Claudin tight junction proteins have been identified to primarily determine intestinal epithelial barrier properties. Whereas functional contribution of single claudins have been characterized in detail, information on the interplay with secretory mechanisms in native intestinal epithelium is scarce. Therefore, effects of cholera toxin and theophylline on rat colon were analyzed, including detection of sealing claudins. Tissue specimens were stripped off submucosal tissue layers and mounted in Ussing chambers. Short circuit current (ISC) and transepithelial resistance (TER) were reported, and expression and localization of claudins was analyzed. Moreover, histological analyses were performed, employing hematoxylin-eosin staining and light and electron microscopy. Theophylline induced a strong increase of ISC in colon tissue specimens. In parallel, a decrease of TER was observed. In contrast, cholera toxin did not induce a significant increase of ISC, whereas an increase of TER was detected after 120 min. West