Effects of acetylcholine on A and B cells of the lizard endocrine pancreas

Abstract

The effects of acetylcholine (ACh) administration on the endocrine pancreas of the lizard Podarcis sicula were studied by histological and immunocytochemical methods at light and electron microscopy, and in situ hybridization, and the results were evaluated by image analysis. In control lizards, the endocrine islet cells, especially B cells, displayed heterogeneity in the immunoreactive content of secretory granules in both the splenic and duodenal pancreatic lobes; however, this feature was more evident in the former. Following ACh administration, the differences in B cells between the splenic and duodenal lobes were more marked. In fact, splenic lobe B cells showed a bubbling aspect caused by a massive exocytosis testified by the greatly swollen ß secretory granules devoid of their content. The duodenal lobe B cells exhibited images of exocytosis to a lesser extent and ‘disorganized’ granules. The secretion of A cells was also stimulated by ACh. These morphological observations were supported by optical density of immunostaining, cell surface areas, granular size, and labeling density measurements of B and A cells carried out using the Ks 300 image analysis system, which showed a reduction of these parameters in the treated lizards. In situ hybridization with proinsulin and proglucagon probes revealed that ACh stimulated not only release but also synthesis of insulin and glucagon, as can also be seen from the enlarged RER cisternae and Golgi complexes of B and A cells.