A possible role for somatostatin in depression of insulin and glucagon levels during hemorrhage

The hypothesis that depression of insulin and glucagon levels during rapid, acute hemorrhage is controlled by somatostatin was supported by hormonal changes measured in the cat. By 5 min of hemorrhage to 50 mmHg (1 mmHg = 133.322 Pa) arterial blood pressure, insulin and glucagon were severely depressed and somatostatin levels rose to 232% of basal levels. Insulin and glucagon suppression was maintained for the 30-min period of hemorrhage. Following return of the blood, somatostatin levels remained high and insulin and glucagon suppression was maintained. The data support, but do not prove, the hypothesis.     

Acute Glucagon Induces Postprandial Peripheral Insulin Resistance

Glucagon levels are often moderately elevated in diabetes. It is known that glucagon leads to a decrease in hepatic glutathione (GSH) synthesis that in turn is associated with decreased postprandial insulin sensitivity. Given that cAMP pathway controls GSH levels we tested whether insulin sensitivity decreases after intraportal (ipv) administration of a cAMP analog (DBcAMP), and investigated whether glucagon promotes insulin resistance through decreasing hepatic GSH levels.Insulin sensitivity was determined in fed male Sprague-Dawley rats using a modified euglycemic hyperinsulinemic clamp in the postprandial state upon ipv administration of DBcAMP as well as glucagon infusion. Glucagon effects on insulin sensitivity was assessed in the presence or absence of postprandial insulin sensitivity inhibition by administration of L-NMMA. Hepatic GSH and NO content and plasma levels of NO were measured after acute ipv glucagon infusion. Insulin sensitivity was assessed in the fed state and after ipv glucagon infusion in the presence of GSH-E. We founf that DBcAMP and glucagon produce a decrease of insulin sensitivity, in a dose-dependent manner. Glucagon-induced decrease of postprandial insulin sensitivity correlated with decreased hepatic GSH content and was restored by administration of GSH-E. Furthermore, inhibition of postprandial decrease of insulin sensitivity L-NMMA was not overcome by glucagon, but glucagon did not affect hepatic and plasma levels of NO. These results show that glucagon decreases postprandial insulin sensitivity through reducing hepatic GSH levels, an effect that is mimicked by increasing cAMP hepatic levels and requires physiological NO levels. These observations support the hypothesis that glucagon acts via adenylate cyclase to decrease hepatic GSH levels and induce insulin resistance. We suggest that the glucagon-cAMP-GSH axis is a potential therapeutic target to address insulin resistance in pathological conditions.     

Influence of prior exercise and liver glycogen content on the sensitivity of the liver to glucagon.

The purpose of the present study was to test the hypothesis that a prior period of exercise is associated with an increase in hepatic glucagon sensitivity. Hepatic glucose production (HGP) was measured in four groups of anesthetized rats infused with glucagon (2 microg. kg(-1). min(-1) iv) over a period of 60 min. Among these groups, two were normally fed and, therefore, had a normal level of liver glycogen (NG). One of these two groups was killed at rest (NG-Re) and the other after a period of exercise (NG-Ex; 60 min of running, 15-26 m/min, 0% grade). The two other groups of rats had a high hepatic glycogen level (HG), which had been increased by a fast-refed diet, and were also killed either at rest (HG-Re) or after exercise (HG-Ex). Plasma glucagon and insulin levels were increased similarly in all four conditions. Glucagon-induced hyperglycemia was higher (P < 0.01) in the HG-Re group than in all other groups. HGP in the HG-Re group was not, however, on the whole more elevated than in the NG-Re group. Exercised rats (NG-Ex and HG-Ex) had higher hyperglycemia, HGP, and glucose utilization than rested rats in the first 10 min of the glucagon infusion. HG-Ex group had the highest HGP throughout the 60-min experiment. It is concluded that hyperglucagonemia-induced HGP is stimulated by a prior period of exercise, suggesting an increased sensitivity of the liver to glucagon during exercise.     

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.     

Hemorrhage induces the rapid development of hepatic insulin resistance

Hyperglycemia is an early metabolic response to trauma and hemorrhage. The role of hepatic insulin resistance to the development of this hyperglycemia is not well understood. The aim of this study was to determine whether the liver becomes insulin resistant and to identify the particular hepatic insulin signaling pathways that may be compromised following trauma and hemorrhage. Male adult rats were bled to a mean arterial pressure of 40 mmHg and maintained at that pressure for 90 min followed by resuscitation with Ringer lactate. Data showed that trauma and hemorrhage rapidly induced profound hyperinsulinemia in combination with significant hyperglycemia, suggesting the development of insulin resistance. After trauma and hemorrhage, hepatic insulin signaling via the insulin-induced phosphatidylinositol 3 (PI3)-kinase-Akt pathway was abolished, whereas ERK1/2 signaling was relatively normal. The regulation (inhibition) of a hepatic-, insulin-, and the PI3-kinase-dependent gene, IGF binding protein-1, was also lost. The present study provides convincing evidence of a rapid onset hepatic insulin resistance following a combination of trauma and hemorrhage.     

Increments in glucose, glucagon and insulin after morphine in rats, and naloxone blocking of this effect.

In awake rats adapted to experimental conditions and allowed food ad libitum, hyperglycemia was induced by the administration of morphine 10 mg/kg through indwelling catheters in the external jugular vein. High glucose values were measured at 5, 15 and 25 min. Glucagon values were high at 5 and 15 min, and again at basal level at 25 min. Insulin was increased after morphine both at 5, 15 and 25 min, whereas somatostatin levels did not change after morphine. When morphine was administered together with naloxone after an initial 10 min period of naloxone administration, there was no increment in glucose, insulin or somatostatin values; neither at 5, 15 or 25 min. There was a remarkable glucagon decrease after naloxone and morphine remaining from 5 to 25 min. Then, one of the possible mechanisms for the hyperglycemic response after morphine may be an opioid effect on pancreas, stimulating glucagon and thereby causing hepatic glucose output.     

Perturbation of glucose flux in the liver by decreasing F26P2 levels causes hepatic insulin resistance and hyperglycemia

Hepatic insulin resistance is one of the characteristics of type 2 diabetes and contributes to the development of hyperglycemia. How changes in hepatic glucose flux lead to insulin resistance is not clearly defined. We determined the effects of decreasing the levels of hepatic fructose 2,6-bisphosphate (F26P(2)), a key regulator of glucose metabolism, on hepatic glucose flux in the normal 129J mice. Upon adenoviral overexpression of a kinase activity-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, the enzyme that determines F26P(2) level, hepatic F26P(2) levels were decreased twofold compared with those of control virus-treated mice in basal state. In addition, under hyperinsulinemic conditions, hepatic F26P(2) levels were much lower than those of the control. The decrease in F26P(2) leads to the elevation of basal and insulin-suppressed hepatic glucose production. Also, the efficiency of insulin to suppress hepatic glucose production was decreased (63.3 vs. 95.5% suppression of the control). At the molecular level, a decrease in insulin-stimulated Akt phosphorylation was consistent with hepatic insulin resistance. In the low hepatic F26P(2) states, increases in both gluconeogenesis and glycogenolysis in the liver are responsible for elevations of hepatic glucose production and thereby contribute to the development of hyperglycemia. Additionally, the increased hepatic gluconeogenesis was associated with the elevated mRNA levels of peroxisome proliferator-activated receptor-gamma coactivator-1alpha and phosphoenolpyruvate carboxykinase. This study provides the first in vivo demonstration showing that decreasing hepatic F26P(2) levels leads to increased gluconeogenesis in the liver. Taken together, the present study demonstrates that perturbation of glucose flux in the liver plays a predominant role in the development of a diabetic phenotype, as characterized by hepatic insulin resistance.     

Acute hemorrhage causes hepatic insulin sensitizing substance (HISS)-dependent insulin resistance

Hepatic insulin sensitizing substance (HISS) has been shown to account for 55% of the action of insulin in the fed state. HISS blockade leads to HISS-dependent insulin resistance (HDIR). The objective of this study was to test the hypothesis that insulin resistance produced by hemorrhage was HDIR. Insulin sensitivity was measured using the rapid insulin sensitivity test (RIST), which can identify HISS-dependent and independent components. Hemorrhage was performed in anesthetized rats by removing blood to reduce mean arterial pressure to 50 mmHg. Subsequent to blood removal, a RIST was performed. The results show that hemorrhage caused complete HDIR as subsequent administration of atropine failed to further reduce insulin sensitivity. However, the post-hemorrhage RIST was reduced by 34% and not the anticipated 55%. The lesser reduction of the RIST index by hemorrhage was related to reduced apparent volume of distribution and clearance of insulin, since occlusion of the superior mesenteric artery, which caused a similar decrease in portal venous flow as did hemorrhage, resulted in a similar degree of reduction of insulin clearance. The response to administered insulin was confounded by the impact of reduced hepatic blood flow on insulin metabolism that resulted in an increase in the HISS independent (direct) action of injected insulin against a background of complete HDIR. HDIR represents a useful hormonal response to assure a hyperglycemic response to hemorrhage.     

The effects of insulin replacement and withdrawal on hepatic ultrastructure and biochemistry

This study examines the early hepatic biochemical and ultrastructural responses to insulin replacement in streptozotocin-diabetic rats and insulin withdrawal from insulin-maintained diabetic rats. Insulin administration rapidly lowered plasma glucose and the elevated glucose-6-phosphatase (G-6-Pase) specific activity of the diabetic rats. However, hepatic glycogen did not increase until after 3 hr of insulin treatment. Hepatic ultrastructure responded to insulin replacement after the decline in glucose and G-6-Pase. This was seen in periportal hepatocytes as a reduction in the close association between smooth endoplasmic reticulum (SER) and glycogen particles in the diabetic animals. The treated rats showed hepatic SER restricted to the periphery of glycogen masses, as is characteristic of these cells from normal rats, in many cells by 6 hr and all cells by 18 hr. Insulin withdrawal from insulin-treated diabetic rats elicited nearly a total reversal of the above events. Plasma insulin declined to a value half that of the normal rats by 6 hr after withdrawal; concurrently, plasma glucose rose sharply to hyperglycemic values as hepatic glycogen content dropped. Following the rise in plasma glucose and fall in glycogen content, G-6-Pase specific activity increased and by 16 hr reached the high values characteristic of the diabetic animal. Hepatic ultrastructure was also changed as evidenced by an intrusion of elements of the SER into the dense glycogen masses; the result was dispersed glycogen closely associated with SER as seen in the diabetic animal. It is concluded that the hepatic response to insulin replacement in diabetic animals and diabetic onset in insulin-withdrawn animals is rapid and occurs through defined stages.     

Regulation by insulin of gluconeogenesis in isolated rat hepatocytes.

Insulin (10nM) completely suppressed the stimulation of gluconeogenesis from 2 mM lactate by low concentrations of glucagon (less than or equal to 0.1 nM) or cyclic AMP (less than or equal to 10 muM), but it had no effect on the basal rate of gluconeogenesis in hepatocyctes from fed rats. The effectiveness of insulin diminished as the concentration of these agonists increased, but insulin was able to suppress by 40% the stimulation by a maximally effective concentration of epinephrine (1 muM). The response to glucagon, epinephrine, or insulin was not dependent upon protein synthesis as cycloheximide did not alter their effects. Insulin also suppressed the stimulation by isoproterenol of cyclic GMP. These data are the first demonstration of insulin antagonism to the stimulation of gluconeogenesis by catecholamines. Insulin reduced cyclic AMP levels which had been elevated by low concentrations of glucagon or by 1 muM epinephrine. This supports the hypothesis that the action of insulin to inhibit gluconeogenesis is mediated by the lowering of cyclic AMP levels. However, evidence is presented which indicates that insulin is able to suppress the stimulation of gluconeogenesis by glucagon or epinephrine under conditions where either the agonists or insulin had no measurable effect on cyclic AMP levels. Insulin reduced the glucagon stimulation of gluconeogenesis whether or not extracellular Ca2+ were present, even though insulin only lowered cyclic AMP levels in their presence. Insulin also reduced the stimulation by epinephrine plus propranolol where no significant changes in cyclic AMP were observed without or with insulin. In addition, insulin suppressed gluconeogenesis in cells that had been preincubated with epinephrine for 20 min, even though the cyclic AMP levels had returned to near basal values and were unaffected by insulin. Thus insulin may not need to lower cyclic AMP levels in order to suppress gluconeogenesis.     

Additive stimulation of hepatic putrescine production by glucagon and insulin after partial hepatectomy in rats

When rats received glucagon or insulin every 2 h after partial hepatectomy (Hx), hepatic putrescine content was increased above control levels at 6 and 12 h, respectively. When the two hormones were combined, the increased levels were additive. Hepatic ornithine decarboxylase activity was above control levels at 12 h after insulin treatment. Hepatic spermidine N1-acetyltransferase activity was enhanced at 6 h only when glucagon was dosed. Putrescine administration from 0 to 4 h or from 6 to 10 h increased hepatic DNA synthesis to similar levels 22 h after Hx. These results suggest that glucagon and insulin additively stimulate hepatic putrescine production after Hx. This may explain the cooperative stimulation of liver regeneration by both hormones.     

Nutritional status and endocrine response to hemorrhage

Hyperglycemia-inducing hyperosmolality has recently been proven beneficial in the maintenance of blood volume and extracellular fluid volume during early hemorrhagic hypotension. Fed animals benefitted from better plasma refill compared with starved ones when subjected to equal blood loss. Using lightly sedated fed and 24-30 h starved rats, hormones with relevance to glucose homeostasis were studied during 90 min of hemorrhagic hypotension of 70 mmHg (1 mmHg = 133.32 Pa). Marked differences in the overall hormonal developments were found between the two groups. In fed rats, insulin and glucagon responses were initially attenuated, while somatostatin increased to an early peak level at 30 min, returning to basal at 90 min. In starved rats, somatostatin increased gradually during the 90 min. Adrenaline release was massive in both groups. Corticosterone showed no increase from basal levels in the fed group during hemorrhage, while starved rats increased their basal level fourfold already at 30 min. These data are presented as evidence that changing nutritional status alters hormonal response to hypovolemic stress.     

Effects of insulin,glucagon and dexamethasone on pyruvate kinase in cultured hepatocytes

Long-term (24–48 h) and short-term (10–30 min) regulation by hormones of hepatic pyruvate kinase activity was investigated in adult rat hepatocytes cultured under serum-free conditions. In the absence of hormones, pyruvate kinase total activity decreased to 83%, 67% and 39% of the initial level at 24, 48 and 72 h of culture. Insulin (100 nM) maintained total activity significantly above control levels throughout this period. In contrast, glucagon (100 nM) and dexamethasone (100 nM) accelerated the gradual decrease within 24 h (glucagon) or 48 h (dexamethasone) of culture. In these long-term experiments, activity at non-saturating concentrations of phosphoenolpyruvate was decreased by glucagon and dexamethasone but not directly modulated by insulin. However, insulin increased the cellular content of the pyruvate kinase activator fructose-1,6-diphosphate. In short-term experiments on cells cultured under serum- and hormone-free conditions for 48 h, both glucagon and dexamethasone independently caused a rapid, dose-dependent increase of the K_0.5 for phosphoenolpyruvate within 10 min, while V_max was not affected. Insulin inhibited this action of glucagon and dexamethasone and, in their absence, significantly increased substrate affinity for phosphoenolpyruvate within 30 min. Cellular fructose-1,6-diphosphate contents remained unchanged under these conditions. The data identify glucocorticoids and insulin - in addition to glucagon - as short-term regulators of the catalytic properties of pyruvate kinase. All three hormones are effective in the long-term control of total enzyme activity.     

Glucagon chronically impairs hepatic and muscle glucose disposal

Defects in insulin secretion and/or action contribute to the hyperglycemia of stressed and diabetic patients, and we hypothesize that failure to suppress glucagon also plays a role. We examined the chronic impact of glucagon on glucose uptake in chronically catheterized conscious depancreatized dogs placed on 5 days of nutritional support (NS). For 3 days of NS, a variable intraportal infusion of insulin was given to maintain isoglycemia (approximately 120 mg/dl). On day 3 of NS, animals received a constant low infusion of insulin (0.4 mU.kg-1.min-1) and either no glucagon (CONT), basal glucagon (0.7 ng.kg-1.min-1; BasG), or elevated glucagon (2.4 ng.kg-1.min-1; HiG) for the remaining 2 days. Glucose in NS was varied to maintain isoglycemia. An additional group (HiG+I) received elevated insulin (1 mU.kg-1.min-1) to maintain glucose requirements in the presence of elevated glucagon. On day 5 of NS, hepatic substrate balance was assessed. Insulin and glucagon levels were 10+/-2, 9+/-1, 7+/-1, and 24+/-4 microU/ml, and 24+/-5, 39+/-3, 80+/-11, and 79+/-5 pg/ml, CONT, BasG, HiG, and HiG+I, respectively. Glucagon infusion decreased the glucose requirements (9.3+/-0.1, 4.6+/-1.2, 0.9+/-0.4, and 11.3+/-1.0 mg.kg-1.min-1). Glucose uptake by both hepatic (5.1+/-0.4, 1.7+/-0.9, -1.0+/-0.4, and 1.2+/-0.4 mg.kg-1.min-1) and nonhepatic (4.2+/-0.3, 2.9+/-0.7, 1.9+/-0.3, and 10.2+/-1.0 mg.kg-1.min-1) tissues decreased. Additional insulin augmented nonhepatic glucose uptake and only partially improved hepatic glucose uptake. Thus, glucagon impaired glucose uptake by hepatic and nonhepatic tissues. Compensatory hyperinsulinemia restored nonhepatic glucose uptake and partially corrected hepatic metabolism. Thus, persistent inappropriate secretion of glucagon likely contributes to the insulin resistance and glucose intolerance observed in obese and diabetic individuals.     

Beta- and alpha-cell dysfunction in type 2 diabetes.

Insulin resistance is a common pathogenetic feature of type 2 diabetes. However, hyperglycemia would not develop if a concomitant defect in insulin secretion were not present. Impaired insulin secretion results from functional and survival defects of the beta-cell. The functional defects can be demonstrated early in the natural history of diabetes and they are hallmarked by abnormal pulsatility of basal insulin secretion and loss of first-phase insulin release in response to a glucose challenge. Moreover, a significant reduction of the beta-cell mass is apparent at the time of the diagnosis of diabetes. The progressive increase in glucose levels, that seems to characterize the natural history of type 2 diabetes, has been claimed to be largely due to progressive reduction of function and mass of beta-cells. Although a genetic predisposition is likely to account for impaired insulin secretion, chronic exposure to hyperglycemia and high circulating FFA is likely to contribute to both functional and survival defects. The disturbance in the endocrine activity of the pancreas is not limited to insulin, since a concomitant increase in fasting plasma glucagon and impaired suppression after the ingestion of an oral glucose load are often observed. This alteration becomes prominent after the ingestion of a mixed meal, when plasma glucagon remains much higher in the diabetic patient as compared to normal individuals. The disproportionate changes in the plasma concentration of the two pancreatic hormones is clearly evident when the insulin:glucagon molar ratio is considered. It is the latter that mainly affects hepatic glucose production. Because of the reduction of the insulin:glucagon molar ratio basal endogenous glucose concentration will be higher causing fasting hyperglycemia, while the hepatic glucose output will not be efficiently suppressed after the ingestion of a meal, contributing to excessive post-prandial glucose rise. Correcting beta- and alpha-cell dysfunction becomes, therefore, an attractive and rational therapeutic approach, particularly in the light of new treatments that may directly act on these pathogenetic mechanisms of type 2 diabetes.     

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside renders glucose output by the liver of the dog insensitive to a pharmacological increment in insulin

This study aimed to test whether stimulation of net hepatic glucose output (NHGO) by increased concentrations of the AMP analog, 5-aminoimidazole-4-carboxamide-1-beta-d-ribosyl-5-monophosphate, can be suppressed by pharmacological insulin levels. Dogs had sampling (artery, portal vein, hepatic vein) and infusion (vena cava, portal vein) catheters and flow probes (hepatic artery, portal vein) implanted >16 days before study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and hyperinsulinemic-euglycemic (0-150 min) periods. At time (t) = 0 min, somatostatin was infused, and basal glucagon was replaced via the portal vein. Insulin was infused in the portal vein at either 2 (INS2) or 5 (INS5) mU.kg(-1).min(-1). At t = 60 min, 1 mg.kg(-1).min(-1) portal venous 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) infusion was initiated. Arterial insulin rose approximately 9- and approximately 27-fold in INS2 and INS5, respectively. Glucagon, catecholamines, and cortisol did not change throughout the study. NHGO was completely suppressed before t = 60 min. Intraportal AICAR stimulated NHGO by 1.9 +/- 0.5 and 2.0 +/- 0.5 mg.kg(-1).min(-1) in INS2 and INS5, respectively. AICAR stimulated tracer-determined endogenous glucose production similarly in both groups. Intraportal AICAR infusion significantly increased hepatic acetyl-CoA carboxylase (ACC, Ser(79)) phosphorylation in INS2. Hepatic ACC (Ser(79)) phosphorylation, however, was not increased in INS5. Thus intraportal AICAR infusion renders hepatic glucose output insensitive to pharmacological insulin. The effectiveness of AICAR in countering the suppressive effect of pharmacological insulin on NHGO occurs even though AICAR-stimulated ACC phosphorylation is completely blocked.     

Insulin and glucagon share the same mechanism of neuroprotection in diabetic rats: role of glutamate

In patients with acute ischemic stroke, diabetes and hyperglycemia are associated with increased infarct size, more profound neurologic deficits and higher mortality. Notwithstanding extensive clinical and experimental data, treatment of stroke-associated hyperglycemia with insulin is controversial. In addition to hyperglycemia, diabetes and even early prediabetic insulin resistance are associated with increased levels of amino acids, including the neurotoxic glutamate, in the circulation. The pleiotropic metabolic effects of insulin include a reduction in the concentration of amino acids in the circulation. In this article, we show that in diabetic rats exposed to transient middle cerebral artery occlusion, a decrease of plasma glutamate by insulin or glucagon reduces CSF glutamate, improves brain histology, and preserves neurologic function. The neuroprotective effect of insulin and glucagon was similar, notwithstanding their opposite effects on blood glucose. The therapeutic window of both hormones overlapped with the short duration (~30 min) of elevated brain glutamate following brain trauma in rodents. Similar neuroprotective effects were found after administration of the glutamate scavenger oxaloacetate, which does not affect glucose metabolism. These data indicate that insulin and glucagon exert a neuroprotective effect within a very brief therapeutic window that correlates with their capacity to reduce glutamate, rather than by modifying glucose levels.     

Diminished glucagon suppression after β-cell reduction is due to impaired α-cell function rather than an expansion of α-cell mass

Impaired suppression of glucagon levels after oral glucose or meal ingestion is a hallmark of type 2 diabetes. Whether hyperglucagonemia after a β-cell loss results from a functional upregulation of glucagon secretion or an increase in α-cell mass is yet unclear. CD-1 mice were treated with streptozotocin (STZ) or saline. Pancreatic tissue was collected after 14, 21, and 28 days and examined for α- and β-cell mass and turnover. Intraperitoneal (ip) glucose tolerance tests were performed at day 28 as well as after 12 days of subcutaneous insulin treatment, and glucose, insulin, and glucagon levels were determined. STZ treatment led to fasting and post-challenge hyperglycemia (P < 0.001 vs. controls). Insulin levels increased after glucose injection in controls (P < 0.001) but were unchanged in STZ mice (P = 0.36). Intraperitoneal glucose elicited a 63.1 ± 4.1% glucagon suppression in control mice (P < 0.001), whereas the glucagon suppression was absent in STZ mice (P = 0.47). Insulin treatment failed to normalize glucagon levels. There was a significant inverse association between insulin and glucagon levels after ip glucose ingestion (r(2) = 0.99). β-Cell mass was reduced by ～75% in STZ mice compared with controls (P < 0.001), whereas α-cell mass remained unchanged (P > 0.05). α-Cell apoptosis (TUNEL) and replication (Ki67) were rather infrequently noticed, with no significant differences between the groups. These studies underline the importance of endogenous insulin for the glucose-induced suppression of glucagon secretion and suggest that the insufficient decline in glucagon levels after glucose administration in diabetes is primarily due to a functional loss of intraislet inhibition of α-cell function rather than an expansion of α-cell mass.     

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)     

Weaning and metabolic regulation in the rat

Premature weaning of rats to high carbohydrate diets causes a variety of short- and long-term changes in lipid metabolism, but the mechanisms involved are unclear. It is likely that interaction of diet with certain emerging hormonal control patterns during weaning might condition metabolic control and (or) subsequent adaptations in the adult organism. This implies that the adaptive responses of infant animals to diet may differ from those of the mature organism. For example, premature weaning leads to early appearance of rat liver malic enzyme (ME), even when fat supplies as much as 65% of the dietary energy; the same diet suppresses ME activity in 45-day-old rats. The levels of plasma glucagon and thyroid hormones are elevated during the weaning period. Several studies have shown that triiodothyronine evokes hepatic ME in suckling rats. Conversely, glucagon infusion into prematurely weaned rats suppresses the early appearance of the enzyme. Premature weaning, regardless of fat intake, leads to a rapid decline in plasma glucagon levels. Since glucagon is known to antagonize the actions of triiodothyronine on liver ME, the interaction of diet with glucagon and thyroid hormones is conceivably part of the mechanism responsible for the early appearance of hepatic malic enzyme, whereby the decline of plasma glucagon permits triiodothyronine to act on liver ME. Insulin probably exerts a permissive action subsequently. The manner in which these events relate to the long-term consequences of premature weaning is unknown.