An electrophysiological method for measuring the potassium permeability of the nerve perineurium

An electrophysiological method is described for measuring the potassium permeability ( P K) 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 (ΔDC), 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 ΔDC in response to raised [K +] in the range 5–100 mM [K +] in desheathed nerves; from this calibration curve relating ΔDC 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 P K of