The effect of insulin-induced hypoglycaemia on the secretion of glucagon and hepatic glucose production in pancreatectomized dogs

The concentration of plasma glucose in insulin deprived pancreatectomized dogs was decreased from the basal 385 +/- 44 to 65 +/- 12 mg/dL by the infusion of 7 mU X kg-1 X min-1 insulin. During the infusion, the plasma concentration of immunoreactive glucagon (IRG) did not change and hepatic glucose production was decreased. This is in contrast to earlier findings in alloxan diabetic dogs in which plasma IRG decreased in hypoglycaemia. The hypothesis is put forward that, in contrast to pancreatic alpha cells in which the effect of insulin prevails, neither insulin nor a decrease in the ambient concentration of glucose exerts any effect on the secretion of glucagon from extrapancreatic alpha cells.     

Role of insulin and glucagon in oxytocin effects on glucose metabolism.

Oxytocin (OT) infusion in normal dogs increases plasma insulin and glucagon levels and increases rates of glucose production and uptake. The purpose of this study was to determine whether the effects of OT on glucose metabolism were direct or indirect. The studies were carried out in normal, unanesthetized dogs in which OT infusion was superimposed on infusion of either somatostatin, which suppresses insulin and glucagon secretion, or clonidine, which suppresses insulin secretion only. Infusion of 0.2 microgram/kg/min of somatostatin suppressed basal levels of plasma insulin and glucagon and inhibited the OT-induced rise of these hormones by about 60-80% of that seen with OT alone. The rates of glucose production and uptake by tissues, measured with [6-3H] glucose, were significantly lower than those seen with OT alone, and the rise in glucose clearance was completely inhibited. Clonidine (30 micrograms/kg, sc), given along with an insulin infusion to replace basal levels of insulin, completely prevented the OT-induced rise in plasma insulin and markedly reduced the glucose uptake seen with OT alone, but did not reduce the usual increase in plasma glucose and glucagon levels or glucose production. To determine whether the OT-induced rise in plasma insulin was in response to the concomitant increase in plasma glucose, similar plasma glucose levels were established in normal dogs by a continuous infusion of glucose and an OT infusion was superimposed. OT did not raise plasma glucose levels further, but plasma insulin levels were increased, indicating that OT can stimulate insulin secretion independently of the plasma glucose changes. Studies by others have shown that the addition of OT to pancreatic islets or intact pancreas can stimulate insulin and glucagon secretion, indicating a direct effect. Our studies agree with that and suggest that in vivo, OT raises plasma insulin levels, at least in part, through a direct action on the pancreas. These studies also show that OT increases glucose production by increasing glucagon secretion and, in addition, a direct effect of OT on glucose production is likely. The OT-induced increase in glucose uptake is mediated largely by increased insulin secretion.     

Plasma glucagon, insulin and glucose responses to intravenous arginine infusion in the rat

Plasma glucagon (IRG), insulin and glucose responses to intravenous arginine infusion in the rat were studied. Three doses of arginine hydrochloride were infused into fasted rats: 0.2 gm/kg b.w., 0.5 gm/kg b.w., and 1 gm/kg b.w. The 0.2 gm/kg dose did not result in significant elevation of plasma IRG or insulin. Both the 0.5 and 1 gm/kg doses produced a significant increase in glucagon and insulin levels within 5 minutes of starting the infusion. The 1 gm/kg dose was most effective in stimulating secretion of both hormones. This dose produced a 250% rise in the plasma IRG compared to 80% peak rise with the 0.5 gm/kg dose (p less than .01) and 1055% rise in insulin levels compared to a peak level of 225% above baseline with the 0.5 gm/kg dose (p less than .001). These results demonstrate the effectiveness of intravenous arginine in the stimulation of glucagon and insulin secretion in the rat.     

Islet-activating protein (IAP)-induced adrenergic modulation of pancreatic A and B cell in dogs

Islet-activating protein (IAP) is a substance purified from the culture medium of Bordetella pertussis, and its main action is characterized by the enhancement of secretory response to glucose and other stimuli in pancreatic islet. In this experiment, the effect of IAP on epinephrine-induced secretion of immunoreactive insulin (IRI) and glucagon (IRG) was investigated in normal dogs. Epinephrine suppressed IRI secretion and it had a little increment to IRG secretion in control group, while IRI and IRG secretions were significantly increased by epinephrine in IAP pretreated group. Using beta-blocker (Propranolol) with epinephrine, these increments of IRI and IRG secretions in IAP pretreated group were abolished. However, using alpha-blocker (Phentolamine) with epinephrine, these secretions of IRI and IRG in IAP pretreated group were much more increased than epinephrine alone induced secretions. Blood glucose levels were lower in IAP pretreated group than in control group throughout the loading tests in all of the experiments. These findings suggest that (1) IAP decreases blood glucose level and (2) IAP enhances epinephrine-induced secretion of insulin and glucagon by acceleration of beta-adrenergic effect and by reduction of alpha-adrenergic suppression in dogs.     

Response of extrapancreatic immunoreactive glucagon to intraluminal nutrients in pancreatectomized dogs

To investigate the response of extrapancreatic glucagon to intraluminal stimuli, nutrients were administered to normal and pancreatectomized dogs through a stomach tube in a fully conscious state after an overnight fast. Plasma immunoreactive glucagon was determined with antisera specific and nonspecific to glucagon and was designated as IRG and total IRG, respectively. Oral glucose load elicited a decrease in plasma IRG and a remarkable rise of plasma total IRG in a group of 6 pancreatectomized dogs, as in the control dogs. When arginine was given, both plasma IRG and total IRG significantly increased in a group of 5 pancreatectomized dogs, while only total IRG rose significantly in the normal control dogs. Butter load did not reveal any changes in plasma IRG and total IRG in a group of 5 pancreatectomized dogs, whereas only total IRG increased in the normal control dogs. It is concluded that extrapancreatic glucagon responds to intraluminal administration of nutrients, as pancreatic glucagon does. In addition, gut glucagon-like immunoreactivity increased following glucose or arginine ingestion in pancreatectomized dogs. Furthermore, the failure in response of plasma IRG and total IRG to butter load in pancreatectomized animals suggests that its intraluminal hydrolysis is important in the secretion of extrapancreatic immunoreactive glucagon.     

Insulin as a physiological modulator of glucagon secretion

Glucose homeostasis is regulated primarily by the opposing actions of insulin and glucagon, hormones that are secreted by pancreatic islets from beta-cells and alpha-cells, respectively. Insulin secretion is increased in response to elevated blood glucose to maintain normoglycemia by stimulating glucose transport in muscle and adipocytes and reducing glucose production by inhibiting gluconeogenesis in the liver. Whereas glucagon secretion is suppressed by hyperglycemia, it is stimulated during hypoglycemia, promoting hepatic glucose production and ultimately raising blood glucose levels. Diabetic hyperglycemia occurs as the result of insufficient insulin secretion from the beta-cells and/or lack of insulin action due to peripheral insulin resistance. Remarkably, excessive secretion of glucagon from the alpha-cells is also a major contributor to the development of diabetic hyperglycemia. Insulin is a physiological suppressor of glucagon secretion; however, at the cellular and molecular levels, how intraislet insulin exerts its suppressive effect on the alpha-cells is not very clear. Although the inhibitory effect of insulin on glucagon gene expression is an important means to regulate glucagon secretion, recent studies suggest that the underlying mechanisms of the intraislet insulin on suppression of glucagon secretion involve the modulation of K(ATP) channel activity and the activation of the GABA-GABA(A) receptor system. Nevertheless, regulation of glucagon secretion is multifactorial and yet to be fully understood.     

Hormone secretion and glucose metabolism in islets of Langerhans of the isolated perfused pancreas from normal and streptozotocin diabetic rats.

The glucose responsiveness of alpha- and beta-cells of normal as well as untreated and insulin-treated streptozotocin diabetic rats was tested in the extracorporeal perfusion system. Also assessed was the possible in vitro effect of added insulin on the glucose sensitivity of islets from untreated diabetic animals. Insulin and glucose responsiveness of the two cell types. The rate of glucose entry islet tissue was estimated, and the effect of glucose on the tissue supply of ATP and lactate and the cyclic 3':5'-AMP level of islets was measured under the above in vitro conditions. It was demonstrated that beta-cells are more accessible to glucose than alpha-cells, that glucose entry into islet cells is not significantly modified by insulin and that glucose had no effect on ATP, lactate and cyclic 3':5'-AMP levels of islet tissue under any of the conditions investigated. High insulin in vitro elevated ATP levels of alpha-cell islets independent of extracellular glucose. Glucose caused insulin release from normal but not from diabetic islets and rapidly and efficiently suppressed stimulated glucagon secretion of the pancreas from normal and insulin treated diabetic rats. Glucose was less effective in inhibiting stimulated glucagon secretion by the pancreas from untreated diabetic rats whether insulin was added to the perfusion media or not. Therefore, profound differences of glucose responsiveness of alpha-cells fail to manifest themselves in alterations of basic parameters of glucose and energy metabolism in contrast to what had been postulated in the literature. It is however, apparent that the glucose responsiveness of alpha-cells is modified by insuling by an as yet undefined mechanism.     

Stimulation of gastric glucagon secretion by epinephrine administration in dogs

To determine the response of gastric A-cells to adrenergic substances, immunoreactive glucagon was determined simultaneously in the jugular vein and in the left gastroepiploic vein of totally depancreatized dogs. Under basal conditions a significant gradient of glucagon concentrations between the jugular and gastric veins was observed, whereas plasma insulin values were almost undetectable. Intravenous administration of epinephrine elicits a prompt and significant increase in glucagon concentrations in the gastric vein which persist during the time of hormone infusion. To ensure adequate adrenergic blockade, blockers were infused before epinephrine administration. Accordingly, after phentolamine, the infusion of epinephrine failed to increase gastric glucagon concentrations, while after propranolol, epinephrine induced a significant release of gastric glucagon. These results indicate that epinephrine stimulates gastric glucagon secretion and that this effect is mediated through alpha-adrenergic receptors.     

Hepatic glucagon clearance during insulin induced hypoglycemia

Studies concerning the importance of glucagon secretion in hypoglycemic counterregulation have assumed that peripheral levels of glucagon are representative of rates of pancreatic glucagon secretion. The measurement of peripheral levels of this hormone, however, may be a poor reflection of secretion rates because of glucagon's metabolism by the liver. Therefore, in order to understand the relationship between pancreatic glucagon secretion and levels of glucagon in the peripheral blood during hypoglycemia, we evaluated hepatic glucagon metabolism during insulin induced hypoglycemia. Four dogs received an insulin infusion to produce glucose levels less than 50 mg/dl for 45 minutes. In response to this, the delivery of glucagon to the liver increased from 36.7 +/- 5.9 ng/min in the baseline to 322.6 +/- 6.3 ng/min during hypoglycemia. Hepatic glucagon uptake increased proportionally from 13.6 +/- 7.2 ng/min to 103.1 +/- 28.3 ng/min and the percentage of delivered hormone that was extracted did not change (30.8 +/- 13.8% vs 32.9 +/- 11.6%). The absolute amount of glucagon metabolized by the liver was dependent on the rate of delivery and was not directly affected by plasma glucose level per se. To directly study the effect of hypoglycemia on hepatic glucagon metabolism, five dogs were given an exogenous infusion of somatostatin followed by an infusion of glucagon and then administered insulin to produce hypoglycemia. The percent of glucagon extracted by the liver (19.5 +/- 4.9% and 21.3 +/- 6.4%) was not affected by a fall in the plasma glucose level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increase by somatostatin of the arginine induced rise in blood glucose in untreated insulin requiring diabetics.

We have demonstrated previously that cyclic somatostatin (GH-RIH) exerts a diabetogenic action in healthy subjects. To further examine the impact of this phenomenon studies of blood glucose (BG), immunoreactive insulin (IRI), glucagon (IRG) and growth hormone (GH) were performed in insulin requiring diabetics (n = 6) receiving i.v. arginine (0.5 g/kg) both in the absence and presence of i.v. GH-RIH (500 microgram/h). The infusion of GH-RIH-resulted in a persistent diminution in plasma IRI, IRG and GH. BG fell during i.v. GH-RIH during the initial 30 min and was below control values up to 45 min after initiation of i.v. arginine, but subsequently exceeded control levels (p less than 0.05 - less than 0.025). The excess rise in BG occurred in spite of suppression by somatostatin of the ariginine induced release of IRG, IRI and GH. A fall in BG was seen following cessation of i.v. GH-RIH and during a rebound of insulin release with glucagon levels remaining in the basal range. These findings indicate a diabetogenic action of somatostatin also in insulin requiring diabetics as long as some residual capacity for insulin release is retained.     

Pancreatic glucagon secretion during a short period of hemorrhage in anesthetized dogs

Glucagon has been implicated in the hormonal metabolic response to hemorrhage. However, evidence for this has been obtained largely from observations of circulating plasma glucagon concentration. A clear increase in the pancreatic glucagon secretion remains to be demonstrated. Plasma concentrations of pancreatic immunoreactive glucagon (IRG) and insulin (IRI) were determined in portal venous and aortic blood, and plasma glucose in aortic blood. Dogs were bled (approximately 15 mL/kg) until aortic systolic blood pressure dropped to approximately 50% (70.5 +/- 8.1 mmHg, n = 7) (1 mmHg = 133.32 Pa) of its control value (135 +/- 7.1 mmHg, n = 7), and the hemorrhagic hypotension was maintained for 10 min. The net portal venous IRG delivery rate rose significantly and continued to increase during the hemorrhagic hypotension despite a significant fall in the portal venous blood flow. Aortic IRG increased significantly along with the increase in portal venous IRG delivery rate (r = 0.838, n = 42, p less than 0.01). The portal venous delivery rate of IRI decreased significantly in response to hemorrhage. The aortic IRG/IRI concentration ratio increased significantly during the hemorrhage-induced hypotension. Aortic glucose concentration increased significantly 5 min after hemorrhage and continued to rise until the end of the hemorrhagic hypotension. The present study demonstrates that the secretion of pancreatic glucagon actually increases during the early phase of hemorrhage. The results also indicate that the increase in aortic IRG during the hemorrhagic hypotension is due to the increased pancreatic glucagon secretion. It is suggested that the pancreatic glucagon may be involved in the early hyperglycemic response to hemorrhage.     

Stimulatory effect of xenin-8 on insulin and glucagon secretion in the perfused rat pancreas

Xenin is a 25-amino acid peptide of the neurotensin/xenopsin family identified in gastric mucosa as well as in a number of tissues, including the pancreas of various mammals. In healthy subjects, plasma xenin immunoreactivity increases after meals. Infusion of the synthetic peptide in dogs evokes a rise in plasma insulin and glucagon levels and stimulates exocrine pancreatic secretion. The latter effect has also been demonstrated for xenin-8, the C-terminal octapeptide of xenin. We have investigated the effect of xenin-8 on insulin, glucagon and somatostatin secretion in the perfused rat pancreas. Xenin-8 stimulated basal insulin secretion and potentiated the insulin response to glucose in a dose-dependent manner (EC(50)=0.16 nM; R(2)=0.9955). Arginine-induced insulin release was also augmented by xenin-8 (by 40%; p<0.05). Xenin-8 potentiated the glucagon responses to both arginine (by 60%; p<0.05) and carbachol (by 50%; p<0.05) and counteracted the inhibition of glucagon release induced by increasing the glucose concentration. No effect of xenin-8 on somatostatin output was observed. Our observations indicate that the reported increases in plasma insulin and glucagon levels induced by xenin represent a direct influence of this peptide on the pancreatic B and A cells.     

Effect of beta-hydroxybutyrate and acetoacetate on insulin and glucagon secretion from perfused rat pancreas

To elucidate the physiological significance of ketone bodies on insulin and glucagon secretion, the direct effects of beta-hydroxybutyrate (BOHB) and acetoacetate (AcAc) infusion on insulin and glucagon release from perfused rat pancreas were investigated. The BOHB or AcAc was administered at concentrations of 10, 1, or 0.1 mM for 30 min at 4.0 ml/min. High-concentration infusions of BOHB and AcAc (10 mM) produced significant increases in insulin release in the presence of 4.4 mM glucose, but low-concentration infusions of BOHB and AcAc (1 and 0.1 mM) caused no significant changes in insulin secretion from perfused rat pancreas. BOHB (10, 1, and 0.1 mM) and AcAc (10 and 1 mM) infusion significantly inhibited glucagon secretion from perfused rat pancreas. These results suggest that physiological concentrations of ketone bodies have no direct effect on insulin release but have a direct inhibitory effect on glucagon secretion from perfused rat pancreas.     

Effects of histamine, of histidine and of anti-histaminic agents on the release of glucagon and insulin from the rat pancreas.

We have studied the effect of histamine (HA) and histidine (HIS) on the release of immunoreactive glucagon (IRG) and insulin (IRI) by isolated rat islets and pieces of pancreas. In isolated islets, HA and HIS stimulated IRG release at a glucose concentration of 3.0 mg/ml and IRI release at a glucose concentration of 0.5 mg/ml. In pieces of pancreas incubated in the presence of glucose (3.0 mg/ml), HA at a 2mM concentration stimulated IRG release and had no effect on the release of IRI; however, when the concentration of HA was doubled (4 mM), an inhibition of IRI release could also be demonstrated. HIS was ineffective. Perphenazine and dexchlorpheniramine, two anti-histaminic agents, inhibited IRG and stimulated IRI release. These results indicate that histamine, whether endogenous or exogenous, directly stimulated IRG and inhibits IRI release and suggest that some of the in vivo effects of histamine, such as hyperglycemia and lipid mobilization, may be mediated, at least in part, by these endocrinologic effects.     

Response of plasma somatostatin-like immunoreactivity to the administration of alloxan in dogs.

The present study was designed to examine the effects of intravenously injected alloxan (75 mg/kg) upon plasma somatostatin-like immunoreactivity (SLI), glucagon (IRG), insulin (IRI) and glucose levels in 6 dogs. Within 2 hours of the injection of alloxan, SLI and IRI levels decreased significantly below their respective baselines, while IRG and plasma glucose concentrations increased. At 8 hours SLI levels had increased significantly by 55 pg/ml, together with a rise in IRI and a decrease in IRG and glucose concentrations. After 24 hours, marked hyperglycemia and hyperglucagonemia had developed whereas SLI levels were not different from preinjection values.     

Plasma glucagon response to intravenous alanine in obese and non-obese subjects

To investigate glucagon (IRG) and insulin (IRI) responses to alanine infusion in obesity and to assess the effect of body weight reduction with respect to hormonal balance, we compared six obese subjects with nine normal weight controls. None of the subjects were diabetic by OGTT criteria. Plasma IRI and IRG were measured following IV alanine at a rate of 0.1 g/kg over a period of 2 min. Our obese subjects had an increase in IRG response to alanine, which was due to decreased suppression of alpha-cell function due to insulin resistance. Weight reduction via calorie restriction reduced insulin demand, resulting in reduced plasma IRI by restoring beta-cell function, and the IRG response was paradoxically decreased as compared with that before weight loss. It is conceivable that improvements in insulin sensitivity after body weight reduction may re-establish the normalization of pancreatic beta-cell function and the insulin-induced inhibition of IRG secretion. Our obese subjects were characterized by decreased IRG secretion which was reflected in a change in body weight reduction.     

Immunological specificity and biological action of biliary glucagon-like substance in the rabbit

A glucagon-like substance named biliary IRG2000 whose molecular weight is approximately 2,000 was isolated by gel filtration from rabbit bile. This substance showed a strong crossreactivity as equivalent to 25.7 +/- 5.1 ng/ml of porcine glucagon in RIA with antiserum 30K specificity. Biliary IRG2000 brought about a significant increase and delayed the response of blood glucose level in coexistence with porcine glucagon, though it has no appreciable effect on the glucose level when administered singly to the mouse intraperitoneally. The response with the coexistence of these materials was far greater than when porcine glucagon was given alone. In Mortimore's type rat liver perfusion, a significant rise in glucose concentration in effluent was also observed when a mixture of biliary IRG2000 and porcine glucagon was perfused. The rate of 125I-glucagon degradation was found delayed in the presence of biliary IRG2000 when examined in the rat. Thus the increase and delayed response of glucose level in coexistence of porcine glucagon with biliary IRG2000 may be explained by a suppressive effect of glucagon degradation due to biliary IRG2000.     

Splanchnic glucagon kinetics in exercising alloxan-diabetic dogs.

The purpose of this study was to define the relationship between arterial immunoreactive glucagon (IRG) and IRG that perfuses the liver via the portal vein during exercise in the diabetic state. Dogs underwent surgery >16 days before the experiment, at which time flow probes were implanted in the portal vein and the hepatic artery, and Silastic catheters were inserted in the carotid artery, portal vein, and hepatic vein for sampling. Dogs were made diabetic with alloxan injected intravenously approximately 3 wk before study (AD) or were studied in the nondiabetic state (ND). Each study consisted of a 30-min basal period and a 150-min moderate-exercise period on a treadmill. The findings from these studies indicate that the exercise-induced increment in portal vein IRG can be substantially greater in AD compared with ND, even when arterial and hepatic vein increments are not different. The larger IRG gradient from the portal vein to the systemic circulation in AD dogs is a function of a twofold greater increase in nonhepatic splanchnic IRG release and a fivefold greater hepatic fractional IRG extraction during exercise. In conclusion, during exercise, arterial IRG concentrations greatly underestimate the IRG levels to which the liver is exposed in ND, and this underestimation is considerably greater in dogs with poorly controlled diabetes.     

Effect of pinealectomy on plasma glucose, insulin and glucagon levels in the rat

In an attempt to know the role of the pineal gland on glucose homeostasis, the blood plasma concentrations of glucose, insulin and glucagon under basal conditions or after the administration of nutrients were studied in the jugular vein of conscious pinealectomized (Pn), melatonin-treated pinealectomized (Pn + Mel) and control (C) rats. Glucose levels were smaller in C than in Pn rats, while immunoreactive insulin (IRI) concentrations were significantly greater in C than in Pn rats. Contrary to this, immunoreactive glucagon (IRG) levels were significantly greater in Pn than in C animals. Melatonin treatment of Pn rats induces an increase of IRI concentrations and a reduction in IRG levels. Similar changes were obtained when hormonal determinations were carried out in portal blood plasma. Although ether anesthesia increases circulating glucagon levels in the porta and cava veins, the qualitative changes of plasma insulin and glucagon in Pn and Pn + Mel were similar to those found in conscious rats. To determine the effects of nutrients on pancreatic hormone release, intravenous arginine or oral glucose were administered to the animals of the three experimental groups. In C rats, both glucose and IRI levels reached a peak 30 minutes after glucose ingestion, decreasing thereafter. However, in Pn rats a glucose intolerance was observed, with maximum glucose and insulin concentrations at 60 minutes, while in Pn + Mel animals, glucose and IRI concentrations were in between the data obtained with the other two groups. Furthermore, glucose ingestion induced a significant reduction of IRG levels in all the groups.(ABSTRACT TRUNCATED AT 250 WORDS)