Identification of glucagon in the gastrointestinal tract

Revisiting the role of glucagon in health, diabetes mellitus and other metabolic diseases

Insulin and glucagon exert opposing effects on glucose metabolism and, consequently, pancreatic islet β-cells and α-cells are considered functional antagonists. The intra-islet hypothesis has previously dominated the understanding of glucagon secretion, stating that insulin acts to inhibit the release of glucagon. By contrast, glucagon is a potent stimulator of insulin secretion and has been used to test β-cell function. Over the past decade, α-cells have received increasing attention due to their ability to stimulate insulin secretion from neighbouring β-cells, and α-cell-β-cell crosstalk has proven central for glucose homeostasis in vivo. Glucagon is not only the counter-regulatory hormone to insulin in glucose metabolism but also glucagon secretion is more susceptible to changes in the plasma concentration of certain amino acids than to changes in plasma concentrations of glucose. Thus, the actions of glucagon also include a central role in amino acid turnover and hepatic fat oxidation. This Review provides insights into glucagon secretion, with a focus on the local paracrine actions on glucagon and the importance of α-cell-β-cell crosstalk. We focus on dysregulated glucagon secretion in obesity, non-alcoholic fatty liver disease and type 2 diabetes mellitus. Lastly, the future potential of targeting hyperglucagonaemia and applying dual and triple receptor agonists with glucagon receptor-activating properties in combination with incretin hormone receptor agonism is discussed.

Glucagon 100 years. Important, but still enigmatic

Glucagon was discovered in 1923 as a contaminant of early insulin preparations, and its hormonal status was not established until its structure was established in the 1950 s and when the first radioimmunoassay was developed by Roger Unger, providing information about its secretion. Its role in hepatic glucose production was soon established and it was proposed as an essential factor in diabetic hyperglycemia. However, even today a number of issues remain unsolved. For instance, the assays for glucagon are not straightforward, although the development of sandwich ELISAs allowed reasonably accurate measurements also in rodents. The tools for evaluation of glucagon physiology include pancreatectomy, but studies in both humans and experimental animals pointed towards extrapancreatic sources of glucagon. It was demonstrated that glucagon receptor knockout animals do not develop diabetes upon destruction of their beta cells with streptozotocin. However, in patients with type 1 diabetes, glucagon antagonists do not normalize glucose levels; but antagonists do lower glucose levels in patients with in type 2 diabetes. Recent studies in animals and humans have confirmed the essential role of glucagon in glucose metabolism, but have suggested that it may be at least equally important for amino acid and lipid metabolism. In spite of the 100 years, glucagon research is very much alive.

Pseudo-hyperglucagonemia was observed in pancreatectomized patients when measured by glucagon sandwich enzyme-linked immunosorbent assay

Glucagon is detected in plasma even after total pancreatectomy, and it is debated whether this glucagon is derived from the gastrointestinal tract. Here, we applied sandwich enzyme-linked immunosorbent assay (ELISA) and liquid chromatography-high-resolution mass spectrometry to measure plasma glucagon levels in one patient after partial pancreatectomy (one-seventh of the pancreas remaining) and three patients after total pancreatectomy. Sandwich ELISA detected higher glucagon levels in pancreatectomy patients than in healthy individuals. In contrast, liquid chromatography-high-resolution mass spectrometry showed that plasma glucagon levels in pancreatectomy patients were below the lower limit of quantification. Plasma glucagon measured by sandwich ELISA showed a striking correlation with plasma glicentin, suggesting cross-reaction with this gastrointestinal glucagon-related peptide. These results indicated that pancreatectomized patients falsely showed pseudo-hyperglucagonemia when measured by glucagon sandwich ELISA.

Methods and Guidelines for Measurement of Glucagon in Plasma

Glucagon circulates in concentrations in the low picomolar range, which is demanding regarding the sensitivity of the methods for quantification applied. In addition, the differential and tissue specific proteolytic processing of the glucagon precursor and the presence in of several glucagon-like sequences, not only in the precursor of glucagon, but also in a number of other peptides of the glucagon-secretin family of peptides, put special demands on the specificity of the assays. Finally, experience has shown that unspecific interference of plasma components has presented additional problems. All of these problems have resulted in a lot of diverging results concerning measured and reported glucagon responses in both humans and experimental animals that have and still are causing considerable debate and controversy. There is very solid evidence that glucagon is an important hormone in human and mammalian metabolism, but its precise physiological role in glucose and lipid metabolism and in metabolic disease has been difficult to establish, not least because of these difficulties. It was our purpose with this review to discuss the methods of glucagon quantification and discuss pitfalls and sources of error. We also reviewed some of the dogmas regarding glucagon secretion in the light of the methodological difficulties.

Insulin deficiency with and without glucagon: A comparative study between total pancreatectomy and type 1 diabetes

AIMS/INTRODUCTION

Patients with a total pancreatectomy and type 1 diabetes are similar in regard to absolute insulin deficiency, but different in regard to glucagon, providing a unique opportunity to study the contribution of glucagon to glucose metabolism in an insulin-dependent state. The aim of the present study was to investigate the contribution of glucagon to glucose homeostasis in complete insulin deficiency in vivo.

METHODS

A total of 38 individuals with a complete lack of endogenous insulin (fasting C-peptide <0.0066 nmol/L) and whose glycemic control was optimized with an insulin pump during hospitalization were retrospectively studied. The basal insulin requirement, time-to-time adjustment of the basal insulin infusion rate, prandial insulin requirement and fasting plasma glucagon were compared between patients with a total pancreatectomy (n = 10) and those with type 1 diabetes (n = 28) after achievement of optimal glycemic control.

RESULTS

Total daily insulin (P = 0.03) and basal insulin (P = 0.000006), but not prandial insulin requirements, were significantly lower in total pancreatectomy patients than in type 1 diabetes patients. The basal percentage (basal insulin/total daily insulin) was also significantly lower in total pancreatectomy patients than in type 1 diabetes patients (15.8 ± 7.8 vs 32.9 ± 10.1%, P = 0.00003). An increase in the insulin infusion rate early in the morning was not necessary in most patients with a pancreatectomy. The fasting plasma glucagon concentration was significantly lower in total pancreatectomy patients than in type 1 diabetes patients (P = 0.00007), and was positively correlated with the basal insulin requirement (P = 0.038).

CONCLUSIONS

The difference in insulin requirements between total pancreatectomy and type 1 diabetes patients suggests a contribution of glucagon to the basal insulin requirement and dawn phenomenon.

Growth-promoting actions of peptide hormones

In common with other growth-promoting hormones, peptide hormones evoke multiple biochemical responses in their target tissues. These can be divided into two groups: (a) rapid effects involving permeability properties of the target cell to amino acids, sugars and ions or changes in key intracellular metabolites like cyclic nucleotides; (b) slow responses based on the stimulation of RNA and protein synthesis. The impossibility of explaining all the late events as the results of early changes raises the possibility that more than one species of hormone receptor exists. It is proposed that the final expression of growth and maturation results from the cooperative interaction of rapid and slow responses of the target cell to the hormone.

Gastrointestinal Function

This chapter describes the normal biochemical processes of intestinal secretion, digestion, and absorption. The digestive system is composed of the gastrointestinal (GI) tract, or the alimentary canal, salivary glands, the liver, and the exocrine pancreas. The principal functions of the gastrointestinal tract are to digest and absorb ingested nutrients, and to excrete waste products of digestion. Most nutrients are ingested in a form that is either too complex for absorption or insoluble, and therefore, indigestible or incapable of being digested. Within the GI tract, much of these substances are solubilized and further degraded enzymatically to simple molecules, sufficiently small in size, and in a form that permits absorption across the mucosal epithelium. This chapter explains in detail the mechanisms of salivary secretions, compositions of saliva, and the functions of saliva. The chapter also elaborates properties of bile as well as the synthesis of bile acids. The chapter explores the pathogenesis of the important gastrointestinal diseases of domestic animals, and the biochemical basis for their diagnosis and treatment. The chapter concludes with a discussion on disturbances of gastrointestinal function such as vomition, acute diarrheas, malabsorption, bacterial overgrowth, and ulcerative colitis.

Evolution of the restorative proctocolectomy and its effects on gastrointestinal hormones

Gastrointestinal (GI) peptide hormones are chemical messengers that regulate secretory, mechanical, metabolic, and trophic functions of the gut. Restorative proctocolectomy (RPC) or resection of the colon and rectum with maintenance of intestinal continuity through the construction of an ileal pouch reservoir and preservation of the anal sphincters has become the standard of care for the surgical treatment of ulcerative colitis and familial adenomatous polyposis. The manipulation of the digestive system to create the ileal pouch involves altering gut-associated lymphoid tissue among other anatomic changes that lead to changes in GI peptides. In addition, the ileal pouch epithelium responds to a wide variety of stimuli by adjusting its cellularity and function. These adaptive mechanisms involve systemic factors, such as humoral and neural stimuli, as well as local factors, such as changes in intestinal peristalsis and intraluminal nutrients. There have been conflicting reports as to whether the alterations in GI hormones after RPC have actual clinical implications. What the studies on alterations of GI peptides' response and behavior after RPC have contributed, however, is a window into the possible etiology of complications after pouch surgery, such as pouchitis and malabsorption. Given the possibility of pharmacologically modifying GI peptides or select components of adaptation as a therapeutic strategy for patients with ileal pouch dysfunction or pouchitis, a clear understanding of human pouch mucosal adaptation is of paramount importance. In this review, we summarize the evolution of the RPC and its effects on the GI hormones as well as their possible clinical implications.

Somatostatin restrains the secretion of glucagon-like peptide-1 and -2 from isolated perfused porcine ileum

Suspecting that paracrine inhibition might influence neuronal regulation of the endocrine L cells, we studied the role of somatostatin (SS) in the regulation of the secretion of the proglucagon-derived hormones glucagon-like peptide-1 and -2 (GLP-1 and GLP-2). This was examined using the isolated perfused porcine ileum stimulated with acetylcholine (ACh, 10(-6) M), neuromedin C (NC, 10(-8) M), and electrical nerve stimulation (NS) with or without alpha-adrenergic blockade (phentolamine 10(-5) M), and perfusion with a high-affinity monoclonal antibody against SS. ACh and NC significantly increased GLP secretion, whereas NS had little effect. SS immunoneutralization increased GLP secretion eight- to ninefold but had little influence on the GLP responses to ACh, NC, and NS. Basal SS secretion (mainly SS28) was unaffected by NS alone. Phentolamine + NS and NC abstract strongly stimulated release mainly of SS14, whereas ACh had little effect. Infused intravascularly, SS14 weakly and SS28 strongly inhibited GLP secretion. We conclude that GLP secretion is tonically inhibited by a local release of SS28 from epithelial paracrine cells, whereas SS14, supposedly derived from enteric neurons, only weakly influences GLP secretion.

Reduced insulin secretion by subtotal pancreatectomy: preservation of insulin sensitivity and glucose tolerance in postoperative patients

This study investigated insulin sensitivity and glucose tolerance after subtotal pancreatectomy for carcinoma of the head of the pancreas. Twelve consecutive, non-diabetic patients were studied after potentially curative surgery at which the distal pancreas was stapled off, leaving approximately 15% of the pancreas in situ. Brief infusions of insulin (10 mU kg-1) and glucose (25 g) were given before and 4 days after operation. Postoperatively, blood glucose levels remained unchanged, whereas fasting levels of insulin. C-peptide, and pancreatic glucagon were decreased, although significantly (p less than 0.01) only for glucagon. The early and late phases of the insulin and C-peptide responses to glucose were severely reduced. Notably, the hypoglycemic action of insulin and the glucose tolerance were similar to those observed before operation. It is concluded that an acute reduction in pancreatic mass does not impair insulin action or glucose tolerance shortly after surgery. This contrasts with the insulin resistance and glucose intolerance seen shortly after pancreas-preserving intra-abdominal procedures of similar size. It is suggested that the decrease in glucagon levels is at least partly responsible for the preservation of insulin action after subtotal pancreatectomy.

Clinical ketosis

A diagnosis of primary ketosis is based on clinical signs, clinical pathology, and ruling-out disorders that cause secondary ketosis. Various treatments can be used alone or in combination during the management of clinical ketosis. A treatment should be based on drugs with a mechanism of action that will eliminate the pathogenesis of the clinical signs observed.

Effect of stimulation of endogenous glucagon secretion by amino acid administration on canine hepatic bile flow

Exogenous glucagon administration is associated with stimulation of hepatic bile flow. The physiologic role that glucagon plays in the control of hepatic bile flow remains indeterminant. The purpose of this study was to evaluate amino acid administration, a stimulus of endogenous glucagon release, on canine hepatic bile flow. The experiments were performed utilizing cholecystectomized dogs with chronic biliary fistulas. The enterohepatic circulation of bile salts was artificially maintained by intravenous bile salt administration. Intravenous L-arginine stimulated endogenous glucagon release and hepatic bile secretion. Intravenous amino acid administration produced significant increases in hepatic bile flow and plasma glucagon and was significantly more potent than intravenous arginine. Intravenous amino acid administration produced small but significant increases in serum insulin but did not significantly change plasma concentrations of cholecystokinin. The results of this study suggest that endogenous glucagon secretion produces a choleresis and supports a role for glucagon in the physiologic control of canine hepatic bile flow.

Immunohistochemical distribution of glucagon, substance P and vasoactive intestinal polypeptide in hepatic vasculature of the rat

The distribution of immunoreactive glucagon, substance P (SP) and vasoactive intestinal polypeptide (VIP)-like structures was investigated in the rat liver, with special reference to the hepatic vasculature by means of the indirect immunofluorescence method. Immunoreactive structures of glucagon were seen in the walls of the portal vein, hepatic artery and hepatic vein, but not in the central vein. Immunoreactive glucagon was localized in the smooth muscle cells of these blood vessels. SP and VIP-like immunoreactive (SPI and VIPI) structures were seen in the neuronal elements. In the porta hepatis, thick, compact SPI and VIPI fibers, which were dissociated from their fiber bundles, reached the tunica adventitia where they were distributed. No SPI and VIPI structures were seen in the tunica media or the tunica interna. No SPI- and VIPI-containing cell bodies could be detected in the liver. These observations suggest that these peptides may have an important role in the neural regulation of hepatic hemodynamics.

Evidence of glucagon biosynthesis involving protein intermediates in rat salivary glands

In an attempt to determine the ability of rat submaxillary glands to synthesise glucagon via protein intermediates, isolated cells from these glands were incubated in vitro with 3H-L-tryptophan and the acid-ethanol extracts of the cells were purified on Bio-Gel P-30 columns. Aliquots of the eluates were incubated with a C-terminal glucagon antiserum (30K) and the radioactivity bound to the glucagon antibody appeared to be distributed among proteins of Molecular weight greater than 40.14 and 3.5 Kdaltons. A similar elution pattern was obtained in the presence of urea (7 mol/l) and guanidine hydrochloride (6 mol/l). To determine the molecular weight of the immunoreactive material eluting before the 3.5 Kdalton polypeptide, aliquots of the cell extracts were immunoprecipitated and analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Polypeptides of 125.8, 63.1, 42.6 and 14.4 Kdaltons were obtained. These polypeptides incorporate more radioactive tryptophan with increase in the time of incubation. Pulse-chase experiments with unlabelled tryptophan, cycloheximide-treatment of isolated cells and limited tryptic digestion of the larger glucagon immunoreactive component, transform it into a 3.5 Kdalton polypeptide with immunological characteristics indistinguishable from pancreatic glucagon. These results suggest that the larger molecule contains glucagon and thus may serve as a precursor or an intermediate of extrapancreatic glucagon biosynthesis.

Metabolic responses to intraduodenal glucose loading in insulin-infused diabetic dogs

The hormonal and metabolic responses to an intraduodenal glucose load (0.5 g/kg) were first determined in eight normal dogs before diabetes (alloxan/streptozotocin) was induced and then comparison made of the responses to the glucose load when normal plasma glucose profiles were recreated by preprogrammed infusion of insulin via the portal or peripheral circulations. Basal intraportal and peripheral insulin infusions at 0.021 +/- 0.001 and 0.022 +/- 0.000 U . kg-1 . h-1, respectively, for 16 h to fasting diabetic dogs normalized peripheral plasma levels of glucose (5.5 +/- 0.3 and 5.6 +/- 0.6 mmol/liter, respectively), immunoreactive insulin (IRI) (11.5 +/- 1.2 and 16.4 +/- 1.6 microU/ml), glucagon (65 +/- 7 and 62 +/- 5 pg/ml), lactate (0.63 +/- 0.04 and 0.54 +/- 0.03 mmol/liter), and alanine (0.236 +/- 0.037 and 0.191 +/- 0.008 mmol/liter). However, peripheral but not intraportal infusion of insulin depressed levels of glycerol, nonesterified fatty acids (NEFA), and 3-hydroxybutyrate (0.074 +/- 0.006 vs. 0.109 +/- 0.013, P less than 0.01; 0.67 +/- 0.04 vs. 0.84 +/- 0.09, P less than 0.05; and 0.018 +/- 0.004 vs. 0.059 +/- 0.015 mmol/liter, P less than 0.01, respectively). With the preprogrammed insulin infusions used to normalize plasma glucose profiles to the intraduodenal glucose load, all hormonal and metabolic responses were normalized during intraportal infusion (IRI, 72.5 +/- 4.2 microU/ml; glucagon, 66 +/- 10 pg/ml; lactate, 1.06 +/- 0.10 mmol/liter; alanine, 0.251 +/- 0.042 mmol/liter; glycerol, 0.043 +/- 0.013 mmol/liter; NEFA, 0.24 +/- 0.03 mmol/liter; and 3-hydroxybutyrate, 0.012 +/- 0.007 mmol/liter) but marked hyperinsulinemia (103.2 +/- 6.1 microU/ml) and depressed glycerol, NEFA, and 3-hydroxybutyrate responses at 2 h (0.056 +/- 0.005, 0.52 +/- 0.10, and 0.019 +/- 0.010 mmol/liter, respectively) resulted during peripheral infusion. Therefore, only the portal route of insulin infusion achieved complete metabolic normalization during glucose loading in diabetic dogs.

Ontogeny of immunoreactive glicentin in the human gastrointestinal tract and endocrine pancreas

The gestational time of appearance and distribution of immunoreactive glicentin was compared to that of immunoreactive glucagon in the gastrointestinal tract and endocrine pancreas of human fetuses, aged between 5 and 24 weeks, by an indirect immunoperoxidase method. With the glicentin antiserum No. R 64, the first immunoreactive cells were detected at the 10th week of gestation in the oxyntic mucosa and proximal small intestine, at the 8th week in the ileum and at the 12th week in the colon. In the endocrine pancreas, the first immunoreactive cells were observed as early as 8 weeks within the walls of the primitive pancreatic ductules. At a more advanced stage of development (12 weeks), they were found interspersed among the islet cell clusters and still later (16 weeks) inside the recognizable islets of Langerhans. With the glucagon antiserum No. GB 5667, no immunoreactive cells were demonstrated in the gastrointestinal tract whatever the age of the fetuses. In the endocrine pancreas, the first immunoreactive cells were observed at the 8th week of gestation in the pancreatic parenchyma. The distribution of glucagon-containing cells in the pancreas was similar to that of glicentin immunoreactivity throughout ontogenesis. In the pancreatic islets of one 18-week-old human fetus, the study of consecutive semithin sections treated by both antisera showed that the same cells were labelled. The significance of these findings concerning the role of glicentin as a glucagon precursor is discussed.

Pancreatic glucagon: possible implications of the hyperglycemic hormone in diabetes control

Pancreatic glucagon, the hyperglycemic hormone secreted by the alpha cells of the islets of Langerhans, promotes glycogenolysis, neoglucogenesis, lipolysis, and ketogenesis. Several abnormalities of glucagon secretion have been described in diabetes mellitus. These include absolute or relative hyperglucagonemia, exaggerated response to a protein load, and insufficient response to hypoglycemia. Although glucagon's role in diabetes has been challenged, the bulk of the evidence suggests that while insufficiency of insulin is the major abnormality involved, inappropriately elevated glucagon levels contribute to worsening of hyperglycemia. It is suggested that lowering of glucagon levels will result in better control of diabetes. To some extent, this can be achieved by continuous infusion of insulin in insulin-dependent diabetics. In addition, development of analogs of somatostatin holds promise of therapeutic benefit in both insulin-dependent and non-insulin-dependent types of the disease.

Responses to mixed meals in pancreatectomized dogs deprived of postprandial insulin

Insulin plays a central role in metabolic control after a mixed meal. In the absence of adequate meal insulin release, abnormal circulating concentrations of most meal-derived metabolic substrates can be expected. To quantify these abnormalities in depth, responses of six pancreatectomized dogs on long-term intravenous insulin replacement were compared to those of five normal control dogs. Blood samples were drawn hourly for 24 h via a chronic indwelling catheter, and all animals ate a single mixed meal. To establish whether there were route-related differences, insulin was delivered into either the portal or the peripheral circulation of the diabetic animals at constant rates. These insulin infusion rates resulted in premeal fasting normoglycemia and in normal levels of insulin, glucagon, lactate, pyruvate, 3-hydroxybutyrate, nonesterified fatty acids, and 9 of 13 amino acids. In the absence of enhanced meal insulin infusion, the subsequent responses of glucose, lactate, pyruvate, alanine, and 10 of 13 other blood amino acids were exaggerated in terms of both amplitude and duration. Only minor or transient differences were attributable to the routes of insulin infusion. Remarkably, in spite of these abnormal postmeal responses, basal insulin alone (with constant circulating levels) succeeded in restoring all metabolite and hormonal levels during the postabsorptive period 16-23 h after the meal. Thus, with intravenous insulin infusions, the requirements for fasting metabolic normalization may be considered independently of those for metabolic control following caloric intake. It remains to be shown how prolonged deprivation of the postprandial insulin supplement results in metabolic decompensation under these conditions.

Absence of islet alpha cell function in pancreatectomized patients

Plasma immunoreactive glucagon, C-peptide and substrates (glucose, lactate, and alanine) were measured in 21 pancreatectomized patients and 28 patients with chronic calcifying pancreatitis during arginine infusion. Results were compared with those obtained in control and in insulin-dependent diabetic subjects, and in pancreatectomized subjects receiving a combined infusion of glucagon and arginine or somatostatin and arginine. Plasma immunoreactive glucagon in the pancreatectomized patients was 230 +/- 26 pg/ml (control subjects 100 +/- 13 pg/ml, p less than 0.001), but was unchanged following arginine or somatostatin. Following ethanol extraction of plasma it became undetectable. Similar results were obtained in patients with chronic pancreatitis. In contrast to the insulin-dependent diabetic subjects, no changes in blood glucose, lactate, and alanine concentrations were found during arginine infusion in the pancreatectomized or pancreatitis patients. Addition of glucagon restored the metabolic response to arginine in the pancreatectomized patients. Our results confirm previous smaller studies that in pancreatectomized patients, A cell function is absent or insignificant.

Glucagon and glicentin immunoreactive cells in human colon

An immunohistochemical study of glucagon and glicentin immunoreactive endocrine cells in the human colon epithelium was performed. Serial sections and qualitative analysis show a cell population containing both immunoreactivities. However, there is another cell population exhibiting only an immunoreactivity with glicentin. The exact distribution of these immunoreactive endocrine cells within the colon crypt segments is also analysed. The significance of these findings concerning the synthesis of glucagon and glicentin and their function is discussed.

The metabolic and hormonal responses to a mixed meal in unrestrained pancreatectomised dogs chronically treated by portal or peripheral insulin infusion

The metabolic and hormonal consequences of long term intravenous insulin replacement were studied in 11 pancreatectomised dogs. Insulin was delivered into the portal circulation of six animals for 164-224 days and into the peripheral circulation of the remainder for 123-365 days. Infusion rates were initially adjusted to achieve normoglycaemia in the fasting (0.37 +/- 0.01 mU Kg-1 min-1 portal; 0.45 +/- 0.03 mU kg-1 min-1 peripheral) and post-prandial states (2.57 +/- 0.07 mU kg-1 min-1 for 7 1/2 h portal; 3.16 +/- 0.18 mU kg-1 min-1 for 7 h peripheral). Animals were fed their usual mixed diet and blood samples were drawn from indwelling catheters at regular intervals for 24 h. A matched group of six normal dogs was similarly studied. Significantly less insulin was needed for glycaemic normalisation with portal (1.05 +/- 0.03 U kg-1 day-1) compared with peripheral (1.27 +/- 0.08 U kg-1 day-1) infusions, but post-prandial insulin levels were not normalised. Glucagon levels were normal and unaffected by the route of insulin infusion. Lactate and pyruvate responses were exaggerated post-prandially in the diabetic compared with the normal dogs. Fasting non-esterified fatty acid levels were suppressed with peripheral but normal with portal insulin infusion. There were only minor differences in the branched chain, essential and other non-essential amino acids except for alanine which was significantly above normal in the diabetic animals. Fasting levels of insulin, lactate, pyruvate and non-esterified fatty acids were normalised only with portal infusion while glucose, glucagon, 3-hydroxybutyrate and most amino acids were normalised regardless of the route of infusion. We conclude that the metabolic regulation achieved with portal insulin replacement is closer to normal than that achieved with peripheral infusion.

Glucagon and carbohydrate disorder in a totally pancreatectomized man (a study with the aid of an artificial endocrine pancreas)

The effect of insulin withdrawal and exogenous glucagon infusion upon blood glucose concentration was investigated in a totally pancreatectomized patient with the aid of an artificial endocrine pancreas. Blood glucose remained unchanged at about 100 mg/100 ml, when insulin infusion was stopped, but rose up to 300 mg/100 ml, during a 12-h period of exogenous glucagon infusion at a rate of 3 ng/kg/min. Fractionation of whole plasma on Bio Gel P-30 revealed no immunoreactive glucagon in the region of true glucagon. This study seems to reinforce the hypothesis that true glucagon is essential in the fasting condition at least in the short term to produce hyperglycemia in insulin deprived diabetics.

The amino acid sequence of porcine glicentin

Glicentin or gut GLI-1 has previously been isolated from porcine small intestine. On the basis of the available chemical data, the molecule was thought to contain 100 amino acid residues. A redetermination of the amino acid composition of the molecule has shown it to contain 69 amino acid residues, and the full sequence has been established. The sequence of glicentin can be outlined as: GRPP1--30-Lys-Arg-Glucagon33--61-Lys-Arg-Hexapeptide64--69 where GRPP1--30 probably corresponds to the glicentin related pancreatic peptide previously isolated from porcine pancreas. In the pancreas, the two dibasic sequences (Lys31-Arg32 and Lys62-Arg63) probably represent sites of post-synthetic enzymatic cleavages by analogy with the two dibasic sequences of proinsulin. Glicentin thus fulfills the structural requirements for being all or a part of porcine proglucagon. In the intestine, glicentin could be the precursor of oxyntomodulin, a small molecular weight gut GLI presumably identical to glicentin 33--69, i.e., glucagon extended at the C-terminal end by an octapeptide.

The role of gastric glucagon immunoreactivity in pancreatectomized dogs

Immunoreactive glucagon (IRG) or glucagon immunoreactivity is known to be increased in the plasma of insulin-deprived pancreatectomized dogs, most of it originating in the stomach. We attempted to clarify the extent to which gastric IRG is involved in glycogenolysis in the liver of insulin-deprived, pancreatectomized dogs. Mongrel dogs underwent total pancreatectomy. IRG levels in portal vein blood increased to 760 +/- 186 pg/ml on the 4th postoperative day while the insulin levels were negligible. On the 4th postoperative day some of the dogs underwent total gastrectomy. IRG levels in the portal vein blood of pancreatectomized dogs decreased from 760 +/- 186 pg/ml to 135 +/- 44 pg/ml one hour after gastrectomy. Glucose containing insulin was then infused into both pancreatectomized and pancreatectomized-gastrectomized grups of dogs. Glycogen synthesis in the liver during glucose and insulin infusion was much the same in both groups. However, glycogen degradation after glucose and insulin infusion was completely suppressed in pancreatectomized dogs without a stomach while pancreatectomized dogs alone showed marked glycogenolysis in the liver. No difference in portal IRI and blood sugar level was found in both groups while a marked difference in portal IRG were observed. These findings indicate that the IRG released from the stomach plays a significant role i glycogen metabolism in pancreatectomized dogs.

Synthesis and release of glucagon by human salivary glands

Pieces of human salivary glands were homogenised with acid-ethanol or acid-saline solutions immediately after surgical removal. With both extraction procedures the immunoreactive glucagon (IRG) content in the submaxillary glands was greater than in parotid glands as determined with a C-terminal reactive glucagon antiserum (30K). Higher amounts of IRG were determined in acid-saline extracts of submaxillary (18.5 +/- 2.5 VS 8.9 +/- 1.2 ng/g wet weight) and parotid (3.5 +/- 0.3 VS 2.9 +/- 0.3 ng/g wet weight) glands compared with concentrations obtained with acid-ethanol extracts. IRG material extracted with the latter procedure has similar immunological and biological characteristics as pancreatic glucagon. After fractionation of the acid-ethanol extracts on P-30 columns or gel electrophoresis, an immunoreactive peak of 3500 daltons was always obtained. Arginine, ephinephrine and low glucose concentrations stimulated glucagon release from both salivary glands. Active glucagon biosynthesis by these glands was established by the incorporation of 3H-L-tryptophan into a 3500 daltons polypeptide with specific immune reaction with 30K antiserum. These findings indicate that human salivary glands represent a source of extrapancreatic glucagon in man and may therefore contribute to the circulating levels of this hormone.

The physicochemical nature and biological function of extrapancreatic immunoreactive glucagon

The physicochemical nature and biological function of extrapancreatic immunoreactive glucagon in the plasma of totally pancreatectomized dogs were studied. The experimental animals were divided into two groups. Group A received only total pancreatectomy while group B had total pancreatectomy plus total gastrectomy. The blood was taken from both groups of dogs and glucagon was extracted using acid alcohol. The amount of IRG contained in the plasma extract was measured and the glycogenolytic activity of the plasma extract was studied using a rat-liver perfusion. The plasma extract from group A showed almost the same glycogenolytic activity as a commercial beef-pork glucagon but the plasma extract from group B manifested no glycogenolytic activity. The plasma from both groups were fractionated using column chromatography and their physicochemical nature was compared. The IRG in the plasma from group A was fractionated into 3 types, molecular weight 9000, 7000 and 3500, while the IRG in the plasma from group B was found to be of M.W. 9000 and 7000. These results lead to the following conclusions: 1) Extrapancreatic IRG in the plasma is fractionated into three types. 2) The glycogenolytic activity of extrapancreatic IRG is assumed to depend on the IRG fraction of molecular weight 3500. 3) Most of the extrapancreatic IRG fraction of M.W. 3500 originates in the stomach.

The metabolic and hormonal responses to glucose infusion in anaesthetized normal and diabetic dogs controlled by an artificial B-cell

The metabolic response to glucose infusion in anaesthetized normal and pancreatectomized dogs has been assessed. Normoglycaemia was achieved in the diabetic dogs with an external artificial B-cell which administered insulin into the peripheral circulation. No differences were found in the levels of blood glucose, glucagon, lactate, pyruvate and plasma non-esterified fatty acids, either in the fasting state or in response to glucose infusion. However, compared to normal animals normoglycaemic diabetic dogs had significantly elevated circulating levels of insulin and alanine at all times. Fasting levels of the same hormones and metabolites were also measured in conscious dogs. Blood pyruvate levels were higher, and plasma non-esterified fatty acid levels lower, in the anaesthetized animals. There were also minor but consistent changes in blood glucose and plasma insulin while glucagon, lactate and alanine levels were unaffected by anaesthesia. In conclusion, controlled barbiturate anaesthesia has relatively minor effects on the metabolic and hormonal status of the dog. The metabolic and hormonal response to glucose infusion in pancreatectomized dogs treated with an artificial B-cell was almost entirely normalized, except for peripheral hyperinsulinaemia and hyperalaninaemia.

Glucagon levels and ketogenesis in human diabetes following total or partial pancreatectomy and severe chronic pancreatitis

In three groups of patients with insulin-dependent diabetes following total (n = 5) or partial (n = 5) pancreatectomy or chronic pancreatitis (n = 7) and in a group of idiopathic diabetics, ketogenic capacity following insulin withdrawal and during a 24-h fast was studied. Basal glucagon values were significantly increased in all diabetic groups with no significant intergroup differences. Basal ketone body values and their increase during starvation and insulin withdrawal were high and not different in totally pancreatectomized and primary diabetics, both showing unmeasurable C-peptide levels. On the contrary, ketogenesis was reduced in partially pancreatectomized and in pancreatitis diabetics with persistent levels of C-peptide. Our data confirmed the persistence of immunoreactive glucagon after pancreatectomy and demonstrated that ketogenesis is not suppressed in pancreatectomized diabetics and depends above all on residual B-cell function. A possible ketogenic effect of extra-pancreatic glucagon-like substances cannot be excluded.

Tissue distribution of glucagon, glucagonlike immunoreactivity, and insulin in the rat

To show that glucagon, glucagonlike immunoreactivity (GLI), and insulin are synthetized by organs other than the pancreas and the gastrointestinal tract, different rat tissue acid-ethanol extracts were obtained and analyzed by immunoassay using specific antisera. Significant amounts of glucagon were found in the gastrointestinal tract (44.77 +/- 5.4 ng), salivary glands (1.50 +/- 0.17 ng), thymus (2.80 +/- 0.46 ng), thyroid (0.25 +/- 0.02 ng), and adrenal glands (0.25 +/- 0.06 ng). Whereas GLI appeared in the gut mucosa, adrenal and salivary glands, genuine insulin was detected only in the pancreas. Aliquots of the tissue extracts, fractionated on Bio Gel P 30 columns, gave a 3,500 mol wt immunoreactive (30 K) peak that behaved as pancreatic glucagon on acrylamide gel electrophoresis and displaced 125I-labeled glucagon previously bound to its hepatic receptors. Arginine, epinephrine, and low glucose concentrations stimulated glucagon release from parotid, thymus, and thyroid. Active glucagon biosynthesis by these organs was established by the incorporation of L-[3H]tryptophan into a 3,500 mol wt polypeptide with specific immune reaction with 30 K antiserum. These results suggest that different rat tissues can contribute to the circulating levels of glucagon and GLI and therefore to metabolic homeostasis.

Glucagon deficiency and hyperaminoacidemia after total pancreatectomy

The first goal of this study was to investigate whether totally pancreatectomized patients are glucagon deficient and if so, to what degree. Immunoreactive glucagon (IRG) concentrations in peripheral plasma of nine pancreatectomized patients were not significantly different from those of 10 normal controls as measured by two antisera (30-K and RCS-5) both detecting the COOH-terminal portion of the molecule and one (RCS-5) postulated to be specific for pancreatic glucagon. Plasma from six of nine pancreatectomized patients were fractionated over Sephadex G-50 and IRG was measured with both antisera in the column eluates. Using 30-K, 80.8 +/- 9% of the IRG eluted within the void volume. This material was rechromatographed on Sephadex G-200 and found to have an apparent mol wt of approximately 200,000. Only 18.3 +/- 9% eluted in the IRG3500 region. IRG3500 was significantly reduced in pancreatectomized patients as compared to normal controls (49 +/- 9 vs. 18 +/- 9 pg/ml, P less than 0.05). Using RCS-5, all IRG (corresponding to 20 +/- 6 pg/ml of plasma) eluted in the IRG3500 region. The second goal of this study was to investigate the effects of chronic glucagon deficiency on plasma amino acids. In the nine pancreatectomized patients studied, postabsorptive plasma concentrations of serine, alanine, arginine, glycine, threonine, citrulline, alpha-aminobutyrate, and tryosine were significantly elevated compared to values obtained from 20 normal controls. Physiological glucagon increments produced in two pancreatectomized patients by infusion of glucagon (6.25 and 8.0 microgram/h, respectively) resulted in normalization of the hyperaminoacidemia within 22 h. We conclude (a) that pancreatectomized patients are partially glucagon deficient because of diminished basal as well as diminished stimulated glucagon secretion; (b) that fasting concentrations of certain glucogenic amino acids are elevated in pancreatectomized patients probably as result of reduce; hepatic gluconeogenesis; and (c) that the RCS-5 antiserum is not "pancreatic glucagon" specific.

Gastrointestinal Function

This chapter discusses the functions of gastrointestinal tract. The principal functions of the gastrointestinal tract are assimilation of nutrients and excretion of the waste products of digestion. Within the gastrointestinal tract, these substances are solubilized and degraded enzymatically to simple molecules, sufficiently small in size and in a form that permits absorption across the mucosal epithelium. The distribution of the different types of secretory cells in the salivary glands varies among species. The mandibular and sublingual glands are mixed salivary glands containing both mucous and serous types of cells, and produce a viscous secretion that contains large amounts of mucus. The cytoplasm of the secretory cells contains numerous zymogen granules that vary in size and number depending on the activity of the gland. These granules contain the precursors of the hydrolytic enzymes responsible for digestion of the major dietary components. The cells of the terminal ducts probably secrete the bicarbonate ion responsible for neutralizing hydrochloric acid that enters the duodenum from the stomach.

How concentrate feeding affects glucoregulatory hormones in ruminants: implications in bovine ketosis

Twenty-four feedlot steers and four sheep were fed roughage or grain rations while three other sheep received intraduodenal infusions of amino acids, glucose, volatile fatty acids, and proteins to determine whether the release of plasma glucagon-like immunoreactivity is influenced by dietary factors and the composition of duodenal chyme. Plasma glucagon-like immunoreactivity rose in both cattle and sheep as the proportion of grain in their rations was increased. In response to intraduodenal infusions, only glucose stimulated glucagon-like immunoreactivity release. These experiments demonstrate that plasma glucagon-like immunoreactivity concentrations change in cattle and sheep in response to the amount of grain consumed and that glucagon-like immunoreactivity release is triggered by glucose in intestinal chyme.

Distribution of the gut hormones in the primate intestinal tract

Reliable and specific radioimmunoassays have been developed for the gut hormones secretin, gastrin, cholecystokinin, pancreatic glucagon, VIP, GIP, motilin, and enteroglucagon. Using these assays, the relative pattern of distribution of the gut hormones has been determined using the same bowel extracts for all measurements. VIP occurred in high concentration in all regions of the bowel, whereas secretin, GIP, motilin, and CCK were predominantly localised in the proximal small intestine. Pancreatic glucagon was almost exclusively confined to the pancreas. Like VIP, enteroglucagon also exhibited a wide pattern of distribution but was maximal in the ileum. The acid ethanol extraction method that was used was found to be unsuitable for gastrin. On gel chromatography of the extracts, motilin and VIP eluted as single molecular species in identical position to the pure porcine peptides. CCK, pancreatic glucagon, enteroglucagon and GIP were all multiform.

Physicochemical and biological properties of glucagon-like polypeptides from porcine colon

Polypeptide material displaying glucagon-like immunoreactivity was isolated from porcine colon using immunoaffinity chromatography. The immunoreactive material was tightly bound to high molecular weight proteins but was dissociated by 0.1% w/v sodium dodecyl sulphate solution into immunoreactive components of approximate molecular weights 12,000,8000,5000 and 3000. These components reacted at least 50 times more strongly with antibodies specific for the N-terminal region of glucagon than with antibodies specific for the C-terminal region of glucagon. While the 8000 and 3000 dalton fractions were homogeneous, the 12,000 and 5000 dalton fractions were resolved into multiple bands by isoelectric focusing. The 12,000 dalton fraction was devoid of glycogenolytic and lipolytic activity, was not insulin releasing and showed no ability to bind to receptor sites specific for glucagon on hepatic plasma membranes and to active hepatic adenylate cyclase. The 8000 and 5000 dalton components showed weak lipolytic activity. The possible significance of colonic glucagon-like immunoreactivity relative to pancreatic glucagon and immunoreactivity from other tissues is discussed.

Effects of selective insulin or glucagon deficiency on glucose turnover

To study the importance of glucagon and insulin in diabetes, somatostatin (ST) was infused, alone or with insulin or glucagon, in 11 conscious dogs. Plasma immunoreactive insulin (IRI) and glucagon (IRG) levels fell 65 +/- 4% and 33 +/- 3%, respectively, with somatostatin infusion. Glucose production (Ra) assessed by [3-3H]glucose, [2-3H]glucose, or [1-14C]glucose decreased transiently. This is in contrast to the rise in Ra seen after insulin withdrawal in depancreatized dogs, which have normal levels of IRG. Thus, suppression of IRG with somatostatin prevented an increase in Ra in spite of suppression of IRI. When near basal IRG levels were provided during ST infusion in normal dogs, Ra increased, indicating that glucagon contributes to the acute development of diabetes. When basal IRI levels were provided with ST, suppression of Ra was maintained, suggesting that the transience of the metabolic effects of ST-induced glucagon suppression requires concomitant insulin suppression. A comparison of glucose turnover measured using different tracers showed that ST-related hormonal changes did not alter the rate of futile cycling in the liver. ST induced a rise in plasma free fatty acid (FFA) levels, attributed solely to insulin deficiency, as glucagon suppression did not significantly alter FFA concentrations when normal insulin levels were maintained.