Pharmacological Blockade of IKs Destabilizes Spiral-Wave Reentry Under β-Adrenergic Stimulation in Favor of Its Early Termination

Selective activation of adrenoceptors potentiates IKs current in pulmonary vein cardiomyocytes through the protein kinase A and C signaling pathways

Delayed rectifier K⁺ current (I_Ks) is a key contributor to repolarization of action potentials. This study investigated the mechanisms underlying the adrenoceptor-induced potentiation of I_Ks in pulmonary vein cardiomyocytes (PVC). PVC were isolated from guinea pig pulmonary vein. The action potentials and I_Ks current were recorded using perforated and conventional whole-cell patch-clamp techniques. The expression of I_Ks was examined using immunocytochemistry and Western blotting. KCNQ1, a I_Ks pore-forming protein was detected as a signal band approximately 100 kDa in size, and its immunofluorescence signal was found to be mainly localized on the cell membrane. The I_Ks current in PVC was markedly enhanced by both β₁- and β₂-adrenoceptor stimulation with a negative voltage shift in the current activation, although the potentiation was more effectively induced by β₂-adrenoceptor stimulation than β₁-adrenoceptor stimulation. Both β-adrenoceptor-mediated increases in I_Ks were attenuated by treatment with the adenylyl cyclase (AC) inhibitor or protein kinase A (PKA) inhibitor. Furthermore, the I_Ks current was increased by α₁-adrenoceptor agonist but attenuated by the protein kinase C (PKC) inhibitor. PVC exhibited action potentials in normal Tyrode solution which was slightly reduced by HMR-1556 a selective I_Ks blocker. However, HMR-1556 markedly reduced the β-adrenoceptor-potentiated firing rate. The stimulatory effects of β- and α₁-adrenoceptor on I_Ks in PVC are mediated via the PKA and PKC signal pathways. HMR-1556 effectively reduced the firing rate under β-adrenoceptor activation, suggesting that the functional role of I_Ks might increase during sympathetic excitation under in vivo conditions.

Inhibitory Effect of Vascular Endothelial Growth Factor on the Slowly Activating Delayed Rectifier Potassium Current in Guinea Pig Ventricular Myocytes

Background

Vascular endothelial growth factor (VEGF) exerts a number of beneficial effects on ischemic myocardium via its angiogenic properties. However, little is known about whether VEGF has a direct effect on the electrical properties of cardiomyocytes. In the present study, we investigated the effects of different concentrations of VEGF on delayed rectifier potassium currents (I_K) in guinea pig ventricular myocytes and their effects on action potential (AP) parameters.

Methods and Results

I_K and AP were recorded by the whole‐cell patch clamp method in ventricular myocytes. Cells were superfused with control solution or solution containing VEGF at different concentrations for 10 minutes before recording. Some ventricular myocytes were pretreated with a phosphatidylinositol 3‐kinase inhibitor for 1 hour before the addition of VEGF. We found that VEGF inhibited the slowly activating delayed rectifier potassium current (I_Ks) in a concentration‐dependent manner (18.13±1.04 versus 12.73±0.34, n=5, P=0.001; 12.73±0.34 versus 9.05±1.20, n=5, P=0.036) and prolonged AP duration (894.5±36.92 versus 746.3±33.71, n=5, P=0.021). Wortmannin, a phosphatidylinositol 3‐kinase inhibitor, eliminated these VEGF‐induced effects. VEGF had no significant effect on the rapidly activating delayed rectifier potassium current (I_Kr), resting membrane potential, AP amplitude, or maximal velocity of depolarization.

Conclusions

VEGF inhibited I_Ks in a concentration‐dependent manner through a phosphatidylinositol 3‐kinase–mediated signaling pathway, leading to AP prolongation. The results indicate a promising therapeutic potential of VEGF in prevention of ventricular tachyarrhythmias under conditions of high sympathetic activity and ischemia.

Mechanism of Proarrhythmic Effects of Potassium Channel Blockers

Any disturbance of electrical impulse formation in the heart and of impulse conduction or action potential (AP) repolarization can lead to rhythm disorders. Potassium (K(+)) channels play a prominent role in the AP repolarization process. In this review we describe the causes and mechanisms of proarrhythmic effects that arise as a response to blockers of cardiac K(+) channels. The largest and chemically most diverse groups of compound targets are Kv11.1 (hERG) and Kv7.1 (KvLQT1) channels. Finally, the proarrhythmic propensity of atrial-selective K(+) blockers inhibiting Kv1.5, Kir3.1/3.4, SK, and K2P channels is discussed.