2-APB Induces Instability in Rat Left Atrial Mechanical Activity

2-Aminoethyldiphenyl Borinate: A Multitarget Compound with Potential as a Drug Precursor

BACKGROUND

Boron is considered a trace element that induces various effects in systems of the human body. However, each boron-containing compound exerts different effects.

OBJECTIVE

To review the effects of 2-Aminoethyldiphenyl borinate (2-APB), an organoboron compound, on the human body, but also, its effects in animal models of human disease.

METHODS

In this review, the information to showcase the expansion of these reported effects through interactions with several ion channels and other receptors has been reported. These effects are relevant in the biomedical and chemical fields due to the application of the reported data in developing therapeutic tools to modulate the functions of the immune, cardiovascular, gastrointestinal and nervous systems.

RESULTS

Accordingly, 2-APB acts as a modulator of adaptive and innate immunity, including the production of cytokines and the migration of leukocytes. Additionally, reports show that 2-APB exerts effects on neurons, smooth muscle cells and cardiomyocytes, and it provides a cytoprotective effect by the modulation and attenuation of reactive oxygen species.

CONCLUSION

The molecular pharmacology of 2-APB supports both its potential to act as a drug and the desirable inclusion of its moieties in new drug development. Research evaluating its efficacy in treating pain and specific maladies, such as immune, cardiovascular, gastrointestinal and neurodegenerative disorders, is scarce but interesting.

Evidence that 2-aminoethoxydiphenyl borate provokes fibrillation in perfused rat hearts via voltage-independent calcium channels

We tested whether 2-aminoethoxydiphenyl borate (2-APB) induces arrhythmia in perfused rat hearts and whether this arrhythmia might result from the activation of voltage-independent calcium channels. Rat hearts were Langendorff perfused and beat under sinus rhythm. An isovolumic balloon inserted into the left ventricle was used to record mechanical function while bipolar electrograms were recorded from electrodes sutured to the base and the apex of hearts. Western and immunofluorescence analyses were performed on rat left ventricular protein extracts and left ventricular frozen sections, respectively. Rat ventricular myocytes express Orai 1 and Orai 3, and ventricle also contains the Orai regulator Stim1. Rat hearts (n=5) perfused with Krebs-Henseleit (KH) alone maintained sinus rhythm at 4.8 ± 0.1 Hz and stable mechanical function. By contrast, perfusing hearts (n=5) with (KH+22 μM 2-APB) provoked a period of tachycardic ectopy at rates of up to 10.8 ± 0.2 Hz. As perfusion with (KH+22 μM 2-APB) continued, the rate of spontaneous ventricular depolarization increased to 21.8 ± 1.2 Hz and became disorganized. Heart mechanical function collapsed as developed pressure decreased from 87 ± 8.8 to 3.5 ± 1.9 mm Hg. Flow rate did not change between normal (16.6 ± 0.9 ml/min) and fibrillating (17.4 ± 0.8 ml/min) hearts. The addition of 20 μM 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy]ethyl-1H-imidazole (SKF-96365) to (KH+22 μM 2-APB) perfusates (n=4) restored sinus rhythm and heart mechanical output. These data indicate that activating myocardial voltage-independent calcium channels, possibly the Orais, may be a novel cause of ventricular arrhythmia.

Pharmacological evidence for Orai channel activation as a source of cardiac abnormal automaticity

Calcium transport through plasma membrane voltage-independent calcium channels is vital for signaling events in non-excitable and excitable cells. Following up on our earlier work, we tested the hypothesis that this type of calcium transport can disrupt myocardial electromechanical stability. Our Western and immunofluorescence analyses show that left atrial and ventricular myocytes express the Orai1 and the Orai3 calcium channels. Adding the Orai activator 2-aminoethoxydiphenyl borate (2-APB) to the superfusate of rat left atria causes these non-automatic muscles to contract spontaneously and persistently at rates of up to 10 Hz, and to produce normal action potentials from normal resting potentials, all in the absence of external stimulation. 2-APB likewise induces such automatic activity in superfused rat left ventricular papillary muscles, and the EC(50)s at which 2-APB induces this activity in both muscles are similar to the concentrations which activate Orais. Importantly, the voltage-independent calcium channel inhibitor 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy]ethyl-1H-imidazole (SKF-96365) suppresses this automaticity with an IC(50) of 11 ± 0.6 μM in left atria and 6 ± 1.6 μM in papillary muscles. 1-(5-Iodonaphthalene-1-sulfonyl)-hexahydro-1,4-diazepine (ML-7), a second voltage-independent calcium channel inhibitor, and two calmodulin inhibitors also prevent 2-APB automaticity while two calmodulin-dependent protein kinase II inhibitors do not. Thus an activator of the Orai calcium channels provokes a novel type of high frequency automaticity in non-automatic heart muscle.

2-Aminoethoxydiphenyl borate, a inositol 1,4,5-triphosphate receptor inhibitor, prevents atrial fibrillation

The expression of the inositol 1,4,5-triphosphate receptor (IP3R) is upregulated and the function of IP3R also increases during atrial fibrillation (AF). 2-Aminoethoxydiphenyl borate (2-APB) is a membrane-permeable inhibitor of IP3R. However, the effect of 2-APB on AF is unknown. The aim of the present study is to explore the effects of 2-APB on AF. In vitro rabbit heart models of ischemia-, stretch- and cholinergic agitation-induced AF were developed. Fura-2-acetoxymethyl (Fura-2-AM) and Mg2+-Fura-2-AM were used to monitor alterations of intracellular Ca2+ and ATP, respectively, in HL-1 cells, an atrial muscle cell line, under chemical ischemia or cholinergic agitation. The results showed that inhibition of IP3R significantly reduced the incidence and its probability of being sustained in all three types of AF. IP3R inhibition ameliorated the cytoplasmic Ca2+ overload and energy compromise resulting from chemical ischemia or cholinergic agitation. Thus, IP3R inhibition may be a novel target for AF treatment, and IP3R may be an important molecule in the context of different kinds of AF.

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.