Transfer of nonelectrolytes from blood into peripheral nerve endoneurium

Permeability-surface area (PA) products were determined for the transfer of seven nonelectrolytes across the blood-nerve barrier (BNB) of rat tibial nerve using a quantitative in vivo injection technique. PA values at the BNB for slowly penetrating nonelectrolytes such as urea, mannitol, L-glucose, and sucrose differed by less than threefold from values at the blood-brain barrier in the same animals. Permeability coefficients for transfer across the BNB were calculated assuming both endoneurial capillaries and perineurium contribute to solute flux into endoneurium. Total BNB surface area was determined as 175 cm2/g with morphometric techniques. Calculated permeability coefficients for slowly penetrating nonelectrolytes ranged from 7 X 10(-8) cm/s for sucrose to 4 X 10(-7) cm/s for urea and were directly proportional to solute lipid solubility as measured by the octanol-water partition coefficient. BNB permeability coefficients for sucrose, mannitol, L-glucose, and urea were within 60% of values at cerebral capillaries, of the same order of magnitude as values at aporous lipid membranes, and 100-1,000 times less than values at most nonneural capillaries, such as in skeletal muscle. These results demonstrate that the BNB markedly restricts the transfer of hydrophilic nonelectrolytes between plasma and endoneurium and that diffusion restriction of the BNB is comparable to that of the blood-brain barrier.