title: An electrophysiological method for measuring the potassium permeability of the nerve perineurium.
abstract: An electrophysiological method is described for measuring the potassium permeability (PK) of the perineurium of the sciatic nerve of the frog. The method is based on the principle of grease-gap recording, in which an insulating compartment separates two surface recording electrodes. The sciatic nerves of frogs Rana temporaria and R. pipiens were isolated and mounted across a five compartment chamber, with Vaseline grease seals on the partitions between compartments. Compartments #1, #2 and #5 contained frog Ringer solution, #4 was filled with Vaseline and formed the grease gap, and #3 was the test compartment in which solutions could be changed. The nerve was stimulated via platinum electrodes in compartments #1 and #2, and DC potentials and compound action potentials (CAP) were recorded between Ag/AgCl electrodes connected through Ringer-agar bridges to compartments #3 and #5. In nerves with undamaged perineurium, changing from normal Ringer to high [K+] Ringer (100 mM, KCl replacing NaCl) for 2 min caused negligible change in DC potential or CAP, indicating that raised [K+] was not reaching the axon surface, and hence that the perineurium was exerting a diffusional restriction on K+ entry. In nerves damaged by stretching or drying, K+ pulses caused a depolarising change in DC potential (delta DC), and corresponding decline in CAP amplitude, consistent with a leaky perineurium allowing K+ entry and axonal depolarisation. Ringer made hypertonic by the addition of 2.5 M sucrose or 5 M NaCl caused increased perineurial permeability to K+. The method was calibrated by measuring the delta DC in response to raised [K+] in the range 5-100 mM [K+] in desheathed nerves; from this calibration curve relating delta DC to endoneurial [K+] it was possible to calculate the change in endoneurial [K+] occurring in intact preparations. The calculations showed that the undamaged perineurium had a PK of < 6.3 x 10(-7) cm.s-1, similar to the value calculated for in situ nerves using radioisotopic techniques, but less than the value reported for isolated perineurial cylinders. The method gives real-time information on the K+ permeability of the nerve perineurium and its modulation by experimental treatments.
authors.affiliation: Biomedical Sciences Division, King's College London, UK. nj.abbott@kcl.ac.uk
authors.forename: N J
authors.initials: NJ
authors.lastname: Abbott
authors.forename: G
authors.initials: G
authors.lastname: Mitchell
authors.forename: K J
authors.initials: KJ
authors.lastname: Ward
authors.forename: F
authors.initials: F
authors.lastname: Abdullah
authors.forename: I C
authors.initials: IC
authors.lastname: Smith
mesh_terms: D000818:Animals; D002199:Capillary Permeability / Q000502:physiology*; D004058:Diffusion; D017097:Electric Impedance; D004594:Electrophysiology; D006982:Hypertonic Solutions; D007552:Isotonic Solutions; D008839:Microelectrodes; D008954:Models, Biological; D009995:Osmosis; D010525:Peripheral Nerves / Q000378:metabolism*; D011188:Potassium / Q000493:pharmacokinetics*; D011894:Rana pipiens; D011896:Rana temporaria; D000077331:Ringer's Solution; D012584:Sciatic Nerve / Q000378:metabolism / Q000648:ultrastructure; D012965:Sodium Chloride; D013395:Sucrose
chemical_list: D006982:Hypertonic Solutions; D007552:Isotonic Solutions; D012965:Sodium Chloride; D013395:Sucrose; D000077331:Ringer's Solution; D011188:Potassium