Ionic permeabilities of the frog perineurium.

Ionic permeation was investigated across the perineurium of the frog sciatic nerve, under normal conditions and following treatment by hypertonic Ringer, ouabain or amiloride. A cylindrical segment of perineurium removed from the nerve and mounted in vitro on two cannulae was continuously perfused. Permeation rates of 22Na and 42K across the perineurium were the same in either direction and were unaffected by the drugs. The mean 22Na permeability coefficient at the perineurium equaled 1.68 +/- 0.08 (S.E.M.) X 10(-6) cm/sec. Simultaneous measurement of transperineurial fluxes of 22Na, 42K and 36Cl indicated that the K/Na permeability ratio exceeded the ratio of limiting conductances of these ions in free solution, but that the Cl/K permeability ratio did not differ from the respective limiting conductance ratio. Immersion of the perineurial cylinder in Ringer, made hypertonic by addition of NaCl, increased the absolute permeability coefficients of the three ionic tracers but did not affect their permeability ratios. The flux ratio of 22Na/[14C]sucrose, however, was decreased by hypertonic treatment. It is concluded that there is no evidence of active Na or K transport across the perineurium and that the paracellular path in the perineurium exhibits size-dependent permselectivity properties. In addition, the low rates of transperineurial permeation of ions and water-soluble non-electrolytes (e.g. sucrose) are comparable to those in epithelia with tight junctions. These permeability coefficients provide quantitative estimates of the diffusion barrier properties of the perineurium.