Membrane adenylate cyclase from rat heart was activated by the two gut peptides secretin and vasoactive intestinal peptide (VIP), glucagon, and the beta-adrenergic drug isoproterenol, in the presence of guanosine 5'-triphosphate (GTP). With all the stimuli tested, the optimal magnesium concentration was 5 mM, i.e. in excess over the 0.5 mM ATP substrate concentration and 0.01 mM GTP used as cofactor. Under these conditions, half-maximal adenylate cyclase activation with glucagon, secretin, and VIP was achieved at concentrations of 0.5, 0.5 and 1.0 microM, respectively. Data obtained with the secretin (7--27) fragment, a secretin antagonist, indicate that secretin and VIP acted on the same binding sites, which differed from glucagon binding sites. Structural requirements for secretin activation of cardiac adenylate cyclase were evaluated by comparing the potency and efficacy of parent peptides and synthetic analogs. The gastric inhibitory peptide GIP was inactive. When using 13 mono-or bi-substituted analogs, it appeared that amino acids in positions 1, 2, 3, 4 and 6 were of major importance while those in position 5 and 11 played a relatively minor role.

[3H]Caerulein was bound to dispersed acinar cells from rat pancreas in a rapid, reversible, specific, saturable, and temperature-dependent manner. Binding decreased above pH 6.5. lein within 1 min at 37 "C. The dissociation of remainder followed much slower kinetics. .The results suggested that intact rat pancreatic acinar cells have one class of caerulein binding sites existing in two states: one with high affinity and another with low affinity, the proportion of sites in each state depending on the degree af site occupancy (negative cooperativity), and on the intracellular concentration of nucleotides.Caerulein, a decapeptide isolated from the skin of the Australian amphibian Hyla cacrulea, is similar in chemical structure and in biologic activity to cholecystokinin-pancreozymin [l -41. In general the potency of caerulein is approximately the same as that of the C-terminal octapeptide of cholecystokininpancreozymin.The interaction of monotritiated [3H]caeruleiii with rat pancreatic plasma membranes has been reported in the preceding paper [5]. A study with dispersed intact acinar cells was undertaken to observe the effect of extramembrane events on caerulein binding, e. with the ability to stimulate secretion, calcium outflux and adenylate cyclase activity. Some of the present data have been published in abstract form [7]. MATERIALS AND METHODS Preparation of Isolated Rat Pancreatic Acinar CellsMale Wistar albino rats (200-250 g) fed ad libitum were sacrificed by decapitation. Isolated pancreatic acinar cells were prepared following the procedure of Amsterdam and Jamieson [8] using crude collagenase and hyaluronidase, EDTA and mild shearing forces. The digestion of the rat pancreas and the isolation of acinar cells offered no more difficulty than that of the guinea pig. The final three washings of the cells were conducted in media containing 0.5 mM calcium concentration. Light microscopy revealed that 96 -98 % of the cells were zymogen-containing exocrine cells. Their viability was demonstrated by their ability to exclude trypan blue. Cell concentration was determined by counting an appropriately diluted suspension in a conventional Neubauer hemocytometer. Unless otherwise specified, the dispersed acinar cells