Effects of bile salts on transport rates and routes of FITC-labelled compounds across porcine buccal epithelium in vitro

In this study the penetration enhancing effect of bile salts on the transport of hydrophilic macromolecular compounds across porcine buccal mucosa was investigated in-vitro. Coadministration of 100 mM of the trihydroxy bile salts sodium glycocholate (GC) and sodium taurocholate (TC) and the dihydroxy bile salts sodium glycodeoxycholate (GDC) and sodium taurodeoxycholate (TDC) increased the in-vitro transport of fluorescein isothiocyanate (FITC) by a factor of a hundred or more, without a significant difference between the four bile salts. The concentration dependence of the enhancing effect of GDC was studied using FITC-labelled dextrans of increasing molecular weight as permeants (FD4, MW 4400; FD10, MW 9400; FD20, MW 19 600). The maximal enhancement was observed when GDC was coadministered in a concentration of 10 mM, resulting in an enhancement ratio of about 2000 for FD4. Using confocal laser scanning microscopy the effects of bile salts on the penetration pathways of hydrophilic compounds were investigated. The uniform distribution of FITC throughout the epithelium was changed by coadministration of 100 mM of bile salt to an increased amount of the fluorescent probe present in the intercellular domains. The intercellular distribution of both FD4 and FD10 was not changed by a low, but effective, concentration of GDC (2 mM, enhancement ratio of 72 for FD4). Increasing the concentration of GDC to 10 and 100 mM resulted in uptake of the fluorescent probe in the epithelial cells. From these results we conclude that the di- and trihydroxy bile salts studied increase the transport of hydrophilic compounds across buccal epithelium in vitro, below 10 mM by increasing the intercellular transport and at 10 mM and higher concentrations by opening up a transcellular route.