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In these structure activity studies, the 46 analogs of the 27‐amino‐acid form of the pituitary‐adenylate‐cyclase‐activating peptide, PACAP(1–27), and the 38‐amino‐acid form, PACAP(1–38), were either monosubstituted or bisubstituted at positions 1–3, 20 and 21 or N‐terminally shortened. All analogs were compared on human neuroblastoma NB‐OK‐1 cell membranes for their ability to occupy 125I‐[AcHis1]PACAP(1–27)‐labelled receptors (AcHis, Nα‐acetylhistidine) and to activate adenylate cyclase (in terms of potency and intrinsic activity). The monophasic slope of dose/effect curves on both parameters suggested interaction with one class of PACAP receptor. Residues 28–38 in the C‐terminally extended peptide, PACAP(1–38), played a favorable role in recognition, in that receptors coupled to adenylate cyclase were, in general, more sensitive to PACAP(1–38) analogs than to the corresponding PACAP(1–27) analogs. At variance with PACAP(6–27), PACAP(6–38) was well recognized and acted as a potent competitive antagonist (Ki 1.5 nM). Residues 1–3 were all important in enzyme activation: modification of the β‐turn potential gave full agonists (the LAla2 and DAla2 derivatives) or partial agonists (LPhe2 and DPhe2; LArg2 and DArg2; Glu3 and Asn3). Finally, a proper α‐helix was also important: the combined substitution of Lys21/Lys22 by Gly21/Gly22 decreased the binding affinity sharply.

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Pancreatic islets contain calmodulin. The protein binds to a particulate fraction derived from the islets and stimulates adenylate cyclase activity in this subcellular fraction, both phenomena being activated by ionized calcium. A calcium-dependent stimulation of adenylate cyclase by endogenous calmodulin may contribute to the accumulation of adenosine 3',5'-monophosphate evoked by insulin releasing agents in the islet cells.

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