Chemically homogenous compounds with antagonistic properties at all α1-adrenoceptor subtypes but not β1-adrenoceptor attenuate adrenaline-induced arrhythmia in rats

Studies proved that among all α₁-adrenoceptors, cardiac myocytes functionally express only aα_1A- and α_1B-subtype. Scientists indicated that α_1A-subtype blockade might be beneficial in restoring normal heart rhythm. Therefore, we aimed to determine the role of α₁-adrenoceptors subtypes (i.e., α_1A and α_1B) in antiarrhythmic effect of six structurally similar derivatives of 2-methoxyphenylpiperazine. We compared the activity of studied compounds with carvedilol, which is β₁- and α₁-adrenoceptors blocker with antioxidant properties. To evaluate the affinity for adrenergic receptors, we used radioligand methods. We investigated selectivity at α₁-adrenoceptors subtypes using functional bioassays. We tested antiarrhythmic activity in adrenaline-induced (20 μg/kg i.v.), calcium chloride-induced (140 and 25 mg/kg i.v.) and barium chloride-induced (32 and 10 mg/kg i.v.) arrhythmia models in rats. We also evaluated the influence of studied compounds on blood pressure in rats, as well as lipid peroxidation. All studied compounds showed high affinity toward α₁-adrenoceptors but no affinity for β1 receptors. Biofunctional studies revealed that the tested compounds blocked α_1A-stronger than α_1B-adrenoceptors, but except for HBK-19 they antagonized α_1A-adrenoceptor weaker than α_1D-subtype. HBK-19 showed the greatest difference in pA2 values-it blocked α_1A-adrenoceptors around seven-fold stronger than α_1B subtype. All compounds showed prophylactic antiarrhythmic properties in adrenaline-induced arrhythmia, but only the activity of HBK-16, HBK-17, HBK-18, and HBK-19 (ED₅₀ = 0.18-0.21) was comparable to that of carvedilol (ED₅₀ = 0.36). All compounds reduced mortality in adrenaline-induced arrhythmia. HBK-16, HBK-17, HBK-18, and HBK-19 showed therapeutic antiarrhythmic properties in adrenaline-induced arrhythmia. None of the compounds showed activity in calcium chloride- or barium chloride-induced arrhythmias. HBK-16, HBK-17, HBK-18, and HBK-19 decreased heart rhythm at ED84. All compounds significantly lowered blood pressure in normotensive rats. HBK-18 showed the strongest hypotensive properties (the lowest active dose: 0.01 mg/kg). HBK-19 was the only compound in the group, which did not show hypotensive effect at antiarrhythmic doses. HBK-16, HBK-17, HBK-18, HBK-19 showed weak antioxidant properties. Our results indicate that the studied 2-methoxyphenylpiperazine derivatives that possessed stronger α_1A-adrenolytic properties (i.e., HBK-16, HBK-17, HBK-18, and HBK-19) were the most active compounds in adrenaline-induced arrhythmia. Thus, we suggest that the potent blockade of α_1A-receptor subtype is essential to attenuate adrenaline-induced arrhythmia.