Apamin interacts with all subtypes of cloned small-conductance Ca2+-activated K+ channels

The purpose of the present study was to examine how apamin interacts with t he three cloned subtypes of small-conductance Ca2+-activated KC channels (h SK1, rSK2 and rSK3). Expression of the SK channel subtypes in Xenopus laevi s oocytes resulted in large outward currents (0.5-5 muA) after direct injec tion of Ca2+. In all three cases the Ca2+-activated K+ currents could be to tally inhibited by 500 nM apamin. Dose-response curves revealed a subtype-s pecific 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. Consis tent with these results, membranes prepared from oocytes expressing the SK channel subtypes bound I-125-labelled apamin with distinct dissociation con stants (Kd values) of approx. 390 pM for hSK1, 4 pM for rSK3 and II 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 th at apamin may be useful for determining the expression pattern of SK channe l subtypes in native tissue.