A compartmental analysis of solute transfer and exchange across blood-nerve barrier

A three-compartment model was derived to analyze solute exchange among plasma, peripheral nerve epineurium, and endoneurium. The model was fit to measured tibial-nerve epineurial and endoneurial contents of [14C]sucrose after intravenous bolus injection of tracer in pentobarbital sodium-anesthetized rats. The transfer constant (K) for tracer at epineurial vessels approximated 1.1 X 10(-3) ml.s-1.g whole-nerve-1. K at the blood-nerve barrier (BNB) equaled 1.36-1.51 X 10(-5) ml.s-1.g endoneurium-1. The endoneurial uptake data were analyzed also by a simplified two-compartment model incorporating solute exchange between plasma and endoneurium. K at the BNB (1.46 X 10(-5) ml.s-1.g endoneurium-1), determined by multiple uptake time graphic analysis, was only threefold greater than K at the blood-brain barrier. The transfer constant at the perineurium alone was determined in situ and equaled (1 X 10(-6) ml.s-1.g endoneurium-1. Calculated permeability coefficients (X 10(-8) cm/s) equaled 3.2 at the perineurium, 23 at the endoneurial capillaries, and 3.2 at the brain vasculature. The results demonstrate that for hydrophilic nonelectrolytes 1) flux across the perineurium contributes significantly to solute uptake from plasma into the endoneurium; 2) although nerve capillaries are more permeable than brain capillaries, the tissues of the BNB function as a unit to markedly restrict solute diffusion into the endoneurium; and 3) a two-compartment analysis accurately describes hydrophilic solute transfer from plasma into peripheral nerve.