Mast cell degranulating peptide and dendrotoxin selectively inhibit a fast-activating potassium current and bind to...

Dendrotoxin and mast cell degranulating peptide are highly potent convulsant polypeptides from mamba snake and bee venoms, respectively. Electrophysiological techniques and binding assays were used to study their interaction with fast-activating, voltage-dependent potassium channels in rat neurons. Intracellular recordings in sensory ganglion cells showed that mast cell degranulating peptide blocks the same slowly inactivating potassium current as dendrotoxin but with lower potency, the respective IC50 values in sensory A neurons of nodose ganglion being 2.1 nM and 37 nM. In contrast, the transient potassium current (IA) in superior cervical ganglion neurons was unaffected by either toxin, highlighting the heterogeneity of these potassium channels and the selective action of the toxins. Using biologically active 125I-labelled derivatives of dendrotoxin and beta-bungarotoxin (a related snake protein), the binding of mast cell degranulating peptide to two subtypes of high-affinity acceptors in rat cerebrocortical synaptosomal preparations was examined. Mast cell degranulating peptide antagonized the specific binding of both radioiodinated toxins to each of the acceptor species, in the membrane-bound state; additionally, [125I]dendrotoxin binding in detergent-solubilized extracts was, likewise, blocked by mast cell degranulating peptide. Notably, the observed inhibitory constants (KI) for mast cell degranulating peptide were appreciably larger than for dendrotoxin, consistent with their different efficacies in blocking the potassium conductances. It is concluded that the specific interaction of this apian polypeptide with dendrotoxin acceptors must underlie its selective action on potassium conductances, emphasizing a functional relationship between these membrane acceptors and the potassium channel variants, sensitive to both dendrotoxin and mast cell degranulating peptide.