Maurotoxin, a 34-amino acid toxin from Scorpio maurus scorpion venom, was examined for its ability to inhibit cloned human SK (SK1, SK2, and SK3), IK1, and Slo1 calcium-activated potassium (K Ca ) channels. Maurotoxin was found to produce a potent inhibition of Ca 2ϩ -activated 86 Rb efflux (IC 50 , 1.4 nM) and inwardly rectifying potassium currents (IC 50 , 1 nM) in CHO cells stably expressing IK1. In contrast, maurotoxin produced no inhibition of SK1, SK2, and SK3 small-conductance or Slo1 large-conductance K Ca channels at up to 1 M in physiologically relevant ionic strength buffers. Maurotoxin did inhibit 86 Rb efflux (IC 50 , 45 nM) through, and 125
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