Mechanism Of Ephedrine Directly Activating Cardiac I_Ks Currents That Control Heart Rhythms

Ephedra is a kind of famous and special local product which has been used to perspire, wheeze and cough for two thousand years in China. The dried medicinal herb of Mahuang is the spigeal stem of Ephedra sinica stapf. Ephedra is composed of three pairs of stereo-isomers alkaloid, including L-ephedrine, D-pseudoephedrine, L-norephedrine, D-norpseudoephedrine, L-methylephedrine, D-methylpseudoephedrine. As the major components, ephedrine (Eph) is about 80 percent of the total alkaloid. Ephedra and Eph have been used in clinical for many years and to treat for obesity as the food assistants. The wide application of Ephedra and Eph induces a series of side effects, especially in cardiovascular system. It is well-known that physiological function and side effects induced by Eph may be through a or (3-adrenoceptor directly or indirectly. However, our findings indicated that Eph could directly activate ion channel.Firstly, Eph could activate markedly the potassium channel in PC12 cells. But, Eph had no influences on the calcium channel in PC12 cells and sodium channel of DRG cells by whole-cell recording. Oppositely, pseudoephedrine (Pse) could not change the current of K⁺, Ca²⁺ and Na⁺ channel.Secondly, the effects of Eph on KCNQ1 channel were also investigated in this paper. KCNQ1 (K_V7.1, K_VLQTl), a kind of voltage-gated potassium channel, which coexpressed with the anxiliary subunit KCNE1 in cardia formed slowly activated-rectified-potassium currents (Iks) and has key contribution on the phase II of repolarization of cardiac action potential. In this study, we found Eph could increase the current of I_Ks in vitro (both in Xenopus oocytes and HEK293 cells) by means of patch clamp or two electrode voltage clamp. Then, we demonstrated that Eph, but not pseudoephedrine (Pse), could activate cardiac I_Ks currents with EC₅₀ = 50 nM. The onset and offset time constants of Eph activation of the I_Ks current wereÏ„_on = 49 s andÏ„_off = 400 s, respectively. A pair of Eph binding sites was also shown to occur at F296 and Y299 in the S5-S6 P-loop of the KCNQ1 channel by using of site-mutation, microinjection, transfection, and so on. As the binding sites are conserved highly in the KCNQ family and the M currents in the brain are formed by KCNQ2/KCNQ3 heterotetramer which belonged to the KCNQ family, Eph may have more profound significance on learning and memory. The mechanism of activation by Eph may provide a clue for drug design in the future.Thirdly, to further corroborate our findings, influences of Eph on the electrocardiographic (ECG) of BALB/c have been studied in the paper. The rats injected Eph with the dose of 5 mg/kg showed an increment of about 49 % in the heart rate and a decrement of about 39 % in the QTc interval. Besides, effects of Eph on defecation of C. elegans have been investigated in the paper also because KQT-3, the homologization of KCNQ1 channel, located mainly at intestine of C.elegans. Eph increased the ration of Exp/pBoc 20 %, whereas had no difference on the cycle period and interval period of defecation.At last, ephdra has been used to treate as common cold, bronchitis, asthma and arthritis. Until now, it is the first time that the influences of Eph on proliferation of bronchial smooth muscle cells (BSMCs) have been investigated in the paper. There was slight difference on the proliferation of BSMCs in the presence of 300μg/mL Eph. On the contrary, with the dose of 600μg/mL for 24 hours and 48 hours, Eph inhibited the proliferation of BSMCs (25.9±0.43) % and (40.7±0.35) %, respectively.