A pharmacological model for calcium overload-induced tachycardia in isolated rat left atria.

Few experimental models produce spontaneous tachycardia in normal left atria to allow the study of the cellular mechanisms underlying this contributor to atrial fibrillation. We reported 2-aminoethoxydiphenyl borate (2-APB) that provokes sporadic spontaneous mechanical activity and calcium leak in isolated rat left atria. Since sarcoplasmic reticulum calcium leak in the presence of high calcium load may trigger tachyarrhythmias, we tested how conditions that increase calcium load affect 2-APB-induced ectopic activity. Exposing superfused rat left atria to (i) 30 nM isoproterenol, (ii) 3 microM forskolin, (iii) 300 nM (-)BayK 8644 ((4S)-1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluormethyl)phenyl]-3-pyridinecarboxylic acid methyl ester), (iv) 300 nM FPL-64176 (2,5-Dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester) or (v) 120 microM ouabain increases their force of contraction, evidence of calcium loading, but does not produce ectopic activity. Spontaneous mechanical activity occurs in left atria superfused with 20 microM 2-APB at 47+/-6 contractions/min in the absence of pacing. Any of these five agents increase rates of 2-APB-induced spontaneous mechanical activity to >200 contractions/min in the absence of pacing. Washing tachycardic left atria with superfusate lacking 2-APB restores normal function, demonstrating the reversibility of these effects. Decreasing superfusate sodium reverses this tachycardia and two hyperpolarization-activated current (I(f)) inhibitors blunt this ectopic activity. Thus conditions that increase atrial calcium load increase the frequency of spontaneous mechanical activity. Decreasing extracellular sodium and I(f) inhibitors suppress this spontaneous tachycardia suggesting forward-mode sodium-calcium exchange and I(f)-like activities underlie this activity. This model may help define cell pathways that trigger atrial tachyarrhythmias.

2-APB induces instability in rat left atrial mechanical activity.

Atrial contractile abnormalities are common clinical disorders but few pharmacological models can reliably produce such abnormalities in isolated atrial muscle. Since sarcoplasmic reticulum (SR) calcium leak may underlie these contractile irregularities, we investigated whether 2-aminoethoxydiphenyl borate (2-APB), a calcium leak-inducer, affects mechanical function in isolated, superfused rat left atria. Exposing left atria paced at 3 Hz to >10 microM 2-APB produced sporadic mechanical events that occurred in the absence of pacing stimulus. Prolonging atrial diastole in the presence of 2-APB produced spontaneous mechanical activity (SMA) defined as numerous mechanical events occurring in the absence of pacing stimulus. SMA depends on atrial sodium and chloride gradients as decreasing superfusate concentration of either ion suppressed SMA. Increasing superfusate potassium to produce an EK of approximately -74mV reversed SMA, revealing possible membrane potential sensitivity. Mechanical function decreased with time in left atria treated with 2-APB and low sodium or the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) compared with atria exposed to low sodium or DIDS alone, suggesting 2-APB may decrease left atrial SR activator calcium. Thus, 2-APB produces instability in regular left atrial mechanical activity that may require forward-mode sodium-calcium exchange and chloride channel activities. This data identify a new model for studying atrial contractile abnormalities.