title: Potassium and the excitability properties of normal human motor axons in vivo.
abstract: Hyperkalemia is an important cause of membrane depolarization in renal failure. A recent theoretical model of axonal excitability explains the effects of potassium on threshold electrotonus, but predicts changes in superexcitability in the opposite direction to those observed. To resolve this contradiction we assessed the relationship between serum potassium and motor axon excitability properties in 38 volunteers with normal potassium levels. Most threshold electrotonus measures were strongly correlated with potassium, and superexcitability decreased at higher potassium levels (P = 0.016), contrary to the existing model. Improved modelling of potassium effects was achieved by making the potassium currents obey the constant-field theory, and by making the potassium permeabilities proportional to external potassium, as has been observed in vitro. This new model also accounted well for the changes in superexcitability and other excitability measures previously reported in renal failure. These results demonstrate the importance of taking potassium levels into account when assessing axonal membrane dysfunction by excitability testing, and provide evidence that potassium currents are activated by external potassium in vivo.
authors.affiliation: Department of Neurology, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland.
authors.forename: Delphine
authors.initials: D
authors.lastname: Boërio
authors.affiliation: Department of Neurology, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland; Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, United Kingdom.
authors.forename: Hugh
authors.initials: H
authors.lastname: Bostock
authors.affiliation: Department of Neurology, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland.
authors.forename: Romana
authors.initials: R
authors.lastname: Spescha
authors.affiliation: Department of Neurology, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland; Department of Neurosurgery, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland.
authors.forename: Werner J
authors.initials: WJ
authors.lastname: Z'Graggen
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000328:Adult; D000368:Aged; D001369:Axons / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008959:Models, Neurological; D009046:Motor Neurons / Q000166:cytology* / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D011188:Potassium / Q000097:blood* / Q000378:metabolism*; D012684:Sensory Thresholds; D055815:Young Adult
chemical_list: D011188:Potassium