A K+ channel in Xenopus nerve fibres selectively blocked by bee and snake toxins: binding and voltage-clamp experiments Journal Article

Author(s): Bräu, Michael E; Dreyer, Florian W; Jonas, Peter; Repp, Holger; Vogel, Werner
Article Title: A K+ channel in Xenopus nerve fibres selectively blocked by bee and snake toxins: binding and voltage-clamp experiments
Abstract: The effects of mast cell degranulating peptide (MCDP), a toxin from the honey bee, and of dendrotoxin (DTX), a toxin from the green mamba snake, were studied in voltage-clamped experiments with myelinated nerve fibres of Xenopus. MCDP and DTX blocked part of the K+ current. About 20% of the K+ current, however, was resistant to the toxins even in high concentrations. In Ringer solution half-maximal block was reached with concentrations of 33 nM MCDP and 11 nM DTX. In high-K+ solution the potency of both toxins was lower. β-Bungarotoxin (β-BuTX), another snake toxin, also blocked part of the K+ current, but was less potent than MCDP and DTX. Tail currents in high-K+ solution were analysed and three K+ current components were separated according to Dubois (1981b). Both MCDP and DTX selectively blocked a fast deactivating, slowly inactivating K+ current component which steeply activates between E = -60 mV and E = -40 mV (component f1). In concentrations around 100 nM, MCDP and DTX blocked neither the slow K+ current (component s) nor the fast deactivating, rapidly inactivating K+ current which activates between E = -40 mV and E = 20 mV (component f2). Similar results could be derived from K+ outward currents in Ringer solution. In high-K+, IC50 of MCDP for component f1 was 99 nM, whereas it was 7.6 μM for f2. Corresponding values for DTX are 68 nM and 1.8 μM. Binding studies with nerve fibre membranes of Xenopus reveal high-affinity binding sites for 125I-labelled DTX )K(D) = 22 pM in Ringer solution and 81 pM in high-K+ solution). 125I-labelled DTX can be displaced from its sites completely by unlabelled DTX, toxin I (black mamba toxin), MCDP, and partially by β-BuTX. Immunocytochemical staining demonstrates that binding sites for DTX are present in nodal and paranodal regions of the axonal membrane. The axonal membrane of motor and sensory nerve fibres is equipped with three types of well-characterized K+ channels and constitutes so far the best preparation to study MCDP- and DTX-sensitive K+ channels with electrophysiological and biochemical methods.
Keywords: Animals; Binding Sites; Dose-Response Relationship, Drug; Potassium Channels; Membrane Potentials/drug effects; Xenopus laevis; Bee Venoms/pharmacology; Bungarotoxins/pharmacology; Elapid Venoms/pharmacology; Nerve Fibers, Myelinated/drug effects; Neurotoxins/pharmacology; Peptides/pharmacology; Potassium Channels/drug effects; Bee Venoms; Bungarotoxins; Elapid Venoms; Neurotoxins; Peptides; mast cell degranulating peptide; dendrotoxin
Journal Title: Journal of Physiology
Volume: 420
ISSN: 1469-7793
Publisher: Wiley-Blackwell  
Date Published: 1990-01-01
Start Page: 365
End Page: 385
DOI: 10.1113/jphysiol.1990.sp017918
Open access: yes (repository)