Bidirectional fluxes of Cl- across isolated and stripped goldfish intestinal epithelium mounted in Ussing-type chambers increased after addition of 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP), suggesting an increase of the paracellular permeability for Cl-. Confirming this, the addition of 8-Br-cAMP to the stripped intestine reduced the diffusion potential generated by isosmotic serosal or mucosal replacement of part of the NaCl by mannitol. The addition of the protein kinase C (PKC) activator 4 beta-phorbol 12,13-dibutyrate (PDB), 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), or the Ca2+ ionophore A23187 was without effect on the Cl- permeability. The cAMP-specific reduction of the diffusion potential was used to screen the epithelium for the presence of receptors coupled to adenylyl cyclase. The results indicate the presence of a serotonin (5-HT) receptor, positively coupled to adenylyl cyclase but insensitive to 5-HT1-, 5-HT2-, 5-HT3-, and nonclassical 5-HT4-receptor antagonists. Addition of vasoactive intestinal polypeptide (VIP) also reduced the diffusion potential in a dose-dependent way. Epinephrine restored the diffusion potential after its reduction by 5-HT or VIP. This effect could be mimicked by the partial alpha 2-adrenergic receptor agonist clonidine and blocked by the alpha 2-antagonists yohimbine and idazoxan. The Rp diastereoisomer of cAMP, (Rp)adenosine 3',5'-cyclic phosphorothioate [(Rp)cAMPS], counteracted the effect of VIP. The results indicate that in goldfish enterocytes VIP and 5-HT reduce the ion selectivity of the tight junctions through elevation of cAMP and that activation of alpha 2-adrenergic receptors antagonize these effects.(ABSTRACT TRUNCATED AT 250 WORDS)