ATP, released e.g. after cell damage or during inflammation, can alter ion transport across the intestinal mucosa via stimulation of purinergic receptors in the basolateral as well as in the apical membrane of epithelial cells. When ATP acts from the serosal side, it induces an increase in short-circuit current (I sc ) via Cl - secretion across the colonic epithelium. In contrast, mucosal ATP or its derivative, BzATP, predominantly stimulating ionotropic P2X 4 and P2X 7 receptors, evoke an increase in I sc , which could not be explained by Cl - secretion. The underlying ion currents after stimulation of apical purinergic receptors in rat distal colon are still unclear and were investigated in the present study. Ussing chamber experiments revealed that the I sc induced by mucosal ATP was dependent on the presence of mucosal Ca 2+ and inhibited by the K + channel blocker, Ba 2+ , indicating the involvement of Ca 2+ -dependent K + channels. Blockade of the transepithelial I sc by lanthanides (La 3+ , Gd 3+ ) suggests that Ca 2+ enters the epithelium via nonselective cation channels. Experiments with basolaterally depolarized epithelia confirmed the activation of apical lanthanide-sensitive Na + - and Ca 2+ -permeable cation channels by ATP. Putative candidates might be TRP channels, from which several subtypes were detected in colonic tissue in RT-PCR experiments. In addition, the activation of an apical Cl - conductance was observed when suitable Cl - concentration gradients were applied. Consequently, mucosal ATP, acting as 'danger signal', stimulates cation and anion channels in the apical membrane to induce a secretory response as part of the local defence mechanism in the intestinal epithelium.