Somatostatin in epithelial cells of intestinal mucosa is present primarily as somatostatin 28

Abstract

Region-specific antisera to [Tyr14]-SS28(1–14) were used to identify cells containing immunoreactivity to the SS28(1–14) fragment of somatostatin 28 (SS28) in gastric and intestinal mucosal epithelium and in pancreatic islets by immunoperoxidase staining. Radioimmunoassay with iodinated [Tyr 14]-SS28(1–14) identified one antiserum (F4) to SS28(1–14) that cross-reacted equally with SS28(1–12), SS28(1–14) and SS28. Two other antisera (F3 and F8) to SS28(1–14) did not cross-react with SS28(1–12) and showed insignificant cross-reactivity to SS28. Immunostaining results showed that F4 stained the same cells that reacted with antiserum AS-10, which is specific for the cyclic tetradecapeptide somatostatin, SS28(15–28). Antisera F3, F4, and F8 all reacted with islet D cells and with somatostatin cells in the antral mucosa. However, only antiserum F4 detected immunoreactivity in mucosal epithelial cells: F3 and F8 did not bind to these cells. After sections of intestine were exposed to trypsin, however, epithelial cells containing immunoreactivity to SS28(1–14) were detected in intestinal mucosa with antisera F3 and F8. These results were obtained for duodenum, jejunum, ileum, and colon, but most of the epithelial cells with immunoreactivity to SS28(1–14) were in the duodenum. Both radioimmunoassay and immunostaining results suggest that F3 and F8 bind to a region of SS28(1–14) that is unavail-Both radioimmunoassay and immunostaining results suggest that F3 and F8 bind to a region of SS28(1–14) that is unavailable to antibodies in the intact SS28 molecule. The positive immunocytochemical staining for SS28(1–14) with F3 and F8 after trypsin treatment indicates that the somatostatin-containing epithelial cells of intestinal mucosa probably do not process the somatostatin prohormone completely to SS28(15–28) (e.g., to cyclic somatostatin tetradecapeptide), but produce a larger somatostatin immunoreactive molecule as a terminal biosynthetic product. These results suggest that intestinal mucosal epithelial cells containing somatostatin-like immunoreactivity may be a major source of circulating SS28-like peptides.