The effects of two absorption promoters, (sodium caprate (C(10)) and melittin), on intestinal permeability and viability were measured in intact rat and human colonic epithelia mounted in Ussing chambers. Apical-side addition of C(10) (10 mM) and melittin (10-50 microM) rapidly reduced the transepithelial electrical resistance (TEER) and increased the apparent permeability coefficient (Papp) of [(14)C]-mannitol and FITC-dextran-4 kDa (FD4) across colonic mucosae from both species. Effects of C(10) on flux were greater than those of melittin at the concentrations selected. C(10) irreversibly decreased TEER, but the effects of melittin were partially reversible. Enhanced permeability of polar sugars (0.18-70 kDa) in colonic mucosae with C(10) was accompanied by significant release of lactate dehydrogenase (LDH) from the luminal surface as well as by inhibition of electrogenic chloride secretion induced by the muscarinic agonist, carbachol (0.1-10 microM). Although melittin did not alter electrogenic chloride secretion in rat or human colonic mucosae, it caused leakage of LDH from rat tissue. Gross histology and electron microscopy of rat and human colonic mucosae demonstrated that each permeation enhancer can induce colonic epithelial damage at concentrations required to increase marker fluxes. C(10) led to more significant mucosal damage than melittin, characterised by sloughing and mucosal erosion. Overall, these results indicate that while C(10) and melittin increase transport of paracellular flux markers across isolated human and rat colonic mucosae in vitro, these effects are associated with some cytotoxicity.