Somatostatin may not be a hormonal messenger of fat-induced inhibition of gastric functions

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.

Effects of lanreotide, a somatostatin analogue, on postprandial gastric functions and biliopancreatic secretions in humans

AIMS

Lanreotide is a novel synthetic somatostatin analogue. A long-acting formulation of lanreotide has been shown to be effective for the treatment of gastroentero-pancreatic hormone-producing tumours but effects on postprandial digestive and absorptive functions remain obscure. The aim of the present study was to evaluate the effects of intravenous lanreotide on gastric and biliopancreatic secretions in man as well as the absorption of nutrients and the duodeno-caecal transit time after ingestion of an homogenized meal (500 kcal, 55% carbohydrates, 15% proteins, 30% lipids).

METHODS

Eight healthy male volunteers were studied on two occasions within a 2 weeks interval, using a perfusion method. They received in single-blind and random order continuous i.v. infusion of either placebo or lanreotide (100 micrograms h-t after a bolus of 100 micrograms 15 min before the beginning of the study).

RESULTS

Lanreotide significantly decreased gastric acid secretion (90%) for the initial 3 h period. Gastric emptying was not significantly modified by lanreotide infusion. Compared with placebo, lanreotide almost completely abolished both bile salts and lipase responses to the meal. It largely increased the duodeno-caecal transit time and decreased significantly the duodenal absorption of carbohydrates and triglycerides.

CONCLUSIONS

Since lanreotide has powerful effects on gastrointestinal functions, it could be useful in the prevention or in the treatment of pancreatic and bowel fistulas as well as short bowel syndrome.

The effect of octreotide on human gastric compliance and sensory perception

Somatostatin or its analogue octreotide (OCT) has previously been shown to modulate gastric emptying, intestinal motor activity and visceral sensation. In the current study we sought to determine the effect of a single dose of OCT (1.25 micrograms kg-1 s.c.), which has previously been shown to have both motor and sensory effects, on proximal gastric compliance and on conscious perception of gastric distention. Gastric distention was performed in 13 healthy male volunteers, by either slow ramp distention (60 ml min-1) or by intermittent pressure steps (phasic distention; 4-20 mmHg) using an electronic distention device. Compliance curves (pressure-volume relationship), and thresholds for innocuous (fullness) and noxious sensations (discomfort, pain) were determined following vehicle or OCT injection. OCT consistently and significantly reduced the rate of the gastric accommodation reflex by 50%, resulting in a reduced compliance at distention pressures greater than 10 mmHg during phasic distention. In contrast, no effect was observed on the compliance curve obtained during ramp distention. OCT selectively increased the threshold for fullness during both ramp and phasic distention. During phasic distention, OCT decreased the volume thresholds for noxious (pain) sensations experienced at volumes greater than 300 ml, without affecting the corresponding pressure threshold. These findings suggest that at low distension volumes, OCT in the dosage used has a direct inhibitory effect on afferents mediating innocuous gastric sensations. The hyperalgesic effect observed during phasic distention may be secondary to OCT's inhibitory effect on the gastric accommodation reflex.

The effect of intravenous lipids on gastric emptying in rats subjected to total parenteral nutrition

Total parenteral nutrition (TPN) is indicated in a number of clinical situations involving obstructive disorders of the gastrointestinal tract or when total bowel rest is required. However, little is known of its effects upon the physiology of gastric storage and emptying. A study of the effects of lipid nutrient content of TPN on the gastric emptying of a non-energy liquid test meal in male and female conscious rats (250-280 g) was undertaken. Five experimental groups were set up according to the percentage of total non-protein energy administered in the form of different lipids: A (lipid-free: 100% of non-protein energy as glucose); B (5% long-chain triglycerides (LCT), 95% glucose); C (40% LCT, 60% glucose); D (5% LCT and medium-chain triglycerides (MCT) in 1:1 proportion, 95% glucose); and E (40% LCT and medium-chain triglycerides (MCT) in 1:1 proportion, 60% glucose). Animals were maintained on TPN for 24 h and for 3 and 4 days, after which gastric emptying was measured by the phenol red method. 40% lipid TPN was found to accelerate gastric emptying as infusion was prolonged, although the type of lipid had no effect.

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.

Somatostatin is released in response to cholecystokinin by activation of type A CCK receptors

Cholecystokinin is a principal mediator of intestinal fat-induced inhibition of gastric acid secretion, indicating that it is an important physiological enterogastrone. Cholecystokinin has been shown to inhibit acid secretion by activation of type A CCK receptors and through a mechanism involving somatostatin. In the present study, we investigated the possibility that these two mechanisms are directly related such that activation of type A CCK receptors by CCK causes the release of somatostatin. We tested this hypothesis in vivo in a study of CCK-stimulated release of somatostatin in dogs and in vitro in a study of CCK-stimulated release of somatostatin from an enriched culture of canine fundic D cells. In dogs, IV infusion of CCK (50 pmol/kg/h, IV) significantly increased circulating somatostatin concentrations above basal. Further, systemic administration of somatostatin MAb F(ab)1 fragments of a somatostatin monoclonal antibody prevented most of CCK-induced inhibition of meal-stimulated acid secretion. In canine fundic D cells in culture, CCK-stimulated somatostatin release was blocked in a dose-dependent fashion by application of a type A CCK receptor antagonist. This study indicates that CCK activates type A CCK receptors to release somatostatin from canine fundic mucosal D cells, and accounts for somatostatin-dependent CCK-induced inhibition of acid secretion.

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

In awake dogs with chronic gastric, duodenal, and jejunal fistulas, F(ab)1 fragments of somatostatin monoclonal antibody (mAb S607) were administered intravenously (IV) to test the hypothesis that intraintestinal lipid inhibits peptone-stimulated gastric acid secretion and emptying by a hormonal somatostatin mechanism. Plasma somatostatin was increased significantly by duodenal and jejunal perfusion with 20% lipid. Somatostatin administered IV caused dose-dependent inhibition of meal-stimulated gastric acid secretion and gastric emptying similar to that seen after intestinal perfusion with lipid. Administration of mAb S607 F(ab)1 fragments significantly reversed somatostatin (400 pmol.kg-1.h-1, IV)-induced inhibition of peptone-stimulated acid output and gastric emptying. Acid output inhibited by intraduodenal lipid was reversed partially after F(ab)1 administration, but the inhibitory effect of intrajejunal lipid was not altered. Inhibition of acid secretion by IV somatostatin and by intraintestinal fat was not caused by a decrease in circulating gastrin concentrations. Gastric emptying delayed by intraintestinal lipid was unaffected by antibody administration. Somatostatin does not appear to be a major hormonal mediator of intestinal fat-induced inhibition of gastric acid secretion or delayed gastric emptying in dogs.

The effect of continuous enteral nutrition on gastric acidity in humans

The effect of continuous intraduodenal enteral nutrition on gastric pH was compared with the effects of fasting and of parenteral and standard nutrition control regimens containing equal amounts of carbohydrate, protein, and lipid. Eleven healthy volunteers underwent four 24-hour intragastric pH-metry studies; serum glucose, calcium, immunoreactive insulin and gastrin levels were determined during fasting and enteral and parenteral regimens. Median 24-hour gastric pH during enteral nutrition (group median pH 1.4) was lower than during parenteral nutrition (pH 1.9; P = 0.0039 vs. enteral) but was not different from fasting (pH 1.4) or standard nutrition (pH 1.6) values. Median 24-hour serum glucose levels during enteral nutrition (group median, 4.8 mmol/L) were higher than during fasting (4.0 mmol/L; P = 0.00098 vs. enteral) and lower than during parenteral nutrition (5.3 mmol/L; P = 0.0039 vs. enteral). Median 24-hour serum insulin levels during enteral nutrition (group median, 22.9 mU/L) were higher than during fasting (group median, 9.2 mU/L; P = 0.00098 vs. enteral) but similar to levels during parenteral nutrition (23.3 mU/L). Neither median 24-hour gastrin levels nor calcium levels were affected by any nutrition regimen. Thus, continuous enteral nutrition produces gastric pH values similar to those seen with fasting or standard nutrition, suggesting that, under most physiological conditions, gastric acidity is subject to close feedback control. Parenteral nutrition increases gastric pH, suggesting that systemic nutrients may influence this feedback mechanism.

Role of endogenous secretin and cholecystokinin in intraduodenal oleic acid-induced inhibition of gastric acid secretion in rats

We investigated a possible role of endogenous secretin and cholecystokinin (CCK) in inhibition of gastric acid secretion induced by intraduodenal administration of oleic acid in rats. Intraduodenal administration of oleic acid emulsion in a dose of 1 mmol/hr resulted in significant inhibition of gastric acid secretion stimulated by intravenous infusion of pentagastrin (0.3 micrograms/kg/hr), and this was accompanied by an increase in the plasma concentration of both secretin and CCK, from 1.2 +/- 0.08 pM and 20.6 +/- 1.2 pM to 4.3 +/- 0.18 pM and 31.6 +/- 0.9 pM, respectively (P less than 0.001). Intravenous infusion of secretin (0.05 CU/kg/hr) inhibited pentagastrin-stimulated gastric acid secretion, but CCK-8 (0.03 micrograms/kg/hr) failed, although intravenous infusion of secretin and CCK in those doses produced plasma levels comparable to the levels achieved in response to oleic acid administration. Furthermore, the oleic acid-induced suppression of gastric acid secretion was blocked significantly by intravenous injection of rabbit anti-secretin serum (0.1 ml), but not by intravenous infusion of a CCK-receptor antagonist, CR 1409 (5 mg/kg/hr). Thus, the results of this study indicate that endogenous secretin rather than CCK is involved in the hormonal mechanism regulating the inhibition of gastric acid secretion by intestinal fat in rats.

Short report: plasma somatostatin concentrations in the irritable bowel syndrome

The description of a patient with the irritable bowel syndrome whose symptoms were completely relieved by the administration of somatostatin raised the possibility that a deficiency of somatostatin may be involved in the pathogenesis of the disorder. We have examined this possibility by studying 11 healthy controls (35 +/- 12 years; mean +/- S.D. 8 female) and 10 irritable bowel syndrome patients (39 +/- 14 years; 7 female) complaining of frequency of defaecation of 4 or more times a day. Plasma somatostatin concentrations were determined by specific radioimmunoassay, fasting and at 15, 30, 45, 60, 90, 120 and 180 min after a standard breakfast. Irritable bowel syndrome patients and controls had similar fasting (27.4 +/- 5.1 vs. 35.2 +/- 4.3 pg/ml; mean +/- S.E.M. and integrated increment of post-prandial (5105 +/- 858 vs. 3885 +/- 793 pg.min/L) plasma concentrations of somatostatin, as assessed by student's t-test. These observations do not support the idea that a state of somatostatin deficiency exists in the irritable bowel syndrome.

Effects of some gastrointestinal peptides on isolated human and rabbit gastric glands

The isolated gastric gland preparation, with aminopyrine accumulation as an index of the parietal cell response, has been used to study the effects of somatostatin (S-14), gastrin-releasing peptide (GRP), cholecystokinin (CCK-8), vasoactive intestinal peptide (VIP), and peptide YY (PYY) on the in vitro acid secretion in human and rabbit oxyntic mucosa. Somatostatin was able to inhibit the parietal cell response to histamine in both human and rabbit isolated gastric glands (maximal inhibition, 22% and 34%, respectively) but failed to inhibit the parietal cell response to db-cAMP. However, other peptides capable of inhibiting gastric acid secretion in vivo, such as CCK, VIP, and PYY, were unable to induce any inhibition of the parietal cell response to db-cAMP or histamine in the isolated gastric gland preparation irrespective of the species studied. GRP was not able to induce a parietal cell response, a finding that is in accord with the assumption that the stimulatory effect of GRP on gastric acid secretion in vivo is by releasing gastrin from antral G-cells.

Effect of octreotide on mouth-to-caecum transit time in healthy subjects and in the irritable bowel syndrome

The effect of a single subcutaneous injection of octreotide (50 micrograms) on mouth-to-caecum transit time was determined in patients with the irritable bowel syndrome who complained of bowel frequency, and in healthy volunteers. The assessment of mouth-to-caecum transit time was performed by monitoring breath hydrogen concentration and noting a sustained 10 p.p.m. rise after ingestion of lactulose 40 ml. Measurements were performed fasting, and on a separate day, after a standard breakfast which included 40 ml lactulose. The studies were performed double-blind in a pre-determined random order. Octreotide prolonged mouth-to-caecum transit time in irritable bowel syndrome patients and healthy subjects by factors of 2.4 and 2.6 after lactulose when fasting, respectively, and by factors of 2.8 and 2.6 after the breakfast which contained lactulose. The upper gastrointestinal transit rate was similar in irritable bowel syndrome patients and healthy controls.

Short-term somatostatin infusion affects T lymphocyte responsiveness in humans

Peripheral blood lymphocytes (PBL) function was analysed in 16 young men with duodenal ulcers after one-hour intravenous infusion of somatostatin (SMS) at a dose of 250 micrograms/h. Proliferative responses of PBL from SMS-treated patients were significantly diminished compared with pre-treatment values, after stimulation with PHA, PWM or Con A. Spontaneous IL-2R expression was moderately increased after SMS infusion but PHA-induced IL-2R expression was not affected by this drug. Alloantigen and autoantigen stimulation of PBL showed no significant changes in the proliferative response after SMS infusion. NK cell activity was similarly unaffected. These observations establish a link between SMS exposure and possible development of immune dysfunction.