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 pancrea s and the relationship of this distribution to lobular variation in in sulin content and secretion. Pancreases from adult male toad Bufo aren arum were separated into their five lobes: free, gastric, hepatic, duo denal, and jejunal. Pieces of each lobe were incubated with glucose, a rginine, and K+ and the insulin concentration in the medium was measur ed by radioimmunoassay. In the presence of 2 or 8 mM glucose, 10 mM ar ginine, and 10 mM K+ the free lobe released a significantly greater am ount 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 amo unt of insulin than at 2 mM. The insulin content of the free lobe was significantly higher than that of the others, whereas this parameter w as comparable among the latter. These pancreases contained islets of v ariable size and irregular shape. B and non-B cells, detected by immun operoxidase 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 reveale d the following sequence of endocrine cell percentage: free lobe > gas tric lobe = duodenal lobe > jejunal lobe = hepatic lobe. Some 48% of t he endocrine cells were present in the islets, while the remaining 52% were found throughout the exocrine pancreas. In the free lobe, more e ndocrine cells were located within the islets (65%) than outside and i n the gastric lobe the proportion was almost equal (48% within, 52% ou tside), but in the hepatic, duodenal, and jejunal lobes the majority l ay outside the islets (61, 63, and 70% extrainsular, respectively). Th e area covered by B and D cells was far larger within the islets than outside, with the relative magnitude of this difference being free lob e > gastric lobe > duodenal lobe > hepatic lobe = jejunal lobe. In the free lobe, this relative distribution was more skewed than in the rem aining lobes. PP and A cells occupied a more extensive area outside th e islets than inside in every lobe. There were no significant differen ces among the extrainsular areas occupied by each type of endocrine ce ll 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 dat a further suggest that the nonuniform secretory capacities of amphibia n pancreatic lobes reflect localized differences in their insulin cont ent, which heterogeneity in turn stems from the dissimilar distributio n and organization of their constituent endocrine-cell populations. (C ) 1996 Academic Press, Inc.