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.

The effects of increasing concentrations of leucine (0.2, 2.0, and 15.0 mmol/liter) on glucagon secretion from the perfused rat pancreas were examined at various glucose levels (0, 3.3, or 8.3 mmol/liter) and in the absence or presence of either arginine (5.0 mmol/liter) or glutamine (10.0 mmol/liter). At a low glucose concentration (3.3 mmol/liter), leucine caused a dose-related biphasic increase in glucagon output in the absence of arginine, but only a transient increase in the presence of the latter amino acid. These positive responses were markedly reduced and, on occasion, abolished at a high glucose concentration (8.3 mmol/liter). Moreover, at a low glucose concentration (3.3 mmol/liter) and in the presence of arginine, the highest concentration of leucine (15.0 mmol/liter) provoked a sustained and reversible inhibition of glucagon release. Likewise, leucine (15.0 mmol/liter) reversibly inhibited glucagon secretion evoked by glutamine in the absence of glucose. Thus, leucine exerted a dual effect on the secretion of glucagon, the inhibitory effect of leucine prevailing at a high concentration of the branched chain amino acid and when glucagon secretion was already stimulated by arginine or glutamine. At a physiological concentration (0.2 mmol/liter), however, leucine was a positive stimulus for glucagon release, especially in the absence of another amino acid. Concomitantly, leucine was always a positive stimulus for both insulin and somatostatin secretion. The intimate mechanisms involved in the dual effect of leucine on glucagon secretion remain to be elucidated.

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The existence of specific receptors for the two PACAPs (Pituirary Adenylate Cyclase Activating Peptides of 27 and 38 amino acids) was previously demonstrated on membranes from the pancreatic acinar cell line AR 4‐2J (Buscail et al., FEBS Lett. 202, 77–81, 1990) by [125I]PACAP‐27 binding. Here we demonstrate, by comparing Scatchard analysis of saturation curves and competition binding curves obtained with [125I]PACAP‐27 and [125I]PACAP‐38 as radioligands, the coexistence of two classes of receptors : 1/ PACAP‐A receptors that recognize PACAP‐27 and PACAP‐38 with the same high affinity (K d 0.3 nM) and 2/ PACAP‐B receptors that recognize PACAP‐38 with a high affinity (K d 0.3 nM) and PACAP‐27 with a lower affinity (K d 30 nM). These two receptors are coupled to adenylate cyclase but can be clearly distinguished by the ability of PACAP(6–27) to specifically inhibit PACAP‐27 adenylate cyclase activation.

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