![]() We conclude that TsTX-K alpha unblocks the voltage-gated inactivating K+ channels in synaptosomes when they are blocked by alpha-DTX, but not when they are blocked by ChTX. ![]() The inactivating component could then be blocked by ChTX, which is structurally homologous to TsTX-K alpha. When TsTX-K alpha was added in the presence of alpha-DTX, however, only the noninactivating component of the K(+)-stimulated efflux was blocked. The effects of TsTX-K alpha and ChTX on 86Rb efflux were also additive. Both the inactivating and the noninactivating components of the 100 mM K(+)-stimulated 86Rb efflux were completely blocked when 200 nM TsTX-K beta and either 600 nM alpha-DTX or 200 nM ChTX were present. Both alpha-DTX and ChTX blocked the same inactivating component of the K(+)-stimulated 86Rb efflux in synaptosomes. TsTX-K alpha and TsTX-K beta blocked the same noninactivating component of 100 mM K(+)-stimulated 86Rb efflux in synaptosomes. Both TsTX-K alpha and ChTX completely displaced specifically bound 125I-alpha-DTX from synaptic membranes, but TsTX-K beta had no effect on bound alpha-DTX. ![]() We studied interactions among these toxins in 125I-alpha-DTX binding and 86Rb efflux experiments. In contrast, alpha-dendrotoxin (alpha-DTX) and charybdotoxin (ChTX) block voltage-gated inactivating K+ channels in synaptosomes (IC50 values of 90 nM and 40 nM, respectively). Two nonhomologous polypeptide toxins, tityustoxin K alpha (TsTX-K alpha) and tityustoxin K beta (TsTX-K beta), purified from the venom of the Brazilian scorpion Tityus serrulatus, selectively block voltage-gated noninactivating K+ channels in synaptosomes (IC50 values of 8 nM and 30 nM, respectively). ![]()
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