Topographic differences in cell populations and insulin secretion in the endocrine pancreas of the toad Bufo arenarum

We analyzed the endocrine cell topography within the amphibian pancreas and the relationship of this distribution to lobular variation in insulin content and secretion. Pancreases from adult male toad Bufo arenarum were separated into their five lobes: free, gastric, hepatic, duodenal, and jejunal. Pieces of each lobe were incubated with glucose, arginine, and K+ and the insulin concentration in the medium was measured by radioimmunoassay. In the presence of 2 or 8 mM glucose, 10 mM arginine, and 10 mM K+ the free lobe released a significantly greater amount of insulin than the other lobes, while the output of the gastric lobe was greater than that of the duodenal, hepatic, and jejunal. At 8 mM glucose, every pancreatic lobe released a significantly higher amount of insulin than at 2 mM. The insulin content of the free lobe was significantly higher than that of the others, whereas this parameter was comparable among the latter. These pancreases contained islets of variable size and irregular shape. B and non-B cells, detected by immunoperoxidase staining, were located at the central and peripheral zones of the islets, respectively. A large number of non-B cells were also scattered over the exocrine component. Morphometrical analyses revealed the following sequence of endocrine cell percentage: free lobe > gastric lobe = duodenal lobe > jejunal lobe = hepatic lobe. Some 48% of the endocrine cells were present in the islets, while the remaining 52% were found throughout the exocrine pancreas. In the free lobe, more endocrine cells were located within the islets (65%) than outside and in the gastric lobe the proportion was almost equal (48% within, 52% outside), but in the hepatic, duodenal, and jejunal lobes the majority lay outside the islets (61, 63, and 70% extrainsular, respectively). The area covered by B and D cells was far larger within the islets than outside, with the relative magnitude of this difference being free lobe > gastric lobe > duodenal lobe > hepatic lobe = jejunal lobe. In the free lobe, this relative distribution was more skewed than in the remaining lobes. PP and A cells occupied a more extensive area outside the islets than inside in every lobe. There were no significant differences among the extrainsular areas occupied by each type of endocrine cell within a given pancreatic lobe. These results constitute the first demonstration of the heterogeneity in morphology, insulin content, and secretory function among the pancreatic lobes in B. arenarum. The data further suggest that the nonuniform secretory capacities of amphibian pancreatic lobes reflect localized differences in their insulin content, which heterogeneity in turn stems from the dissimilar distribution and organization of their constituent endocrine-cell populations.