Intestinal fat does not inhibit gastric function through a hormonal somatostatin mechanism in dogs

Active immunoneutralization of somatostatin in the sheep: effects on gastrointestinal somatostatin expression, storage and secretion

In the absence of somatostatin antagonists, somatostatin antisera administered acutely or animals chronically immunized against somatostatin have been used to define the functions of somatostatin. However, the circulating immunoglobulins from immunized animals may contain substantial quantities of endogenous hormones. This has not been examined for somatostatin. We have measured the amount of free somatostatin bound to circulating immunoglobulins in somatostatin-immunized animals and the effect of this sequestering of the free peptide on somatostatin secretion and gastric somatostatin synthesis and storage. The average concentration of somatostatin bound to the antisera was 6.9 nmol/l, about 1000-fold higher than normal circulating levels. Compared to control animals, there was a doubling of somatostatin mRNA in the fundus and a 4-fold increase in fundic somatostatin peptide. Similar increases were seen in pancreas, but the antrum was not significantly affected providing further evidence of distinct regulatory mechanisms between the antrum and fundus. We suggest that withdrawal of active somatostatin activates a regulatory loop to increase fundic somatostatin biosynthesis and storage. The data support the concept that somatostatin autoregulates its own expression at both the RNA and peptide level.

Role of the vagus nerve in mediating proximal nutrient-induced glucagon-like peptide-1 secretion

Plasma levels of glucagon-like peptide-1 (GLP-1) rise rapidly after nutrient ingestion, suggesting the existence of a proximal gut signal regulating GLP-1 release from the L cells of the distal small intestine. Glucose-dependent insulinotropic peptide (GIP) has been shown to be one such proximal signal; however, the dependence of GIP on gastrin-releasing peptide, a neuromodulator, suggested a role for the nervous system in this proximal-distal loop. Investigations into the nature of this proximal signal were therefore conducted in an in situ model of the rat gastrointestinal system. Infusions of corn oil into a 10-cm segment of duodenum that was isolated by loose ligation (to ensure that the luminal contents did not progress to the ileal L cell) increased the secretion of GLP-1 in parallel with that of gut glucagon-like immunoreactivity (gGLI; r = 0.85; P < 0.05). Infusion of fat into a transected segment of duodenum also significantly raised gGLI secretion compared with saline infusion, reaching a peak value of 132 +/- 37 pg/ml above basal (P < 0.05). However, peak secretion was significantly delayed when the gut was transected compared with that after ligation alone (19 +/- 4 vs. 6 +/- 1 min, respectively; P < 0.05). Furthermore, bilateral subdiaphragmatic vagotomy in conjunction with gut transection completely abolished the fat-induced rise in gGLI secretion (P < 0.001). Consistent with a role for the vagus in the regulation of the L cell, stimulation of the distal end of the celiac branch of the subdiaphragmatic vagus nerve significantly stimulated the secretion of gGLI to a level of 71 +/- 14 pg/ml above basal (P < 0.05). As found previously, supraphysiological infusion of GIP significantly increased gGLI secretion in control animals by 123 +/- 32 pg/ml (P < 0.05); this was not prevented by hepatic branch vagotomy (96 +/- 25 pg/ml; P < 0.05). In contrast, although infusion of GIP at physiological levels into sham-vagotomized animals also increased gGLI secretion, by 40 +/- 6 pg/ml (P < 0.05), selective hepatic branch vagotomy abolished GIP-induced gGLI secretion (P < 0.05). The results of these experiments therefore demonstrate that the secretion of GLP-1 and gGLI from the ileal L cell in response to fat is regulated by a complex neuroendocrine loop, involving the enteric nervous system, the afferent and efferent vagus nerves, as well as the duodenal hormone GIP.

Active immunization against somatostatin alters regulation of gastrin in response to gastric acid secretagogues

We have examined the coupling between somatostatin, gastrin, and gastric acidity, using sheep chronically immunized against somatostatin. All immunized sheep had high-titer (3.2 x 10(5) +/- 1.1 x 10(4) M), high-affinity (1.5 x 10(11) +/- 1.2 x 10(10) l/mol) antibodies. However, basal gastrin and gastric acidity were similar to those in control animals, indicating that an inhibitory somatostatin tone was not required for the maintenance of normal basal gastrin and gastric acidity. Omeprazole (a proton pump inhibitor) increased gastric pH to a similar extent in both the control and immunized groups but resulted in a smaller increase in plasma gastrin in the immunized sheep, thus calling into question the assumption that hypergastrinemia associated with hypochlorhydria is the result of somatostatin withdrawal. Pentagastrin- or histamine-stimulated somatostatin secretion reversed or attenuated the omeprazole-induced hypergastrinemia in control but not immunized sheep, demonstrating a functional role for somatostatin and the biological efficacy of the somatostatin immunization. In a separate series of omeprazole-treated sheep, restoration of an acidic gastric pH with intragastric HCl reversed the hypergastrinemia in both control and immunized animals. We conclude that somatostatin is not essential for the acid-mediated regulation of gastrin. The use of a chronically immunized model as opposed to the acute administration of somatostatin antibodies has important advantages in determining the steady-state regulatory role of somatostatin.

Esvaziamento gástrico e jejunal proximal em ratos submetidos ou não à gastrectomia parcial com reconstrução do trânsito intestinal em Y de Roux ou a Billroth II

Estudou-se os efeitos da reconstrução gastrojejunal em Y de ROUX (YR) ou a BILLROTH II (BII) no esvaziamento gástrico (EG) e jejunal proximal de líquido nutriente. Constitui-se quatro grupos de dez ratos Wistar machos: simulado (grupo I), antrectomia em YR (grupo II), gastrectomia subtotal em YR (grupo III) e antrectomia a BII (grupo IV). O EG e jejunal proximal foram avaliados no sétimo dia de pós-operatório, 60 minutos após a introdução gástrica de ovo cru mexido marcado com Tc99m. Após esse tempo, retirou-se separadamente o estômago, intestino proximal e restante do intestino, seguindo-se da quantificação do radioisótopo presente em cada segmento, numa câmara de ionização. O EG foi mais rápido nos animais gastrectomizados do que naqueles sem gastrectomia (α < 0,05). O EG no grupo antrectomia em YR foi mais lento do que no grupo gastrectomia subtotal em YR (α < 0,05), mas não apresentou diferença significante com relação ao grupo antrectomia a BII. O esvaziamento da alça de ROUX (grupos TI e III) não mostrou diferença significante em relação ao da alça eferente de igual comprimento (grupo IV). Concluiu-se que o EG de liquido nutriente foi semelhante na antrectomia em YR ou a BII e que nas reconstruções em YR, foi mais rápido na gastrectomia subtotal do que na antrectomia.

Somatostatin: physiology and clinical applications

Somatostatin (SOM) was originally isolated as the hypothalamic inhibitor of growth hormone release but was subsequently shown to have a widespread distribution including the gastrointestinal tract. In fact the gastrointestinal tract contains about 70% of the total body SOM. SOM has inhibitory actions on gastrointestinal exocrine and endocrine secretions, motility and blood flow. Within the gut it functions as an endocrine, paracrine, autocrine and neurocrine factor. SOM is released by a meal, and a number of neurotransmitters and regulatory peptides also influence SOM release. SOM is a key component of the gastrin-acid feedback loop as luminal acid releases SOM, which in turn has inhibitory effects on both gastrin and gastric acid. Consistent with the diverse functions of SOM, a number of different although related SOM receptors with distinct distribution patterns and intracellular mediators have been cloned and sequenced. SOM is the first of the gut regulatory peptides to have a significant therapeutic use. By inhibiting both the target cell (e.g. parietal cell) and the release of the active agent (e.g. gastrin) the therapeutic potential of SOM is magnified. To date most of the clinical experience has been with the one analogue, octreotide. This analogue has a longer half-life than SOM (hours versus minutes) but has only minimal oral activity, therefore requiring subcutaneous injections several times a day. The definite gastrointestinal applications include treatment of gastroenteropancreatic tumours. It is also becoming a favoured treatment for gastrointestinal fistulae, variceal bleeding and diarrhoea. However, octreotide has no consistent effect on tumour growth. The high density of SOM receptors on tumours has allowed localization of tumours using in vivo scintography with labelled octreotide. The sequencing of a variety of SOM receptors with different distributions and differing cellular effector systems raises the likelihood of developing SOM analogues for specific clinical applications.

Gut hormones in gastric function

This chapter has focused on many of the gut hormones that regulate gastric function. Gastrin remains the principal, and only, gastric hormone controlling gastric acid secretion during the cephalic, gastric and intestinal phases of secretion. Several other hormones, including cholecystokinin, peptide YY and secretin, released from intestinal endocrine cells in response to food substrates, have significant inhibitory effects on gastric acid secretion. Many of these hormones, including enteroglucagon and glucagon-like peptide, may act through paracrine release of somatostatin, which in turn acts as the final mediator of acid inhibition. In addition, several peptides contained in nerves, including gastrin releasing peptide and vasoactive intestinal peptide, have been shown to regulate gastric acid secretion and motor function. With the creation of specific monoclonal antibodies for use in in vivo immunoneutralization studies, and the development of selective chemical antagonists for use in receptor blockade experiments, the specific contributions of the different gut hormones in the regulation of gastric function, can be assessed.

Regulation of somatostatin-14 and -28 secretion by gastric acid in dogs: differential role of cholecystokinin

BACKGROUND

Prosomatostatin-derived peptides include two principle bioactive molecular forms, somatostatin 28 (S-28) and somatostatin 14 (S-14). This study examined whether there is a functional relationship between gastric acid secretion and the release of S-28 and S-14 into the circulation.

METHODS

In conscious dogs with gastric and duodenal cannulas, S-28 and S-14 responses, measured after extraction of acidified plasma and separation by gel chromatography, were evaluated by administration of nutrients and acid-inducing secretagogues without and with omeprazole.

RESULTS

Ingestion of a solid meal caused equivalent plasma elevations of S-28 and S-14, whereas infusions of histamine and gastrin selectively increased plasma S-14. Omeprazole decreased meal-stimulated S-28 (-67% +/- 8%; P < 0.01) and S-14 (-56 +/- 9%; P < 0.01) and abolished S-14 increases to histamine and gastrin. Intraduodenal perfusions of a liquid protein meal increased S-28 above S-14, comprising approximately 71% of total somatostatin-like immunoreactivity released, and omeprazole suppressed S-28 (-87% +/- 5%; P < 0.01) without influencing S-14. Similar responses occurred after exogenous cholecystokinin. Moreover, pretreatment of the intraduodenal protein meal with the cholecystokinin-A receptor antagonist MK-329 abolished increases of S-28 and S-14 and caused a further twofold increase of gastric acid (P < 0.025).

CONCLUSIONS

In the fed state, gastric acid causes direct release of S-14 from the stomach, but the acid-dependent component of S-28 secretion requires cholecystokinin as a cofactor. Negative feedback regulation between somatostatin and gastric acid secretory responses to nutrients may include S-28 modulated, in part, by cholecystokinin.