Eel atrial natriuretic peptide inhibited the serosa-negative transepithelial potential difference and short-circuit current, accompanied by a decrease in NaCl and water absorption across the seawater eel intestine. Similar effects were obtained after treatment with N-terminally truncated eel atrial natriuretic peptide (5-27), indicating that N-terminal amino acids are not essential for the action of eel atrial natriuretic peptide. Although mammalian atrial natriuretic peptides also inhibited the short-circuit current, a 100-fold higher concentration was required to obtain the same effect as with eel atrial natriuretic peptide, indicating that eel atrial natriuretic peptide is 100 times as potent in eel intestine as the mammalian atrial natriuretic peptides. Similarly, in mammalian atrial natriuretic peptide, the four N-terminal amino acids had no significant effects. However, when the C-terminal tyrosine was removed, the potency of rat atrial natriuretic peptide was lowered. Compared with the effects of acetylcholine, serotonin and histamine, eel atrial natriuretic peptide was the most potent inhibitor, with 100% inhibition at 10(-7) M; 50% inhibition was obtained at 10(-2) M in acetylcholine, and 30% inhibition in serotonin (10(-5) M) and histamine (10(-3) M). These inhibitory effects of eel atrial natriuretic peptide were not diminished even in the presence of tetrodotoxin, and were mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate. Based on these results, structure-activity relationships of eel atrial natriuretic peptide and a possible mechanism of action of eel atrial natriuretic peptide are discussed.