Fig 1.
Induction of AF by transesophageal atrial burst pacing in mice.
Representative lead II body surface electrocardiogram (ECG) recordings (A). Simultaneous recordings of Lead II body surface ECG (upper) and esophageal ECG (lower) (B-D). (A) AF was induced by transesophageal atrial burst pacing (BP). An AF lasted about 32 seconds before spontaneous conversion into normal sinus rhythm (NSR). (B) Spontaneous conversion from AF to NSR. (C) Representative example of AF episode with disorganized fibrillatory atrial activities and irregular ventricular responses. (D) Conversion from AF to Afl with 4:1 atrioventricular-nodal conduction. Asterisks, arrows and circles indicate P-waves, atrial- and ventricular-electrograms, respectively. All R-R intervals are expressed in milliseconds.
Fig 2.
Norepinephrine strikingly elongates the duration of AF.
(A) Schematic diagrams of experimental protocol to induce AF after sympathetic activation in mice. Mice were treated with 1.5 mg/kg of norepinephrine (NE) by intraperitoneal injection followed by transesophageal atrial burst pacing to induce AF. The rectangle represents the period from the start of burst pacing to the termination of AF. Note that, for each individual animal, the longest duration among 10 trials was taken to be the duration of AF after NE administration. (B) The duration of AF was strikingly increased after NE administration in a dose-dependent manner. NE was intraperitoneally injected into each mouse at one of several doses as indicated below, and was followed by transesophageal atrial burst pacing (n = 6–8, *P<0.05 vs CTRL, †P<0.05 vs 2 μg/kg, ‡P<0.05 vs 500 μg/kg) (n = 6–8, ***P<0.001 vs CTRL, †††P<0.001 vs 2 μg/kg, ‡‡‡P<0.001 vs 500 μg/kg, §§P<0.01 vs 1000 μg/kg).
Fig 3.
Norepinephrine elongates AF duration through β1- and α1-adrenergic receptor-mediated signaling.
(A) 2 mg/kg of metoprolol, (B) 1 mg/kg of prazosin or natural saline (CTRL) was intraperitoneally injected into conscious mice 45 min before the administration of NE (1.5 mg/kg). Both metoprolol and prazosin treatment significantly shortened the NE-elongated AF. (n = 10–13, *P<0.05, #P<0.1 vs CTRL) (n = 10–15, *P<0.05).
Fig 4.
Measurement of Ca2+ transient in atrial myocytes.
Representative Ca2+ traces of atrial myocytes in the absence (A) or presence (B) of 1 μM NE. Fluo-4 loaded myocytes were electrically paced at 1 Hz for 15 seconds followed by a rapid switch of the extracellular solution from normal Tyrode to 0Na+/0Ca2+ Tyrode. The spontaneous Ca2+ release (SCR) was counted for 30 seconds. The diastolic Ca2+ leak from sarcoplasmic reticulum (SR), SR Ca2+ leak, was estimated by measuring the downward shift in fluorescence after 1 mM tetracaine treatment. Finally, 10 mM caffeine was administered rapidly to estimate the SR Ca2+ content.
Fig 5.
Norepinephrine induces SR Ca2+ leak and spontaneous Ca2+ releases via β1- and α1-AR mediated signaling.
Quantification of spontaneous Ca2+ release (A) and SR Ca2+ leak (B). Atrial myocytes were incubated with metoprolol or prazosin in the absence (-) or presence (+) of 1 μM NE. Metoprolol and prazosin significantly reversed the NE-enhanced SCR (n = 14–22, **P<0.01, ***P<0.001) and SR Ca2+ leak (n = 14–22, *P<0.05). The magnitude of SR Ca2+ leak is expressed as a relative value to SR Ca2+ content. Values under the graph represent the concentrations of adrenergic receptor antagonists (μM).