Apamin interacts with all subtypes of cloned small-conductance Ca 2+ -activated K + channels

The purpose of the present study was to examine how apamin interacts with the three cloned subtypes of small-conductance Ca2+-activated K+ channels (hSK1, rSK2 and rSK3). Expression of the SK channel subtypes in Xenopus laevis oocytes resulted in large outward currents (0.5–5 µA) after direct injection of Ca2+. In all three cases the Ca2+-activated K+ currents could be totally inhibited by 500 nM apamin. Dose–response curves revealed a subtype-specific affinity for the apamin-induced inhibition with IC50 values of 704 pM and 196 nM (biphasic) for hSK1, 27 pM for rSK2 and 4 nM for rSK3. Consistent with these results, membranes prepared from oocytes expressing the SK channel subtypes bound 125I-labelled apamin with distinct dissociation constants (K d values) of approx. 390 pM for hSK1, 4 pM for rSK2 and 11 pM for rSK3. These results show that apamin binds to and blocks all three subtypes of cloned SK channels, and the distinct values for IC50 and K d suggest that apamin may be useful for determining the expression pattern of SK channel subtypes in native tissue.