We have already reported that poly-l-arginine (PLA) remarkably enhanced the in vivo nasal absorption of hydrophilic macromolecules without producing any significant epithelial damage in rats. In the present study, we examined whether PLA could enhance the absorption of a model hydrophilic macromolecule, fluorescein isothiocyanate-dextran (FD-4), across the intestinal mucosa, as well as the nasal mucosa, by an in situ closed-loop method using the rat intestine. PLA was found to enhance the intestinal absorption of FD-4 in a concentration-dependent manner within the concentrations investigated in this study, but segment-specific differences were found to be associated with this effect (ileum>jejunum>duodenum≧colon). The factors responsible for the segment-specific differences were also investigated by intestinal absorption studies using aprotinin, a trypsin inhibitor, and an analysis of the expression of occludin, a tight junction protein. In the small intestine, the differences in the effect of PLA on the absorption of FD-4 may be related to the enzymatic degradation of PLA. In the colon, the reduced effect of PLA on the absorption of FD-4 may be related to the smaller surface area for absorption and the higher expression of occludin compared with other segments.Key words poly-l-arginine; intestinal absorption; enhancer; fluorescein isothiocyanate-dextran (FD-4); occludin; aprotininWe have already reported that poly-l-arginine (PLA), a polycation, remarkably enhanced the in vivo nasal absorption of hydrophilic macromolecules in rats and increased their in vitro permeation across the rabbit nasal epithelium without producing any significant epithelial damage. [1][2][3][4][5] This effect is caused by the opening of tight junctions (TJ) in the epithelial cells. PLA plays an important role in opening the TJ in these cells, and the reaction induced by PLA appears to be reversible.4,6) Therefore, PLA is considered to be promising as an absorption enhancer for transnasal drug delivery. Although some studies have examined whether PLA can also enhance the absorption of hydrophilic macromolecules across the nasal epithelium, conjunctiva, alveolar epithelium, and Caco-2 cell sheets, [7][8][9][10] there have been no studies looking at PLA᾿s absorption enhancing effect across the intestinal mucosa. The intestinal mucosa is an important tissue particularly for the absorption of orally administered drugs.The intestinal absorption of hydrophilic macromolecules is well known to be typically very poor.11) The small intestine has an extensive surface area for absorption, but various digestive enzymes exist in the gastrointestinal tract. The property of intestinal mucosa differs from that of the nasal mucosa in some respects. Therefore, in order to improve the absorption of hydrophilic macromolecules, various strategies such as the use of absorption enhancers, protease inhibitors and chemical modification have been examined in the intestinal mucosa as well as the nasal mucosa. [12][13][14][15] It was recently reported that the TJ ba...