Apple procyanidins induce hyperpolarization of rat aorta endothelial cells via activation of K ⁺ channels

Apple procyanidins (AP), one of the polyphenol-rich compounds, showed an endothelial-dependent vasorelaxation in rat aorta, but the mechanisms of beneficial effects are still unclear. The present study was designed to clarify the potential role of AP in rat aorta endothelial cells (RAECs). The treatment of RAECs with AP (1-10 μg/ml) resulted in a dose-dependent hyperpolarization with a maximum effect at 10 μg/ml, and for this reason, AP (10 μg/ml) was used in all the following experiments. AP-induced hyperpolarization was significantly inhibited by pretreatment of nonspecific K ⁺ inhibitor, tetraethyl ammonium chloride or specific K ⁺ channel inhibitors, iberiotoxin, glibenclamide, 4-aminopyridine and BaCl ₂, as well as by high KCl or Ca ²⁺-free solution. AP-induced hyperpolarization was also proved using 64-channel multielectrode dish system that can monitor a direct and real-time change of membrane potential. Furthermore, AP treatment caused a significant increase of nitric oxide (NO) production and cyclic guanosine monophosphate levels via endothelial NO synthase messenger RNA expression. The NO production was inhibited by N ^G-monoethyl-l-arginine or Ca ²⁺-free solution and was completely abolished by their combination. Also, AP inhibited endothelial proliferation, while the effect was significantly abolished by N ^G-monoethyl-l-arginine or tetraethyl ammonium chloride. These findings suggest that AP induces both hyperpolarization of RAECs via multiple activation of K ⁺ channels and activation of NO/cyclic guanosine monophosphate pathway via increasing NO production or is responsible for antiangiogenic effect. Diminishment of hyperpolarization as well as NO production of AP in Ca ²⁺-free solution implicated that AP would play a crucial role in promoting Ca ²⁺ influx into endothelial cells so as to promote both actions.