Rat pancreatic adenylate cyclase. IV. Effect of hormones and other agents on cyclic AMP level and enzyme release

Abstract

1. The effects of secretin and pancreozymin-C-octapeptide and phosphodiesterase inhibitors on the concentration of adenosine 3',5'-cyclic monophosphate (cyclic AMP) and on the release of enzymes from rat pancreas have been studied. 2. In determininging cyclic AMP by means of the saturation assay of Brown et al. ((1971) Biochem. J. 121, 561-563) it is found essential to purify the pancreatic tissue extract by ion-exchange chromatography prior to the assay. 3. Injection of synthetic secretin or pancreozymin-C-octapeptide in anaesthetized rats in a secretory active dose (0.1 nmol) has no effect on the pancreatic cyclic AMP level. 4. Incubation for up to 10 min of pancreatic slices in Krebs-Ringer bicarbonate glucose medium containing 10(-2) M theophylline as phosphodiesterase inhibitor does not result in an increase of the cyclic AMP level. With 10(-2) M 1-methyl-3-isobutylxanthine as phosphodiesterase inhibitor the level is more than doubled after the first min of incubation and remains constant thereafter. 5. Addition of 3-10(-7) M secretin to slices incubated in the presence of 10(-2) M theophylline causes 84% increase of the cyclic AMP level above control, whereas the addition of 3-10(-7) M pancreozymin-C-octapeptide has no significant effect. In the presence of 10(-2) M 1-methyl-3-isobutylxanthine the latter hormone causes significant increases of up to 34% above control during 10 min of incubation. Secretin in this condition augments the cyclic AMP level by up to 296% above control during a 10 min incubation period. Addition of secretin and pancreozymin-C-octapeptide together has no greater effect than of secretin alone. 6. A broken cell fraction of rat pancreas contains adenylate cyclase activity which can be stimulated to 457 and 600% above the basal activity by 3-10(-7) M pancreozymin-C-octapeptide and secretin, respectively. Incubation of pancreatic slices with either hormone has no effect on the cyclic AMP phosphodiesterase activity in the homogenate of these slices. 7. Pancreozymin-C-octapeptide, dibutyryl cyclic AMP, 1-methyl-3-isobutylxanthine and carbamylcholine cause an elevated release of chymotrypsin from pancreatic slices incubated for 2 h in Krebs-Ringer bicarbonate medium, containing 10 mM glucose, while secretin, cyclic AMP and butyric acid have no significant effect. The release of the cytoplasmic enzyme lactate dehydrogenase is also elevated by dibutyryl cyclic AMP, 1-methyl-3-isobutylxanthine and carbamylcholine, but not significantly by pancreozymin-C-octapeptide. 8. The results support the role of cyclic AMP in the action of secretin, and do not exclude a mediating function of this nucleotide in the actions of pancreozymin in rat pancreas.