Calcium-handling abnormalities underlying atrial arrhythmogenesis and contractile dysfunction in dogs with congestive heart failure.

Background: Congestive heart failure (CHF) is a common cause of atrial fibrillation. Focal sources of unknown mechanism have been described in CHF-related atrial fibrillation. The authors hypothesized that abnormal calcium (Ca(2+)) handling contributes to the CHF-related atrial arrhythmogenic substrate.
Methods and Results: CHF was induced in dogs by ventricular tachypacing (240 bpm x2 weeks). Cellular Ca(2+)-handling properties and expression/phosphorylation status of key Ca(2+) handling and myofilament proteins were assessed in control and CHF atria. CHF decreased cell shortening but increased left atrial diastolic intracellular Ca(2+) concentration ([Ca(2+)](i)), [Ca(2+)](i) transient amplitude, and sarcoplasmic reticulum (SR) Ca(2+) load (caffeine-induced [Ca(2+)](i) release). SR Ca(2+) overload was associated with spontaneous Ca(2+) transient events and triggered ectopic activity, which was suppressed by the inhibition of SR Ca(2+) release (ryanodine) or Na(+)/Ca(2+) exchange. Mechanisms underlying abnormal SR Ca(2+) handling were then studied. CHF increased atrial action potential duration and action potential voltage clamp showed that CHF-like action potentials enhance Ca(2+)(i) loading. CHF increased calmodulin-dependent protein kinase II phosphorylation of phospholamban by 120%, potentially enhancing SR Ca(2+) uptake by reducing phospholamban inhibition of SR Ca(2+) ATPase, but it did not affect phosphorylation of SR Ca(2+)-release channels (RyR2). Total RyR2 and calsequestrin (main SR Ca(2+)-binding protein) expression were significantly reduced, by 65% and 15%, potentially contributing to SR dysfunction. CHF decreased expression of total and protein kinase A-phosphorylated myosin-binding protein C (a key contractile filament regulator) by 27% and 74%, potentially accounting for decreased contractility despite increased Ca(2+) transients. Complex phosphorylation changes were explained by enhanced calmodulin-dependent protein kinase IIdelta expression and function and type-1 protein-phosphatase activity but downregulated regulatory protein kinase A subunits.
Conclusions: CHF causes profound changes in Ca(2+)-handling and -regulatory proteins that produce atrial fibrillation-promoting atrial cardiomyocyte Ca(2+)-handling abnormalities, arrhythmogenic triggered activity, and contractile dysfunction.