title: Apamin as a selective blocker of the calcium-dependent potassium channel in neuroblastoma cells: voltage-clamp and biochemical characterization of the toxin receptor.
abstract: This paper describes the interaction of apamin, a bee venom neurotoxin, with the mouse neuroblastoma cell membrane. Voltage-clamp analyses have shown that apamin at low concentrations specifically blocks the Ca2+-dependent K+ channel in differentiated neuroblastoma cells. Binding experiments with highly radiolabeled toxin indicate that the dissociation constant of the apamin-receptor complex in differentiated neuroblastoma cells is 15-22 pM and the maximal binding capacity is 12 fmol/mg of protein. The receptor is destroyed by proteases, suggesting that it is a protein. The binding capacity of neuroblastoma cells for radiolabeled apamin dramatically increases during the transition from the nondifferentiated to the differentiated state. The number of Ca2+-dependent K+ channels appears to be at most 1/5th the number of fast Na+ channels in differentiated neuroblastoma. The binding of radiolabeled apamin to its receptor is antagonized by monovalent and divalent cations. Na+ inhibition of the binding of 125I-labeled apamin is of the competitive type (Kd(Na+) = 44 mM). Guanidinium and guanidinated compounds such as amiloride or neurotensin prevent binding of 125I-labeled apamin, the best antagonist being neurotensin.
mesh_terms: D000818:Animals; D001030:Apamin / Q000378:metabolism / Q000494:pharmacology*; D001514:Bee Venoms / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D002412:Cations / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D007473:Ion Channels / Q000187:drug effects*; D009447:Neuroblastoma; D009474:Neurons / Q000187:drug effects*; D009496:Neurotensin / Q000494:pharmacology; D018377:Neurotransmitter Agents / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels; D011956:Receptors, Cell Surface / Q000378:metabolism*
chemical_list: D001514:Bee Venoms; D002412:Cations; D007473:Ion Channels; D018377:Neurotransmitter Agents; D015221:Potassium Channels; D011956:Receptors, Cell Surface; C033799:apamin receptor; D001030:Apamin; D009496:Neurotensin; D011188:Potassium; D002118:Calcium