Regulation of ion and water transport across the eel intestine: effects of acetylcholine and serotonin

Summary Both acetylcholine (ACh) and serotonin (5-HT) lowered the serosa-negative transepithelial potential difference (PD) and the short-circuit current (I_sc), accompanied by a decrease in NaCl and water absorption across the eel intestine. These inhibitory effects of ACh and 5-HT were blocked by atropine, a muscarinic receptor antagonist, and ICS-205930, a 5-HT₃ receptor antagonist, respectively. Even after blocking the ACh receptor with atropine, 5-HT inhibited the PD and I_sc, and ACh lowered them after blocking the 5-HT receptor with ICS-205930, indicating that ACh and 5-HT act independently. Similar inhibition in the PD and the I_sc was observed after electrical field stimulation (EFS) which is expected to release endogenous regulators. These effects of EFS were reduced by 70% after simultaneous addition of atropine and ICS-205930. Since atropine and ICS-205930 block ACh and 5-HT receptors, respectively, these results suggest that endogenous ACh and 5-HT are released by EFS.Abbreviations ACh acetylcholine - EFS electrical field stimulation - 5-HT serotonin - I_sc short-circuit current - PD transepithelial potential difference - R_t tissue resistance - TTX tetrodotoxin