Maurotoxin:: A potent inhibitor of intermediate conductance Ca2+-activated potassium channels

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 SIo1 calcium-activated potassium (K-Ca) channels. Maurotoxin was found to produce a potent inhibition of Ca2+-activated Rb-86 efflux (IC50, 1.4 nM) and inwardly rectifying potassium currents (IC50, 1 nM) in CHO cells stably expressing IK1. In contrast, maurotoxin produced no inhibition of SK1, SK2, and SK3 small-conductance or SIo1 large-conductance K-Ca channels at up to 1 muM in physiologically relevant ionic strength buffers. Maurotoxin did inhibit Rb-86 efflux (IC50, 45 nM) through, and I-125-apamin binding (K-i, 10 nM) to SK channels in low ionic strength buffers (i.e., 18 mM sodium, 250 mM sucrose), which is consistent with previous reports of inhibition of apamin binding to brain synaptosomes. Under similar low ionic strength conditions, the potency for maurotoxin inhibition of IK1 increased by similar to100-fold (IC50, 14 pM). In agreement with its ability to inhibit recombinant IK1 potassium channels, maurotoxin was found to potently inhibit the Gardos channel in human red blood cells and to inhibit the K-Ca in activated human T lymphocytes without affecting the voltage-gated potassium current encoded by Kv1.3. Maurotoxin also did not inhibit Kv1.1 potassium channels but potently blocked Kv1.2 (IC50, 0.1 nM). Mutation analysis indicates that similar amino acid residues contribute to the blocking activity of both IK1 and Kv1.2. The results from this study show that maurotoxin is a potent inhibitor of the IK1 subclass of K-Ca potassium channels and may serve as a useful tool for further defining the physiological role of this channel subtype.