title: Evidence for a sodium-dependent potassium conductance in frog myelinated axon.
abstract: After blockade of the voltage-dependent potassium conductances by intracellular application of 4-aminopyridine and tetraethylammonium in frog myelinated axons, a set of brief (0.1 ms) intracellular depolarizing pulses or a long (200 ms) depolarizing pulse evoked a train of action potentials. Under both experimental conditions a hyperpolarizing afterpotential appeared (duration 367 ms +/- 34, mean +/- S.E., n = 15). The purpose of this study was to investigate the properties of this hyperpolarizing afterpotential. It was found that the hyperpolarizing afterpotential increases in amplitude with: (1) the number of sodium-dependent action potentials; (2) action potential broadening (following potassium channels blockade); and (3) the level of depolarization during a current step. Application of tetrodotoxin prevented the activation of the hyperpolarizing afterpotential by any of the above stimuli. The hyperpolarizing afterpotential was unaffected by: (1) 8-acetyl-strophanthidin, an agent that poisons the electrogenic pumping in the axon; (2) blocking calcium influx with extracellular 10 mM magnesium or 2 mM manganese; and (3) buffering of the intracellular calcium, using EGTA in the recording microelectrode. Extracellular application of tetraethylammonium, but not 4-aminopyridine, reduced the hyperpolarizing afterpotential. The hyperpolarizing afterpotential reversed at >> -92 mV. Increasing the external potassium concentration from 2 to 10 mM shifted the reversal potential +14.5 mV, indicating that the hyperpolarizing afterpotential is a potassium mediated conductance.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D001369:Axons / Q000187:drug effects / Q000378:metabolism*; D004558:Electric Stimulation; D005727:Ganglia, Spinal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D008094:Lithium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008839:Microelectrodes; D009413:Nerve Fibers, Myelinated / Q000187:drug effects / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011894:Rana pipiens; D012964:Sodium / Q000502:physiology*; D013327:Strophanthidin / Q000031:analogs & derivatives / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D015221:Potassium Channels; D013779:Tetrodotoxin; D013327:Strophanthidin; D008094:Lithium; D012964:Sodium; C004681:acetylstrophanthidin