Systemic Distribution of Somatostatin Receptor Subtypes in Human: An Immunohistochemical Study

Somatostatin is well known to inhibit the hormone secretion of various peptides. This action has been considered to be generally mediated via six different specific somatostatin receptors (sstr), sstr1, sstr2A, sstr2B, sstr3, sstr4, and sstr5. It then becomes very important to demonstrate the localization of these sstr subtypes in order to elucidate the possible biological and/or clinical significance of somatostatin actions. These sstr subtypes have been demonstrated to be expressed throughout the human body, including the central nervous system, gastrointestinal tract, pancreas, kidney, and other organs, but its details, especially its systemic distribution and localization in tissue compartments, have yet to be examined thoroughly in human. Therefore, in this study, we examined the systemic localization of all six somatostatin receptors in normal human organs using immunohistochemistry with recently developed specific antibodies against these receptor subtypes. In all of the human tissues examined, various sstr subtypes were detected not only in parenchymal cells but also in various stromal cells such as lymphocytes, fibroblasts, and endothelial cells. Among human tissues in which the presence of sstr has not been previously reported, the parotid gland demonstrated immunoreactivity for sstr2B and sstr5, bronchial gland for sstr1, 2B, 3, 4, 5, parathyroid gland for sstr1, 3, 4, and duodenum for all subtypes immunoreactivity. The great majority of other organs examined demonstrated results consistent with those of previously reported biochemical studies. In pancreatic islet cells, only sstr2A was positive in all the cases but other sstr subtypes were associated with marked intraislet heterogeneity in their distribution. In stomach, all subtypes of receptor were detected in various cell types of the mucosa, but none in ECL cells of fundic gland. These findings demonstrated the broad systemic actions of somatostatin in non-endocrine cells.