Activation of Large-Conductance Calcium-Activated Potassium Channels by Puerarin:The Underlying Mechanism of Puerarin-Mediated Vasodilation

摘要

Puerarin is the main isoflavone found in Pueraria lobata(Willd)Ohwi,which has been used in therapy for various cardiovascular diseases.The present study examined the effects of puerarin on the large-conductance voltage-and Ca~(2+)-acti-vated potassium(BK_(Ca))channel and on rat thoracic aortas.BK_(Ca)channels encoded with either a(BK-alpha)or alpha/beta subunits(BK-alpha+beta 1)were heterologously expressed in Xenopus oocytes or human embryonic kidney 293 cells.The activities of BK_(Ca)channels were measured using excised patch-clamp recordings.Puerarin activated BK-alpha+beta 1 currents with a half-maximal concentration(EC50)of 0.8 nM and a Hill coefficient of 1.11 at 10 mu M Ca~(2+)and with an EC50 of 12.6 nM and a Hill coefficient of 1.08 at 0 mu M Ca~(2+).Puerarin(1 nM)induced a 16-mV leftward shift in the conductance-voltage curve for BK-alpha+beta 1 currents at 10 mu M Ca~(2+)and at 100 nM induced a 26-mV leftward shift at 0 mu M Ca~(2+).Puerarin mainly increased the BK-alpha+beta 1 channel open probability without changing the unitary conductance.Activation was also detected in the absence of the beta 1 subunit.A deglycosylated analog of puerarin,daidzein,also activated BK_(Ca)channels with weaker potency.In addition,puerarin(0.1 to 1000 mu M)caused concentration-dependent relaxations of rat thoracic aortic rings contracted with 1 mu M noradrenaline bitar-trate(EC50=1.1 mu M).These were significantly inhibited by 50 nM iberiotoxin,a specific blocker of BK_(Ca)channels.This is the first study demonstrating that puerarin activates BK_(Ca)channels,especially BK-alpha+beta 1 channels.The activation of the BK_(Ca)channel probably contributes to the puerarin-mediated vasodilation action.