Alpha- and beta-cell interrelationships in health and disease

The entero-insular axis: a journey in the physiopathology of diabetes

Glycemic homeostasis is an essential mechanism for the proper working of an organism. However, balance in blood lipid and protein levels also plays an important role. The discovery of the hormone insulin and the description of its function for glycemic control made fundamental scientific progress in this field. However, since then our view of the problem has been deeply influenced only in terms of glucose and insulin (in an insulin-centric and glucose-centric way). Based on recent scientific discoveries, a fine and sophisticated network of hormonal and metabolic interactions, involving almost every apparatus and tissue of the human body, has been theorized. Efficient metabolic homeostasis is founded on these intricate interactions. Although it is still not fully defined, this complex network can undergo alterations that lead to metabolic disorders such as diabetes mellitus (DM). The endocrine pancreas plays a crucial role in the metabolic balance of an organism, but insulin is just one of the elements involved and each single pancreatic islet hormone is worthy of our concern. Moreover, pancreatic hormones need to be considered in a general view, concerning both their systemic function as direct mediators and as hormones, which, in turn, are regulated by other hormones or other substances. This more complex scenario should be taken into account for a better understanding of the pathophysiology and the therapeutic algorithms of DM. As a consequence, improvements in modern medicine could help to contemplate this new perspective. This review is focused on some aspects of gut-pancreas interaction, aiming to integrate this synergy into a wider context involving other organs and tissues.

Free-Range and Low-Protein Concentrated Diets in Iberian Pigs: Effect on Plasma Insulin and Leptin Concentration, Lipogenic Enzyme Activity, and Fatty Acid Composition of Adipose Tissue

Simple Summary

Recently, it has been shown that reducing dietary crude-protein levels during the final fattening period prior to slaughter is a suitable strategy to increase intramuscular fat content in Iberian pig meat, without affecting pig growth. Investigating the effect of a low-protein diet on the metabolism, development, and composition of the adipose tissue of Iberian pigs, and the obese porcine breed, was the objective of this study. Three groups of pigs fed under free-range conditions and in confinement with concentrated diets with low- and standard-protein contents were studied. All three groups exhibited the same backfat thickness at the end of the fattening period. The level of hormones and activities of enzymes related to adipogenic metabolism were affected by diet, with differences between free-range and intensive feeding systems. Therefore, we suggest that feeding Iberian pigs on low-protein diets did not result in fatter carcasses, and is thus a useful strategy to improve Iberian pig meat quality.

Abstract

The purpose of this study was to investigate the effect of diets with different protein contents on carcass traits, plasma hormone concentration, lipogenic enzyme activities, and fatty acid (FA) composition in the adipose tissue of Iberian pigs. Twenty-four castrated male Iberian pigs (eight per feeding diet) were fed under free-range conditions with acorns and grass (FR), and in confinement with concentrated diets with standard (SP) and low-protein contents (LP) from 116.0 to 174.2 kg live weight. Backfat thickness was not affected by diet. The plasma leptin concentration was higher (p < 0.001) in the FR group than in the LP and SP groups, while insulin concentration was higher in the SP group than in the LP and FR groups. The lipogenic enzyme activities of glucose-6-phosphate dehydrogenase, malic enzyme, and glycerol-3-phosphate dehydrogenase were lower in the FR group compared to the LP and SP pigs. The activities of these enzymes were adipose-tissue-specific. No differences were found in FA composition of adipose tissue between the SP and LP groups, while the FR pigs had lower proportions of saturated FA and higher proportions of monounsaturated and polyunsaturated FA than the SP and LP pigs. In conclusion, feeding low-protein diets in Iberian pigs does not seem to affect adipose carcass traits, strengthening previous findings that indicate that this is a good strategy to improve meat and dry-cured product quality.

Glucose Counter-regulation After Acute Pancreatitis

OBJECTIVES

Persistent hyperglycemia is a common sequela of acute pancreatitis (AP). The role of counter-regulatory hormones in maintaining glucose homeostasis has been largely studied during the course of AP, but not after clinical resolution of the disease. The objectives of this study were to investigate the associations between circulating levels of glucagon, cortisol, and human growth hormone and glucose homeostasis after AP as well as their associations with a comprehensive panel of pancreatic hormones, gut peptides, and proinflammatory cytokines.

METHODS

Participants with no history of pre-existing prediabetes or diabetes were categorized into hyperglycemia and normoglycemia after AP groups. Binary logistic regression and linear regression analyses were conducted.

RESULTS

Eighty-three individuals were included, of whom 19 had hyperglycemia. Glucagon, cortisol, and human growth hormone did not differ significantly between the groups. Glucagon explained up to 86% of the variance in glucagon-like peptide 1, whereas cortisol explained up to 89% of the variance in interleukin 6 in hyperglycemia after AP.

CONCLUSIONS

Counter-regulatory hormones do not appear to play a direct role in the mechanisms underlying hyperglycemia after AP. However, significant associations between glucagon and glucagon-like peptide 1, as well as between cortisol and interleukin 6, suggest that that these hormones may be involved indirectly in the pathophysiology of hyperglycemia after AP.

Defining the importance of daily glycemic control and implications for type 2 diabetes management

Glycemic control remains an elusive goal for most patients with type 2 diabetes. Questions concerning glucose targets that have emerged from recent outcomes studies further complicate glucose control strategies. Navigating through these challenges requires an understanding of the relationship between hyperglycemia, glycemic variability, and risk, as well as how to combine antidiabetic agents safely and effectively to minimize complications. Relevant data were selected from recently published major outcomes studies and peer-reviewed articles discussing glycemic variability, incretins, and dipeptidyl peptidase-4 inhibition. Incretin hormones play a premier role in maintaining normal glucose homeostasis. In type 2 diabetes, however, incretin functioning is impaired and glucose homeostasis is disturbed, contributing to hyperglycemia and both acute and chronic glucose fluctuations. Glycemic control efforts should involve quarterly glycated hemoglobin assessments, routine monitoring of daily blood glucose values, and combination therapy that targets both fasting and postprandial hyperglycemia. Dipeptidyl peptidase-4 inhibitors, which enhance endogenous incretin function, are well suited for combination with other agents to promote daily glycemic control without increasing the risk of hypoglycemia or weight gain. Results of recent outcomes studies suggest that a lifetime strategy for diabetes management might involve aggressive efforts to control glycemia daily and early in type 2 diabetes, with less stringent glucose targets and avoidance of hypoglycemia as patients acquire comorbidities, such as advanced cardiovascular disease. Dipeptidyl peptidase-4 inhibitors have the potential to play a vital role in diabetes management at all stages of the disease.

New approaches to treating type 2 diabetes mellitus in the elderly: role of incretin therapies

The increasing proportion of elderly persons in the global population, and the implications of this trend in terms of increasing rates of chronic diseases such as type 2 diabetes mellitus, continue to be a cause for concern for clinicians and healthcare policy makers. The diagnosis and treatment of type 2 diabetes in the elderly is challenging, as age-related changes alter the clinical presentation of diabetic symptoms. Once type 2 diabetes is diagnosed, the principles of its management are similar to those in younger patients, but with special considerations linked to the increased prevalence of co-morbidities and relative inability to tolerate the adverse effects of medication and hypoglycaemia. In addition, there are many underappreciated factors complicating diabetes care in the elderly, including cognitive disorders, physical disability and geriatric syndromes, such as frailty, urinary incontinence and pain. Available oral antihyperglycaemic drugs include insulin secretagogues (meglitinides and sulfonylureas), biguanides (metformin), alpha-glucosidase inhibitors and thiazolidinediones. Unfortunately, as type 2 diabetes progresses in older persons, polypharmacy intensification is required to achieve adequate glycaemic control with the attendant increased risk of adverse effects as a result of age-related changes in drug metabolism. The recent introduction of the incretins, a group of intestinal peptides that enhance insulin secretion after ingestion of food, as novel oral antihyperglycaemic treatments may prove significant in older persons. The two main categories of incretin therapy currently available are: glucagon-like peptide-1 (GLP-1) analogues and inhibitors of GLP-1 degrading enzyme dipeptidyl peptidase-4 (DPP-4). The present review discusses the effect of aging on metabolic control in elderly patients with type 2 diabetes, the current treatments used to treat this population and some of the more recent advances in the field of geriatric type 2 diabetes. In particular, we highlight the efficacy and safety of GLP-1 and DPP-4 inhibitors, administered as monotherapy or in combination with other oral antihyperglycaemic agents, especially when the relevant clinical trials included older persons. There is strong evidence that use of incretin therapy, in particular, the DPP-4 inhibitors, could offer significant advantages in older persons. Clinical evidence suggests that the DPP-4 inhibitors vildagliptin and sitagliptin are particularly suitable for frail and debilitated elderly patients because of their excellent tolerability profiles. Importantly, these agents lack the gastrointestinal effects seen with metformin and alpha-glucosidase inhibitors taken alone, and have a low risk of the hypoglycaemic events commonly seen with agents that directly lower blood glucose levels.

Pancreatic islet dysfunction in type 2 diabetes: a rational target for incretin-based therapies

BACKGROUND

Insulin resistance alone does not result in the development of type 2 diabetes; progressive dysfunction of pancreatic islet alpha and beta cells, which results in inadequate control of hyperglycemia, must be present for the disease to develop. Because of these defects, meal-stimulated insulin secretion from beta cells is reduced and fails to meet the demands of the insulin-resistant state; in addition, glucagon production by alpha cells, which normally maintains hepatic glucose production during fasting periods, is not suppressed. This increased glucagon secretion leads to inappropriate levels of hepatic glucose output in the post-prandial state and consequently to hyperglycemia.

SCOPE

This review will examine the pathophysiologic processes of type 2 diabetes and provide an overview of some of the new and emerging treatments targeting pancreatic islet dysfunction. A MEDLINE search was performed for literature published in the English language from 1966-August 2006. Abstracts and presentations from the American Diabetes Association Scientific Sessions (2002-2006) and the European Association for the Study of Diabetes Annual Meetings (1998-2006) were also searched for relevant studies.

FINDINGS

Key factors in maintaining the normal balance between insulin and glucagon levels are the incretins--glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Intestinal absorption of glucose stimulates secretion of these hormones, which act to increase insulin and decrease glucagon secretion. Studies demonstrating that incretin activity is impaired in type 2 diabetes have led to investigations into incretin-based therapies such as incretin mimetics (analogues of GLP-1) and inhibitors of the enzyme dipeptidyl peptidase-4 (DPP-4), which inactivates native incretins.

CONCLUSIONS

Pancreatic islet dysfunction is a rational target for the treatment of type 2 diabetes. Incretin mimetics and DPP-4 inhibitors have been shown to improve glucose tolerance and may also hold the potential for improving overall pancreatic islet health. It should be noted, however, that the long-term effect of these agents on glycemic control has not yet been established, and their potential impact on beta-cell function in humans remains an area of active investigation.

Syndrome-AC: non-insulin-dependent diabetes mellitus and the anabolic/catabolic paradox

When an organism is starving, infected or injured, the body initiates a catabolic response that, among other things, creates insulin resistance. While many possible mechanisms and numerous loci of insulin resistance have been studied, a theme of inappropriately activated components of catabolic chemistry emerges. Patients with non-insulin-dependent diabetes mellitus (NIDDM) have elevated blood levels of glucagon, cortisol, fatty acids, protein, glucose and possibly acute phase reactants. Recent studies hint that this can occur with meals, resulting in a multilevel, multiorgan interference with glucose handling. This implies NIDDM is the result of the dietary activation of catabolic chemistry simultaneously with that of anabolic chemistry. We review the possibility that this is caused by the consumption of body tissue, the substance ordinarily endogenously released in starvation or injury. Activating the catabolic pathways when eating creates a hormonal paradox, forcing exaggerated insulin levels to compensate. Five case studies are reviewed.

Disturbed anabolic hormonal patterns in burned patients: the relation to glucagon

OBJECTIVE

Complex changes in the anabolic regulators of metabolism occur after major injury. We have studied the time course for IGF-I and IGFBP-1 after burn injury and their relations to circulating levels of other anabolic and catabolic hormones. The hormonal patterns during the onset of sepsis were also investigated.

PATIENTS

Eight patients (age 36 (6) years, mean (SEM)) with major burn injury (burn area 42 (6) %) were studied. The first 2 days since the burn were used for rehydration therapy (rehydration period), after which a complete total parenteral nutrition (TPN) period was initiated. Seven positive blood cultures, during the study period. Six of the eight survived.

MEASUREMENTS

The hormonal changes determined in the morning during the first 7 days after the burn and from day 22 to 24 were investigated. The superimposed effects of sepsis were studied by normalizing all data to the day of positive blood cultures and clinical onset of sepsis.

RESULTS

On admission, plasma levels of glucagon, IGFBP-1 and GH were elevated while levels of IGF-I were low. During the first week after the burn, morning levels of glucagon and insulin increased while levels of GH and IGF-I decreased. GH levels were still elevated compared to healthy subjects. Despite the increase in insulin levels, IGFBP-1 remained elevated. Three weeks after the burn injury, IGF-I levels were increased but still markedly below normal, while IGFBP-1 levels remained unchanged. Persistent elevations of insulin levels were combined with reductions in glucagon levels. Admission levels of IGFBP-1 correlated to nitrogen loss (negative nitrogen balance) during the first 24 hours after the burn (r = 0.84, P < 0.05). A correlation between negative nitrogen balance and glucagon levels was found during early catabolic period in the rehydration period (i.e. days 2-3, r = 0.84, P < 0.01). The relative change in IGFBP-1 levels in the rehydration period correlated to changes in glucagon levels (days 2-3 vs admission, r = 0.85, P < 0.05). The insulin/glucagon molar ratio correlated to the IGF-I/IGFBP-1 ratio during both the rehydration period (days 2-3, r = 0.77, P < 0.05) and the third week after the burn (r = 0.77, P < 0.05). During the most catabolic phase in the first week after the burn (TPN period) there was an inverse relation between IGF-I and IGFBP-I and glucagon (r = 0.83, P < 0.05). During the less catabolic third week after the burn, an inverse correlation was found between IGF-I and glucagon (r = -0.83, P < 0.05). Sepsis, superimposed upon the burn trauma, was associated with transient elevations in IGFBP-1 and reductions in insulin despite elevated levels of glucose and a further 50% increase in nitrogen losses.

CONCLUSIONS

The present findings show that marked changes is important anabolic regulating factors occur after major burn injury. Uncoupling of the GH-IGF-I axis, and the attenuation of the inhibitory effects of insulin on IGFBP-1, both contribute to the reduction in IGF-I levels and bioavailability, factors which may play an important role in post injury metabolism. Furthermore, these data suggest that the catabolic hormones (catecholamines, cortisol and glucagon), primarily glucagon seem to be involved in the modulation of IGF-I and IGFBP-1 levels following burn injury.

Diabetes and adrenal disease

Disorders of the adrenal cortex and medulla can result in glucose intolerance or overt diabetes mellitus. Cushing's syndrome, characterized by excessive secretion of glucocorticoids, impairs glucose tolerance primarily by causing insulin resistance at the post-receptor level. On the other hand, phaeochromocytoma and hyperaldosteronism, via the respective actions of catecholamines and hypokalaemia on the pancreatic beta-cell, impair glucose tolerance primarily by inhibiting insulin release. The glucose intolerance associated with these adrenal disorders is usually only mild to moderate in severity. Marked hyperglycaemia, glycosuria, and polyuria are uncommon and ketosis is rare. Moreover, the late complications of diabetes mellitus are distinctly uncommon in patients with these disorders, and the prognosis for morbidity and death is usually that of the underlying disease and not that of diabetes mellitus. The impaired glucose tolerance induced by all three of these adrenal disorders usually returns to normal once the underlying aetiology has been cured. These factors must guide the clinician in treatment of these secondary forms of diabetes, and suggest that tight (near normal) blood glucose control may not be an appropriate goal in patients with these disorders. The relationship between adrenal androgens and glucose tolerance is more uncertain. Several studies in humans have demonstrated an acute decline in serum concentrations of the adrenal steroids DHEA and DHEA-sulfate in response to experimentally-induced hyperinsulinaemia, but the regulatory role of insulin on adrenal androgen metabolism in normal physiology or disease remains speculative. In several animal models DHEA appears to exert potent anti-obesity and anti-diabetogenic actions, but such effects have yet to be demonstrated in humans. Human studies of DHEA are limited, and more research needs to be conducted to determine whether the observations made in animal models will prove applicable to man.

Increasing dextrose concentrations in total parenteral nutrition (TPN) causes alterations in hepatic morphology and plasma levels of insulin and glucagon in rats

Hepatic steatosis is an ongoing problem in total parenteral nutrition (TPN). The etiology of this deranged hepatic morphology is unclear, but lack of enteral stimulation and excess carbohydrate calories have been suggested as altering the intestinal hormone profile. In this study, adult male Sprague-Dawley rats (n = 28) received internal jugular catheters: group 1 (n = 7) received saline (3 cc/hr) and chow ad libitum; groups 2, 3, and 4 (n = 7) received 15, 20, and 25% dextrose-base TPN solutions, respectively, in quantities matching the caloric intake of paired ad libitum animals. At 7 days portal and peripheral venous blood samples were drawn for insulin and glucagon radioimmunoassay, and livers were removed for histologic examination and determination of total hepatic lipid content. Portal and peripheral insulin levels rose in a linear fashion with increasing dextrose concentration. Lipid content increased with elevated portal venous insulin/glucagon ratio in groups 3 and 4. Periportal fatty infiltration increased with increasing dextrose concentrations. The results suggest that the liver's response to altered portal insulin/glucagon ratio may play an important role in changes of hepatic morphology associated with TPN.

Insulin and glucagon response of the diabetic Chinese hamster in the Asahikawa colony

The relationship between pancreatic hormone content and pattern of hormone release has not been completely elucidated because of heterogeneity in diabetes. Accordingly, this study was performed to establish the relationship, using spontaneously diabetic Chinese hamsters in the Asahikawa colony, a newly discovered experimental model resembling insulin-deficient diabetes in humans. As a result of investigations of insulin and glucagon responses to glucose or arginine in vivo and in vitro using isolated islets obtained by the collagenase procedure, a decreased insulin response and paradoxical glucagon response to glucose, and an excessive glucagon response to arginine were found in the diabetic animals. While the yield of isolated islets tended to decrease, a decreased pancreatic insulin content and increased pancreatic glucagon content were found as the diabetic state advanced. It may be suggested, therefore, that the relationship between pancreatic hormone content and pattern of hormone release in diabetic animals in the Asahikawa colony is based on the disruption of islets, disruption or dysfunction of B-cells and hyperplasia or hypertrophy of A-cells by some cause genetically determined.

Plasma glucagon and catecholamines during exhaustive short-term exercise

Plasma glucagon and catecholamine levels were measured in male athletes before and after exhaustive 15 min continuous running and strenuous intermittent short-term exercise (3 X 300 m). Blood lactate levels were higher after the intermittent exercise (mean 16.7 mmol X 1(-1)) than after the continuous running (mean 7.1 mmol X 1(-1)). Plasma glucagon concentration increased during continuous running and intermittent exercise by 41% and 55%, respectively, and the increases in plasma noradrenaline concentration were 7.7- and 9.1-fold compared with the respective pre-exercise values. Immediately after the exercises plasma cyclic AMP, blood glucose and alanine levels were elevated significantly. The data suggest that the sympathoadrenal system is of major importance for liver glucose production during high-intensity exercises. Catecholamines directly stimulate liver glucose production and may indirectly stimulate it by enhancing the secretion of glucagon.

Effect of oral contraceptives on plasma glucose, insulin, and glucagon levels

Effects of oral contraceptive agents (mestranol and norethindrone) on carbohydrate metabolism were evaluated in a group of 18 healthy young women. Plasma glucose, insulin, and glucagon responses were evaluated after a glucose load (oral and intravenous) and an amino acid challenge (oral and intravenous). The oral glucose tolerance was normal and was unaltered by the use of oral contraceptive agents. However, following intravenous administration of glucose, plasma glucose levels were slightly but significantly elevated when subjects were using oral contraceptives. Plasma insulin concentrations were slightly but significantly higher than control values in response to oral and intravenous administration of glucose while subjects were using oral contraceptives. Plasma glucagon concentrations in response to oral and intravenous glucose were similar whether the subjects were using oral contraceptive agents or not. No significant differences from control values were observed after oral and intravenous amino acid challenges when subjects were using oral contraceptive agents. Mild elevations of glucose and insulin without any significant change in glucagon concentrations suggest that glucagon levels do not play a major role in the development of insulin resistance seen in some patients using oral contraceptive agents.

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.

Amino acid transport in isolated rat hepatocytes

Improvements in the collagenase perfusion techniques have made isolated rat hepatocytes a popular model in which to study hepatic function. Our knowledge of hepatic amino acid transport has been advanced as a result of this methodology. Translocation across the hepatocyte plasma membrane can, in some instances, represent the rate-limiting step in the overall metabolism of certain amino acids. Furthermore, regulation of amino acid uptake by hepatocytes appears to play a role in diabetes, and perhaps in malignant transformation. Comparisons between normal adult hepatocytes and several hepatoma cell lines show basic differences in amino acid transport. There are at least eight distinct systems in normal hepatocytes for transport of the hormones. Systems A and N exhibit enhanced uptake rates after the cells have been maintained in the absence of extracellular amino acids, a phenomenon termed adaptive control. Further studies using isolated hepatocytes will increase our basic understanding of membrane transport processes and their regulation.

Hyperglucagonemia and alpha-adrenergic receptor in acute hypoxia

The plasma immunoreactive glucagon (IRG) response to hypoxia was studied in puppies. Three groups of paired experiments were performed. In group I, 8% O2:92% N2 ventilation (PaO2 20--30 torr) produced a rise in plasma IRG and glucose as well as hypotension and bradycardia. However, when group I was air ventilated (PaO2 greater than 70 torr) and given glucose infusions producing hyperglycemia of similar degree, plasma IRG was unchanged. Group II received alpha-adrenergic blockade (phenoxybenzamine). When made hypoxic, group II developed no significant IRG rise and less hyperglycemia than with hypoxia alone. Hypotension was more severe with hypoxia plus alpha-blockade. Phenoxybenzamine itself did not change plasma IRG or glucose during air breathing. Group III receivi developed hyperglucagonemia and hyperglycemia not significantly different from that with hypoxia alone. However, hypoxia-caused hypotension and bradycardia was more pronounced with beta-blockade. No change in plasma IRG or glucose occurred in group III animals breathing air. These data suggest that a) glucagon release is caused by acute oxygen deficiency, and b) the hypoxic response is largely adrenergically mediated with the major role played by the alpha-receptor.

Pancreatic endocrine response to the first feed in term newborn infants

Blood glucose, insulin, C-peptide and glucagon were evaluated in 36 newborn term infants at birth, and before and 60 min after the first feed during the first day of life. Under basal conditions glycaemia diminished during the first day of life and glucagon increased, while insulin and C-peptide did not show any variation. The C-peptide: insulin molar ratio was higher in the newborn than in adults because of the longer half-life of C-peptide, probably due to reduced renal function in the neonatal period. The subjects were divided into two groups: 18 newborn infants were given a feed of commercially available milk powder reconstituted in water at 10% (5 ml/kg); the other 18 were given a feed of 5 ml/kg 10% glucose solution. In each group 6 were given the first feed after a fast of 6 h, 6 after a fast of 12 h and 6 after a fast of 24 h from birth. After the first feed with milk, the average increase of glycaemia was 19.83 mg%, of insulin 6.06 muU/ml, and of C-peptide 1.88 ng/ml. After the first feed with glucose the average increase of glycaemia was 13.59 mg%, of insulin 2.46 muU/ml, and of C-peptide 0.59 ng/ml. Glucagon did not show significant changes after the first feed.

Effects of oral phenylalanine load on plasma glucagon, insulin, amino acid and glucose concentrations in man

An oral phenylalanine load provokes a significant drop in serum tyrosine levels in children with phenylketonuria [8]. The aim of the present investigation was to examine the response of insulin and glucagon to oral phenylalanine loading as these hormones are known to have a hypoaminoacidaemic effect. Six adult normal weight and healthy men were loaded orally with 0.6 mmol L-phenylalanine per kg body weight after an overnight fast. Serum phenylalanine increased within 10 min after the load and reached a maximum concentration at 30 min. Serum tyrosine increased within 10 min after the load and reached a maximum concentration at 2 h. Plasma glucagon and insulin increased during the first 10 min after the load and reached a peak twice the fasting levels at 30 min after the load. The molar insulin/glucagon ratio remained unchanged during the first 20 min after the load but then declined by 50% at 2 h. Associated with this decline plasma amino acid concentration (except phenylalanine and tyrosine) declined by approximately 15%. The decline was most marked for isoleucine, leucine, methionine and valine. As the hypoaminoacidaemic effect of insulin and glucagon is known to be most marked for these four amino acids plus phenylalanine and tyrosine, the response of insulin and glucagon to a phenylalanine load may influence not only the fate of phenylalanine given but also the blood tyrosine level.

Effect of starvation on hepatic glycogen metabolism and glucose homeostasis

The effects of starvation on the hepatic glycogen synthase and phosphporylase systems were sequentially assessed in fed and 24-120-hr-fasted rats. Enzymic changes before and after glucose were correlated with simultaneous measurements of hepatic cyclic AMP and glycogen concentrations and glucose, insulin, and glucagon concentrations in the portal vein plasma. Fasting caused parallel changes in plasma glucose and hepatic glycogen concentrations with decreases by 24 hr and subsequent increases, which correlated with increases in hepatic synthase l and decreases in phosphorylase activites. Hepatic cyclic AMP levels increased as 24-48 hr, decreased below fed levels at 96 hr, and increased again at 120 hr. Fasting caused progressive impairment of glucose disposal, decreased basal and postglucose insulin concentrations, and decreased basal glucagon levels at 48-72 hr. Hepatic synthase l increments following glucose were exaggerated in 48-120-hr-fasted rats, although consistent phosphorylase decrements were seen only in fed rats. There was no clearcut relationship between synthase activation and phosphorylase inactivation following glucose in fed or fasted rats.

Gluconeogenesis in the suckling rat

Gluconeogenesis from lactate was increased more than twofold in perfused livers and isolated hepatocytes from 10- and 15-day-old suckling rats compared with adult rats and was not increased by fasting. Glucagon (10(-19)-10(-7) M) induced minimal activation of gluconeogenesis in hepatocytes from such animals even though it promoted accumulation of cAMP. Suckling rats had low plasma insulin levels and high plasma glucagon levels. The presented very low insulin/glucagon ratios were associated with high liver cAMP levels. Contrary to the situation in adult rats, glucagon levels fell with fasting. Insulin levels fell proportionally further so that the insulin/glucagon ratios declined. On the basis of these findings, it is proposed that the high glucagon and low insulin levels in young suckling rats result in high liver cAMP levels with resultant maximal stimulation of hepatic gluconeogenesis so that it cannot be further increased by fasting or exogenous glucagon. It is also suggested that milk is an important stimulus to the hyperglucagonemia of suckling rats in view of its high protein and low carbohydrate content.

Hyperglucagonemia and altered responsiveness of hepatic adenylate cyclase-adenosine 3',5'-monophosphate system to hormonal stimulation during chronic ingestion of DL-ethionine

Basal activity and hormonal responsiveness of the adenylate cyclase-adenosine 3',5'-monophosphate system were examined in premalignant liver from rats chronically fed the hepatic carcinogen DL-ethionine, and these data were correlated with endogenous levels of plasma glucagon. By 2 weeks basal hepatic cyclic AMP levels, determined in tissues quick-frozen in situ, were 2-fold higher in rats ingesting ethionine than in the pair-fed control. Enhanced tissue cyclic AM content was associated with an increase in the adenylate cyclase activity of whole homogenates of fresh liver from rats fed ethionine (68 +/- 5 pmol cyclic AMP/10 min per mg protein) compared to control (48 +/- 4). Cyclic AMP-dependent protein kinase activity ratios were also significantly higher (control, 0.38 +/- 0.04; ethionine 0.55 +/- 0.05) and the percent glycogen synthetase activity in the glucose 6-phosphate-independent form was markedly reduced (control, 52 +/- 7%; ethionine, 15 +/- 1.5%) in the livers of ethionine-fed rats compared to the controls, suggesting that the high total hepatic cyclic AMP which accompanied ethionine ingestion was bilogically effective. These changes persisted throughout the 38 weeks of drug ingestion. Immunoreactive glucagon levels, determined in portal venous plasma, were 8-fold higher than control after 2 weeks of the ethionine diet (control, 185 +/- 24 pg/ml; ethionine, 1532 +/- 195). Analogous to the changes in hepatic parameters, plasma glucagon levels remained elevated during the entire period of drug ingestion until the development of hepatomas. The hepatic cyclic AMP response to a maximal stimulatory dose of injected glucagon was blunted in vivo in ethionine-fed rats (control, 14 -fold increase over basal, to 8.63 +/- 1.1 pmol/mg wet weight; ethionine, 4.6-fold rise over basal, to 5.42 +/- 0.9). Reduced cyclic AMP responses to both maximal and submaximal glucagon stimulation were also evident in vitro in hepatic slices prepared from rats fed the drug, and the reduction was specific to glucagon. Absolute or relative hepatic cyclic AMP responses to maximally effective concentrations of protaglandin E1 or isoproterenol in hepatic slices from ethionine-fed rats were greater than or equal to those observed in control slices. Parallel alterations in hormonal responsiveness were observed in adenylate cyclase activity of whole homogenates of these livers, implying that the changes in cyclic AMP accumulation following hormone stimulation were related to an alteration in cyclic AMP generation in the premalignant tissue. In view of the recognized hepatic actions of glucagon and the desensitization of adenylate cyclase which can occur during sustained stimulation of the liver with this hormone, the endogenous hyperglucagonemia that accompanies ethionine ingestion could play a role in the pathogenesis of both the basal alterations in hepatic cyclic AMP metabolism and the reduced responsiveness to glucagon observed in liver from rats fed this carcinogen.

Suppressibility of glucagon secretion by glucose in juvenile diabetes

The suppressibility of plasma glucagon concentrations by glucose was investigated in normal and diabetic children. Fasting concentrations of plasma glucagon were similar in normal and in diabetic children despite the hyperglycemia of the latter. Infusion of glucose promptly suppressed glucagon values in the normal as well as in the diabetic children pretreated with half of their usual morning dose of insulin. Glucose alone, however, did not suppress plasma glucagon in diabetic patients, despite the attainment of significant hyperglycemia. Administration of insulin during an ongoing glucose infusion in the diabetic patients lowered their blood glucose concentration; the concentration of glucagon rose transiently when the glucose concentration fell. These data confirm the existence of relative hyperglucagonemia inappropriate for the degree of blood glucose concentration in diabetic children deprived of insulin. The data also suggest that this hyperglucagonemia is secondary to insulin deficiency and suppressibility of glucagon by glucose can be restored by insulin therapy.

Hormonal regulation of hepatic amino acid transport

The transport of 2-aminoisobutyric acid (AIB) into liver tissue was increased by both insulin and glucagon. We have now shown that these hormones do not stimulate the same transport system. Glucagon, possibly via cAMP, increased the hepatic uptake of AIB by a mechanism which resembled system A. This glucagon-sensitive system could be monitored by the use of the model amino acid MeAIB. In contrast, the insulin-stimulated system exhibited little or no affinity for MeAIB and will be referred to as system B. On the basis of other reports that the hepatic transport of AIB is almost entirely Na+ dependent and the present finding that the uptake of 2-aminobicyclo [2,2]heptane-2-carboxylic acid (BCH) was not stimulated by either hormone, we conclude that system B is Na+ dependent. Furthermore, insulin added to the perfusate of livers from glucagon-pretreated donors suppressed the increase in AIB or MeAIB uptake. Depending upon the specificities of systems A and B, both of which are unknown for liver tissue, the insulin/glucagon ratio may alter the composition of the intracellular pool of amino acids.

Influence of physiologic hyperglucagonemia on basal and insulin-inhibited splanchnic glucose output in normal man

To evaluate the effects of physiologic hyperglucagonemia on splanchnic glucose output, glucagon was infused in a dose of 3 ng/kg per min to healthy subjects in the basal state and after splanchnic glucose output had been inhibited by an infusion of glucose (2 mg/kg per min). In the basal state, infusion of glucagon causing a 309 +/- 25 pg/ml rise in plasma concentration was accompanied by a rapid increase in splanchnic glucose output to values two to three times basal by 7-15 min. The rise in arterial blood glucose (0.5-1.5 mM) correlated directly with the increment in splanchnic glucose output. Despite continued glucagon infusion, and in the face of stable insulin levels, splanchnic glucose output declined after 22 min, returning to basal levels by 30-45 min. In the subjects initially receiving the glucose infusion, arterial insulin concentration rose by 5-12 muU/ml, while splanchnic glucose output fell by 85-100%. Infusion of glucagon causing an increment in plasma glucagon concentration of 272 +/- 30 pg/ml reversed the inhibition in splanchnic glucose production within 5 min. Splanchnic glucose output reached a peak increment 60% above basal levels at 10 min, and subsequently declined to levels 20-25% below basal at 30-45 min. These findings provide direct evidence that physiologic increments in plasma glucagon stimulate splanchnic glucose output in the basal state and reverse insulin-mediated inhibition of splanchnic glucose production in normal man. The transient nature of the stimulatory effect of glucagon on splanchnic glucose output suggests the rapid development of inhibition or reversal of glucagon action. This inhibition does not appear to depend on increased insulin secretio.

Studies of hormonal regulation of metabolism using isolated hepatocytes

A number of enzymatic methods have been developed to prepare hepatocytes using collagenase and hyaluronidase. However, best cell preparations are obtained by using only low concentrations of collagenase and exposing the liver to the enzyme for a very short period of time. These isolated cells with intact cell membranes and large numbers of microvilli on the cell surface respond to hormones at physiological concentrations suggesting that these microvilli contain hormone receptors. In addition, high glycogen content is essential to maintain the in vivo metabolic characteristics of the hepatocytes suggesting that intracellular glycogen plays an important role in the hormonal regulation of metabolism in hepatocytes. Studies with glucagon and insulin on carbohydrate metabolism show that the molar ratios of these hormones control gluconeogenesis and glycogenolysis. Furthermore, in vitro addition of insulin stimulates glycogen synthesis and activates glycogen synthase. Insulin also stimulates protein synthesis in cells containing high glycogen and maintains more normal parallel strands of polyribosomes. Studies with isolated hepatocytes from diabetic, hypophysectomized and adrenalectomized animals show a reduced glucagon response to glycogenolysis. This lack of glucagon response was not due to reduction in glycogen levels. Other hormones such as somatostatin and parathyroid also give rise to alterations in carbohydrate metabolism in isolated hepatocytes.

[Metabolic differences between males and females and between normal and obese subjects during total fast]

In 24 normal and 24 obese subjects of both sexes circulating substrates (blood sugar, free fatty acids, ketone bodies) and hormones (insulin, growth hormone, pancreatic glucagon) were determined during 6 days of total fast. In normals the blood sugar fell to lower levels than in the obese. Plasma free fatty acids and ketone concentrations rose faster in normal than in obese subjects, and faster in females than in males. Plasma insulin concentrations declined to a greater extent in obese than in normal subjects. In all groups studied a significant increase of the pancreatic glucagon level within 1-3 days of fasting was observed, however, its rise occurred faster in normal females than in males. Growth hormone (GH) rose significantly in normal males but not in obese males. Following high overnight fasting values in some normal females showed no significant increase in GH levels but significantly higher GH values than obese females after 1-6 days of fasting. After summarizing starvation-induced metabolic changes common to all study groups the respective differences found between males and females and between normal and obese subjects are discussed.

Insulin and the glucose-glucagon feedback mechanism in the duck

The relationship between insulin and the glucose-glucagon feedback mechanism was studied by testing the effectiveness of various routes, doses and timing of insulin administration prior to and during a glucose tolerance test in Peking ducks made transiently diabetic by subtotal pancreatectomy. Insulin injections or infusions given either before, or only during the glucose load, did not restore the A-cell response to glucose. Yet, if given both before and during the glucose test, in conditions which mimic the physiological basal insulin level and its variations (with, initially, intramuscular injections of 0.2 IU/kg and 8 mug/kg glucagon, every six hours, and then an intravenous injection of 3.6 mU/kg plus an infusion of 0.9 mU/kg/minute for one hour), the normal glucagon response to glucose was re-established. Insulin must therefore be present, both before and during glucose stimulation, for glucose to be effective as an A-cell suppressor.

Glucagon responses to hypoglycemia in adrenalectomized man

In order to examine the possibility that epinephrine is involved in either the mediation or modulation of the enhanced glucagon release during glucopenia, glucagon responses to insulin-induced hypoglycemia were evaluated in four men with bilateral adrenalectomy in comparison with ten normal men. In the adrenalectomized patients, the mean nadir of plasma glucose and the rate of ascent to baseline levels were indistinguishable from those observed in normal subjects. Similarly, glucagon responses in the adrenalectomized group were not different from those encountered in the normal volunteers. We conclude that epinephrine does not contribute significantly to the augmented glucagon release during abrupt glucopenia in normal man and is not necessary for normal recovery from hypoglycemia.

Effects of reduced ATP concent on hepatic responses to glucagon

Expression of the glycogenolytic action of glucagon in liver requires ATP for cAMP formation and for several subsequent phosphorylation reactions. To assess the extent to which ATP availability is rate-limiting to this hormonal action, responses to glucagon of intact liver and of liver with marked reductions in ATP content induced by ethionine was examined in female Wistar rats in vivo and in vitro. Compared to values in quick-frozen liver samples from control rats, basal hepatic ATP was 75% lower and cAMP, two fold higher in rats treated with ethionine. Activation of glycogen phosphorylase and inactivation of glycogen synthetase, phosphorylation reactions which require ATP and are initiated by cAMP, were also evident in basal liver samples from ethionine-treated rats. These hepatic alterations were associated with portal glucose and insulin levels which were significantly lower and portal glucagon levels which were four fold higher than values in controls. In ethionine-treated rats, glucose infusion decreased hepatic cAMP content and phosphorylase activity and increased synthetase activity. This and other observation suggested that the higher cAMP and the altered enzyme activities seen in vivo after ethionine administration were mediated by the hyperglucagonemia and/or by other endogenous glycogenolytic stimuli, and accordingly implied that liver remained responsive to such stimuli despite reduced ATP. Pharmacologic doses of exogenous glucagon clearly increased cAMP in vivo and in vitro in livers with decreased ATP. However, the lower ATP of liver exposed to ethionine was associated with a significantly blunted cAMP response to maximal glucagon stimulation. By contrast, alterations in phosphorylase and synthetase activities were not similarly blunted, suggesting that the smaller increases in cAMP seen in liver with reduced ATP content were adequate for the expression of these actions of the hormone. It is concluded that the actions of glucagon to increase cAMP and to activate phosphorylase and inactivate synthetase are not abolished by marked reductions in hepatic APT.

Role of carnitine in hepatic ketogenesis

The enhancement of long-chain fatty acid oxidation and ketogenesis in the perfused rat liver, whether induced acutely by treatment of fed animals with anti-insulin serum or glucagon, or over the longer term by starvation or the induction of alloxan diabetes, was found to ba accompanied by a proportional elevation in the tissue carnitine content. Moreover, when added to the medium perfusing livers from fed rats, carnitine stimulated ketogenesis from oleic acid. The findings suggest that the increased fatty acid flux through the carnitine acyltransferase (carnitine palmitoyl-transferase; palmitoyl-CoA:L-carnitine O-palmitoyltransferase; EC 2.3.1.21) reaction brought about by glucagon excess, with or without insulin deficiency, is mediated, at least in part, by elevation in the liver carnitine concentration.

A morphological basis for intercellular communication between alpha- and beta-cells in the endocrine pancreas

By degranulating beta-cells in the islets of Langerhans of the rat with sulfonylurea, it has been possible to distinguish unambiguously alpha-cells from beta-cells in freeze-fracture replicas. In such preparations, we found morphologically typical tight and gap junctions occurring between alpha- and beta-cells. The presence of gap junctions offers indirect evidence that these cells are coupled with one another; coupling may influence the secretory behavior of alpha- and beta-cells maintaining glucose homeostasis within tightly constricted limits.

A combined radioimmunoassay for glucagon and insulin

A combined radioimmunoassay for glucagon and insulin in biological fluids is presented. It is based on the use of 131I-glucagon and 125I-insulin tracers and a charcoal-dextran separation procedure. Standard curves, sample determinations and recovery studies gave comparable results whether in the combined or individual assay for glucagon and insulin. The combined assay, especially if supported by a decoding and calculating computer program, offers the advantages that it requires a smaller volume of the material to be sampled, is more economical and less time-consuming.

Serum and pancreatic immunoreactive insulin (IRI) and proinsulin-like component (PLC), serum IRI and PLC response to different stimuli in normal subjects and organic hyperinsulinism

The serum levels of total immunoreactive insulin (IRI) and proinsulin-like component (PLC) in the fasting state and following the administration of insulin secretagogues in 5 patients with organic hyperinsulinism and age and sex matched normal subjects are reported. Diagnosis of organic hyperinsulinism could be established in all instances on the basis of the inappropriately high total serum IRI levels for the corresponding blood glucose values; such an abnormal relationship was not seen in normal subjects, and was further enhanced by insulin secretagogues. Unrestrained insulin secretion in organic hyperinsulinism was enhanced following the administration of glucose, tolbutamide, glucagon or amino acids; the last 2 stimuli are known to be ineffective in causing insulin secretion in the presence of hypoglycemia in normal subjects. Four patints had insulinomas and one probably had islet cell hyperplasia or abnormal function of islet cells. Chromatography of serum IRI to quantitate PLC is a useful adjunct to the diagnosis of organic hyperinsulinism as in the fasting state the proportion of PLC is always elevated, above the normal range of 5-22%. Following the administration of insulin secretagogues there was pronounced increase in total serum IRI in organic hyperinsulinism but the proportion of PLC generally decreased, suggesting thereby that mojor increase in IRI was due to release of stored granular IRI which is known to have a low proportion of PLC.

Hormonal control of ketogenesis. Rapid activation of hepatic ketogenic capacity in fed rats by anti-insulin serum and glucagon

The enhanced capacity for long-chain fatty acid oxidation and ketogenesis that develops in the rat liver between 6 and 9 h after the onset of starvation was shown to be inducible much more rapidly by administration of anti-insulin serum or glucagon to fed rats. After only 1 h of treatment with either agent, the liver had clearly switched from a "nonketogenic" to a "ketogenic" profile, as determined by rates of acetoacetate and b-hydroxybutyrate production on perfusion with oleic acid. As was the case after starvation, the administration of insulin antibodies or glucagon resulted in depletion of hepatic glycogen stores and a proportional increase in the ability of the liver to oxidize long-chain fatty acids and (-)-octanoylcarnitine, suggesting that all three treatment schedules activated the carnitine acyltransferase system of enzymes. In contrast to anti-insulin serum, which produced marked elevations in plasma glucose, free fatty acid, and ketone body concentrations, glucagon treatment had little effect on any of these parameters, presumably due to enhanced insulin secretion after the initial stimulation of glycogenolysis. Thus, after treatment with glucagon alone, it was possible to obtain a "ketogenic" liver from a nonketotic animal. The results are consistent with the possibility that the activity of carnitine acyltransferase, and thus ketogenic capacity, is subject to bihormonal control through the relative blood concentrations of insulin and glucagon, as also appears to be the case with hepatic carbohydrate metabolism.