Combination of Sildenafil and Ba2+ at a Low Concentration Show a Significant Synergistic Inhibition of Inward Rectifier Potassium Current Resulting in Action Potential Prolongation

Abstract

Sildenafil (Viagra) is a vasodilator mainly used in the treatment of erectile dysfunction. Atrial or ventricular fibrillation may rarely occur as a side effect during sildenafil therapy. Although changes in inward rectifier potassium currents including IK1 are known to contribute to the pathogenesis of fibrillation, the effect of sildenafil on IK1 has not been studied. In experiments, Ba2+ is used as a specific inhibitor of IK1 at high concentrations (usually 100 µM). Being an environmental contaminant, it is also present in the human body; Ba2+ plasmatic concentrations up to 1.5 µM are usually reported in the general population. This study was primarily aimed to investigate changes of IK1 induced by sildenafil in a wide range of concentrations (0.1–100 µM). Additionally, the effect of combination of sildenafil and Ba2+ at selected clinically-relevant concentrations was tested, at 0.1 µM both on IK1 and on the action potential duration (APD). Experiments were performed by the whole-cell patch-clamp technique on enzymatically isolated rat ventricular cardiomyocytes, mostly at 23°C with the exception of APD measurements which were performed at 37°C as well. Sildenafil caused a significant, reversible, and concentration-dependent inhibition of IK1 that did not differ at −50 and −110 mV. Simultaneous application of sildenafil and Ba2+ at 0.1 µM revealed a massive inhibition of both inward and outward components of IK1 (this synergy was missing at other tested combinations). The combined effect at 0.1 µM (45.7 ± 5.7 and 43.0 ± 6.9% inhibition at −50 and −110 mV, respectively) was significantly higher than a simple sum of almost negligible effects of the individual substances and it led to a significant prolongation of APD at both 23 and 37°C. To our knowledge, similar potentiation of the drug-channel interaction has not been described. The observed massive inhibition of IK1 induced by a combined action of the vasodilator sildenafil and environmental contaminant Ba2+ at a low concentration and resulting in a significant APD prolongation may contribute to the genesis of arrhythmias observed in some patients treated with sildenafil.

Sildenafil affects the human Kir2.1 and Kir2.2 channels at clinically relevant concentrations: Inhibition potentiated by low Ba2+

“…Arrhythmogenesis related to the use of sildenafil is likely complex. Besides the effect of sildenafil on the Kir channels demonstrated in this study as well as in our previous study (Macháček et al, 2022), changes of other cardiac ionic currents should be considered. A study on the rapid component of delayed rectifier potassium current (I Kr ) demonstrated that sildenafil exerted a reversible inhibitory effect on I Kr channels expressed in a cell line, but the inhibition was below 10% at the therapeutic sildenafil concentration of 1 µM (the half inhibitory concentration of ~100 µM); the supratherapeutic concentration (30 µM) inhibiting ~44% of I Kr in the cell line showed a prolongation cardiac repolarization by 15% in isolated guinea pig heart (Geelen et al, 2000).…”

“…The potentiation of the sildenafil inhibitory effect on Kir2.2 was observed only in the cells with low reactivity to Ba 2+ -induced inhibition (Figures 5A, B). On average, these changes were not significant because the inhibitory effect of Ba 2+ on Kir2.2 was too high, preventing the potentiation as has been also observed at higher Ba 2+ inhibition in our recent study on rat I K1 (Macháček et al, 2022). The inhibition of Kir2.2 induced by Ba 2+ was higher than that of Kir2.1 (at −110 mV: 24.5% ± 7.0% in Kir2.2 vs. 6.31% ± 1.34% in Kir2.1, n = 6, p < 0.01; at −50 mV: 16.0% ± 8.1% in Kir2.2 vs. 5.95% ± 0.75% in Kir2.1, n = 8, p > 0.05).…”

“…The effect of sildenafil on the Kir channel I K1 has been recently reported in freshly enzymatically isolated rat ventricular myocytes by our group ( Macháček et al, 2022 ). Sildenafil caused a significant and reversible inhibition of both inward and outward components of the rat ventricular I K1 even at therapeutic concentrations.…”

“…The Tyrode solution of the following composition (in mM) was used during measurements: NaCl 135, KCl 5.4, MgCl 2 0.9, HEPES 10, NaH 2 PO 4 0.33, CaCl 2 0.9, glucose 10 (pH was adjusted to 7.4 with NaOH). The solution was supplemented by specific inhibitors of several ionic currents to be the same one used during analogical measurements in rat cardiomyocytes (2 mM CoCl 2 , 50 mM tetraethylammonium chloride, 1 μM atropine, and 10 μM glibenclamide to inhibit calcium, delayed rectifier potassium, acetylcholine-sensitive potassium, and ATP-sensitive potassium currents, respectively; for details, see Macháček et al, 2022 ).…”

“…Impact of low Ba 2+ concentration on the effect of sildenafil on Kir2.1 and Kir2.2 channels Considering our recently published data on the combined effect of 0.1 µM sildenafil and 0.1 µM Ba 2+ on I K1 in rat ventricular myocytes Frontiers in Pharmacology frontiersin.org (Macháček et al, 2022), we decided to test this effect in the human Kir2.1 and Kir2.2 channels as well. In agreement with the data published in rats, we observed a slight inhibition of Kir2.1 in both 0.1 µM sildenafil and 0.1 µM Ba 2+ when applied separately (8.70% ± 2.10% and 6.31% ± 1.34%, respectively, at −110 mV, and 12.2% ± 2.6% and 5.95% ± 0.75%, respectively, at −50 mV; Figures 4A, B, left panels).…”