Two splice variants of the pituitary adenylate cyclase activating polypeptide (PACAP) type I receptor (PACAP receptor and PACAP/HOP receptor isoform) were stably expressed in Chinese hamster ovary (CHO) cells that did not express constitutively receptors for this family of peptides. The PACAP/HOP receptor protein had a 28 amino acid extension in the C-terminal part of the third intracellular loop. The two cell lines studied, CHO 2-10 (PACAP receptor) and CHO 4-12 (PACAP/HOP receptor) expressed a receptor density of 4.6 +/- 0.3 and 2.6 +/- 0.2 pmol/mg protein, respectively, with corresponding Kd values of 14.2 +/- 2.0 and 8.2 +/- 1.0 nM for [Ac-His1]PACAP-27 used as a tracer. Tracer binding was slightly decreased by GTP in both clones. The Kd values of PACAP-27, PACAP-38, vasoactive intestinal peptide (VIP), PACAP-27 fragments and analogues evaluated by binding competition curves, were higher in CHO 2-10 than in CHO 4-12, whereas the Kd for PACAP-38 fragments did not differ. The receptors were coupled to adenylate cyclase and the EC50 values were lower than the Kd values in both cell lines, suggesting an amplification process due to the existence of spare receptors.(ABSTRACT TRUNCATED AT 250 WORDS)