Anti-secretory properties of non-peptide somatostatin receptor agonists in isolated rat colon: luminal activity and possible interaction with P-glycoprotein.

1. The diverse physiological actions of somatostatin are mediated by a family of G-protein coupled receptors (SSTRs). Several peptide analogues of somatostatin such as octreotide have been developed for therapeutic use, including treatment of gastrointestinal disorders such as secretory diarrhoea. However, their development as anti-diarrhoeal agents has been limited by poor oral bioavailability, necessitating parenteral administration. This in vitro study investigated the anti-secretory potential of a group of novel, non-peptide, somatostatin-receptor agonists that selectively activate specific SSTR subtypes to assess their potential for oral administration. 2. The ability of the agonists to inhibit forskolin-stimulated chloride secretion was measured using a sensitive bioassay system in isolated rat colonic mucosa. 3. The SSTR-2 selective agonist, L-779,976 was 10-times more potent than octreotide as an inhibitor of secretion when added to the basolateral surface of rat colon. Non-peptide agonists selective for SSTR1 (L-797,591), SSTR3 (L-796,778), SSTR4 (L-803,087) or SSTR5 (L-817,818) showed little or no anti-secretory activity in this preparation. 4. L-779,976 was able to inhibit secretion when applied to the luminal surface at sub-micromolar concentrations suggesting that it can cross the colonic epithelium. The anti-secretory potency of luminal L-779,976 was increased 3 fold in the presence of GF120918, a known inhibitor of P-glycoprotein. 5. Non-peptide somatostatin receptor agonists may provide a basis for the development of new, orally available anti-diarrhoeal therapies.

Interferon gamma induces differential upregulation of alpha and beta chemokine secretion in colonic epithelial cell lines.

BACKGROUND: Production of chemoattractant factors by the intestinal epithelium may contribute to mucosal infiltration by inflammatory cells in inflammatory bowel disease. Secretion of the alpha chemokine interleukin 8 (IL-8), a neutrophil chemoattractant, has been widely studied, but little is known about epithelial secretion of beta chemokines, which are preferentially involved in recruiting monocytes. AIMS: To investigate the profiles of alpha and beta chemokine secretion in colonic cell lines and their differential modulation by interferon gamma (IFN-gamma), a product of activated T lymphocytes and natural killer cells. METHODS AND RESULTS: HT29-19A, a model of the CT secretory crypt cell, exhibited a parallel secretion of the alpha chemokines IL-8 and GRO alpha, which could be markedly upregulated by tumour necrosis factor alpha (TNF-alpha) and IL-1 beta. These cells showed no significant expression of the beta chemokines RANTES (regulated upon activation T cell expressed and secreted), MIP-1 alpha (macrophage inflammatory protein 1 alpha), and MCP-1 (monocyte chemotactic protein 1) under these conditions, but IFN-gamma in combination with TNF-alpha caused a dose dependent induction of RANTES and MCP-1 secretion. This was accompanied by a marked increase of RANTES mRNA. In contrast, IFN-gamma had no significant effect on TNF-alpha stimulated IL-8 secretion. Caco-2 cells, with features more typical of villus absorptive cells, were relatively poor secretors of alpha chemokines but secreted high levels of MCP-1 in response to IL-1 beta. IFN-gamma did not influence alpha or beta chemokine secretion in these cells. CONCLUSIONS: These studies suggest that intestinal epithelial cells may produce chemokines capable of attracting both neutrophils and monocytes. The ability of IFN-gamma to activate the expression of beta chemokines preferentially could facilitate the development of chronic inflammatory infiltrates.

Somatostatin receptor subtype 2 mediates somatostatin inhibition of ion secretion in rat distal colon.

BACKGROUND & AIMS: Somatostatin peptides are potent inhibitors of intestinal ion secretion, providing the rationale for their use in treating secretory diarrhea. However, the nature of the receptors that mediate these effects is unclear. The aims of this study were to investigate expression of somatostatin receptor subtypes (SSTRs) 1-5 in rat colonic epithelium and to identify which subtype(s) mediate inhibition of adenosine 3', 5'-cyclic monophosphate (cAMP)-activated secretion using SSTR-selective analogues. METHODS: SSTR expression was determined by reverse-transcription polymerase chain reaction and immunoblotting. Effects of somatostatin analogues on electrogenic ion secretion were studied in isolated colonic mucosa mounted in Ussing chambers. RESULTS: Crypt epithelium expressed messenger RNA for SSTR1 and SSTR2 and low levels of SSTR5. A splice variant of SSTR2 (SSTR2B) was also detected. The SSTR2 selective analogue NC-812 was a potent inhibitor of forskolin-activated secretion and cAMP accumulation. In contrast, peptides selective for SSTR3 (DC-25/12) and SSTR5 (DC-23/99) were weak inhibitors of secretion. NC-812 also inhibited dibutyryl cAMP-activated secretion, indicating a site of action distal to cAMP production. Immunoblot analysis confirmed expression of a 93-kilodalton SSTR2 protein in crypt cell membranes. CONCLUSIONS: SSTR2 receptors expressed by colonocytes mediate somatostatin's antisecretory actions in rat colon. Somatostatin analogues directed to specific SSTRs may provide the basis for more selective antidiarrheal drugs.