The effect of gastrin on basal and aminoacid-stimulated insulin and glucagon secretion in man

Four sidenotes about glucagon peptides

Century old glucagon is a classic pancreatic hormone. But today we also know that the glucagon gene is expressed at high levels at extrapancreatic sites - particularly so in the gut. Major hormonal glucagon gene products in the digestive tract are the two glucagon-like peptides (GLP-1 and -2). Of these, truncated GLP-1 has in recent decades attracted massive interest due to its incretin effect, and the subsequent GLP-1 derived design of potent diabetes and obesity drugs. Truncated GLP-1 has consequently become an important contributor to gastrointestinal endocrinology. The gastrointestinal branch of endocrinology today includes more than 100 bioactive peptides encoded by some 30 different hormone genes. Therefore, the gut is the largest endocrine organ in the body. In addition to a general discussion of glucagon peptides in the hierarchy of gut hormones, this review also includes three short notes about glucagon studies from the 1970s. These studies dealt with reactive hypoglycemia, chronic liver disease, and the secretory response of pancreatic glucagon to gastrin/cholecystokinin stimulation. Considering today's possibilities in molecular endocrinology, revisits to the questions raised by these studies might be worthwhile.

Premises for Cholecystokinin and Gastrin Peptides in Diabetes Therapy

Gastrin and cholecystokinin (CCK) are classical gastrointestinal peptide hormones. Their biogenesis, structures, and intestinal secretory patterns are well-known with the striking feature that their receptor-bound 'active sites' are highly homologous and that this structure is conserved for more than 500 million years during evolution. Consequently, gastrin and CCK are agonists for the same receptor (the CCK2 receptor). But in addition, tyrosyl O-sulphated CCK are also bound to the specific CCK1 receptor. The receptors are widely expressed in the body, including pancreatic islet-cell membranes. Moreover, CCK and gastrin peptides are at various developmental stages and diseases expressed in pancreatic islets; also in human islets. Accordingly, bioactive gastrin and CCK peptides stimulate islet-cell growth as well as insulin and glucagon secretion. In view of their insulinotropic effects, gastrin and CCK peptides have come into focus as drug targets, either alone or in combination with other insulinotropic gut hormones or growth factors. So far, modified CCK and gastrin peptides are being examined as potential drugs for therapy of type 1 as well as type 2 diabetes mellitus.

Nonclassical Islet Peptides: Pancreatic and Extrapancreatic Actions

The pancreas has physiologically important endocrine and exocrine functions; secreting enzymes into the small intestine to aid digestion and releasing multiple peptide hormones via the islets of Langerhans to regulate glucose metabolism, respectively. Insulin and glucagon, in combination with ghrelin, pancreatic polypeptide and somatostatin, are the main classical islet peptides critical for the maintenance of blood glucose. However, pancreatic islets also synthesis numerous ‘nonclassical’ peptides that have recently been demonstrated to exert fundamental effects on overall islet function and metabolism. As such, insights into the physiological relevance of these nonclassical peptides have shown impact on glucose metabolism, insulin action, cell survival, weight loss, and energy expenditure. This review will focus on the role of individual nonclassical islet peptides to stimulate pancreatic islet secretions as well as regulate metabolism. In addition, the more recognised actions of these peptides on satiety and energy regulation will also be considered. Furthermore, recent advances in the field of peptide therapeutics and obesity-diabetes have focused on the benefits of simultaneously targeting several hormone receptor signalling cascades. The potential for nonclassical islet hormones within such combinational approaches will also be discussed.

Expression of Gastrin Family Peptides in Pancreatic Islets and Their Role in β-Cell Function and Survival

OBJECTIVES

Modulation of cholecystokinin (CCK) receptors has been shown to influence pancreatic endocrine function.

METHODS

We assessed the impact of the CCKA and CCKB receptor modulators, (pGlu-Gln)-CCK-8 and gastrin-17, respectively, on β-cell secretory function, proliferation and apoptosis and glucose tolerance, and investigating alterations of CCK and gastrin islet expression in diabetes.

RESULTS

Initially, the presence of CCK and gastrin, and expression of their receptors were evidenced in β-cell lines and mouse islets. (pGlu-Gln)-CCK-8 and gastrin-17 stimulated insulin secretion from BRIN-BD11 and 1.1B4 β-cells, associated with no effect on membrane potential or [Ca]i. Only (pGlu-Gln)-CCK-8 possessed insulin secretory actions in isolated islets. In agreement, (pGlu-Gln)-CCK-8 improved glucose disposal and glucose-induced insulin release in mice. In addition, (pGlu-Gln)-CCK-8 evoked clear satiety effects. Interestingly, islet colocalization of CCK with glucagon was elevated in streptozotocin- and hydrocortisone-induced diabetic mice, whereas gastrin coexpression in α cells was reduced. In contrast, gastrin colocalization within β-cells was higher in diabetic mice, while CCK coexpression with insulin was decreased in insulin-deficient mice. (pGlu-Gln)-CCK-8 and gastrin-17 also augmented human and rodent β-cell proliferation and offered protection against streptozotocin-induced β-cell cytotoxicity.

CONCLUSIONS

We highlight the direct involvement of CCKA and CCKB receptors in pancreatic β-cell function and survival.

The uncovering and characterization of a CCKoma syndrome in enteropancreatic neuroendocrine tumor patients

OBJECTIVE

Neuroendocrine tumors in the pancreas and the gastrointestinal tract may secrete hormones which cause specific syndromes. Well-known examples are gastrinomas, glucagonomas, and insulinomas. Cholecystokinin-producing tumors (CCKomas) have been induced experimentally in rats, but a CCKoma syndrome in man has remained unknown until now.

MATERIAL AND METHODS

Using a panel of immunoassays for CCK peptides and proCCK as well as for chromogranin A, we have examined plasma samples from 284 fasting patients with gastroenteropancreatic neuroendocrine tumors. In hyperCCKemic samples, plasma CCK was further characterized by chromatography.

RESULTS

One of the patients displayed gross hyperCCKemia. She was a 58-year old woman with a pancreatic endocrine tumor, liver metastases, 500-1000-fold elevated basal CCK concentration in plasma, diarrhea, severe weight loss, recurrent peptic ulcer and bilestone attacks from a contracted gallbladder. The CCK concentrations in plasma were not affected by resection of the pancreatic tumor, but decreased to normal after hemihepatectomy with removal of the metastases.

CONCLUSION

A CCKoma syndrome with severe hypersecretion of CCK exists in man. The duodenal ulcer disease and diarrhea with permanently low gastrin in plasma suggest that CCKomas may mimic gastrinoma-like symptoms, because CCK peptides are full agonists of the gastrin/CCK-B receptor.

An atlas and functional analysis of G-protein coupled receptors in human islets of Langerhans

G-protein coupled receptors (GPCRs) regulate hormone secretion from islets of Langerhans, and recently developed therapies for type-2 diabetes target islet GLP-1 receptors. However, the total number of GPCRs expressed by human islets, as well as their function and interactions with drugs, is poorly understood. In this review we have constructed an atlas of all GPCRs expressed by human islets: the 'islet GPCRome'. We have used this atlas to describe how islet GPCRs interact with their endogenous ligands, regulate islet hormone secretion, and interact with drugs known to target GPCRs, with a focus on drug/receptor interactions that may affect insulin secretion. The islet GPCRome consists of 293 GPCRs, a majority of which have unknown effects on insulin, glucagon and somatostatin secretion. The islet GPCRs are activated by 271 different endogenous ligands, at least 131 of which are present in islet cells. A large signalling redundancy was also found, with 119 ligands activating more than one islet receptor. Islet GPCRs are also the targets of a large number of clinically used drugs, and based on their coupling characteristics and effects on receptor signalling we identified 107 drugs predicted to stimulate and 184 drugs predicted to inhibit insulin secretion. The islet GPCRome highlights knowledge gaps in the current understanding of islet GPCR function, and identifies GPCR/ligand/drug interactions that might affect insulin secretion, which are important for understanding the metabolic side effects of drugs. This approach may aid in the design of new safer therapeutic agents with fewer detrimental effects on islet hormone secretion.

Incretin physiology beyond glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide: cholecystokinin and gastrin peptides

Gastrin and cholecystokinin (CCK) are homologous hormone systems known to regulate gastric acid secretion, gallbladder emptying, and cell growth in the pancreas and stomach. They are, however, also involved in the development and secretory functions of pancreatic islet cells. For instance, foetal and neonatal islets express significant amounts of gastrin, and human as well as porcine islet cells express the gastrin/CCK-B receptor abundantly. Therefore, exogenous gastrin and CCK peptides stimulate insulin and glucagon secretion in man. Accordingly, endogenous hypergastrinaemia is accompanied by islet cell hyperplasia and increased insulin secretion. Conventionally, the effect of gastrointestinal hormones on insulin secretion (the incretin effect) has been defined and quantified in relation to oral versus intravenous glucose loadings. Under these unphysiological conditions, the release of gastrin and CCK and, hence, their effect on insulin secretion are modest in comparison with the effects of glucose-dependent insulinotropic polypeptide and glucagon-like peptide 1 (GLP-1). Consequently, the interest of CCK and gastrin in incretin research has for decades been limited. A few years ago, however, it was suggested that gastrin together with epidermal growth factor or later GLP-1 might stimulate beta cell growth and secretion. Recent studies have shown that the combination of gastrin and GLP-1 actually restores normoglycaemia in diabetic mice. Therefore, a short review of the incretin system in a broader functional context that includes gastrin and CCK peptides may be timely.

Control of glucose homeostasis by incretin hormones

In humans as well as in other animal species, the ingestion of food provides the fundamental source of energy for various cellular activities. The intake of food and the ability of controlling the plasma levels of substrates for energy production involve complex mechanisms that ensure a constantly adequate supply of metabolites both in the fasting and in the fed state. A number of hormonal peptides released from the gastrointestinal (GI) tract in response to the ingestion of food have been shown to play a critical role in the postprandial control of glucose homeostasis. They are known to act through three main mechanisms of action. These include; (1) stimulation of insulin secretion of pancreatic islet (beta) cells; (2) inhibition of hepatic gluconeogenesis by suppression of glucagon secretion; and (3) inhibition of GI motility. While for some of these hormones all three mechanisms of action are utilized under physiological conditions, others preferentially use one or a combination of two mechanisms for lowering postprandial hyperglycemia. Although the term glucoincretins (or incretins, or insulinotropic hormones) etymologically only describes factors capable of inducing insulin secretion, it is more frequently used to identify a larger class of peptides that, rather than manifesting a specific mechanism of action (i.e., insulin secretion), share the ability of controlling glucose excursion in the fed state (with or without a direct insulinotropic effect). The latter more inclusive meaning, incretins, is used in this article. This review summarizes recent advances on synthesis, secretion, blood plasma patterns, and metabolism of some of the major GI regulatory peptides acting in the postprandial state.

Possible role of the stomach in enteroinsular axis in rats

To elucidate the possible role of the stomach in enteroinsular axis, rats had both an inflow gastric cannula and an outflow diversion cannula and a duodenal inflow cannula. The effects of intragastric infusion of glucose (1 mL in 10% solution) or mannitol (1 mL in 10% solution) on blood and plasma insulin responses to subsequent intraduodenal glucose (1.5 g/kg in 10% solution) or amino acids (1.0 g/kg in 10% solution) infusion were investigated. Blood glucose and plasma insulin responses to intraduodenal amino acids were not altered by intragastric infusion of glucose or mannitol. However, higher blood glucose and lower plasma insulin responses to intraduodenal glucose were observed in the rat with intragastric infusion of glucose or mannitol compared with controls (intragastric infusion of distilled water). This phenomenon was abolished in the rat with preadministration of phentolamine. These results suggest that intragastric tonicity may suppress glucose-induced insulin secretion, probably through the alpha-adrenergic mechanism in the rat.

Inhibition of gastric acid secretion in humans by glucagon during euglycemia, hyperglycemia, and hypoglycemia

The effect of intravenous infusion of glucagon in a dose of 85 pmol/kg/hr on submaximal pentagastrin-stimulated gastric acid secretion was studied in eight healthy volunteers. The study was repeated four times in each subject. By a glucose-insulin clamp technique blood glucose levels were kept constant during the studies at 5.0 mmol/liter (euglycemic clamp), 2.5 mmol/liter (hypoglycemic clamp), or 7.0 mmol/liter (hyperglycemic clamp) on three different days. Glucose and insulin were not infused during one control day study. During glucagon infusion, plasma glucagon levels increased but the level reached was lower during the hyperglycemic condition when compared to euglycemic and hypoglycemic conditions. Glucagon infusion inhibited gastric acid secretion during hyper- and euglycemic conditions but not during hypoglycemic conditions. Hyperglycemia caused a modest but significant inhibition of acid secretion. Serum gastric concentrations were unaltered during glucagon infusion regardless of the level of blood glucose. The present observations indicate that the inhibitory effect of glucagon is independent of the glucagon-induced hyperglycemia, but the effect is lost when blood glucose is below a certain limit, suggesting that blood glucose may have a modulating effect on gastric acid secretion.

Effect of glucagon on vagally induced gastric acid secretion in humans

The effect of a physiologically relevant dose of pancreatic glucagon, 85 pmol/kg/hr, or saline on gastric acid secretion induced by modified sham feeding (chew and spit), was studied in 10 healthy volunteers. Gastric pH was held constant (pH 5.5) by intragastric titration. Glucagon infusion inhibited gastric acid secretion significantly, from 19.6 +/- 1.5 mmol H+ per hour during saline, to 10.4 +/- 1.4 mmol H+ per hour. Blood glucose increased during glucagon infusion and remained constant during saline infusion. Serum gastrin concentrations increased significantly by sham feeding, during saline as well as glucagon infusion, and no difference between the gastrin response during saline or glucagon infusion was found. Thus glucagon, in a physiologic dose, reduces vagally mediated acid secretion by a gastrin-independent mechanism.

Effects of intraduodenal nutrient infusion on insulin response to subsequent intravenous glucose in rats

To elucidate the so-called incretin effect, the effects of intraduodenal bolus injection of glucose, 0.9% NaCl (saline), amino acids, and triglyceride on insulin response to subsequent intravenous injection of glucose (0.5 g/kg in 10% solution) was investigated in rats. Portal and femoral venous insulin and glucose concentration were simultaneously measured. Significantly higher portal insulin and lower femoral glucose values were observed in intraduodenal glucose or saline injection rats compared with controls (nothing was infused into the duodenum). Peripheral removal of insulin in these rats was not decreased. In the rats with intraduodenal injection of amino acids or triglyceride, slightly higher portal insulin and lower femoral glucose values were observed, but they were not significantly different from controls. These results indicate that incretin effect is provoked by intraduodenal infusion of glucose and saline, and may have some important physiologic role on glucose metabolism in rats in vivo.

Insulin and glucagon production in experimental cirrhosis

The release of insulin and glucagon in cirrhotic rats was examined. Rats were made cirrhotic by a combination treatment of carbon tetrachloride (CCl4) and phenobarbitone. Liver cirrhosis was verified by histologic findings. Both basal and stimulated release of insulin from isolated pancreatic islets, in vitro, were decreased significantly in cirrhotic rats, as compared with control rats. Basal, but not stimulated, levels of glucagon, in vitro, were reduced significantly in cirrhotic rats. Circulating levels of plasma insulin, glucagon, glucose, bilirubin, and amylase levels were unaffected in cirrhotic rats when compared with control rats. There were no signs of pancreatitis. The results indicated that the release of insulin and glucagon is depressed in cirrhotic rats and in rats treated with phenobarbitone and CCl4. Clearance of circulating insulin and glucagon by the liver was apparently reduced, since circulating levels of insulin and glucagon were unaltered in all treated rats.

Enhanced intestinal insulinotropic effect in streptozotocin-diabetic rats

To study the intestinal insulinotropic effects in the diabetic state, an investigation was made into the release of insulin from isolated rat pancreas perfused with portal venous effluent (PVE) obtained from the isolated perfused intestine of streptozotocin-induced diabetic rats. Rat intestine was perfused with Krebs-Ringer bicarbonate medium for 1 h, and the PVE was collected from both untreated and glucose-treated intestines of control and diabetic rats. The PVE, after adjusting its glucose concentration to the desired level, was used as the perfusing medium for the pancreas of a different rat. Glucose concentration in the perfusion medium was maintained at 5.5 mM for 20 min and at 16.7 mM for the next 30 min. Insulin output from pancreas perfused with PVE from untreated intestine of diabetic rats (252 +/- 45 ng/30 min, mean +/- SD) was similar to that of controls (269 +/- 72 ng/30 min). Pancreatic insulin output with PVE from glucose-treated intestine of diabetic rats (579 +/- 100 ng/30 min) was significantly greater compared with either that using PVE from untreated intestine of diabetic rats (P less than 0.01) or that from glucose-treated intestine of control rats (368 +/- 57 ng/30 min) (P less than 0.02). These results indicate that the insulinotropic effect is markedly enhanced in streptozotocin-induced diabetic rats.

Hypogastrinemia in streptozotocin diabetes with islet transplantation--reconstitution

Gastrin is present in normal mammalian pancreatic islets, as well as in the antrum and duodenum. Serum gastrin levels are responsive to many physiologic and pharmacologic factors including hyperglycemia, somatostatin, and glucagon. To evaluate the effects of streptozotocin diabetes and islet transplantation on gastrin homeostasis, young, adult, male Lewis rats underwent streptozotocin diabetes alone (N = 14), diabetes plus intraperitoneal islet isografts (N = 22), or sham operation alone (N = 18). Streptozotocin reduced fasting gastrin immunoreactivity (107 pg/ml +/- 26 mean +/- SEM) compared to controls (256 pg/ml +/- 31) (P less than 0.001). Islet isotransplantation resulted in restoration of fasting gastrin immunoreactivity (230 pg/ml +/- 19) to levels significantly greater than diabetics (P less than 0.001) and comparable to control animals (P = NS). Normalization of serum gastrin levels occurred within one month of transplantation and persisted for up to 14 months. In addition, media from cultures of 4/7 dispersed neonatal rat pancreas cultures contained gastrin immunoreactivity. Streptozotocin diabetes produces hypogastrinemia; intraperitoneal islet isotransplantation normalizes fasting gastrin immunoreactivity. Rat islet culture medium contains gastrin-like immunoreactivity. Pancreatic islets would appear to produce or control a portion of circulating gastrin immunoreactivity.

Interaction of glucagon and pentagastrin on pepsin secretion in healthy subjects

The effect of pentagastrin in step-wise increasing doses of 0 . 02, 2 . 0 and 20 nmol/kg/h (0 . 01, 1 . 0, and 10 . 0 micrograms/kg/h) on pepsin and acid secretion was studied in seven healthy subjects. The study was repeated on another day during infusion of glucagon in a dose of 103 pmol/kg/h (0 . 36 micrograms/kg/h) which results in plasma-glucagon concentrations comparable with those seen after a protein-rich meal. Pepsin output was maximal after 0 . 2 nmol/kg/h (0 . 1 microgram/kg/h) of pentagastrin and 20 nmol/kg/h (10 micrograms/kg/h) resulted in a marked decrease. The dose of pentagastrin required for half-maximal pepsin output was less than 0 . 1 nmol/kg/h (0 . 05 micrograms/kg/h). When the study was repeated during infusion of glucagon, the dose-response curve was shifted to the right. The highest pepsin output was obtained with 20 nmol/kg/h (10 micrograms/kg/h) of pentagastrin and D50 increased to well over 1 microgram/kg/h. The dose of pentagastrin required for half-maximal acid secretion was about 0 . 3 nmol/kg/h (0 . 15 micrograms/kg/h) indicating that the sensitivity of the chief cells to pentagastrin is more than three times that of the parietal cells.

Hyperglucagonemia after removal of lower bowel in rats

Surgical removal of the jejunum, ileum, and colon from rats (LBX) results in greatly elevated levels of plasma immunoreactive glucagon (pIRG) 24 h after surgery (0.98 +/- 0.07 ng/ml, n = 51 vs. 0.20 +/- 0.02 ng/ml, n = 34 in sham-operated controls). Such elevations in pIRG were not noted after gut transection or the removal of ileum, jejunum, or colon alone, ileum plus jejunum, or stomach plus duodenum. Coupled with the failure of adrenal demedullation, adrenalectomy or ganglionic blockade to lower pIRG in LBX animals, these findings suggest that surgical stress alone is an unlikely cause for LBX-induced hyperglucagonemia. It was also shown that alpha-cells in LBX animals retained their responsiveness to both the inhibitory effects of somatostatin and glucose infusion as well as the stimulatory effects of arginine infusion. Chromatography revealed a normal pattern of IRG in the plasma of LBX animals. It is postulated that LBX-induced hyperglucagonemia may result from the removal of an inhibitory factor present in the lower bowel.

Effect of arginine infusion on endogenous pancreatic glucagon and gastric acid secretion in duodenal ulcer patients and normal subjects

The effect of intravenous infusion of L-arginine on pentagastrin-stimulated gastric acid secretion was studied in 15 duodenal ulcer patients and 12 healthy subjects. In both groups L-arginine enhanced plasma concentrations of pancreatic glucagon equally and to levels similar to those seen after a protein-rich meal and inhibited the acid response in duodenal ulcer patients and in normal subjects. The study supports previous findings suggesting that pancreatic glucagon is a physiological inhibitor of gastric acid secretion but does not support the hypothesis of a defect in this inhibitory system in duodenal ulcer patients.

The incretin concept today

1. The insulinogenic factor of the gastrointestinal mucosa named "incretin" is only one part of the complex enteroinsular axis. --2. Of the chemically defined gastrointestinal hormones GIP is the strongest incretin candidate. --3. Because of the dual function of GIP as gastrone and insulinotropic substance several safeguards against GIP-mediated insulin hypoglycaemia exist. --4. No pathological condition has yet been found which is causally related to hyper- or hyposecretion of GIP. However, an exaggerated GIP response (usually secondary to the disease) may participate in the pathogenesis of hyperinsulinaemia of patients with obesity and duodenal ulcer. --5. The injection of GIP antibodies only partially abolishes the incretin effect. Therefore, GIP, although important, is not the only incretin.

The effects of endogenous hypergastrinemia and hypogastrinemia on the exocrine and endocrine rat pancreas

The effects of endogenous hypergastrinemia and hypogastrinemia on the exocrine and endocrine pancreas were studied in the rat. Hypergastrinemia was induced by antral exclusion, and hypogastrinemia by antral resection. The studies were made 14 weeks after surgery. The total weight of the pancreas was increased both in hypergastrinemic and hypogastrinemic animals, due to hypertrophy of the exocrine cells. In contrast, the volume and total weight of the pancreatic islets were decreased. There was no numerical difference in the A-, D-, PP-cells between the hyper- and hypogastrinemic animals, respectively, and the controls. The number of insulin-producing (B-) cells was certainly reduced after the induction of hypogastrinemia. There was, however, signs of increased B-cell activity, which might contribute to an underestimation of the number of B-cells with the technique used. These findings do not support the hypothesis that antral gastrin has trophic influence on either exocrine or endocrine pancreas.

Increased glucose disposal after jejuno-ileostomy

Nine patients were studied 1.5--3 years after jejuno-ileostomy for obesity by an intravenous glucose infusion technique designed to imitate blood glucose concentrations after glucose ingestion. Whereas serum insulin and gastrin concentrations were normal, blood glucose concentrations were significantly depressed compared to preoperative levels as well as to levels in matched normal subjects. Thus, in the fasting state mean concentrations (+/- S.E.M.) of blood glucose, serum insulin and gastrin in the patients were, respectively, 3.3 +/- 0.2 mmol/l, 95 +/- 22 pmol/l and 38 +/- 4 pmol/l. The corresponding concentrations in the matched normals were 4.3 +/- 0.2 mmol/l, 70 +/- 18 pmol/l and 39 +/- 6 pmol/l. The glucose concentrations in the patients were low in all situations, i.e. in the fasting state, after oral glucose ingestion and during the intravenous glucose infusion. The results indicate that jejuno-ileostomy in obesity greatly facilitates peripheral glucose disposal. The mechanism behind this phenomenon is not yet known.