Effects of somatostatin-14 and somatostatin-28 on plasma hormonal and gastric secretory responses to cephalic and gastrointestinal stimulation in man

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

Cloning, expression, pharmacology and tissue distribution of the mouse somatostatin receptor subtype 5

The gene encoding the mouse somatostatin receptor subtype 5 has been isolated from a genomic library and the mRNA start point mapped to position -95 relative to the translational start codon. The promoter region is devoid of TATA and CAAT boxes but contains putative binding sites for AP-1, AP-2 and SP1 and response elements for glucocorticoids (GRE) and phorbol esters (TRE). The encoded receptor protein with a predicted molecular weight of 42.5 kDa is comprised of 385 amino acids and thus contains 22 and 21 amino acids more than rat and human counterparts. The extra amino acids are caused by another translational initiation codon located further upstream. In the region of overlap the mouse somatostatin receptor subtype 5 displays 96.7% sequence identity to the rat and 81.7% to the human homologue. Application of somatostatin-14 and -28 to human embryonic kidney cells expressing the recombinant receptor resulted in the inhibition of forskolin-stimulated adenylyl cyclase with comparable EC50 values. Consistent with the observed sequence relationship, the mouse somatostatin receptor subtype 5 displays a pharmacological profile that resembles the rat homologue more closely than the human counterpart. mRNA for the mouse somatostatin type 5 receptor has been detected in pituitary, kidney, spleen and ovary and, to a lesser extent, in brain, stomach, intestine and thymus but was not observed in heart, pancreas and liver.

Role of cholecystokinin in the control of gastric emptying and secretory response to a fatty meal in normal subjects and duodenal ulcer patients

BACKGROUND

Exogenous cholecystokinin (CCK) is known to affect gastric motor and secretory activities, but its physiologic role in the control of gastric functions is unknown.

METHODS

In this study involving 10 young healthy subjects and 10 duodenal ulcer (DU) patients, 500 ml of a standard meal without or with addition of 15% soybean oil was given, and the gastric emptying rate and the pH profile and plasma levels of gastrin, CCK, pancreatic polypeptide (PP), and somatostatin were determined in separate tests with placebo or with antagonism of type-A CCK receptors (loxiglumide, 1200 mg orally).

RESULTS

In healthy controls and DU patients the emptying half-time was 44 and 34 min, respectively, and the addition of oil prolonged the emptying by about 50%. Pretreatment with loxiglumide significantly reduced fat-induced retardation of gastric emptying in both healthy controls and DU patients. A standard meal in healthy subjects resulted in an immediate rise in median gastric pH to about 6.0, and this was followed by gradual decrease within about 3 h to premeal values of about 2.0. After the meal, plasma gastrin rose by 57%, CCK by 177%, PP by 100%, and somatostatin by 39%. Addition of fat significantly attenuated and prolonged the pH decrease after the meal while reducing the increment in plasma gastrin and enhancing plasma CCK and PP levels. Loxiglumide significantly reduced the median postprandial pH (from control 4.8 to 2.5) and reversed the changes in the pH profile caused by the addition of fat. The increments in plasma gastrin and CCK were markedly augmented, whereas those of somatostatin and PP were significantly attenuated. DU patients showed lower postprandial pH (3.0) in tests with or without fat and higher increments in plasma gastrin. CCK antagonism failed to affect significantly the pH profile or the increments in plasma gastrin in DU patients.

CONCLUSIONS

These results indicate that endogenous CCK released by a fatty meal delays gastric emptying and inhibits gastric acid and plasma gastrin responses in healthy subjects, but in DU patients the inhibitory effect of CCK is less pronounced, suggesting a defect in the action of this hormone on gastrin release and gastric acid secretion.

Circulating somatostatin-28 is not a physiologic regulator of gastric acid production in man

Studies were designed to establish the acid inhibitory potency and plasma kinetics of somatostatin-28 (S-28) in humans and to determine whether the amount of S-28 released into the circulation after a meal is sufficient to regulate gastric acid secretion. A liquid meal induced a significant increase of S-28 (P < 0.01) whereas S-14 levels did not change. Postprandial S-28 concentrations were then mimicked by exogenous infusions and tested on basal and pentagastrin-stimulated gastric acid secretion. Expressed in terms of circulating plasma concentrations measured by specific radioimmunoassays, S-14 was 10 times more potent than S-28 in inhibiting gastric acid production. The plasma half-life of S-28 (1.86 min) was longer than that of S-14 (1.00 min) due to a slower plasma clearance rate. S-28 did neither affect basal and stimulated gastric acid secretion nor postprandial intragastric acidity. These studies suggest that postprandial plasma concentrations of S-28 are unlikely to regulate gastric acid secretion in man. They also show that S-28 is several times less potent than S-14 with respect to inhibition of gastric acid output.

Role of cholecystokinin in the inhibition of gastric acid secretion in dogs

1. This study was designed to determine the involvement of cholecystokinin (CCK) in the gastric secretory responses to exogenous and endogenous secretagogues in conscious dogs with chronic gastric fistulae (GF), pancreatic fistulae (PF) and Heidenhain pouches (HP). 2. A meal of meat or intragastric application of peptone (300 mosM) increased secretion of HCl from the HP and pancreatic secretion of protein and plasma levels of gastrin, CCK and somatostatin. 3. The CCK receptor antagonist L-364,718 caused a further increase in the postprandial HCl secretion from the HP and in the plasma levels of gastrin and CCK but pancreatic output of protein and plasma concentration of somatostatin were significantly reduced. 4. Addition to intragastric peptone of 10% oleate or its acidification to pH 3.0 profoundly inhibited the HP secretion and gastrin release but significantly increased pancreatic secretion of protein and plasma levels of CCK and somatostatin. Administration of L-364,718 reversed the fall in the HP secretion and plasma gastrin while significantly attenuating pancreatic protein secretion and plasma somatostatin levels. 5. Intragastric administration of hyperosmolar (1200 mosM) peptone also inhibited HCl secretion from the HP but this was not affected by L-364,718. 6. Exogenous CCK and bombesin (but not gastrin) caused a small increase in HCl secretion from the HP and marked stimulation of pancreatic protein secretion accompanied by a significant rise in plasma levels of gastrin, CCK and somatostatin. Administration of L-364,718 resulted in a further increase in the HCl response of HP to bombesin and in plasma levels of gastrin and CCK but caused a reduction in plasma levels of somatostatin. 7. We conclude that CCK released by a meal of meat, intragastric peptone, oleate or acidified peptone and intravenous bombesin exerts tonic inhibitory influences on gastric acid secretion and that this effect is mediated, at least in part, by somatostatin.

Non-endocrine applications of somatostatin and octreotide acetate: facts and flights of fancy

Somatostatin, originally detected by Krulich and ultimately isolated by Brazeau, was initially described as a growth hormone release-inhibiting factor. Subsequent investigation into the use of native somatostatin and the development of long-acting somatostatin analogues, especially octreotide acetate, have fostered increasing uses of these compounds. Though the clinical and investigational uses of somatostatin and its analogues are varied, one central theme remains constant: the ability of these agents to suppress circulating peptide levels. This article, a review of the current non-endocrine applications of somatostatin and its analogues, covers a wide range of potential applications for somatostatin-like compounds. These include use in cirrhosis and variceal bleeding, peptic ulcer disease, pancreatic fistulas, acute and chronic pancreatitis, dumping syndrome, cancer therapy, small bowel fistulas, psoriasis, pain control, and autonomic hypotension. Somatostatin may also play a role in the development and potential treatment of neurologic disease and may have profound found influence on behavior.

Plasma somatostatin response to an oral mixed test meal in cirrhotic patients

Ten patients with non-alcoholic cirrhosis and ten control subjects were studied in basal conditions and after ingestion of a standard mixed test meal. Plasma somatostatin, blood glucose, plasma insulin, C-peptide and glucagon were determined before and 15, 30, 45, 60, 90, 120 and 180 min after the start of the meal. Basal somatostatin levels in patients (31.9 +/- 1.8 ng/l) were significantly higher (p less than 0.01) than in controls (12.5 +/- 0.9 ng/l). The time-course of the somatostatin secretory response after the meal was similar in the two groups, but the increase, evaluated as incremental area above baseline, was significantly smaller (p less than 0.01) in cirrhotics (804 +/- 134 ng/l per min) than in controls (1482 +/- 149 ng/l per min). Data indicate that elevated basal plasma somatostatin concentrations in cirrhosis may be consequent to elevated gastrointestinal and/or pancreatic secretion, whereas the blunted somatostatin response to the mixed test meal may derive from the hyperinsulinemia which occurs in the postprandial period.

Somatostatin release induced by gastrin-releasing peptide in man. Effect of proximal gastric vagotomy and cholinergic blockade

The influence of intragastric pH on the basal release of somatostatin has been studied in healthy controls and in duodenal ulcer patients. In addition the somatostatin response to gastrin-releasing peptide infusion has been evaluated both regarding the effect of intragastric pH and the influence of vagal innervation and muscarinic blockade. No difference was found in basal blood levels, when changing the intraluminal pH, although a slightly higher basal somatostatin concentration was noticed in patients with duodenal ulcer disease. Neither proximal gastric vagotomy nor cholinergic blockade had any effect on basal somatostatin concentrations. GRP infused in stepwise increasing doses from 20 pmol/kg/h to 400 pmol/kg/h induced a small but significant response. This effect of GRP was most evident, when the stomach was perfused with 0.1 M HCl. The small, somatostatin response to GRP infusion was not influenced by vagal denervation of the parietal cell area, neither by cholinergic blockade. Despite the previously observed effects of vagotomy and cholinergic blockade on gastrin release induced by GRP, a corresponding inverse effect on somatostatin is not apparent.

Prolonged inhibition of meal-stimulated acid secretion and gastrin release following single subcutaneous administration of octreotide (SMS 201-995) in man

Single subcutaneous doses of the somatostatin analogue, SMS 201-995, were evaluated for the degree and duration of effects on acid secretion, serum gastrin levels, and gastric emptying in eight human male subjects (mean age 44 years) over an 8-h period. All the subjects received subcutaneous 50-micrograms and 100-micrograms doses of SMS 201-995 and placebo on three separate days in a double-blind random order. Drug or placebo was administered at time 0 followed by peptone meals at time 0, 2, 4, and 6-h. Peptone meals were evacuated at time 1, 3, 5 and 7-h to create 'basal' conditions between alternate hours. Gastric acid secretion was determined hourly beginning at time--1. Both the 50-micrograms and 100-micrograms doses of SMS 201-995 significantly inhibited 'basal' and peptone meal-stimulated gastric acid secretion throughout the 8-h measurement period. The minimum effective plasma concentration of SMS 201-995 for inhibition of peptone meal-stimulated gastric acid secretion was approximately 1000 pg/ml. Peptone meal-stimulated plasma gastrin concentrations were inhibited for 5 and 7 h after 50-micrograms and 100-micrograms doses of SMS 201-995, respectively, whereas 'basal' plasma gastrins were inhibited for 4 and 6 h, respectively. Gastric emptying determined by marker dilution was not significantly enhanced compared to placebo. These results indicate prolonged and potent effects of single subcutaneous doses of SMS 201-995 on peptone-meal stimulated acid secretion and gastrin release.

A physiologic role for somatostatin 28 as a regulator of insulin secretion

Somatostatin 28 (S-28) is a peptide produced in the intestinal tract which rises in the circulation during nutrient absorption. We tested the hypothesis that S-28 regulates B-cell function by (a) studying the effects on insulin secretion of "physiologic" infusions of S-28 and (b) measuring insulin responses during elevated nutrient-stimulated endogenous S-28 levels. (a) Synthetic S-28 was infused on separate days into six healthy men at rates of 25 and 50 ng/kg per h which mimicked postprandial levels. Subjects were given a bolus of glucose (0.1 g/kg) after 120 min. Insulin responses during S-28 infusions were compared to a control study using a saline infusion in the same individuals. Glucose-stimulated insulin secretion was inhibited during the infusion of 50 ng/kg per h S-28 when compared to control (P less than 0.05). (b) Insulin secretion during elevations of endogenous S-28 was studied in healthy men who received a bolus of 2.5 g arginine (n = 14) or 25 U of secretin (n = 8) 120 min after swallowing 50 g fat, or, on a separate day, an equivalent volume of water. S-28 levels rose significantly after fat ingestion but did not change after water. Arginine and secretin-stimulated insulin secretion was inhibited following ingestion of fat compared with intake of water (P less than 0.05). Arginine-enhanced glucagon secretion was not changed by fat ingestion. We conclude that elevations in plasma S-28 levels, occurring during the postprandial state, attenuate B-cell secretion and this peptide may be a physiologic modulator of nutrient-stimulated insulin release.

Circulating prosomatostatin-derived peptides. Differential responses to food ingestion

Prosomatostatin (pro-S) and its bioactive posttranslational products, somatostatin-14 (S-14), somatostatin-13 (S-13), and somatostatin-28 (S-28), were measured in human plasma by the use of immunoglobulins to the NH2-terminus of S-28 conjugated with agarose to separate them and, thereafter, by RIA with an antiserum recognizing the COOH-terminus of pro-S, and by specific RIA for the NH2-terminus of S-14 and pro-S. In healthy men, mean basal levels of pro-S were 4 pg equivalent S-14/ml; S-14/S-13 combined were 9 pg equivalent S-14/ml; and S-28 levels were 16 pg/ml. After a 700-kcal meal, pro-S, S-14, and S-14/S-13 did not change, whereas S-28 levels doubled by 120 min and remained elevated for 240 min. To evaluate the origins of these peptides, their levels were compared in peripheral, portal, gastric, and mesenteric veins of anesthetized patients and in patients with total resection of stomach and pancreas before and after nutrient intake. The stomach and small intestine were sources of both peptides; however, most S-28 originated in the small intestine. These findings suggest that, in contrast to S-14, S-28 is a hormone and may modulate postprandial nutrient absorption and use.

Gastric secretion and gastrin under progressive doses of somatostatin-14 and -28 administered intraperitoneally to the rat

The actions of progressive doses of intraperitoneally (IP) administered somatostatin-14 (SS-14) and -28 (SS-28) on gastric secretion (acid, pepsin) and mucosal blood flow (MBF) were studied in conscious gastric fistula rats both under basal conditions and under additional administration of pentagastrin. Also, somatostatin-like immunoreactivity was measured in aortal blood in all groups as well as aortal gastrin levels under basal conditions. IP infusion of equimolar doses of SS-14 and SS-28 resulted in an equal and dose-dependent inhibition of basal as well as pentagastrin-stimulated gastric acid secretion. MBF was reduced by either peptide both in the basal and pentagastrin experiments. Under basal conditions pepsin secretion was significantly increased by infusion of SS-14 at the higher doses, by infusion of SS-28 only at the intermediate dose (3.1 nmole kg-1.hr-1). In the pentagastrin experiments, low and intermediate doses of SS-14 tended to lower pepsin outputs but the highest dose of SS-14 stimulated pepsin secretion, whereas SS-28 had no effect on pepsin. Administration of SS-28 inhibited gastrin only at the highest dose (12.3 nmole kg-1.hr-1), and SS-14 had no influence at all on gastrin. After IP infusion of both peptides, plasma SLI rose dose-dependently under basal and stimulated conditions. Gel chromatography indicated an in-vivo conversion of SS-28 to SS-14 or intermediate fragments. It is concluded that SS-14 and SS-28 delivered by IP infusion, inhibit basal and stimulated gastric acid equally in the rat without suppressing gastrin. The mechanism underlying SS-mediated pepsin stimulation is unknown.

Mechanism of release of gastric luminal somatostatin-like immunoreactivity in response to pentagastrin and sham feeding in man

We studied in five healthy volunteers whether the cholinergic pathway regulated the secretion of gastric intraluminal somatostatin-like immunoreactivity (SLI) in response to stimuli of pentagastrin infusion (0.9 micrograms/kg/h, intravenously) and sham feeding. We measured gastric secretory volume, hydrogen ion output, and SLI at base line, during pentagastrin infusion, after sham feeding, and after applications of atropine (0.0, 0.7, 7.0 micrograms/kg, intramuscularly) given before pentagastrin and sham feeding. The stimuli were given randomly, at separate times on different days. After each stimulus, eight 15-min gastric juice collections were made; samples were adjusted to pH 7, pepstatin-A and aprotinin were added, and samples were extracted with acetone to determine SLI by radioimmunoassay. Pentagastrin and sham feeding significantly increased gastric luminal SLI secretion, which appeared to correlate with the increases in volume and acid output. Atropine at 7 micrograms/kg significantly suppressed gastric volume, acid, and SLI outputs stimulated by sham feeding; however, responses to pentagastrin stimulation remained unchanged. To conclude, the cholinergic mechanism regulates gastric intraluminal SLI response to sham feeding but not to pentagastrin infusion.

Circulating somatostatin. Physiological regulator of pancreatic function?

The present study was designed to determine whether somatostatin is released into the circulation in sufficient amounts to regulate exocrine and endocrine pancreatic function and to evaluate the possible role of somatostatin as a hormonal regulator of the pancreas. Mean plasma somatostatin levels (SLI) increased from 11 +/- 2 pmol liter-1 to peak concentrations of 18 +/- 2 in six healthy male volunteers after a steak meal (P less than 0.05). Infusion of somatostatin inhibited hormone-induced exocrine pancreatic secretion and suppressed cerulein-stimulated pancreatic polypeptide (PP) secretion, but did not significantly change arginine-stimulated insulin and glucagon release at mean plasma somatostatin concentrations within the range seen after a meal. The amount of somatostatin released after a meal thus was of sufficient magnitude to inhibit exocrine pancreatic function and PP release. On the other hand, basal and arginine-stimulated glucagon and insulin secretions were not significantly affected by these plasma concentrations of intravenous somatostatin suggesting that the exocrine pancreas might be more sensitive to somatostatin than the islet cells. We conclude that somatostatin in concentrations within the range seen after a meal is a potent inhibitor of stimulated acinar cell function in man. The findings support the hypothesis that somatostatin acts as a true hormonal regulator.

Somatostatin analogue (SMS 201-995) in the management of gastroenteropancreatic tumors and diarrhea syndromes

SMS 201-995 (Sandostatin) was studied using low doses (50 to 100 micrograms) administered subcutaneously every 12 hours. A single 50-micrograms dose of SMS 201-995 effectively controlled gastric acid and blood gastrin levels for 12 hours in three patients with benign gastrinomas and was useful in their perioperative management. Higher doses of the agent (500 to 800 micrograms per day) had no effect on metastases in one of two patients with metastatic gastrinoma. In the other patient, one tumor shrank but the other continued to grow after three months of treatment while serum gastrin levels did not change. Cultured metastatic tumor tissue from this patient released different forms of gastrin; growth rates varied, independent of uptake of SMS 201-995, and gastrin release increased. A neonate with nesidioblastosis maintained normal blood glucose levels while receiving SMS 201-995 therapy following a 95 percent pancreatic resection. In two elderly patients with organic hypoglycemia--one with a single benign adenoma and one with multiple adenomatosis--the somatostatin analogue did not prolong the hypoglycemia-free interval. In nine patients with carcinoid syndrome, flushing was uniformly controlled with 50 micrograms of SMS 201-995 administered every eight to 12 hours. One of the nine required exocrine pancreatic replacement. After six months of treatment, three of the nine had no change in tumor size and one had remission of symptoms and stopped treatment. In two patients with vipoma, SMS 201-995 controlled diarrhea and reduced levels of vasoactive intestinal peptide; tumor necrosis occurred in one patient. In a patient with diabetic diarrhea unresponsive to all treatments, SMS 201-995 therapy controlled the diarrhea but did not interfere with control of the diabetes.