title: Charybdotoxin, dendrotoxin and mast cell degranulating peptide block the voltage-activated K+ current of fibroblast cells stably transfected with NGK1 (Kv1.2) K+ channel complementary DNA.
abstract: The blocking actions of the K+ channel toxins charybdotoxin, dendrotoxin and mast cell degranulating peptide were studied in B82 mouse fibroblast cells transformed to express NGK1 (Kv1.2) K+ channels. All three toxins were potent blockers of the K+ current in these cells, with KD values of 1.7, 2.8 and 185 nM, respectively. The toxin block exhibited a weak voltage-dependence with the degree of inhibition decreasing at positive membrane potentials. For charybdotoxin and dendrotoxin, reducing [K+]i did not increase the fractional block, demonstrating that the relief of block at positive membrane potentials is not due to displacement of the toxin molecules by outward flow of K+ ions. A voltage-jump protocol was used to determine the rates of binding and unbinding of dendrotoxin and mast cell degranulating peptide; binding of charybdotoxin was too rapid to be quantitatively evaluated in this manner. The binding rates (dendrotoxin, approximately 5 x 10(7)/M per s; mast cell degranulating peptide, approximately 0.8 x 10(7)/M per s) were largely voltage-independent, suggesting that association of the toxin molecules with the channel is diffusion limited. The rates of unbinding (dendrotoxin, approximately 0.3/s; mast cell degranulating peptide, approximately 3/s at +60 mV) of both toxins increased e-fold per approximately 40 mV change in membrane potential, thus accounting for the voltage-dependence of the equilibrium block. Internal perfusion with the three toxins failed to affect the K+ current (in contrast to internal tetraethylammonium which strongly blocked the current), indicating that the toxins exert their blocking action by binding to extracellular sites.
mesh_terms: D000668:Ampicillin Resistance / Q000235:genetics; D000818:Animals; D002478:Cells, Cultured; D018999:Charybdotoxin; D004247:DNA / Q000235:genetics; D004546:Elapid Venoms / Q000494:pharmacology*; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D010455:Peptides / Q000494:pharmacology*; D015221:Potassium Channels / Q000187:drug effects* / Q000378:metabolism; D015183:Restriction Mapping; D012604:Scorpion Venoms / Q000494:pharmacology*; D014162:Transfection
chemical_list: D004546:Elapid Venoms; D010455:Peptides; D015221:Potassium Channels; D012604:Scorpion Venoms; D018999:Charybdotoxin; C008331:mast cell degranulating peptide; C020273:dendrotoxin; D004247:DNA