Biophysical transport properties of the cuticle of Ascaris suum.

The transport properties of isolated cuticle from Ascaris suum were studied using standard two-chamber diffusion cells and a number of radiolabeled permeants which varied in molecular size, lipophilicity and electrical charge. The permeability coefficient of the collagen matrix (lipid-extracted cuticle) vs. molecular radius relationship showed the interdependence of molecular size and electrical charge of the permeants with respect to the aqueous pores of the negatively charged matrix. The permeability of neutral solutes decreased monotonically with size. Protonated amines permeated the aqueous pores faster than neutral solutes of comparable size, while the permeation of anions was slower. The average pore size was estimated to be 1.5 nm in radius. A biophysical model which accounted for diffusion of molecules within a fixed electrostatic field of force and for molecular sieving by the pore channels was used in the mechanistic interpretation of the data. The effective permeability coefficient of the non-lipid-extracted cuticle was delineated into the permeability coefficients of the water-filled collagen matrix and the lipoidal component of the cuticle to determine which layer was the rate-controlling barrier. While each solute was capable of penetrating the water-filled collagen matrix, the rate-determining step for the majority of compounds was passive diffusion across the lipid component, which controlled 75-99% of transport. The exception was water, for which transport kinetics was 75% matrix-controlled. In general, permeation across the lipid-filled tissue was more favorable for small lipophilic compounds because of molecular restriction not only in the aqueous pores, but also in the lipid-filled pores.