Nasal epithelial permeation of thymotrinan (TP3) versus thymocartin (TP4): competitive metabolism and self-enhancement.

Purpose: To investigate concentration dependent permeabilities and metabolism kinetics of thymotrinan (TP3) versus thymocartin (TP4) in nasal epithelium in vitro.

Methods: Excised bovine nasal mucosa was used as an in vitro model. Permeabilities were studied in a diffusion chamber, metabolism kinetics in a reflection kinetics set-up. Studies were performed at various TP3 and TP4 concentrations. The 3H-mannitol flux was measured to monitor junctional permeability. Potential Ca(2+)-complexation was investigated using a Ca(2+)-selective electrode.

Results: Permeability of TP3 was negligible at 0.1 and 0.2 mM and increased drastically above 0.4 mM up to -2 X 10(-5) cm s(-1). In the presence of 2 mM TP4 the TP3 permeabilites were significantly above (approximately 4 x 10(-5) cm s(-1)) the level of TP3 without TP4, and TP3 metabolism was totally inhibited. TP3 and TP4 showed a significant concentration dependent effect on the permeability of 3H-mannitol. A hyperosmolarity effect of the peptide solutions was excluded. Transepithelial electrical resistance (TEER; approximately 30 ohms cm2) was unchanged by either TP3 or TP4. At 1 mM TP3 the mucosal-to-serosal permeability was four times higher than serosal-to-mucosal, indicating enzyme polarization. In reflection kinetics studies, TP3 degradation was slightly higher on the mucosal than on the serosal side. TP3 and TP4 followed the same non-linear metabolism kinetics.

Conclusions: Increase in permeability at high TP concentrations involves competitive enzyme saturation combined with self-enhanced paracellular permeation.