EFFECTS OF BARIUM ON DELAYED RECTIFIER POTASSIUM CURRENT IN BULLFROG SYMPATHETIC NEURONS PRETREATED WITH WORTMANNIN

INTRODUCTIONWortmannin is a microbial product whichhas been identified as a highly potentinhibitor of phosphatidylinositol 3-kinaseand myosin light chain kinase［23, 24］. Wehave previously shown that wortmannin, inhigh concentrations (e.g., 10μM), has directchannel blocking activity on a voltage-dependent potassium current (termed M-cur-rent) in bullfrog sympathetic neurons［18］.Blockage occurred slowly and recovery fromthe block was even slower, implying thatwortmannin either acted within the cell, orat the pore region of the channel but therate constant for binding and unbinding isextremely slow［18］. In any case, once thecells were treated with wortmannin (10μM)for 30-60 min, the M-current was almostnon-existent for the next 30-60 min［18］.We hypothesized that the actions of bari-um on a delayed rectifier potassium currentcould be examined in sympathetic neuronspretreated with wortmannin, for the follow-ing reasons: first, barium is known as apotent M-current blocker［1, 5, 20］with anIC