-term sustained rapid atrial activation as seen clinically during paroxysmal atrial fibrillation (AF). It is the first 'anti-arrhythmic' adaptive response in the setting of AF and appears to counteract the maladaptive changes that lead to intracellular Ca signalling instability and Ca -based arrhythmogenicity. Calcium signalling silencing results in a failed propagation of the [Ca ] signal to the myocyte centre both in patients with AF and in a rabbit model. This adaptive mechanism leads to a substantial reduction in the expression levels of calcium release channels (ryanodine receptors, RyR2) in the sarcoplasmic reticulum, and the frequency of Ca sparks and arrhythmogenic Ca waves remains low. Less Ca release per [Ca ] transient, increased fast Ca buffering strength, shortened action potentials and reduced L-type Ca current contribute to a substantial reduction of intracellular [Na ]. These features of Ca signalling silencing are distinct and in contrast to the changes attributed to Ca -based arrhythmogenicity. Some features of Ca signalling silencing prevail in human AF suggesting that the Ca signalling 'phenotype' in AF is a sum of Ca stabilizing (Ca signalling silencing) and Ca destabilizing (arrhythmogenic unstable Ca signalling) factors. Calcium signalling silencing is a part of the mechanisms that contribute to the natural progression of AF and may limit the role of Ca -based arrhythmogenicity after the onset of AF.