Somatostatin

Effects of octreotide, a somatostatin analogue, on gastric function evaluated by real-time ultrasonography

BACKGROUND

Somatostatin exerts inhibitory effects on physiological functions in the gastrointestinal tract. The actions differ, however, depending on the test meal, dose, and other factors.

AIMS

To determine by use of ultrasonography and scintigraphy the effect of a somatostatin analog, octreotide, on gastric emptying and antral contraction.

SUBJECTS

Twenty healthy men; mean age 23.9 years

METHODS

Subjects were studied for 7 days, once after subcutaneous injection of octreotide, 50 microg, 5 min before the ingestion of a test meal and once after subcutaneous injection of placebo. Ten subjects received a liquid meal, 10 others received a solid meal. With the liquid meal, gastric emptying was measured 15 min after its ingestion and antral contraction was measured for 15 min by ultrasonography. With the solid meal, gastric emptying was measured both by ultrasonography (n = 10) and by simultaneous scintigraphy (n = 6), with antral contraction measured by ultrasonography for 5 h after ingestion of the meal.

RESULTS AND CONCLUSIONS

Octreotide given with a liquid or solid meal inhibited gastric emptying in healthy subjects. A significant suppression of antral contraction occurred only with a solid meal.

The somatostatin-gastrin link of Helicobacter pylori infection

Helicobacter pylori is the new-found cause of duodenal ulcers (DU), but acid secretion remains necessary and is elevated in DU patients. My group and others have asked whether H. pylori itself alters gastric physiology. This infection has been found to decrease local expression of the inhibitory peptide somatostatin, and to increase release of the acid-stimulating hormone gastrin. H. pylori infection can alter acid secretion in both directions. Acid disappears temporarily on first infection, and may dwindle later if H. pylori causes gastric atrophy. DU patients have approximately twice the normal parietal cell mass, which increases their maximal secretory capacity, but it is not clear whether or not this is due to H. pylori. However, the infection certainly does change physiological control of acid secretion, as expected from the endocrine changes. Acid secretion is elevated during fasting, during stimulation with an acidic meal and during infusions of gastrin-releasing peptide. The balance between these opposing effects of H. pylori on acid may be crucial in determining the clinical outcome of H. pylori infection. High-acid secretion leads to DUs whilst low acid secretion is found in patients with gastric ulcers and gastric cancer. Inflammatory cytokines released in H. pylori gastritis may cause some of these changes in gastric physiology.

Effect of somatostatin-14 on duodenal mucosal bicarbonate secretion in guinea pigs

The role of somatostatin-14 in duodenal mucosal HCO3- secretion was investigated in anesthetized, indomethacin-treated guinea pigs. Net HCO3- output from the isolated, perfused (24 mM NaHCO3 + 130 mM NaCl) proximal duodenum was measured during intravenous infusion (alone or in combination) of somatostatin-14, carbachol, vasoactive intestinal peptide (VIP), and prostaglandin E2 (PGE2). In homogenates of duodenal enterocytes, the effect of these agents on adenylate cyclase activity was studied. Basal duodenal HCO3- secretion (3.5 +/- 0.2 mumol/cm/10 min) was reduced dose dependently by somatostatin-14 (10(-11) mol/kg, 10(-9) mol/kg, and 10(-7) mol/kg). Carbachol, VIP, and PGE2 (all 10(-8) mol/kg) increased basal duodenal HCO3- secretion two- to threefold. Somatostatin-14 (10(-7) mol/kg) abolished the stimulatory effect of carbachol and VIP, but not that of PGE2. Basal adenylate cyclase activity in isolated duodenal enterocytes (9.4 +/- 1.0 pmol cAMP/mg protein/min) was unaltered by somatostatin (10(-6) mol/liter) or carbachol (10(-3) mol/liter). VIP (10(-8) mol/liter) and PGE2 (10(-7) mol/liter) increased adenylate cyclase activity two- to threefold, and these effects were unchanged by somatostatin-14 (10(-6) mol/liter). In conclusion, somatostatin-14 inhibits basal and carbachol- and VIP-stimulated duodenal HCO3- secretion, and its mechanism of action is not via inhibition of adenylate cyclase activity in duodenal enterocytes.

The somatostatin receptor subtype on rat enterochromaffinlike cells

BACKGROUND/AIMS

Gastric enterochromaffinlike (ECL) cells play an important role in peripheral regulation of acid secretion. This study investigated the somatostatin receptor subtype on ECL cells.

METHODS

ECL cells were isolated from rat fundic mucosa to a purity of 90%-95% by combining enzymatic digestion, elutriation, density gradient centrifugation, and culture.

RESULTS

Polymerase chain reaction performed with templates from an ECL cell complementary DNA library and primers specific to each of the five known somatostatin receptor subtypes showed that the somatostatin receptor type 2 was significantly enriched in ECL complementary DNA. Single cell videoimaging of highly purified ECL cells in culture showed that only the somatostatin receptor type 2 selective agonist, DC 32-87, inhibited the gastrin-induced calcium signal at 10(-11) mol/L. The type 3 and type 4 selective agonists, DC 25-12 and DC 32-92, and also somatostatin 14 required 100-1000 times higher concentrations (10(-8) mol/L). The somatostatin receptor type 2 analogue also inhibited gastrin-stimulated histamine release with a 50% inhibitory concentration (IC50) value of 2 x 10(-12) mol/L, whereas somatostatin 14 and the type 3 and 4 analogues showed IC50 values of 1 to 5 x 10(-9) mol/L.

CONCLUSIONS

The predominant somatostatin receptors on rat gastric ECL cells are of the somatostatin receptor 2 subtype; they inhibit histamine secretion by interfering with the gastrin-induced calcium signal.

Reciprocal changes in antral gastrin and somatostatin mRNA in pernicious anemia

Somatostatin is involved in the regulation of gastrin by intragastric pH in animal models. To investigate whether this is so in man, we measured gastrin and somatostatin mRNA in endoscopic biopsies from six patients with hypergastrinemia and achlorhydria due to pernicious anemia and 12 age- and sex-matched controls. The pernicious anemia patients had significantly higher fasting plasma gastrin concentrations with a median (range) of 640 (420-3500) pmol/liter compared with 5 (2-58) pmol/liter, P < 0.001. The median gastrin mRNA/rRNA ratio was 10.4 (3.7-38.0) in the pernicious anemia patients compared with 1.7 (0.7-8.3) in the controls (P < 0.02), and it correlated strongly with the plasma gastrin concentration, r = 0.93, P < 0.0001. In contrast, the median somatostatin mRNA/rRNA ratio was lower in the pernicious anemia patients 0.84 (0.58-2.32) versus 2.04 (0.05-6.47) in the controls, P < 0.05. These findings suggest that in pernicious anemia gastric neutralization leads to hypergastrinemia through the modulation of antral gastrin synthesis by somatostatin.

Biochemistry and physiology of gastrointestinal somatostatin

Somatostatin, a tetradecapeptide initially isolated from the ovine hypothalamus, is widely distributed throughout the gastrointestinal tract where it may act as a hormone, local chemical messenger, or neurotransmitter to elicit many physiological actions. Release of somatostatin from D cells in the gut is regulated by mechanisms that are both dependent on and independent of cAMP. In most cases somatostatin acts to inhibit the function of its target cells. It performs this action in part via pertussis-toxin-sensitive inhibitory guanine nucleotide-binding proteins that regulate adenylate cyclase activity. Other mechanisms may involve sites of action distal to intracellular second messenger systems.