Introduction:
Heart failure (HF) frequently causes atrial fibrillation (AF) and focal sources of unknown mechanism have been implicated. Here, we studied the potential role and molecular mechanisms of Ca
2+
handling abnormalities.
Methods:
Ca
2+
handling (microfluorescence, Indo-1 AM) and related protein expression (Western blot) were assessed in left atria of 20 dogs with ventricular tachypacing (240 bpm × 2 wks)-induced HF and 20 controls (CTLs). Whole-cell perforated-patch was used to record action potentials (APs), delayed afterdepolarizations (DADs) and triggered activity.
Results
: HF increased [Ca
2+
]
i
transient amplitude from 239±24 to 444±43* nM (*P<0.05), and [Ca
2+
]
i
release by 10 mM local caffeine puffs (an index of SR Ca
2+
content) from 849±71 (CTL) to 1574±169* nM (HF). Spontaneous Ca
2+
release events increased from 1.8±0.5 (CTL) to 10.7±2.1* events/run (HF). HF significantly increased APD (by ~40% at 1 Hz). DADs and triggered activity were more common in HF (15.2±2.6 triggered APs/run, vs CTL 0.4±0.2*), and were abolished by ryanodine (10 μM), but not by the I
f
-blocker Cs
+
(2 mM). HF caused profound changes in protein expression of key Ca
2+
handling and regulatory proteins (Table
). Calsequestrin, the major SR Ca
2+
-binding protein, was reduced by 32%*. Fractional RYR2 PKA (Ser2809) phosphorylation decreased by 63%*, whereas CaMKII (Ser2815) RYR2 phosphorylation increased by 221%*. The catalytic and regulatory (RII) PKA subunits were downregulated by 15%* and 73%*, whereas expression and autophosphorylation (Thr287) of CaMKIIδ were increased by 45%* and 81%* respectively. NCX1, SERCA and total, PKA and CaMKII phosphorylated SERCA-regulatory phospholamban were unchanged by HF.
Conclusions:
HF causes profound changes in regulation and expression of atrial Ca
2+
handling proteins, producing increased SR Ca
2+
load and release, along with DADs and triggered activity that may account for focal mechanisms that initiate and/or sustain HF-related AF.
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