Effect of amino acids and proteins on insulin secretion in man

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

The Glycemic Index and Human Health with an Emphasis on Potatoes

Diabetes and obesity are associated with the excessive intake of high-glycemic index (GI) carbohydrates, increased glycemic load (GL) foods, and inactive lifestyles. Carbohydrate-rich diets affect blood glucose levels. GI is an indicator of the impact of a specific food on blood glucose, while GL represents the quantity and quality of carbohydrates in the overall diet and their interactions. There are in vitro and in vivo methods for estimating GI and GL. These values are useful human health markers for conditions such as diabetes, obesity, and pregnancy. Potato is a major starchy vegetable, which is consumed widely and is the fourth most important crop globally. However, the GI of diets rich in starchy vegetables such as potatoes has not been studied in detail. The GI values in potatoes are affected by external and internal factors, such as methods of cooking, methods of processing, resistant starches, cultivation methods, mixed meals and food additions, and hormone levels. This review summarizes how these factors affect the GI and GL associated with diets containing potatoes. Understanding the impacts of these factors will contribute to the development of new and improved potato varieties with low GI values. The consumption of low-GI foods will help to combat obesity. The development of low-GI potatoes may contribute to the development of meal plans for individuals living with diabetes and obesity.

Arginine and Endothelial Function

Arginine (L-arginine), is an amino acid involved in a number of biological processes, including the biosynthesis of proteins, host immune response, urea cycle, and nitric oxide production. In this systematic review, we focus on the functional role of arginine in the regulation of endothelial function and vascular tone. Both clinical and preclinical studies are examined, analyzing the effects of arginine supplementation in hypertension, ischemic heart disease, aging, peripheral artery disease, and diabetes mellitus.

Effect of nutrient composition in a mixed meal on the postprandial glycemic response in healthy people: a preliminary study

BACKGROUND/OBJECTIVES

The glycemic index (GI) is a measure of the postprandial glucose response (PPGR) to food items, and glycemic load (GL) is a measure of the PPGR to the diet. For those who need to maintain a healthy diet, it is beneficial to regulate appropriate levels of blood glucose. In reality, what influences the meal GI or GL depends on the macronutrient composition and the physical chemistry reactions in vivo. Thus, we investigated whether different macronutrients in a meal significantly affect the PPGR and the validity of calculated GI and GL values for mixed meals.

SUBJECTS/METHODS

12 healthy subjects (6 male, 6 female) were recruited at a campus setting, and subjects consumed a total of 6 test meals one by one, each morning between 8:00 and 8:30 am after 12 h of fasting. PPGR was measured after each consumed meal and serial finger pricks were performed at indicated times. Test meals included 1) 68 g oral glucose, 2) 210 g rice, 3) rice plus 170 g egg white (RE), 4) rice plus 200 g bean sprouts (RS), 5) rice plus 10 g oil (RO), and 6) rice plus, egg white, bean sprouts, and oil (RESO). The incremental area under the curve (iAUC) was calculated to assess the PPGR. Mixed meal GI and GL values were calculated based on the nutrients the subjects consumed in each of the test meals.

RESULTS

The iAUC for all meals containing two macronutrients (RS, RO, or RE) were not significantly different from the rice iAUC, whereas, the RESO iAUC (2,237.5 ± 264.9) was significantly lower (P < 0.05). The RESO meal's calculated GI and GL values were different from the actual GI and GL values measured from the study subjects (P < 0.05).

CONCLUSIONS

The mixed meal containing three macronutrients (RESO) decreased the PPGR in healthy individuals, leading to significantly lower actual GI and GL values than those derived by nutrient-based calculations. Thus, consuming various macronutrient containing meals is beneficial in regulating PPGR.

Patterns of postmeal insulin secretion in individuals with sulfonylurea-treated KCNJ11 neonatal diabetes show predominance of non-KATP-channel pathways

OBJECTIVE

Insulin secretion in sulfonylurea-treated KCNJ11 permanent neonatal diabetes mellitus (PNDM) is thought to be mediated predominantly through amplifying non-K_ATP-channel pathways such as incretins. Affected individuals report symptoms of postprandial hypoglycemia after eating protein/fat-rich foods. We aimed to assess the physiological response to carbohydrate and protein/fat in people with sulfonylurea-treated KCNJ11 PNDM.

RESEARCH DESIGN AND METHODS

5 adults with sulfonylurea-treated KCNJ11 PNDM and five age, sex and body mass index-matched controls without diabetes had a high-carbohydrate and high-protein/fat meal on two separate mornings. Insulin(i) and glucose(g) were measured at baseline then regularly over 4 hours after the meal. Total area under the curve (tAUC) for insulin and glucose was calculated over 4 hours and compared between meals in controls and KCNJ11 cases.

RESULTS

In controls, glucose values after carbohydrate and protein/fat were similar (median glucose tAUC_0-4h21.4 vs 19.7 mmol/L, p=0.08). In KCNJ11 cases glucose levels were higher after carbohydrate than after protein/fat (median glucose tAUC_0-4h58.1 vs 31.3 mmol/L, p=0.04). These different glycemic responses reflected different patterns of insulin secretion: in controls, insulin secretion was greatly increased after carbohydrate versus protein/fat (median insulin tAUC_0-4h727 vs 335 pmol/L, p=0.04), but in KCNJ11 cases insulin secretion was similar after carbohydrate and protein/fat (median insulin tAUC_0-4h327 vs 378 pmol/L, p=0.50).

CONCLUSIONS

Individuals with sulfonylurea-treated KCNJ11 PNDM produce similar levels of insulin in response to both carbohydrate and protein/fat meals despite carbohydrate resulting in much higher glucose levels and protein/fat resulting in relatively low glucose levels. This suggests in an inability to modulate insulin secretion in response to glucose levels, consistent with a dependence on non-K_ATP pathways for insulin secretion.

TRIAL REGISTRATION NUMBER

NCT02921906.

Use of Ex Vivo Normothermic Perfusion for Quality Assessment of Discarded Human Donor Pancreases

A significant number of pancreases procured for transplantation are deemed unsuitable due to concerns about graft quality and the associated risk of complications. However, this decision is subjective and some declined grafts may be suitable for transplantation. Ex vivo normothermic perfusion (EVNP) prior to transplantation may allow a more objective assessment of graft quality and reduce discard rates. We report ex vivo normothermic perfusion of human pancreases procured but declined for transplantation, with ABO-compatible warm oxygenated packed red blood cells for 1-2 h. Five declined human pancreases were assessed using this technique after a median cold ischemia time of 13 h 19 min. One pancreas, with cold ischemia over 30 h, did not appear viable and was excluded. In the remaining pancreases, blood flow and pH were maintained throughout perfusion. Insulin secretion was observed in all four pancreases, but was lowest in an older donation after cardiac death pancreas. Amylase levels were highest in a gland with significant fat infiltration. This is the first study to assess the perfusion, injury, as measured by amylase, and exocrine function of human pancreases using EVNP and demonstrates the feasibility of the approach, although further refinements are required.

Mitochondrial regulation of β-cell function: maintaining the momentum for insulin release

All forms of diabetes share the common etiology of insufficient pancreatic β-cell function to meet peripheral insulin demand. In pancreatic β-cells, mitochondria serve to integrate the metabolism of exogenous nutrients into energy output, which ultimately leads to insulin release. As such, mitochondrial dysfunction underlies β-cell failure and the development of diabetes. Mitochondrial regulation of β-cell function occurs through many diverse pathways, including metabolic coupling, generation of reactive oxygen species, maintenance of mitochondrial mass, and through interaction with other cellular organelles. In this chapter, we will focus on the importance of enzymatic regulators of mitochondrial fuel metabolism and control of mitochondrial mass to pancreatic β-cell function, describing how defects in these pathways ultimately lead to diabetes. Furthermore, we will examine the factors responsible for mitochondrial biogenesis and degradation and their roles in the balance of mitochondrial mass in β-cells. Clarifying the causes of β-cell mitochondrial dysfunction may inform new approaches to treat the underlying etiologies of diabetes.

Interaction of ingested leucine with glycine on insulin and glucose concentrations

The majority of individual amino acids increase insulin and attenuate the plasma glucose response when ingested with glucose. Objective. To determine whether ingestion of two amino acids simultaneously, with glucose, would result in an additive effect. Leucine (Leu) and glycine (Gly) were chosen because they were two of the most potent glucose-lowering amino acids when given individually. Materials and Methods. Nine subjects received test items on four separate days. The first was a water control, then 25 g glucose, or Leu + Gly (1 mmol/kg fat-free mass each) ±25 g glucose, in random order. Glucose, insulin, and glucagon were measured frequently for 2.5 hours. Net areas were calculated. Results. The glucose area response decreased by 66%. The insulin area response increased by 24% after ingestion of Leu + Gly + glucose compared to ingestion of glucose alone. The decrease in glucose response was not additive; the increase in insulin response was far less than additive when compared to previously published individual amino acid results. The glucagon concentration remained unchanged. Conclusion. There is an interaction between Leu and Gly that results in a markedly attenuated glucose response. This occurred with a very modest increase in insulin response. Changes in glucagon response could not explain the results. The mechanism is unknown.

Ingestion of leucine + phenylalanine with glucose produces an additive effect on serum insulin but less than additive effect on plasma glucose

Most individual amino acids stimulate insulin secretion and attenuate the plasma glucose response when ingested with glucose. We determined whether ingestion of two amino acids simultaneously with glucose would result in an additive effect on the glucose area response compared with ingestion of amino acids individually. Leucine and phenylalanine were chosen because they were two of the most potent glucose-lowering amino acids when given individually. Eight healthy subjects were studied on four separate days. Test meals were given at 0800. The first meal was a water control. Subjects then received 25 g glucose or leucine + phenylalanine (1 mmol/kg fat free body mass each) ±25 g glucose in random order. Glucose, insulin and glucagon were measured frequently for 2.5 hours thereafter. Net areas under the curves were calculated using the mean fasting value as baseline. The insulin response to leucine + phenylalanine was additive. In contrast, the decrease in glucose response to leucine + phenylalanine + glucose was less than additive compared to the individual amino acids ingested with glucose. Interestingly, the insulin response to the combination was largely due to the leucine component, whereas the glucose response was largely due to the phenylalanine component. Glucose was unchanged when leucine or phenylalanine, alone or in combination, was ingested without glucose. This trial is registered with ClinicalTrials.gov NCT01471509.

The antihypertensive effect of arginine

Hypertension is a leading cause of morbidity and mortality worldwide. Individuals with hypertension are at increased risk of stroke, heart disease and kidney failure. Although the etiology of essential hypertension has a genetic component, lifestyle factors such as diet play an important role. Reducing dietary salt is effective in lowering blood pressure in salt-sensitive individuals. Insulin resistance and altered glucose metabolism are common features of hypertension in humans and animal models, with or without salt sensitivity. Altered glucose metabolism leads to increased formation of advanced glycation end products. Insulin resistance is also linked to oxidative stress, and alterations in the nitric oxide pathway and renin angiotensin system. A diet rich in protein containing the semiessential amino acid, arginine, and arginine treatment, lowers blood pressure in humans and in animal models. This may be due to the ability of arginine to improve insulin resistance, decrease advanced glycation end products formation, increase nitric oxide, and decrease levels of angiotensin II and oxidative stress, with improved endothelial cell function and decreased peripheral vascular resistance. The Dietary Approaches to Stop Hypertension (DASH) study demonstrated that the DASH diet, rich in vegetables, fruits and low-fat dairy products; low in fat; and including whole grains, poultry, fish and nuts, lowered blood pressures even more than a typical North American diet with similar reduced sodium content. The DASH diet is rich in protein; the blood pressure-lowering effect of the DASH diet may be due to its higher arginine-containing protein, higher antioxidants and low salt content.

The antihypertensive effect of cysteine

Hypertension is a leading cause of morbidity and mortality worldwide. Individuals with hypertension are at an increased risk for stroke, heart disease and kidney failure. Essential hypertension results from a combination of genetic and lifestyle factors. One such lifestyle factor is diet, and its role in the control of blood pressure has come under much scrutiny. Just as increased salt and sugar are known to elevate blood pressure, other dietary factors may have antihypertensive effects. Studies including the Optimal Macronutrient Intake to Prevent Heart Disease (OmniHeart) study, Multiple Risk Factor Intervention Trial (MRFIT), International Study of Salt and Blood Pressure (INTERSALT) and Dietary Approaches to Stop Hypertension (DASH) study have demonstrated an inverse relationship between dietary protein and blood pressure. One component of dietary protein that may partially account for its antihypertensive effect is the nonessential amino acid cysteine. Studies in hypertensive humans and animal models of hypertension have shown that N-acetylcysteine, a stable cysteine analogue, lowers blood pressure, which substantiates this idea. Cysteine may exert its antihypertensive effects directly or through its storage form, glutathione, by decreasing oxidative stress, improving insulin resistance and glucose metabolism, lowering advanced glycation end products, and modulating levels of nitric oxide and other vasoactive molecules. Therefore, adopting a balanced diet containing cysteine-rich proteins may be a beneficial lifestyle choice for individuals with hypertension. An example of such a diet is the DASH diet, which is low in salt and saturated fat; includes whole grains, poultry, fish and nuts; and is rich in vegetables, fruits and low-fat dairy products.

Antihypertensive effects of dietary protein and its mechanism

Hypertension is a leading cause of morbidity and mortality worldwide. Individuals with hypertension are at increased risk of stroke, heart disease and kidney failure. Both genetic and lifestyle factors, particularly diet, have been attributed an important role in the development of hypertension. Reducing dietary sugar and salt intake can help lower blood pressure; similarly, adequate protein intake may also attenuate hypertension. Observational, cross-sectional and longitudinal epidemiological studies, and controlled clinical trials, have documented significant inverse associations between protein intake and blood pressure. Human and animal studies have shown that specific amino acids within proteins may have antihypertensive effects. Cysteine, glutathione (a tripeptide), glutamate and arginine attenuate and prevent alterations that cause hypertension including insulin resistance, decreased nitric oxide bioavailability, altered renin angiotensin system function, increased oxidative stress and formation of advanced glycation end products. Leucine increases protein synthesis in skeletal muscle and improves insulin resistance by modulating hepatic gluconeogenesis. Taurine and tryptophan attenuate sympathetic nervous system activity. Soy protein helps lower blood pressure through its high arginine content and antioxidant activity exhibited by isoflavones. A diet containing an ample amount of protein may be a beneficial lifestyle choice for individuals with hypertension; one example is the Dietary Approaches to Stop Hypertension (DASH) diet, which is low in salt and saturated fat; includes whole grains, lean meat, poultry, fish and nuts; and is rich in vegetables, fruits and low-fat dairy products, which are good sources of antioxidant vitamins, minerals and fibre. Including an adequate supply of soy in the diet should also be encouraged.

The insulinogenic effect of whey protein is partially mediated by a direct effect of amino acids and GIP on β-cells

BACKGROUND

Whey protein increases postprandial serum insulin levels. This has been associated with increased serum levels of leucine, isoleucine, valine, lysine, threonine and the incretin hormone glucose-dependent insulinotropic polypeptide (GIP). We have examined the effects of these putative mediators of whey's action on insulin secretion from isolated mouse Langerhans islets.

METHODS

Mouse pancreatic islets were incubated with serum drawn from healthy individuals after ingestion of carbohydrate equivalent meals of whey protein (whey serum), or white wheat bread (control serum). In addition the effect of individual amino acid combinations on insulin secretion was also tested. Furthermore, the stimulatory effects of whey serum on insulin secretion was tested in vitro in the absence and presence of a GIP receptor antagonist ((Pro(3))GIP[mPEG]).

RESULTS

Postprandial amino acids, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) responses were higher after whey compared to white wheat bread. A stimulatory effect on insulin release from isolated islets was observed with serum after whey obtained at 15 min (+87%, P < 0.05) and 30 min (+139%, P < 0.05) postprandially, compared with control serum. The combination of isoleucine, leucine, valine, lysine and threonine exerted strong stimulatory effect on insulin secretion (+270%, P < 0.05), which was further augmented by GIP (+558% compared to that produced by glucose, P < 0.05). The stimulatory action of whey on insulin secretion was reduced by the GIP-receptor antagonist (Pro(3))GIP[mPEG]) at both 15 and 30 min (-56% and -59%, P < 0.05).

CONCLUSIONS

Compared with white wheat bread meal, whey causes an increase of postprandial insulin, plasma amino acids, GIP and GLP-1 responses. The in vitro data suggest that whey protein exerts its insulinogenic effect by preferential elevation of the plasma concentrations of certain amino acids, GIP and GLP-1.

Control of blood glucose in type 2 diabetes without weight loss by modification of diet composition

Background

Over the past several years our research group has taken a systematic, comprehensive approach to determining the effects on body function (hormonal and non-hormonal) of varying the amounts and types of proteins, carbohydrates and fats in the diet. We have been particularly interested in the dietary management of type 2 diabetes. Our objective has been to develop a diet for people with type 2 diabetes that does not require weight loss, oral agents, or insulin, but that still controls the blood glucose concentration. Our overall goal is to enable the person with type 2 diabetes to control their blood glucose by adjustment in the composition rather than the amount of food in their diet.

Methods

This paper is a brief summary and review of our recent diet-related research, and the rationale used in the development of diets that potentially are useful in the treatment of diabetes.

Results

We determined that, of the carbohydrates present in the diet, absorbed glucose is largely responsible for the food-induced increase in blood glucose concentration. We also determined that dietary protein increases insulin secretion and lowers blood glucose. Fat does not significantly affect blood glucose, but can affect insulin secretion and modify the absorption of carbohydrates. Based on these data, we tested the efficacy of diets with various protein:carbohydrate:fat ratios for 5 weeks on blood glucose control in people with untreated type 2 diabetes. The results were compared to those obtained in the same subjects after 5 weeks on a control diet with a protein:carbohydrate:fat ratio of 15:55:30. A 30:40:30 ratio diet resulted in a moderate but significant decrease in 24-hour integrated glucose area and % total glycohemoglobin (%tGHb). A 30:20:50 ratio diet resulted in a 38% decrease in 24-hour glucose area, a reduction in fasting glucose to near normal and a decrease in %tGHb from 9.8% to 7.6%. The response to a 30:30:40 ratio diet was similar.

Conclusion

Altering the diet composition could be a patient-empowering method of improving the hyperglycemia of type 2 diabetes without weight loss or pharmacologic intervention.

ATP-sensitive potassium channelopathies: focus on insulin secretion

ATP-sensitive potassium (K(ATP)) channels, so named because they are inhibited by intracellular (ATP), play key physiological roles in many tissues. In pancreatic beta cells, these channels regulate glucose-dependent insulin secretion and serve as the target for sulfonylurea drugs used to treat type 2 diabetes. This review focuses on insulin secretory disorders, such as congenital hyperinsulinemia and neonatal diabetes, that result from mutations in K(ATP) channel genes. It also considers the extent to which defective regulation of K(ATP) channel activity contributes to the etiology of type 2 diabetes.

Changes in the plasma aminogram of parenterally fed infants treated with dexamethasone for bronchopulmonary dysplasia

A substantial increase in the plasma concentration of most amino acids was observed in 59 preterm infants with chronic lung disease soon after the initiation of dexamethasone therapy. The size of increase appeared to be dose related. This phenomenon is likely to be the result of steroid induced protein catabolism. Interestingly, neither phenylalanine nor tyrosine concentrations were significantly increased.

Key role of L-alanine in the control of hepatic protein synthesis

We investigated the effects of administration of single amino acids to starved rats on the regulation of protein synthesis in the liver. Of all the amino acids tested, only alanine, ornithine and proline promoted statistically significant increases in the extent of hepatic polyribosome aggregation. The most effective of these was alanine, whose effect of promoting polyribosomal aggregation was accompanied by a decrease in the polypeptide-chain elongation time. The following observations indicate that alanine plays an important physiological role in the regulation of hepatic protein synthesis. Alanine was the amino acid showing the largest decrease in hepatic content in the transition from high (fed) to low (starved) rates of protein synthesis. The administration of glucose or pyruvate is also effective in increasing liver protein synthesis in starved rats, and their effects were accompanied by an increased hepatic alanine content. An increase in hepatic ornithine content does not lead to an increased protein synthesis, unless it is accompanied by an increase of alanine. The effect of alanine is observed either in vivo, in rats pretreated with cycloserine to prevent its transamination, or in isolated liver cells under conditions in which its metabolic transformation is fully impeded.

The glycaemic index of foods tested in diabetic patients: a new basis for carbohydrate exchange favouring the use of legumes

Recently diabetic patients have been encouraged to increase their carbohydrate intake, but exact details of which foods to use are lacking. To determine whether sufficiently large differences existed to justify more specific dietary advice, we compared the glycaemic responses to 50 g carbohydrate portions of different foods, taken as breakfast test meals by groups of five to seven diabetic patients. Two- to threefold differences were seen amongst the 15 foods tested. The glycaemic responses for spaghetti, 'All-bran', rice and beans were significantly below those for bread, while 'Cornflakes' were above. Factors predicted to influence this were without effect, including: substituting wholemeal for white bread, increasing substantially the simple sugars (using 'All-bran' or bananas instead of wholemeal bread) and doubling meal protein by adding cottage cheese to bread. Paired comparisons of the glycaemic response to the five legumes with those of the seven other starchy foods (breads, spaghetti, rice, Cornflakes, oatmeal porridge and potatoes) showed that the mean peak rise in blood glucose concentration and mean area under the glucose curve after beans were 23 and 28% lower, respectively, than the mean for the other foods (p less than 0.001). Such results suggest a potentially valuable role for dried leguminous seeds in carbohydrate exchanges for individuals with impaired carbohydrate tolerance. These large differences in blood glucose response to different food cannot at present be predicted directly from tables of chemical composition. Nevertheless, physiological testing may both aid in understanding the factors responsible and help selection of the appropriate carbohydrate foods for the diabetic diet.

Effect of free fatty acids and amino acids on glucagon and insulin secretions in normal and diabetic ducks

The relationship between two metabolites free fatty acids (FFA) and amino acids (AA), and the two main pancreatic hormones, insulin and glucagon, was studied by infusing small amounts of these metabolites into normal and diabetic Peking ducks, i.e. two days after subtotal pancreatectomy. Infusion of oleic acid (0.365 g/kg/30 min as an emulsion in plasma) indicated a suppressive effect of free fatty acids on glucagon secretion, but was without effect on insulin secretion, in normal as well as in diabetic ducks, indicating that insulin might not be directly involved in the FFA-glucagon feedback in the duck. Infusions of arginine for one hour (1 g/kg/h) into normal ducks, hyperglycaemic normal birds (as a result of glucose infusion: 1 g/kg/h) and diabetic ducks, suggested the persistence of amino acid effect on glucagon secretion, and a slight reduction of the effect on insulin secretion in diabetes. This suggests that insulin may not be involved in amino acid-induced glucagon secretion in the duck.

Effect of starvation on the turnover and metabolic response to leucine

l-Leucine was administered as a primed continuous 3-4-h infusion in nonobese and obese subjects in the postabsorptive state and for 12 h in obese subjects after a 3-day and 4-wk fast. In nonobese and obese subjects studied in the post-absorptive state, the leucine infusion resulted in a 150-200% rise in plasma leucine above preinfusion levels, a small decrease in plasma glucose, and unchanged levels of plasma insulin and glucagon and blood ketones. Plasma isoleucine (60-70%) and valine (35-40%) declined to a greater extent than other amino acids (P < 0.001). After 3 days and 4 wk of fasting, equimolar infusions of leucine resulted in two- to threefold greater increments in plasma leucine as compared to post-absorptive subjects, a 30-40% decline in other plasma amino acids, and a 25-30% decrease in negative nitrogen balance. Urinary excretion of 3-methylhistidine was however, unchanged. Plasma glucose which declined in 3-day fasted subjects after leucine administration, surprisingly rose by 20 mg/100 ml after 4 wk of fasting. The rise in blood glucose occurred in the absence of changes in plasma glucagon and insulin and in the face of a 15% decline in endogenous glucose production (as measured by infusion of [3-(3)H]glucose). On the other hand, fractional glucose utilization fell by 30% (P < 0.001), thereby accounting for hyperglycemia. The estimated metabolic clearance rate of leucine fell by 48% after 3 days of fasting whereas the plasma delivery rate of leucine was unchanged, thereby accounting for a 40% rise in plasma leucine during early starvation. After a 4-wk fast, the estimated metabolic clearance rate of leucine declined further to 59% below base line. Plasma leucine nevertheless fell to postabsorptive levels as the plasma delivery rate of leucine decreased 65% below postabsorptive values.

CONCLUSIONS

(a) Infusion of exogenous leucine in prolonged fasting results in a decline in plasma levels of other amino acids, improvement in nitrogen balance and unchanged excretion of 3-methylhistidine, thus suggesting stimulation of muscle protein synthesis, (b) leucine infusion also reduces glucose production and to an even greater extent, glucose consumption, thereby raising blood glucose concentration; and (c) the rise in plasma leucine in early starvation results primarily from a decrease in leucine clearance which drops progressively during starvation.

Somatostatin-induced changes in insulin and glucagon secretion in normal and diabetic dogs

In conscious dogs intravenously infused somatostatin (3.3 mug per min for 1 h) caused prompt and sustained declines in mean plasma insulin and glucagon, even during alanine infusion and intraduodenal casein hydrolysate feeding; plasma glucose declined, but not significantly. 6.7 mug per min of somatostatin significantly lowered pancreatoduodenal vein glucagon and insulin within 2.5 min and profoundly suppressed their secretion throughout the infusion. Consistent bihormonal suppression occurred at rates as low as 24 ng per kg per min, but was variable at 12 and 2.4 ng per kg per min. When somatostatin-induced (3.3 mug per min) hypoglucagonemia was corrected by exogenous glucagon, hyperglycemia occurred. In dogs with long-standing insulin-requiring alloxan diabetes 3.3 mug per min of somatostatin suppressed glucagon to 55 pg per ml throughout the 30-min infusion and lowered glucose by 36.4+/-6.1 mg per dl, about 1 mg per dl per min. Glucagon suppression was maintained despite alanine infusion, and glucose, which rose 29 mg per dl during alanine infusion without somatostatin, declined 58 mg per dl in the somatostatin-treated diabetic dogs despite alanine. Continuous infusion of somatostatin for 24 h in five insulin-requiring alloxan-diabetic dogs suppressed glucagon and lowered glucose significantly, usually to below normal. It is concluded that in normal dogs pharmacologic doses of somatostatin virtually abolish insulin and glucagon secretion in the basal state and during hyperaminoacidemia. Hyperglycemia occurs during somatostatin-induced insulin lack only if hypoglucagonemia is corrected. Somatostatin suppresses glucagon in diabetic dogs and lowers their plasma glucose approximately 1 mg per dl per min, even when the gluconeogenic substrate alanine is abundant. Glucagon suppression can be maintained for several hours in such dogs and hyperglycemia is thereby reduced.

Glucose modulation of amino acid-induced glucagon and insulin release in the isolated perfused rat pancreas

Interactions between glucose and arginine and a mixture of 20 amino acids found in normal rat serum were studied in the isolated perfused rat pancreas of normal rats, with release of immunoreactive glucagon and insulin as parameters. Secretion of both pancreatic hormones was low during the steady state, whether glucose (5 mM) was included in the perfusion medium or not. This glucose concentration significantly stimulated insulin release twofold and resulted in an 80% inhibition of basal glucagon release. Arginine and the amino acid mixture were potent stimulants of both hormones. Secretion of both hormones followed identical biphasic response patterns after addition of arginine or the amino acid mixture. However, stimulation of insulin release occurred only when glucose was included, whereas both phases of glucagon release were elicited in the absence of glucose and markedly reduced in its presence. The dose-dependency curves of hormone release due to arginine on one hand and the amino acid mixture on the other differed substantially: with arginine, release of insulin and glucagon was linear between a concentration of 0.3 and 20 mM. In contrast, the amino acid mixture resulted in half-maximal release for both hormones between a concentration of 3 and 4.5 mM, and maximal release between 6 and 8 mM. The dose-dependencies of glucose modulation of alpha- and beta-cell activity were also different: when the amino acid mixture was maintained at 15 mM and glucose varied (0-6.25 nM), no insulin release occurred until glucose was above 2.5 mM, whereas incremental inhibition of glucagon occurred through the complete dose range. It was also observed that glucose inhibition of amino acid-stimulated glucagon release was dissociated from glucose-dependent increase of insulin release. THESE STUDIES INDICATE THAT: (a) the alpha-cell, like the beta-cell, secretes at a low basal rate; (b) hypoglycemia per se is a weak stimulus for glucagon secretion compared to the high efficacy of a physiologic amino acid mixture; (c) glucose plays opposite roles in the mechanisms leading to amino acid-induced hormone release from the alpha- and beta-cells, functioning as an inhibitor in the first case and a permissive agent in the second, and (d) the data are compatible with the postulated existence of glucose and amino acid receptors in both the alpha- and beta-cells.

Characterization of the effects of arginine and glucose on glucagon and insulin release from the perfused rat pancreas

To characterize the mechanisms by which arginine and glucose affect pancreatic alpha and beta cell function, the effects of these agents over their full dose response, both alone and in various combinations, were studied using the perfused rat pancreas. Arginine (0-38 mM), in the absence of glucose, stimulated biphasic glucagon (IRG) secretion (Km approximately 3-4 mM) at concentrations less than 1 mM and caused nonphasic insulin (IRI) release (Km approximately 12-13 mM) but only at concentrations greater than 6 mM. Glucose (0-27.5 mM) alone stimulated biphasic IRI release (Km approximately 9-10 mM) at concentrations in excess of 5.5 mM and caused nonphasic inhibition of IRG secretion (Kt approximately 5-6 mM) at concentrations as low as 4.1 mM. These results demonstrate fundamental differences in pancreatic alpha and beta cell secretory patterns in response to glucose and arginine and suggest that glucagon secretion is more sensitive to the effect of both glucose and arginine. Various concentrations of arginine in the presence of 5.5 mM glucose stimulated biphasic IRG and IRI release: IRG responses were diminished and IRI responses were enhanced compared with those seen with arginine in the absence of glucose. Glucose (0-27.5 mM) in the presence of 3.2 or 19.2 mM arginine caused similar inhibition of IRG secretion (Km approximately 5-6 mM) and stimulation of IRI release (Km approximately 9-10 mM) as that seen with glucose alone, although greater IRG and IRI release occurred. This augmentation of IRI secretion was greater than that expected from mere additive effects of glucose and arginine. Classical Lineweaver-Burk analysis of these results indicates that glucose is a non-competitive inhibitor arginine-stimulated glucagon secretion and suggests that glucose and arginine affect pancreatic alpha and beta cell function via different mechanisms. In addition, comparison of simultaneous insulin and glucagon secretion patterns under various conditions suggests that endogenous insulin per se has little or no direct effect on IRG secretion and that endogenous glucagon does not appreciably affect pancreatic beta cell function.

The different effects on the serum lipids and fecal steroids of high carbohydrate diets given orally or intravenously

The hypothesis that diets high in carbohydrate produce hyperlipidemia in man was tested in new experiments which provided all calories either by the intravenous route or orally. After a base-line general diet, eight healthy men were fed fat-free diets consisting of 80% of the calories from glucose and 20% from an amino acid hydrolysate. The calories were adequate to maintain body weight. The solutions (1 cal/ml) were infused by constant drip over a 24 h period through either a superior vena cava catheter or a nasogastric tube. Each feeding was for 12 days in sequence but assigned in random order. The high CHO diet given orally, as expected, increased the mean base-line serum triglyceride level from 176+/-29 (SE) to 274+/-47. The identical diet given intravenously (i.v.) failed to produce hypertriglyceridemia; triglyceride levels were not significantly changed, 154+/-37, nor were blood glucose levels. Serum insulin levels were higher during the intravenous feeding. In contrast, both i.v. and oral feedings greatly lowered mean serum cholesterol concentration from the base-line value of 220+/-13 mg/100 ml to 135+/-11 and 151+/-13, respectively. However, the serum cholesterol level was significantly lower (P < 0.01) with the intravenous feeding than with the oral feeding. In addition, the fecal excretion of both neutral sterols and bile acids diminished greatly during the period of intravenous feeding. The fecal mass was likewise decreased. The bacterial conversion of cholesterol to conprostanol did not occur with either intravenous or oral feeding, but with both regimens secondary bile acids predominated, as usual, in the bile acid fraction of the stool. These results emphasize the key role of the intestinal mucosa in the etiology of carbohydrate-induced hypertriglyceridemia and as a direct or indirect contributor to plasma triglyceride and cholesterol levels in the absence of dietary lipids. When the gut mucosa was bypassed, carbohydrate-induced hypertriglyceridemia did not occur and both serum triglyceride and serum cholesterol levels decreased greatly at a time when the excretion of steroids in the stool was also reduced.

Glucagon-stimulating activity of 20 amino acids in dogs

The effect of 20 L-amino acids upon pancreatic glucagon secretion has been studied in conscious dogs. Each amino acid was administered intravenously over a 15 min period in a dose of 1 mmole/kg of body weight to a group of four or five dogs. Pancreatic glucagon and insulin were measured by radioimmunoassay. 17 of the 20 amino acids caused a substantial increase in plasma glucagon. Asparagine had the most glucagon-stimulating activity (GSA), followed by glycine, phenylalanine, serine, aspartate, cysteine, tryptophan, alanine, glutamate, threonine, glutamine, arginine, ornithine, proline, methionine, lysine, and histidine. Only valine, leucine, and isoleucine failed to stimulate glucagon secretion, and isoleucine may have reduced it. No relationship between glucagon-stimulating activity and insulin-stimulating activity was observed. The amino acids which enter the gluconeogenic pathway as pyruvate and, which are believed to provide most of the amino acid-derived glucose, had a significantly greater GSA than the amino acids which enter as succinyl CoA or as alpha-ketoglutarate. However, pyruvate itself did not stimulate glucagon secretion. The R-chain structure of the amino acid did not appear to be related to its GSA, except that the aliphatic branched chain amino acids, valine, leucine, and isoleucine, were devoid of GSA.

Hypoalaninemia: a concomitant of ketotic hypoglycemia

The cause of of ketotic hypoglycemia, the commonest form of hypoglycemia in childhood, is not known. The present study was undertaken to determine whether the primary defect in this condition is a deficiency of gluconeogenic precursor(s) or an abnormality in the hepatic gluconeogenic enzyme system. Plasma glucose, alanine, and insulin and blood beta-hydroxybutyrate (beta-OHB), pyruvate, and lactate levels were determined in eight ketotic hypoglycemic children and seven agematched controls maintained on a normal diet and after being fed a provocative hypocaloric low-carbohydrate diet (1200 kcal/1.73 m(2), 15% carbohydrate, 17% protein, and 68% fat). On a normal diet, overnight fasting plasma alanine (211+/-10 muM) and glucose (68+/-4 mg/100 ml) were significantly lower and blood beta-OHB (1.22+/-0.37 mM) significantly higher in ketotic hypoglycemic children than in controls (alanine, 315+/-15 muM; glucose, 81+/-3 mg/100 ml; beta-OHB, 0.18+/-0.08 mM). All ketotic hypoglycemic children developed symptomatic hypoglycemia (33+/-3 mg/100 ml) and ketosis (beta-OHB, 3.70+/-0.32 mM) 8-16 hr after starting the provocative diet and these changes were associated with a further decline in plasma alanine (155+/-17 muM). Normal children, even after 36 hr on this diet, maintained higher plasma glucose (48+/-2 mg/100 ml) and alanine (225+/-5 muM) and lower beta-OHB levels (2.56+/-0.44 mM). Intravenous infusions of alanine (250 mg/kg) uniformly restored the hypoglycemic plasma glucose levels of ketotic hypoglycemic children to normal. Cortisone acetate (300 mg/m(2)), given orally in three divided doses during feeding of the provocative diet, produced a 3- to 4-fold increase in plasma alanine within 4-6 hr after beginning steroid therapy and completely prevented the development of hypoglycemia and ketosis. Quantitative amino acid profiles were performed and demonstrated that alanine was the only gluconeogenic amino acid which differed significantly between the two groups. Plasma insulin and blood lactate and pyruvate levels did not differ significantly between normal and ketotic hypoglycemic children under all conditions examined. These results support the hypothesis that a deficiency in gluconeogenic precursor (e.g., alanine) rather than a defect in the hepatic gluconeogenic enzyme apparatus represents the most likely factor in the pathogenesis of ketotic hypoglycemia.

The effect of alanine on glucagon secretion

If glucagon plays a hormonal role in the regulation of gluconeogenesis from endogenous amino acids, its secretion might be stimulated by an increase in the concentration of alanine, which has recently been identified as a principal gluconeogenic precursor. To determine if this is the case, 0.75 mmole of alanine per kilo was infused into conscious dogs immediately after a priming injection of 0.25 mmole per kg for 15 min. A uniform rise in the plasma level of pancreatic glucagon, as determined by a relatively specific radioimmunoassay for pancreatic glucagon, was observed. The rise, which averaged 90 pg per ml, was highly significant at 7(1/2) and 15 min after the start of the infusion. Insulin rose an average of only 8 muU per ml, while glucose rose an average of 10 mg per 100 ml. A lower dose of alanine, 1 mmole per kg, infused over a 1 hr period without an initial priming injection, also elicited a significant rise in glucagon measured in the pancreaticoduodenal venous plasma; glucagon rose from 350 pg per ml to 1066 pg per ml at the end of the infusion. The insulin response was modest and inconsistent, and glucose, again, rose 10 mg per 100 ml. To determine if the availability of exogenous glucose would abolish the alanine-induced rise in glucagon secretion, dogs were made hyperglycemic by a constant intravenous glucose infusion and were then given the high-dose alanine infusion. Under these circumstances, glucagon did not rise above the mean fasting concentration of 75 pg per ml, whereas mean insulin rose dramatically by more than 100 muU per ml. It was concluded that, in the fasting state, alanine does stimulate the secretion of glucagon, while having very little stimulatory effect on insulin secretion. Glucagon could, therefore, be a humoral mediator of gluconeogenesis from endogenous alanine, responding to hyperalaninemia in the fasting state, but not when exogenous glucose is available.

Effects of glucose and other modifiers of insulin release on the oxidative metabolism of amino acids in micro-dissected pancreatic islets

The oxidation of alanine, arginine, leucine, glucose, and pyruvate was studied in microdissected pancreatic islets of obese-hyperglycaemic mice. The following main observations were made. The oxidation of glucose was enhanced severalfold when its concentration was raised from 3 to 20mm. At the latter concentration the rate was about 65mmol/h per kg dry wt. The oxidation of 17mm-pyruvate amounted to 20mmol/h per kg dry wt. indicating a significant entry of this compound into the beta-cells. Leucine oxidation was little affected by concentration changes above 5mm, the rate at 20mm corresponding to about 25% of that obtained with 20mm-glucose. In the absence of glucose, the oxidation of alanine or arginine was barely significant. Glucose stimulated the oxidation of alanine but depressed that of leucine. These effects of glucose were blocked by mannoheptulose or iodoacetamide but were not influenced by adrenaline, diazoxide, dibutyryl 3':5'-cyclic AMP, or glibenclamide. The rate of alanine oxidation was doubled in the presence of 17mm-pyruvate but was unaffected by citrate or succinate. Succinate depressed the oxidation of leucine. Neither alanine nor leucine significantly affected the oxidation of glucose. It is suggested that the effects of glucose on the oxidation of alanine and leucine were mediated by metabolism of the sugar, and that amino acids do not act as insulin secretagogues by serving as fuels for the beta-cells. The results are consistent with the existence of mechanisms auxiliary to glucose metabolism for control of insulin release.

Studies of pancreatic alpha cell function in normal and diabetic subjects

The development of a glucagon radioimmunoassay with a relatively high degree of specificity for pancreatic glucagon made possible studies of alpha cell function in healthy nondiabetic subjects and in patients with diabetes mellitus. In the former group mean fasting plasma glucagon averaged 108 mumug/ml (SEM +/-10). In 12 juvenile-type diabetics fasting glucagon averaged 110 (+/-9) and in 33 adult-type diabetics the average was 114 (+/-8). The diabetic averages did not differ significantly from the nondiabetic subjects; however, when hyperglycemia was induced by glucose infusion in the nondiabetic subjects so as to simulate the fasting hyperglycemia of the diabetics, mean glucagon fell to 57 mumug (+/-8), which was significantly below the diabetic mean. In 28 healthy subjects the infusion of arginine elicited a rise in glucagon of at least 100 mumug/ml with a peak level averaging 331 mumug/ml (+/-22) at 40 min. This response to arginine was diminished but not abolished during hyperglycemia induced by simultaneous glucose infusion. In everyone of 45 diabetic subjects tested the infusion of arginine elicited a rise in glucagon of at least 140 mumug/ml to levels significantly greater than in nondiabetics. The peak glucagon level in juvenile-type diabetics averaged 458 mumug/ml (SEM +/-36) and in adult-type diabetics averaged 452 mumug/ml (SEM +/-38). The glucagon response to arginine was unrelated to duration of diabetes, to body weight, type of diabetic treatment, or to other known factors. Marked hyperresponsiveness of glucagon to arginine infusion was observed in two patients with advanced Kimmelsteil-Wilson disease. Glucagon levels were markedly elevated in certain patients with severe diabetic ketoacidosis before treatment with insulin. The findings suggest that alpha cell function is inappropriately increased in diabetes mellitus and could play a significant role in the diabetic syndrome.

The role of aminogenic glucagon secretion in blood glucose homeostasis

Hyperaminoacidemia is a powerful stimulus of pancreatic glucagon secretion. These studies were designed to elucidate the role of aminogenic hyperglucagonemia in glucoregulation. Conscious dogs with previously implanted indwelling venous catheters were employed. The results support the view that a role of glucagon is to limit blood glucose decline during hyperaminoacidemia.First, a significant negative correlation between the area of glucagon increment during the 1st 20 min of a 10 amino acid infusion and the maximum fall in glucose concentration was observed. Second, when endogenous glucagon secretion was suppressed by means of a continuous glucose infusion, hyperaminoacidemia induced a maximal glucose decline which averaged 35 mg/100 ml, differing significantly from mean maximal fall of 3 mg/100 ml, which normally occurs in the presence of endogenous hyperglucagonemia. Third, when, during hyperglycemic suppression of endogenous glucagon secretion, 50 mmug of exogenous glucagon/min was infused via the mesenteric vein with the amino acids, the fall in glucose was reduced to an average of 5 mg/100 ml. Similarly when pancreozymin, administered during the combined infusion of glucose and amino acids, overcame glucose suppression of endogenous glucagon secretion, plasma glucose did not fall. Similar results were obtained when aminogenic hyperglucagonemia was prevented by other means. Hyperlipacidemia, induced by infusing a triglyceride emulsion and giving heparin injections, also suppressed aminogenic hyperglucagonemia in two of four experiments; in these two dogs glucose fell 15 and 11 mg/100 ml. In a final group of experiments, the canine pancreas was resected except for the uncinate process, which is virtually devoid of alpha-cells. In two dogs, in which this procedure resulted in zero portal venous glucagon levels, the administration of amino acids and/or pancreozymin resulted in a glucose decline of 14 and 16 mg/100 ml, despite the reduced beta-cell population resulting from the subtotal pancreotectomy. It thus appears that the secretion of pancreatic glucagon during hyperaminoacidemia in association with insulin secretion, serves to limit the decline of glucose concentration.

Characterization of response of circulating glucagon to intraduodenal and intravenous administration of amino acids

Studies were carried out to determine if hyperaminoacidemia stimulates the secretion of pancreatic glucagon, and, if so, to evaluate the effect of endogenous and exogenous pancreozymin and of hyperglycemia upon this response. The intravenous administration to 16 dogs of 1 g/kg of a 10 amino acid mixture over a 60 min period raised amino nitrogen to a mean level of 13.5 mg/100 ml; mean pancreaticoduodenal vein insulin rose from 84 to 459 muU/ml and glucagon from 1.1 to 2.7 mmug/ml. Further augmentation of both insulin and glucagon secretion was achieved during hyperaminoacidemia by infusing pancreozymin. Since endogenous pancreozymin is known to be stimulated by amino acids in the gut, it seemed possible that intraduodenal loading of amino acids would elicit a greater insulin and glucagon response than could be explained by the accompanying hyperaminoacidemia. The intraduodenal administration of 1 g/kg of the amino acid mixture was followed by substantial hyperinsulinemia and hyperglucagonemia, which frequently anticipated the hyperaminoacidemia, and in many of the dogs the ratio of hormone rise to amino nitrogen rise was greater after intraduodenal than after the intravenous route of amino acid administration in the same animal. Intraduodenal administration of amino acids did not cause measurable release of intestinal glucagon-like immunoreactivity into the mesenteric vein plasma. Hyperglycemia induced by constant glucose infusion prevented aminogenic hyperglucagonemia and even suppressed the augmenting action of pancreozymin; sudden termination of the infusion with continued amino acid infusion was associated with a striking rise in glucagon. It is concluded (a) that hyperaminoacidemia stimulates pancreatic glucagon secretion, (b) that aminogenic hyperglucagonemia is augmented by the infusion of pancreozymin, (c) that intraduodenal administration of amino acids stimulates pancreatic glucagon secretion without measurable release of glucagon-like immunoreactivity into the mesenteric vein, and (d) that hyperglycemia prevents aminogenic hyperglucagonemia even during augmentation with pancreozymin. This conclusion suggests that the prevention of hypoglycemia during amino acid-induced insulin secretion may be an important function of glucagon.