Effect of enteric pacing on intestinal motility and hormone secretion in dogs with short bowel.

The present study was designed to examine the role of electrode position during retrograde pacing in dogs with short bowel syndrome and unsevered intact duodenum. In nine beagle dogs a subtotal resection of the small bowel and jejunoileostomy was performed. In five of these dogs, an isolated blind loop (jejunum) with preserved mesenteric connections was left in situ as an additional place for a stimulation electrode. Small intestinal motility and plasma levels of insulin, glucagon, gastrin, somatostatin, and glucose were examined during pacing of the residual jejunum or the isolated loop, respectively, compared with control experiments in the same dogs without pacing. During pacing of the loop a significant (P less than 0.05-0.01) decrease in the postprandial small intestinal motility index was observed combined with a significant (P less than 0.05) increase in plasma insulin levels, whereas the postprandial increase in glucagon, somatostatin, gastrin, and glucose levels was not different from that in controls. In contrast, pacing of the jejunum increased postprandial small intestinal motility index (less than or equal to 68%), whereas the levels of the four hormones and plasma glucose were not different from those in controls. The data suggest that in dogs with intact duodenum, pacing on an excluded loop is required to obtain the desired effect of reduced intestinal motility and improved anabolic pancreatic hormone secretion.     

Independance of glucagon and insulin handling by the isolated perfused dog kidney

Summary The effect of raising arterial plasma glucagon concentrations on kidney glucagon uptake was investigated using an isolated dog kidney perfused with whole blood. In addition, the effect of insulin on the magnitude of glucagon uptake by the kidney was studied at various glucagon concentrations. Renal vein plasma glucagon (V) has been found to be proportional to renal artery plasma glucagon (A). V and A were highly significantly correlated. In the absence of exogenous insulin infusion, V equalled 0.733±0.034 A, while in the presence of insulin V equalled 0.747±0.015 A. When kidney glucagon uptake was measured directly it increased as a function of arterial plasma glucagon. The calculated regression lines were similar in the presence and in the absence of insulin. The mean clearance rate of glucagon by the kidney was similar at low, medium or high concentrations of glucagon and was not affected by the presence of insulin at a mean concentration of 335.7±15.7 U/ml. At this concentration of insulin, kidney insulin uptake was not affected by glucagon at concentrations ranging from 32 to 1600 pg/ml. Comparison of kidney glucagon uptake at similar arterial plasma glucagon concentrations, but with different renal plasma flows, indicated that kidney glucagon uptake is more dependant on arterial plasma glucagon concentration than on the quantity of glucagon entering the kidney per minute. It is concluded that: 1) kidney glucagon uptake increases as a function of arterial plasma glucagon concentration; 2) the clearance rate of glucagon is similar at low, medium or high arterial concentrations of glucagon; 3) at concentration of 300–350 U/ml, insulin does not affect kidney glucagon uptake, and 4) at concentrations of glucagon up to 1600 pg/ml, renal insulin uptake is not affected by glucagon. These studies indicate that insulin and glucagon are handled independantly by the kidney of the dog.     

Factors affecting E-rosette forming ability in peripheral lymphocytes of insulin dependent diabetics

Summary Poorly controlled insulin dependent diabetics showed impaired E-rosette forming ability compared to sex and age matched normal controls (34.8±3.1, n=31 vs 55.5±1.7, n=33; p< 0.001; mean±SEM). The reduction of E-rosette cells % was not related to the duration of diabetes, nor to fasting blood glucose levels. Incubation of lymphocytes from a subsequent series of 17 insulin-dependent diabetics with insulin (100 U/ml) plus glucose (100 mg/100 ml) significantly increased E-rosette formation (37.6±3.3 vs 47.0±2.2; p= 0.01); conversely glucagon (0.1 g/ml) significantly impaired E-rosette forming ability of normal lymphocytes (51.5±3.6 vs 44.5±4.2; n=17; p< 0.01). No difference was observed in cAMP content of normal and diabetic lymphocytes, nor was E-rosette forming ability related to intracellular cAMP content. Incubation with increasing glucose concentrations (up to 500mg/100ml) did not affect E-rosette forming ability of normal lymphocytes. Incubation of normal lymphocytes with diluted (110) serum from sex and age matched insulin dependent diabetics impaired E-rosette forming ability to the level found in diabetics (61.1 ± 2.9 vs 39.7 ± 4.4; p < 0.001). The results of these in vitro experiments show that insulin and glucagon exhibit opposite effects on E-rosette forming ability and that undefined factor(s) present in diabetic serum may affect this T-cell function.     

Efficacy determinants of subcutaneous microdose glucagon during closed-loop control

Background

During a previous clinical trial of a closed-loop blood glucose (BG) control system that administered insulin and microdose glucagon subcutaneously, glucagon was not uniformly effective in preventing hypoglycemia (BG <70 mg/dl). After a global adjustment of control algorithm parameters used to model insulin absorption and clearance to more closely match insulin pharmacokinetic (PK) parameters observed in the study cohort, administration of glucagon by the control system was more effective in preventing hypoglycemia. We evaluated the role of plasma insulin and plasma glucagon levels in determining whether glucagon was effective in preventing hypoglycemia.

Methods

We identified and analyzed 36 episodes during which glucagon was given and categorized them as either successful or unsuccessful in preventing hypoglycemia.

Results

In 20 of the 36 episodes, glucagon administration prevented hypoglycemia. In the remaining 16, BG fell below 70 mg/dl (12 of the 16 occurred during experiments performed before PK parameters were adjusted). The (dimensionless) levels of plasma insulin (normalized relative to each subject’s baseline insulin level) were significantly higher during episodes ending in hypoglycemia (5.2 versus 3.7 times the baseline insulin level, p = .01). The relative error in the control algorithm’s online estimate of the instantaneous plasma insulin level was also higher during episodes ending in hypoglycemia (50 versus 30%, p = .003), as were the peak plasma glucagon levels (183 versus 116 pg/ml, p = .007, normal range 50–150 pg/ml) and mean plasma glucagon levels (142 versus 75 pg/ml, p = .02). Relative to mean plasma insulin levels, mean plasma glucagon levels tended to be 59% higher during episodes ending in hypoglycemia, although this result was not found to be statistically significant (p = .14). The rate of BG descent was also significantly greater during episodes ending in hypoglycemia (1.5 versus 1.0 mg/dl/min, p = .02).

Conclusions

Microdose glucagon administration was relatively ineffective in preventing hypoglycemia when plasma insulin levels exceeded the controller’s online estimate by >60%. After the algorithm PK parameters were globally adjusted, insulin dosing was more conservative and microdose glucagon administration was very effective in reducing hypoglycemia while maintaining normal plasma glucagon levels. Improvements in the accuracy of the controller’s online estimate of plasma insulin levels could be achieved if ultrarapid-acting insulin formulations could be developed with faster absorption and less intra- and intersubject variability than the current insulin analogs available today.     

Adaptive changes in postprandial motility after intestinal resection and bypass

An electromyographic technique was used to study the changes in postprandial motility induced by jejunal and ileal resection and jejunal bypass (50% reduction of total length of small bowel). Electrodes were implanted in rats throughout the intestine. Compared to control animals, the duration of postprandial interruption of the myoelectric complex (DIMC) was rapidly increased after jejunal resection, more gradually augmented after jejunal bypass, and remained constant after ileal resection. The frequency of occurrence of spike bursts during the postprandial period was significantly decreased in the short remaining proximal segment after jejunal resection and was not changed in the ileum. The jejunal bypass induced no change in the frequency throughout the remaining bowel. Ileal resection was followed by a decrease on the jejunum. The percentage of slow waves superimposed by a spike burst remained constant after jejunal resection and bypass but was significantly decreased after ileal resection on the whole remaining intestine. These results show important modifications in postprandial motor activity of the small bowel, which appear rapidly after jejunal resection, more gradually after jejunal bypass, and which are less pronounced after ileal resection. This electromyographic study emphasizes the role of intestinal motility in the development of adaptation after small bowel resection or bypass.     

Possible mechanism by which somatostatin-induced glucagon suppression improves glucose tolerance during insulinopaenia in man

Summary The present study examines the question of whether lowering the basal plasma glucagon concentration alters the response of the liver to an intravenous glucose load under conditions where insulin is present at near-basal concentrations. Acute hyperglycaemia of 220–240 mg/dl was induced by peripheral venous glucose infusion in two groups of normal men who had undergone hepatic vein catheterization. Somatostatin (0.9 mg/h) was infused in both groups together with an infusion of insulin (0.15 mU/kg/min) to maintain arterial insulin levels at 7–12 U/ml. Glucagon (1.5 ng/kg/min) was infused in one group resulting in a rise in plasma glucagon levels from 148±37 to 228±25 pg/ml, thus mimicking basal portal glucagon concentrations, whereas in the second group glucagon was not replaced, resulting in a fall in circulating glucagon levels from 132±21 to 74±15 pg/ml. In the glucagon-deprived group, net splanchnic glucose production (NSGP) fell from 143±31 to –72.5±39 mg/ min (p<0.01), indicating that net splanchnic glucose uptake had occurred. By contrast, NSGP did not change significantly (137±20 vs 151±60 mg/min) in the group in which both insulin and glucagon were replaced during hyperglycaemia. These data thus suggest that during hyperglycaemia, when the insulin concentration is fixed at basal levels, glucagon may play an important role in determining whether or not the liver diminishes its output of glucose and stores glucose in response to a glucose load.     

Somatostatin infusion in liver cirrhosis: Glucagon control of glucose homeostasis

Summary In order to evaluate the role of glucagon in blood glucose homeostasis in liver cirrhosis, ten normal subjects and ten cirrhotic patients were infused with somatostatin (500 g/h for 5 h) with and without glucagon (3 mg/kg/h) administration. Somatostatin infusion brought about a fall in plasma glucose both in normal (37%) and cirrhotic (41%) subjects in the first 90 minutes. In normal subjects, this was followed by a rise in plasma glucose (147±2 mg/dl at 5 h), while in cirrhotics no rise in plasma glucose was observed (50±1 mg/dl at 5 h). Plasma insulin and glucagon levels were suppressed in both normal and cirrhotic subjects. Addition of glucagon to the somatostatin infusion caused a two fold rise in plasma glucose level to 183±12 mg/dl at 4 h in normal subjects; a much smaller increase was found in the cirrhotic group (105±3 mg/dl at 4 h). When the infusion was stopped, plasma glucose fell both in normal and cirrhotic subjects (102±14 and 87±2 mg/dl at 6 h respectively). Subsequently, hyperglucagonaemia recurred in the cirrhotic patients (319 ±31 pg/ml). A rebound of plasma insulin was observed in normal subjects (47±8 U/ml) which did not occur in the cirrhotics (16±2 U/ml). Thus when both insulin and glucagon were suppressed by somatostatin infusion, euglycaemia occurred in cirrhotic subjects only when glucagon concentration was restored exogenously. We conclude that glucagon is important in glucose homeostasis in patients with liver cirrhosis.     

Effect of furosemide on insulin and glucagon responses to arginine in normal subjects

Summary This study aimed at evaluating the influence of furosemide upon insulin and glucagon responses to arginine in healthy subjects. For this purpose, six normal subjects received two consecutive arginine pulses (3 g), 60 min apart, before and after the administration of furosemide (40 mg, IV). The acute insulin response (mean change from 3–10 min) to the second arginine pulse was significantly inhibited by furosemide (mean increase: 14.8 ±3.0 U/ml versus 11.7±2.5 U/ml, p<0.01). By contrast, the acute glucagon response was significantly increased (mean increase: 77±18 pg/ml versus 105±21 pg/ml, p<0.01). No significant changes in plasma glucose levels occurred. In control experiments, in which saline rather than furosemide was administered, the acute insulin and glucagon response to the first arginine pulse did not differ from that observed with the second pulse. The effect of furosemide on insulin and glucagon secretion might be mediated through enhanced release of endogenous prostaglandin E.     

Sulfonylurea-induced inhibition of glucagon secretion from the perfused rat pancreas: evidence for a direct,non-paracrine effect

Summary The effects of sulfonylurea on glucagon secretion were characterized in the perfused rat pancreas using glibenclamide (1 g/ml) or tolazamide (10 g/ml) in the presence of 3.3 mmol/1 glucose. Glucagon release, which was unaffected by glibenclamide at 2.75 mmol/1 calcium, was suppressed at 1.19 and 0.64 mmol/l but transiently stimulated at 0.25 mmol/l extracellular calcium. The insulinogenic effect of glibenclamide at 0.64 and 0.25 mmol/1 calcium was enhanced by 35% and 89%, respectively, compared to the response at 2.75 mmol/1 calcium. The stimulatory effect of the compound on somatostatin secretion, however, was lost at the lower calcium levels. The effects of tolazamide at 2.75 and 0.64 mmol/1 calcium mimicked those of glibenclamide, thus indicating that our results with the latter compound may be representative for all sulfonylureas. In pancreata from insulin-deficient alloxan diabetic rats, glibenclamide completely lost its inhibitory effect on glucagon release at 0.64 mmol/1 calcium. Inhibition was not restored by adding insulin (25 U/1) to the perfusate. However, when diabetic rats had been treated with insulin for 6–7 days, glibenclamide suppressed glucagon release at low calcium levels in the absence of stimulated insulin and somatostatin release. It is concluded that, at low calcium concentrations, sulfonylureas suppress glucagon secretion by a direct action on the A cell and not through paracrine interactions by insulin and somatostatin. Prolonged insulin deficiency impairs the sulfonylurea action on glucagon secretion.     

The role of “diabetogenic” hormones on carbohydrate and lipid metabolism following oral glucose loading in insulin dependent diabetics: Effects of acute hormone administration

Summary To evaluate the relative role of diabetogenic hormones as insulin antagonists in severe derangements of diabetic control, glucagon, cortisol, growth hormone and adrenaline were administered by continuous intravenous infusion, separately and in combination, to ketosis-prone insulin-dependent diabetics (n=11). The amount of insulin required for the assimilation of a 50 g glucose load during the various hormone infusions was determined by means of an automated glucose-controlled insulin infusion system and used as an index of insulin effectiveness. Raising plasma hormone concentrations acutely into the range seen in severe diabetic states (glucagon 517±70 pg/ml; cortisol 32±3 g/dl; growth hormone 14±3 ng/ml) did not alter significantly blood glucose profile and insulin requirement (control 11.3±1.1 U; glucagon 11.6±2.0 U; cortisol 11.1±0.4 U; growth hormone 12.9±1.4U), except for adrenaline (plasma level 550±192 pg/ml), which caused a marked rise in blood glucose levels and a threefold increase in insulin demand (31.1±3.7 U). Combined infusion of all hormones did not potentiate significantly the latter effect (38.3±4.7 U). The effectiveness of metabolic control by insulin was assessed by a marked decrease in plasma non-esterified free fatty acids and ketone bodies upon its administration after glucose ingestion in all groups studied. It is concluded that from the hormones investigated within this study adrenaline exerts the strongest diabetogenic action during its short term administration followed by that of growth hormone. Whereas it may well be that over-insulinization of the patients by the glucose controlled insulin infusion system has overcome and disguised the smaller diabetogenic effects of cortisol and glucagon.     

Expression of an islet regenerating (reg) gene in isolated rat islets: effects of nutrient and non-nutrient growth factors

Summary The expression of a novel regenerating (reg) gene has been reported previously in the regenerating islets of a surgical model of diabetes in rats. We exposed collagenase-isolated rat islets for three days to nutrient and non-nutrient growth factors in minimally supplemented RPMI medium (2.7 mmol/l glucose, 2% fetal calf serum), and investigated the relationship between reg gene expression and islet cell replication. RNA was prepared from half of the islets by homogenisation in guanidinium isothiocyanate followed by phenol/chloroform extraction. Northern/dot blot analyses were used to semi-quantify reg mRNA. Islet cell replication was estimated by culturing the remaining islets in radiolabelled thymidine to determine de novo DNA synthesis. Thymidine uptake was stimulated by the following factors: 11 mmol/l glucose (50% increase); 10% amino acids (126% increase); 10% fetal calf serum (39% increase); 100 ng/ml insulin (45% increase); 250 ng/ml growth hormone (65% increase); 1.5 nmol/l aldosterone (29% increase); 2U/ml platelet derived growth factor (116% increase). The results are expressed as a percentage of the thymidine incorporated into control islets cultured in minimal RPMI (1118±100 (SD) cpm/g protein, n=15). Increased islet cell replication was paralleled in each case by a clear rise in reg mRNA expression compared to controls. Furthermore, the rank order for reg gene expression was the same as that for thymidine uptake (r=0.90). The present findings suggest a clear association between reg gene expression and islet cell replication in vitro, and are the first to demonstrate reg gene expression in response to individual growth factors.     

Previous exposure to glucose enhances insulin and suppresses glucagon responses to argentine in man

Summary The effects of previous exposure to glucose on the insulin, glucagon, growth hormone and blood glucose responses to subsequent stimulation with L-arginine were investigated in normal man. During control conditions (i.e., after 120 min of saline infusion), the i.v. administration of arginine enhanced the release of all three hormones and caused a small and transient rise in blood glucose. When arginine was preceded by i.v. glucose during 0–60 min, followed by a rest period of 60–120 min, the insulin release induced by the amino acid was further enhanced, glucagon and GH release were unaffected and blood glucose depressed below control levels. When arginine was preceded by a small oral glucose load (0.5 g/kg) the initial insulin response to arginine was augmented, the initial glucagon response was slightly but significantly depressed and blood glucose lowered while the growth hormone response was unaffected. Conclusions: (1) a near-physiological intake of glucose increases insulin and depresses glucagon secretion evoked by amino acids resulting in increased glucose disposal; (2) the modifications of the insulin and glucagon responses constitute separate components in the feed-back regulation of glucose homeostasis.     

Dose-kinetics of pancreatic glucagon responses to arginine and glucose in subjects with normal and impaired pancreatic B cell function

Summary Pancreatic glucagon responses to different amounts of intravenous arginine and glucose were studied in 10 insulin-dependent diabetics, 14 healthy controls (high insulin responders) and 15 subjects with decreased insulin response to glucose but normal intravenous glucose tolerance (low insulin responders). The dose-kinetics of the glucagon response was studied by using four different arginine doses. The suppressive effect of glucose was evaluated by infusing three glucose doses during a submaximal stimulation with arginine. The diabetics were tested first when under fair metabolic control and then following intensive treatment with insulin to produce near-normalisation of blood glucose. Finally, five subjects underwent insulin-induced hypoglycaemia. The changes in plasma glucagon and blood -amino-nitrogen in response to the four arginine doses were significantly correlated in all groups but the slope of the dose response curve was steeper in the poorly controlled-diabetics than in the non-diabetics. These diabetics displayed higher fasting plasma glucagon values than healthy controls (high insulin responders) (224±4 versus 151±22 pg/ml, p<0.01), higher plasma glucagon responses to arginine and an absence of inhibition by glucose of the arginine-stimulated glucagon release. In strictly controlled diabetic patients, fasting plasma glucagon levels (176±16 pg/ml) were not significantly different from healthy controls, the glucagon response to arginine returned to the normal range, A cell suppressibility by glucose was restored and A cell stimulation by hypoglycaemia reappeared. In the low insulin responders, fasting plasma glucagon was not different from that of high responders (107±12 pg/ml), the slope of the dose response curve to arginine was similar in both groups and the A cells were inhibited by glucose to a similar extent. These results support the concept that islet A cell dysfunction in diabetes is not a primary phenomenon.     

Distribution of plasma glucagon immunoreactivity in a patient with suspected glucagonoma.

Gel filtration of plasma from a patient with a clinical syndrome of glucagonoma and a total plasma glucagon level of 2600 pg/ml, revealed the four glucagon immunoreactive fractions found in normal subjects. The total hyperglucagonemia observed was due to high levels of true glucagon and proglucagon moieties. The so-called "big plasma glucagon" (BPG) measured 190 pg/ml (normal average 113 +/- 79 pg/ml, Mean +/- SD, N = 10); the large glucagon immunoreactivity, LGI (9000 mol wt), measured 625 pg/ml (normal average 11 +/- 16 pg/ml); the true glucagon accounted for 1435 pg/ml (normal average 31 +/- 29 pg/ml); and the small glucagon immunoreactive fraction (approximately 2000 mol wt) measured 35 pg/ml (normal average 26 +/- 18 pg/ml). The high levels of LGI, considered a candidate for proglucagon, may reflect the increased secretory activity of the tumor.     

Regulation of glucagon release: Effects of insulin on the pancreatic A2-cell of the guinea pig

Summary The effect of exogenous insulin on glucagon release by guinea pig A₂-cells of isolated islets from normal and streptozotocin treated animals has been studied to test the hypothesis that insulin directly affects glucagon secretion. Glucose utilization and ATP concentration were also measured. In addition, the effects of exogenous somatostatin on glucagon release and glucose utilization of these cells have been investigated. In the A₂-cell rich islets from streptozotocin treated guinea pigs glucagon release was 76.9±7.8 pg/g islet dry weight/h in 1.7 mmol/l glucose, not being significantly inhibited when the glucose concentration was increased up to 16.7 mmol/l. Glucagon release was, however, strongly suppressed by 30 mU/ml insulin independent of the glucose concentration, while release from the normal islets was unaffected by exogenous insulin. Glucose utilization increased four-fold in the A₂cell rich islets when the glucose level was raised from 1.7 to 16.7 mmol/l, but was nevertheless at all times less than 70 per cent of that of the normal islets. Addition of exogenous insulin caused a further significant stimulation (40–100 per cent) in the A₂-cell rich islets, but not in the normal islets. The ATP concentration of the A₂-cell rich islets increased in parallel with the glucose concentration. Addition of insulin effected the highest ATP levels, about 15 mmol/kg islet dry weight, irrespective of the glucose concentration. Somatostatin (2.5 g/ml) strongly inhibited glucagon release, but failed to affect glucose utilization. The present observations support the view that the A₂-cell is sensitive to insulin, and suggest that the suppressive effect of insulin on glucagon release is mediated via an increased intracellular ATP concentration generated by stimulated glucose metabolism.This work was supported by grants from the Swedish Medical Research Council (B79-12X-001-15B and B79-12X-0229712C), the Swedish Diabetes Association, the Medical Faculty of the University of Uppsala and the Swedish Society for Medical Research.     

Increasing antidumping effect of intestinal pacing with motor-active agents

The aim was to determine whether cholecystokinin-octapeptide (CCK-OP), bethanechol Cl, or metoclopramide HCl would increase the antidumping effect of intestinal pacing in five dogs with truncal vagotomy and Roux gastrectomy. While recording electrical activity from the conscious animals, the amount of a 100-ml, 25% dextrose gastric instillate emptied in 20 min was determined during control tests, during tests with CCk-OP (500 ng/kg/hr), bethanechol (80 g/kg/hr), or metoclopramide alone (600 g/kg/hr) given intravenously or during tests using combinations of pacing and drugs. In other tests, intraluminal gastrointestinal pressure was measured during control and drug infusions. CCK-OP, which relaxed the proximal stomach, slowed emptying of the dextrose instillates (mean±sem emptied, no pacing, no drug=74±5 ml; CCK-OP alone=34±5 mlP<0.05). CCK-OP also enhanced the slowing effect produced by pacing (pacing alone=41±7 ml; pacing plus, CCK-OP=19±8 ml;P<0.05). In contrast, bethanechol and metoclopramide, which did not alter proximal gastric motility, did not alter emptying or augment or diminish the effect of pacing. The conclusion was that the combination of pacing and CCK-OP slowed gastric emptying of the dextrose more than pacing alone and thus had a greater antidumping effect. In contrast, neither bethanechol nor metoclopramide enhanced the pacing effect.Supported by USPHS NIH grants AM18278 and TW03296, Medtronic Inc., and the Mayo Foundation.     

The effect of insulin and glucagon on systolic properties of the normal and septic isolated rat heart

Summary Controversy exists in the literature concerning the effects of insulin and glucagon on cardiac muscle contractility, in particular during anoxia, ischemia or sepsis. The purpose of the present study was to determine the effects of insulin and glucagon on the systolic function of the normal and the dysfunctioning septic rat myocardium in the Langendorff preparation. In the normal isolated rat heart, neither insulin nor glucagon exhibited any lasting inotropic effect on systolic function or coronary flow. Sepsis (cecal ligation and puncture) resulted in a dramatic reduction of systolic function to 44% of control animals. All insulin-containing formulations tested improved systolic function in septic hearts by a mean of 85% compared to Krebs and glucose only. However, this improvement did not reach statistical significance compared to the use of Krebs and glucose only. Glucagon at 100 g/l was doing as well as Krebs and glucose alone while at 1 mg/l glucagon was only able to maintain pre-perfusion contractility. Our results suggest that neither insulin nor glucagon seem to possess special inotropic properties for the isolated perfused normal or septic rat heart.Supported in part by grants from the Chief Scientist of the Israel Ministry of Health and the Joint Research Fund of the Hebrew University and Hadassah.     

Evidence of a liver–alpha cell axis in humans: hepatic insulin resistance attenuates relationship between fasting plasma glucagon and glucagonotropic amino acids

Aims/hypothesis

The secretion of glucagon is controlled by blood glucose and inappropriate secretion of glucagon contributes to hyperglycaemia in diabetes. Besides its role in glucose regulation, glucagon regulates amino acid metabolism in hepatocytes by increasing ureagenesis. Disruption of this mechanism causes hyperaminoacidaemia, which in turn increases glucagon secretion. We hypothesised that hepatic insulin resistance (secondary to hepatic steatosis) via defective glucagon signalling/glucagon resistance would lead to impaired ureagenesis and, hence, increased plasma concentrations of glucagonotropic amino acids and, subsequently, glucagon.

Methods

To examine the association between glucagon and amino acids, and to explore whether this relationship was modified by hepatic insulin resistance, we studied a well-characterised cohort of 1408 individuals with normal and impaired glucose regulation. In this cohort, we have previously reported insulin resistance to be accompanied by increased plasma concentrations of glucagon. We now measure plasma levels of amino acids in the same cohort. HOMA-IR was calculated as a marker of hepatic insulin resistance.

Results

Fasting levels of glucagonotropic amino acids and glucagon were significantly and inversely associated in linear regression models (persisting after adjustment for age, sex and BMI). Increasing levels of hepatic, but not peripheral insulin resistance (p?>?0.166) attenuated the association between glucagon and circulating levels of alanine, glutamine and tyrosine, and was significantly associated with hyperaminoacidaemia and hyperglucagonaemia. A doubling of the calculated glucagon–alanine index was significantly associated with a 30% increase in hepatic insulin resistance, a 7% increase in plasma alanine aminotransferase levels, and a 14% increase in plasma γ-glutamyltransferase levels.

Conclusions/interpretation

This cross-sectional study supports the existence of a liver–alpha cell axis in humans: glucagon regulates plasma levels of amino acids, which in turn feedback to regulate the secretion of glucagon. With hepatic insulin resistance, reflecting hepatic steatosis, the feedback cycle is disrupted, leading to hyperaminoacidaemia and hyperglucagonaemia. The glucagon–alanine index is suggested as a relevant marker for hepatic glucagon signalling.

     

Effect of indomethacin on the metabolic and hormonal response to a standardized breakfast in normal subjects

Summary We have investigated the influence of a single oral administration of indomethacin on blood glucose, plasma free fatty acids (FFA), -amino-nitrogen, insulin and glucagon concentrations in young healthy subjects. Two groups of 6 subjects were studied, the first received a standardized 500 kcal mixed meal without any previous drug administration (controls) whereas the second group received 50 mg indomethacin 2 h before ingesting an identical meal. Plasma indomethacin concentration reached its maximum (2.36±0.36 g/ml) 15 min after administration and declined to 0.45±0.04 ug/ml after 2 h. Indomethacin ingestion was followed by a significant increase in blood glucose and plasma FFA reaching their maximum value at 45 min and returning to basal levels at 120 min. No simultaneous changes in plasma -amino-nitrogen, insulin or glucagon levels were detected during this period. The meal was followed by a rise in blood glucose and plasma insulin as well as by a decrease in plasma FFA concentration. No significant differences were detected between the controls and the subjects receiving indomethacin. In controls, the meal was followed by a rise in plasma -amino-nitrogen and a modest although significant increase in glucagon levels. In indomethacin-treated subjects, the increment of -amino-nitrogen was less marked and the increase in plasma glucagon was not observed. Thus, indomethacin by itself can exert several metabolic effects; however, it does not deteriorate the blood glucose or insulin profile after a regular meal. The present work is the first to demonstrate that an inhibitor of prostaglandin synthesis inhibits the plasma glucagon rise occurring after a physiological stimulus such as a normal meal. On the basis of previousin vitro experiments, we suggest that this effect results from an inhibition of glucagon secretion by the PG synthesis inhibitor.     

Reinterpretation of the effect of haloperidol and ethanol on insulin secretion

Summary We were unable to confirm the report of haloperidol induced dose-dependent inhibition of insulin and glucagon release from the isolated canine pancreas. The possibility that the inhibition was caused by ethanol, previously used as the solvent for haloperidol, was tested. Infusion of ethanol at increasing concentrations (15.8 to 252 mmol/l) caused a progressive inhibition of insulin (-17±1 to -69 ±2%) and glucagon (-13±3 to -67±3%) secretion, using a perfusate containing 200 mg/dl glucose and 2.65 mmol/l calcium. Haloperidol (5 to 20 mol/l) dissolved in ethanol (252 mmol/l) did not augment the inhibitory effects of ethanol. At a low calcium concentration (1.3 mmol/l) ethanol further inhibited insulin secretion (-83 ± 2%) with no additional inhibition by 20 mol/l haloperidol (-80±3%). At a high calcium concentration (8.8 mmol/l) the inhibitory effect of ethanol on insulin or glucagon secretions was diminished and variable. This strongly suggests that the inhibition of insulin and glucagon secretion previously attributed to haloperidol was caused by the ethanol solvent.