Somatostatin-28 1-14

Somatostatin 28(1-14) immunoreactivity in primary afferent neurons of the rat spinal cord

Somatostatin 28(1-14) immunoreactivity is present in dorsal root ganglion cells and in the superficial laminae of the spinal cord in the rat. The distribution of somatostatin 28(1-14) immunoreactive varicosities in the dorsal horn corresponds well to the distribution of somatostatin 14 immunoreactive elements. Some dorsal root ganglion cells exhibit both somatostatin 14 and somatostatin 28(1-14) immunoreactivities.

Cytochemical demonstration of somatostatin 28-(1-14)-like immunoreactivity in the rat hypothalamus and gastro-entero-pancreatic endocrine system

Somatostatin-28 (SS 28) is a NH2-terminally extended form of somatostatin-14 (SS 14). Using an antiserum whose antigenic determinant is contained in segment 1-14 of SS 28, the cellular localization of a SS 28-(1-14)-like immunoreactivity was examined in the hypothalamus and gastro-enteropancreatic endocrine system of the rat. Immunoreactivity for SS 28-(1-14) was recognized in the SS 14-immunoreactive nerve cells of the hypothalamus and their axons terminating in the median eminence. Electron microscopic immunocytochemistry revealed that SS 28-(1-14)-like immunoreactivity in the median eminence was localized in the secretory granules of a type of nerve terminal. The immunoreactive cells in the gastro-enteropancreatic endocrine system also corresponded to the SS 14-immunoreactive D cells. Only in the pyloric antrum, however, did SS 14-immunoreactive D cells show no or very weak reactivity to the anti-SS 28-(1-14) serum. The wide distribution of SS 28-(1-14)-like immunoreactivity in the same cellular elements as those containing SS 14 supports the view that SS 28 is a precursor of SS 14.

Cysteamine induces a loss of tissue somatostatin-28 when measured as somatostatin-28(15-28)-like immunoreactivity but not when assessed as somatostatin-28(1-14)-like immunoreactivity: evidence for the importance of the disulfide bond for cysteamine action

The reported loss of somatostatin-14 (S-14)-like immunoreactivity (LI) by cysteamine (CSH) could be mediated through an action on the S-14 disulfide bond. If so, then in the case of somatostatin-28 (S-28) (a linear 14 amino acid N-terminally extended form of S-14), it should be possible to demonstrate with region-specific antisera, a selective effect of CSH on the disulfide bond containing C-terminal half of the molecule. To obtain evidence for this, we have determined by RIAs, the effect of CSH on S-28 concentration measured separately as S-28(15-28) LI and S-28(1-14) LI in the jejunal mucosa, a tissue rich in S-28. Four hours after a single sc injection of CSH to rats, mucosal S-28(15-28) LI was reduced from 16.4 +/- 0.6 to 4.6 +/- 0.51 pmol/mg protein (P less than 0.01). By contrast, S-28(1-14) LI sustained no loss and in fact increased from 27.6 +/- 1.9 to 41.6 +/- 2.2 pmol/mg protein (P less than 0.01). On Sephadex G-50 columns (in 6 M urea) approximately 70% of S-28(15-28) LI and S-28(1-14) LI coeluted with synthetic S-28 marker. These data suggest that CSH acts on the 15-28 segment of the S-28 molecule and renders it nonimmunoreactive probably through interaction with the disulfide bond. This mechanism probably also accounts for CSH-induced S-14 loss.

Somatostatin-14 and somatostatin-28 in chicken pancreatic islet D-cells

Somatostatin (SST)-14 and mammalian (m) SST-28[1-14] immunoreactivities of chicken pancreatic islets were investigated by using light microscopic immunohistochemistry. Chicken D-cells in both A- and B-islets showed immunoreactivity to SST-14, but not to mSST-28[1-14]. The acid-extract from both splenic and ventral lobes of pancreas was fractionated by reverse-phase high-performance liquid chromatography, and the SST-like immunoreactivity was measured in the radioimmunoassay using anti-SST-14 serum. In both lobes, the SST-like immunoreactivity was detected in the fraction which corresponded to that of SST-14 standard, but was not found in that of mSST-28 standard. Immunohistochemically, pancreatic endocrine D-cells of 1 amphibian, 4 reptiles and 12 birds showed the same immunostaining property as chicken D-cells. By contrast, both SST-14- and mSST-28[1-14]-immunoreactive D-cells were observed in the pancreatic islets of 16 mammals. From these results, we concluded that chicken islet D-cells contain only SST-14-like peptide, but not SST-28-like peptide, and that this phenomenon may be common to the avian species.

A high molecular weight form of somatostatin-28 (1-12)-like immunoreactive substance without somatostatin-14 immunoreactivity in the rat pancreas. Evidence that somatostatin-14 synthesis can occur independently of somatostatin-28

Synthesis of somatostatin-14 (S-14) could occur through direct enzymatic processing of precursor somatostatin (prosomatostatin) or via sequential breakdown of prosomatostatin leads to somatostatin-28 (S-28) leads to S-14. If direct processing is important, it should theoretically generate S-14 and a molecule equivalent to prosomatostatin without the S-14 sequence. In an attempt to identify such a molecule, I characterized the molecular forms of S-28(1-12)-like immunoreactivity (S-28(1-12) LI) in the rat pancreas and compared the relative amounts of these forms with those of S-14-like immunoreactivity (S-14 LI). Pancreatic extracts were chromatographed on Sephadex G-50 and Sephadex G-75 columns (Pharmacia Fine Chemicals Inc., Piscataway, NJ) under denaturing conditions and immunoreactivity in the eluting fractions was analyzed by region-specific radioimmunoassays (RIAs). For RIA of S-28(1-12) LI we used a newly developed rabbit antibody R 21 B, 125I-Tyr12 S-28(1-14), and S-28(1-12) standards. This system detects S-28, S-28(1-12), high molecular weight forms of S-28(1-12), but not S-14. S-14 LI was measured using antibody R149, which detects S-14, S-28, and higher molecular weight S-14-like substances, but not S-28(1-12). Three forms of S-28(1-12) LI were identified: Mr 9,000-11,000, Mr 1,200 (corresponding to S-28(1-12), and Mr less than 1,000, comprising, respectively, 35, 53, and 12% of total immunoreactivity. The relative abundance of the 9,000-11,000 mol wt S-28(1-12) LI material was unchanged following removal of S-14 LI from pancreatic extracts by affinity chromatography before gel filtration. Serial dilutions of fractions containing 9-11,000 and 1,200 mol wt materials exhibited parallelism with synthetic S-28(1-12). The total pancreatic concentration of S-28(1-12) LI was 1.56 pmol/mg protein, of which S-28(1-12) accounted for 0.83 pmol/mg protein and 9-11,000 S-28(1-12) LI comprised 0.55 pmol/mg protein. Pancreatic S-14 LI concentration was 2.07 pmol/mg protein, of which 98% corresponded to S-14. S-28-related peaks accounted for <1% of immunoreactivity in both RIAs. I concluded that (a) S-14 is the main form of pancreatic S-14 LI; (b) S-28 is present in very small quantities, in the pancreas; (c) S-28(1-12) LI consist mainly of S-28(1-12) and 9-11,000 mol wt S-28(1-12) LI; (d) 9-11,000 l wt S-28(1-12) LI could represent prosomatostatin without the S-14 sequence; (e) the finding of high concentrations of 9-11,000 mol wt S-28(1-12) LI suggests that S-14 synthesis can occur independently of S-28 and that direct processing of prosomatostatin is an important pathway for S-14 synthesis in the pancreas.