Abnormal Calcium Handling in Atrial Fibrillation Is Linked to Changes in Cyclic AMP Dependent Signaling

Both, the decreased L-type Ca2+ current (I-Ca,I-L) density and increased spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), have been associated with atrial fibrillation (AF). In this study, we tested the hypothesis that remodeling of 3 ',5 '-cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) signaling is linked to these compartment-specific changes (up- or down-regulation) in Ca2+-handling. Perforated patch-clamp experiments were performed in atrial myocytes from 53 patients with AF and 104 patients in sinus rhythm (Ctl). A significantly higher frequency of transient inward currents (I-TI) activated by spontaneous Ca2+ release was confirmed in myocytes from AF patients. Next, inhibition of PKA by H-89 promoted a stronger effect on the I-TI frequency in these myocytes compared to myocytes from Ctl patients (7.6-fold vs. 2.5-fold reduction), while the beta-agonist isoproterenol (ISO) caused a greater increase in Ctl patients (5.5-fold vs. 2.1-fold). I-Ca,I-L density was larger in myocytes from Ctl patients at baseline (p < 0.05). However, the effect of ISO on I-Ca,I-L density was only slightly stronger in AF than in Ctl myocytes (3.6-fold vs. 2.7-fold). Interestingly, a significant reduction of I-Ca,I-L and Ca2+ sparks was observed upon Ca2+/Calmodulin-dependent protein kinase II inhibition by KN-93, but this inhibition had no effect on I-TI. Fluorescence resonance energy transfer (FRET) experiments showed that although AF promoted cytosolic desensitization to beta-adrenergic stimulation, ISO increased cAMP to similar levels in both groups of patients in the L-type Ca2+ channel and ryanodine receptor compartments. Basal cAMP signaling also showed compartment-specific regulation by phosphodiesterases in atrial myocytes from 44 Ctl and 43 AF patients. Our results suggest that AF is associated with opposite changes in compartmentalized PKA/cAMP-dependent regulation of I-Ca,I-L (down-regulation) and I-TI (up-regulation).