Altered ability of the liver to produce glucose following a period of glucagon deficiency

It is known that the effectiveness of a physiologic increment in glucagon to stimulate glucose production wanes with time even when counterregulatory insulin secretion is prevented. The aim of the present study was to establish whether the converse is true: does glucagon become a more effective stimulus of glucose production following a period of acute hypoglucagonemia? To determine this, somatostatin was infused along with basal replacement amounts of glucagon (0.65 ng/mg x min) and insulin (228 microU/kg x min) into five postabsorptive conscious dogs. After 1.5 h of basal hormone replacement, the glucagon infusion was terminated and a selective fall in the plasma glucagon level (75 +/- 6 to 30 +/- 4 pg/ml) occurred. This resulted in a drop in tracer (3H-glucose)-determined glucose production from 3.0 +/- 0.4 to 1.5 +/- 0.3 mg/kg x min. The plasma insulin level remained unchanged at 10 +/- 1 microU/ml throughout the experiment. Euglycemia (110 +/- 4 mg/dl) was maintained by exogenous glucose infusion. After 3 h of glucagon lack, restoration of the glucagon infusion returned the IRG level to control values (78 +/- 6 pg/ml) but restored glucose output only partially (42 +/- 9%), necessitating continued glucose infusion to preserve euglycemia. Repetition of the experiment in dogs whose pancreatic glucoregulatory feedback loops were broken surgically (two-stage pancreatectomy) rather than pharmacologically resulted in similar findings. It is concluded that glucagon deficiency of 3-h duration leads to a decrease, rather than an increase, in hepatic sensitivity to glucagon.     

The effect of insulin treatment on insulin secretion and insulin action in type II diabetes mellitus

We have studied the effects of 3 wk of continuous subcutaneous insulin infusion (CSII) on endogenous insulin secretion and action in a group of 14 type II diabetic subjects with a mean (+/-SEM) fasting glucose level of 286 +/- 17 mg/dl. Normal basal and postprandial glucose levels were achieved during insulin therapy at the expense of marked peripheral hyperinsulinemia. During the week of posttreatment evaluation, the subjects maintained a mean fasting glucose level of 155 +/- 11 mg/dl off insulin therapy, indicating a persistent improvement in carbohydrate homeostasis. Adipocyte insulin binding and in vivo insulin dose-response curves for glucose disposal using the euglycemic clamp technique were measured before and after therapy to assess the effect on receptor and postreceptor insulin action. Adipocyte insulin binding did not change. The insulin dose-response curve for overall glucose disposal remained right-shifted compared with age-matched controls, but the mean maximal glucose disposal rate increased by 74% from 160 +/- 14 to 278 +/- 18 mg/m2/min (P less than 0.0005). The effect of insulin treatment on basal hepatic glucose output was also assessed; the mean rate was initially elevated at 159 +/- 8 mg/m2/min but fell to 90 +/- 5 mg/m2/min in the posttreatment period (P less than 0.001), a value similar to that in control subjects. Endogenous insulin secretion was assessed in detail and found to be improved after exogenous insulin therapy. Mean 24-h integrated serum insulin and C-peptide concentrations were increased from 21,377 +/- 2766 to 35,584 +/- 4549 microU/ml/min (P less than 0.01) and from 1653 +/- 215 to 2112 +/- 188 pmol/ml/min (P less than 0.05), respectively, despite lower glycemia. Second-phase insulin response to an intravenous (i.v.) glucose challenge was enhanced from 170 +/- 53 to 1022 +/- 376 microU/ml/min (P less than 0.025), although first-phase response remained minimal. Finally, the mean insulin and C-peptide responses to an i.v. glucagon pulse were unchanged in the posttreatment period, but when glucose levels were increased by exogenous glucose infusion to approximate the levels observed before therapy and the glucagon pulse repeated, responses were markedly enhanced. Simple and multivariate correlation analysis showed that only measures of basal hepatic glucose output and the magnitude of the postbinding defect in the untreated state could be related to the respective fasting glucose levels in individual subjects.(ABSTRACT TRUNCATED AT 400 WORDS)     

Somatostatin impairs clearance of exogenous insulin in humans

Somatostatin has been widely used to suppress endogenous pancreatic hormone secretion in research studies. Many of these studies required the simultaneous infusion of a hormone together with somatostatin. A critical assumption for its use in metabolic investigation is that somatostatin has no effect on the action or clearance of a concomitantly infused hormone. To test whether clearance of an exogenously infused hormone is affected, we infused insulin with or without somatostatin in two sets of studies. Insulin (40 mU X kg-1 X h-1) was infused for 100 min (n = 6). Plasma glucose levels fell to 55 +/- 4.1 mg/dl with insulin alone and significantly lower, to 44 +/- 1.9 mg/dl, when somatostatin (250 micrograms/h) was also infused (P less than .01). Plasma immunoreactive insulin (IRI) rose to 57 +/- 12.5 microU/ml with insulin alone, which was significantly different from 88 +/- 15 microU/ml when insulin was infused together with somatostatin (P less than .01). When a smaller dose of insulin (30 mU X kg-1 X h-1) was infused for 100 min (n = 4), similar results were observed. When somatostatin was infused together with insulin, plasma glucose fell to lower levels (41 +/- 4.2 vs. 62 +/- 9.5 mg/dl; P less than .01) and plasma IRI rose higher (39 +/- 8.5 vs. 27 +/- 5.9 microU/ml; P less than .01) than when insulin was infused alone. C-peptide was equally suppressed by hypoglycemia regardless of whether somatostatin was administered, indicating suppression of endogenous insulin during these studies. We conclude that somatostatin infusion impairs the clearance of exogenous insulin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Suppression and stimulation mechanisms controlling glucagon secretion in a case of islet-cell tumor producing glucagon, insulin, and gastrin.

The mechanisms controlling secretion of glucagon and other pancreatic hormones were studied in a patient affected with multihormone-secreting islet-cell tumor. Fasting glucagon levels (3,000 pg./ml.) rose to 10 ng./ml. following arginine stimulation. While oral glucose load and intravenous glucose infusion did not suppress glucagon secretion, insulin administration induced a prompt depression in glucagon levels. Glucagon, insulin, and gastrin levels were suppressed by somatostatin while calcium infusion caused a paradoxical increase. It is suggested that only some of the stimulation-inhibition mechanisms were conserved in this case of glucagon-secreting pancreatic tumor.     

Beta-endorphin-induced inhibition and stimulation of insulin secretion in normal humans is glucose dependent

This study evaluated the effect of human beta-endorphin on pancreatic hormone levels and their responses to nutrient challenges in normal subjects. Infusion of 0.5 mg/h beta-endorphin caused a significant rise in plasma glucose concentrations preceded by a significant increase in peripheral glucagon levels. No changes occurred in the plasma concentrations of insulin and C-peptide. Acute insulin and C-peptide responses to intravenous pulses of different glucose amounts (0.33 g/kg and 5 g) and arginine (3 g) were significantly reduced by beta-endorphin infusion (P less than .01). This effect was associated with a significant reduction of the glucose disappearance rates, suggesting that the inhibition of insulin was of biological relevance. beta-Endorphin also inhibited glucose suppression of glucagon levels and augmented the glucagon response to arginine. To verify whether the modification of prestimulus glucose level could be important in these hormonal responses to beta-endorphin, basal plasma glucose concentrations were raised by a primed (0.5 g/kg) continuous (20 mg kg-1.min-1) glucose infusion. After stabilization of plasma glucose levels (350 +/- 34 mg/dl, t = 120 min), beta-endorphin infusion caused an immediate and marked increase in plasma insulin level (peak response 61 +/- 9 microU/ml, P less than .01), which remained elevated even after the discontinuation of opioid infusion. Moreover, the acute insulin response to a glucose pulse (0.33 g/kg i.v.) given during beta-endorphin infusion during hyperglycemia was significantly higher than the response obtained during euglycemia (171 +/- 32 vs. 41 +/- 7 microU/ml, P less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of insulin release by cyclosporine and production of peripheral insulin resistance in the dog

Cyclosporine has been shown to cause glucose intolerance in both humans and animals. This can result from alterations in insulin release, insulin metabolism, the sensitivity of peripheral or hepatic tissues to insulin, or a combination of these factors. The present study was designed to simultaneously evaluate the effect of CsA on these variables. A group of chronically catheterized dogs were administered oral CsA (20 mg/kg/day) for a period of 10 weeks. The glucagon stimulation test (GST) and the euglycemic glucose clamp technique, using a primed continuous infusion of 3H-3-glucose and a continuous insulin infusion (0.8 mU/kg/min), were employed to evaluate pancreatic insulin release, peripheral glucose disposal rate (Rd), hepatic glucose output (HGO), and metabolic clearance rate (MCR) of insulin. The dogs were tested before and after 2, 6, and 10 weeks of CsA administration. Serum CsA levels were 358 +/- 85, 244 +/- 48, and 355 +/- 81 ng/ml at 2, 6, and 10 weeks, respectively (P = NS). Elevated fasting glucose and an abnormal glucose response to an i.v. bolus of glucagon (0.25 U) were noted after 2, 6, and 10 weeks of CsA administration. The areas under the glucose curve (AUCG) for 0-60 min were 9605 +/- 773, 11634 +/- 1226, 12380 +/- 719, and 12626 +/- 1560 mg/min/dl at 0, 2, 6, and 10 weeks, P(F3, 15 = 5.1) = 0.012, demonstrating a CsA-induced disturbance of glucose homeostasis. The areas under the insulin curve (AUCI) for 0-20 min of the insulin response curve were 2033 +/- 203, 1089 +/- 187, 1038 +/- 179, and 972 +/- 161 uU/min/dl at 0, 2, 6, and 10 weeks, P(F3, 15 = 13.1) less than 0.001, indicating a 50% reduction during CsA treatment. CsA did not affect basal Rd, but peripheral insulin resistance was noted in the insulin-stimulated state. Rd during the third hour of the insulin infusion decreased from 6.72 +/- 0.69 to 4.42 +/- 0.44, 5.02 +/- 0.64, 4.47 +/- 0.52 mg/kg/min at 0, 2, 6, and 10 weeks, respectively, P(F3, 15 = 6.94) less than 0.004. HGO suppression by insulin and MCR of insulin were not altered by CsA. Similarly, glucagon secretion did not appear to be influenced by CsA. In conclusion, this study has simultaneously evaluated the effect of CsA on several aspects of glucose and insulin metabolism in the dog. CsA administration produces abnormal glucose homeostasis by reducing pancreatic insulin release, in addition to inducing peripheral insulin resistance.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of dibutyryl cyclic AMP and theophylline on biliary cholesterol secretion

Glucagon increases hepatocellular cAMP and decreases biliary cholesterol output. In these experiments, we examined the relation between cAMP and biliary cholesterol secretion. Bile flow and composition were measured in conscious dogs previously prepared by cholecystectomy, ligation of the lesser pancreatic duct, and placement of duodenal and gastric cannulae. Sodium taurocholate (500 mg/hr) was given intravenously to stabilize bile flow. After 2 hr of taurocholate infusion, dibutyryl cyclic AMP (160 mg kg-1 hr-1) or theophylline (20 mg kg-1 hr-1) was administered intravenously. Dibutyryl cAMP caused a decrease in both cholesterol concentration (242 +/- 25 micrograms/ml to 81 +/- 11 micrograms/ml) and cholesterol output (692 +/- 102 micrograms/15 min to 382 +/- 47 micrograms/15 min). Theophylline decreased cholesterol concentration (282 +/- 39 micrograms/ml to 221 +/- 21 micrograms/ml), but there was no significant change in cholesterol output. Bile flow increased significantly with both dibutyryl cAMP (2.8 +/- 0.2 ml/15 min to 4.9 +/- 0.2 ml/15 min) and theophylline (2.6 +/- 0.4 ml/15 min to 4.2 +/- 0.4 ml/15 min). In additional experiments, aminophylline (85% theophylline, 15% ethylenediamine) was administered intravenously (24.7 mg kg-1 hr-1). Aminophylline reduced cholesterol concentration (59 +/- 6 micrograms/ml to 36 +/- 5 micrograms/ml), but cholesterol output was stable. Bile flow increased significantly (3.7 +/- 0.2 ml/15 min to 6.5 +/- 0.4 ml/15 min). The mechanisms of these changes remain unknown. The effect of dibutyryl cAMP on biliary cholesterol secretion supports but does not prove the hypothesis that glucagon decreases biliary cholesterol output via the second messenger, cAMP.     

Somatostatin release from isolated perfused rat pancreas. Possible role of endogenous somatostatin on insulin release.

In order to elucidate the role of endogenous somatostatin in the control of insulin and glucagon secretion, glucagon- or insulin-induced somatostatin release from the isolated perfused rat pancreas was studied. Immunoreactive somatostatin was persistently released for 60 min in response to perfusion by 5.5 mM glucose at concentrations ranging between 10 and 15 pg/ml. The addition of glucagon (10(-8), 10(-7), and 10(-6) M) caused a dose-related increase of somatostatin release. In contrast, insulin release, especially its first phase, was suppressed when concentrations of glucagon were increased. The addition of insulin (10(-7) M and 10(-6) M) had no significant effect on somatostatin and glucagon release. These results raise the possibility that endogenous somatostatin and glucagon together regulate insulin secretion, suggesting a close interrelationship between insulin, glucagon, and somatostatin secretion within the islet.     

Greater efficacy of pulsatile insulin in type I diabetics critically depends on plasma glucagon levels

The aim of this study was to investigate the role of plasma glucagon levels on the blood glucose response to intravenous insulin administered continuously or in a pulsatile manner. Six type I diabetic patients proven to have no residual insulin secretion were investigated. Endogenous glucagon secretion was inhibited by a continuous intravenous infusion of somatostatin (100 micrograms/h) and replaced by exogenous infusions of the hormone at three different rates (7.5, 4.5, and 2.5 micrograms/h), resulting in three different plasma glucagon steady-state levels (i.e., approximately equal to 200, approximately equal to 130, and approximately equal to 75 pg/ml, respectively). Each subject, in random order and on different days, was infused intravenously with regular human insulin either continuously (0.17 mU X kg-1 X min-1) or with the same amount of insulin infused in a pulsatile manner (0.85 mU X kg-1 X min-1 during 2 min followed by 8 min during which no insulin was infused). At plasma glucagon levels approximately equal to 200 pg/ml, blood glucose rose from approximately 10 to approximately 13 mM without any difference between the two modalities of insulin infusion. For plasma glucagon levels approximately equal to 130 pg/ml, plasma glucose remained steady throughout the experiments, but during the last 40 min, plasma glucose levels were significantly lower when insulin was administered intermittently. This greater blood glucose-lowering effect of pulsatile insulin occurred earlier and was more pronounced for plasma glucagon levels averaging 75 pg/ml. We conclude that the greater hypoglycemic effect of insulin administered intravenously in a pulsatile manner in type I diabetics critically depends on plasma glucagon circulating levels.     

24-hour studies of the effects of somatostatin on the levels of plasma growth hormone, glucagon, and glucose in normal subjects and juvenile diabetics

Somatostatin was infused in various doses into normal subjects and juvenile diabetics for a 24-hour period preceded by a 24-hour control period and followed by another three-hour control period. Saline was infused during the first control period. Meals were served during the two 24-hour periods. Blood samples were taken hourly. Five normal males received a total dose of 4 mg. somatostatin. Four male diabetics received 2 mg., four received 4 mg., and four 6 mg. In the diabetics, somatostatin suppressed plasma growth hormone, glucagon, and glucose throughout the infusion. All parameters rebounded at cessation of infusion. In the normals, somatostatin suppressed plasma growth hormone, glucagon, and insulin but increased plasma glucose. It is concluded that the plasma glucose suppression in the diabetics is mainly due to the suppression of the diabetogenic hormones growth hormone and glucagon. A minor effect of decreased and/or delayed absorption of carbohydrates cannot be excluded in these experiments. The elevated plasma glucose levels in normals must be due to the suppressive effects of somatostatin on insulin secretion.     

Influence of arginine on splanchnic glucose metabolism in man.

To examine the mechanism of the arginine-induced rise in blood glucose concentration, splanchnic glucose output (SGO) and precursor uptake were studied during i.v. infusion of arginine (30 g/30 min) with and without somatostatin infusion (500 microgram/h, 90 min) in postabsorptive and in 60-h fasted healthy subjects. The hepatic venous catheter technique was employed. In the postabsorptive state, arginine infusion was accompanied by an eightfold and a fivefold increment, respectively, in the hepatic venous concentration of insulin and glucagon; SGO doubled and blood glucose increased by 30%. After cessation of arginine infusion, SGO and blood glucose returned to basal levels within 30 min. When both arginine and somatostatin were administered, glucagon rose threefold, whereas the insulin response was abolished. And while the rise in SGO during arginine infusion and its subsequent decline were uninfluenced by the simultaneous infusion of somatostatin, the rise in blood glucose was more pronounced and the glucose concentration remained elevated longer than in control studies without somatostatin. Splanchnic uptake of glucogenic precursors was uninfluenced by arginine infusion, with or without simultaneous somatostatin administration. In the 60-h fasted group, arginine infusion was accompanied by a minimal increase in insulin but a fivefold elevation of the glucagon level. Combined arginine and somatostatin infusion did not boost insulin significantly but the glucagon level rose threefold above the basal value. Basal SGO was 55% lower than in the postabsorptive state, and it rose in response to arginine administration (+50%) as well as during combined arginine and somatostatin infusion (+80%). No significant change in splanchnic uptake of glucogenic precursors was observed during arginine infusion with or without somatostatin administration. We conclude that (1) arginine infusion is accompanied by a rise in SGO and blood glucose due to arginine-induced stimulation of glucagon secretion, (2) the rise in SGO is caused primarily by glucagon-stimulated hepatic glycogenolysis, and (3) combined somatostatin and arginine administration is accompanied by a more marked rise in blood glucose due to hypoinsulinemia and reduced peripheral glucose utilization.     

Examination of the role of the pituitary-adrenocortical axis, counterregulatory hormones, and insulin clearance in variable nocturnal insulin requirements in insulin-dependent diabetes

In insulin-dependent diabetics, insulin requirements increase significantly after 0600 h, resulting in prebreakfast hyperglycemia with either conventional insulin therapy or constant insulin infusions with insulin infusion devices. In order to clarify the role of the pituitary-adrenocortical axis and further examine the mechanisms of the phenomenon of nocturnal variability in insulin requirements, we studied five IDDs using a closed-loop insulin infusion device (Biostator, GCIIS). The subjects were given saline (SAL) or dexamethasone (DEX) i.v. from 1800 to 0900 h on successive nights. From 2400-0300 to 0600-0900 h, mean insulin infusion rates required to maintain blood glucose values between 109 and 120 mg/dl increased by 0.21 +/- 0.05 mU/kg/min during the SAL infusion, and 0.16 +/- 0.04 mU/kg/min during the DEX infusion, when plasma cortisols were suppressed to less than or equal to 2 micrograms/dl. Mean free insulin concentrations did not increase and remained constant throughout both study nights in spite of the significantly higher 0600-0900-h insulin infusion rates. Growth hormone, glucagon, epinephrine, and norepinephrine concentrations showed normal nocturnal and early morning patterns during both study nights. We conclude that the nocturnal variability in insulin requirements persists despite suppression of the pituitary-adrenocortical axis, and that increased free insulin clearance or degradation may contribute to the "dawn phenomenon" of rising prebreakfast glucose despite constant insulin infusion.     

Adenosine reversal of in vivo hepatic responsiveness to insulin

Modulation by adenosine of hepatic responsiveness to insulin was investigated in vivo in 10 healthy mongrel dogs of both sexes by determining net hepatic glucose output (NHGO) in response to insulin during the presence or absence of exogenous adenosine infusion. In addition, two separate series of experiments were performed to study the effect of adenosine (n = 7) or glucagon (n = 5) on NHGO. Basal NHGO, quantitated via the Fick principle, was significantly decreased by insulin infusion (4 U/min; 4.8 +/- 0.6 vs. -1.7 +/- 2.6 mg.kg-1.min-1, P less than 0.05). The addition of an intrahepatic arterial infusion of adenosine (10 mumol/min) during insulin infusion caused glucose output to return to basal levels (insulin, -1.7 +/- 2.6 mg.kg-1.min-1; insulin + adenosine, 3.8 +/- 1.6 mg.kg-1.min-1, P less than 0.05). The addition of intrahepatic arterial saline (control) during insulin infusion had no effect on insulin's action (insulin, -1.0 +/- 1.9 mg.kg-1.min-1; insulin + saline, -1.2 +/- 1.6 mg.kg-1.min-1, P greater than 0.05). Hepatic glucose, lactate, and oxygen deliveries were not affected during either insulin or insulin plus adenosine infusion. Intrahepatic arterial infusion of adenosine alone had no effect on NHGO, whereas intrahepatic arterial infusion of glucagon alone stimulated glucose output approximately fivefold (basal, 2.7 +/- 0.4 mg.kg-1.min-1; glucagon, 15.5 +/- 1.2 mg.kg-1.min-1, P less than 0.01). These results show that adenosine completely reversed the inhibition by insulin of NHGO. These data suggest that adenosine may act as a modulator of insulin action on the liver.     

Fuel-induced insulin release in vitro from insulinomas transplanted into the rat kidney

We studied the release of insulin, glucagon, and somatostatin in response to glucose, glyceraldehyde (GA), and alpha-ketoisocaproate (KIC) from rat kidneys containing transplanted insulinomas. Kidneys were perfused about 11 wk after transplantation when the plasma glucose concentration of the fed animals had decreased from 180 +/- 7 to 95.1 +/- 9.9 mg/dl and plasma insulin concentrations had increased from 2.6 +/- 0.5 to 14.2 +/- 2.0 ng/ml. The insulin content of the tumor-containing kidney ranged from 40 to 679 micrograms; the glucagon and somatostatin concentrations ranged from undetectable levels to 3.7 micrograms and 248 ng, respectively. The average response to 30 mM glucose and 10 mM GA was a four- to fivefold increase in insulin secretion, whereas 30 mM KIC caused a 16- to 28-fold increase. In vitro stimulation of the insulinoma with 30 mM glucose primed the beta-cell response to a second stimulus following a short rest period. Cytochalasin B did not enhance this primed glucose response. Diazoxide inhibited glucose, GA, and KIC-stimulated insulin release. Glucose, GA, and KIC stimulated glucagon release in 2 of 17 insulinomas studied here. Somatostatin release was not seen in any of the experiments. These findings show that this islet cell tumor transplanted under the kidney capsule releases insulin in response to physiologic and model fuel substances. Thus, this particular transplantable tumor offers an opportunity to study the biochemistry and biophysics that underlie fuel-stimulated insulin release.     

Exercise-induced changes in insulin and glucagon are not required for enhanced hepatic glucose uptake after exercise but influence the fate of glucose within the liver

To test whether pancreatic hormonal changes that occur during exercise are necessary for the postexercise enhancement of insulin-stimulated net hepatic glucose uptake, chronically catheterized dogs were exercised on a treadmill or rested for 150 min. At the onset of exercise, somatostatin was infused into a peripheral vein, and insulin and glucagon were infused in the portal vein to maintain basal levels (EX-Basal) or simulate the response to exercise (EX-Sim). Glucose was infused as needed to maintain euglycemia during exercise. After exercise or rest, somatostatin infusion was continued in exercised dogs and initiated in dogs that had remained sedentary. In addition, basal glucagon, glucose, and insulin were infused in the portal vein for 150 min to create a hyperinsulinemic-hyperglycemic clamp in EX-Basal, EX-Sim, and sedentary dogs. Steady-state measurements were made during the final 50 min of the clamp. During exercise, net hepatic glucose output (mg x kg(-1) x min(-1)) rose in EX-Sim (7.6 +/- 2.8) but not EX-Basal (1.9 +/- 0.3) dogs. During the hyperinsulinemic-hyperglycemic clamp that followed either exercise or rest, net hepatic glucose uptake (mg x kg(-1) x min(-1)) was elevated in both EX-Basal (4.0 +/- 0.7) and EX-Sim (4.6 +/- 0.5) dogs compared with sedentary dogs (2.0 +/- 0.3). Despite this elevation in net hepatic glucose uptake after exercise, glucose incorporation into hepatic glycogen, determined using [3-3H]glucose, was not different in EX-Basal and sedentary dogs, but was 50 +/- 30% greater in EX-Sim dogs. Exercise-induced changes in insulin and glucagon, and consequent glycogen depletion, are not required for the increase in net hepatic glucose uptake after exercise but result in a greater fraction of the glucose consumed by the liver being directed to glycogen.     

Somatostatin does not increase insulin-stimulated glucose uptake in humans

Somatostatin (SRIF) has been widely used in the study of in vivo carbohydrate metabolism to suppress pancreatic hormone secretion and thereby interrupt the glucoregulatory feedback loops between insulin, glucagon, and glucose. A critical assumption in the use of SRIF is that it has no effect on hepatic or peripheral glucose metabolism other than those mediated through the inhibition of hormone secretion. To assess whether doses of SRIF commonly used in human investigation have any effect on insulin-stimulated glucose disposal rates, we measured the rate in 6 normal subjects (mean fasting serum glucose level, 93 +/- 2 mg/dl) during euglycemic (approximately equal to 85 mg/dl) hyperinsulinemic (40 mU X m-2 X min-1) clamp studies both with and without the concomitant infusion of SRIF (600 micrograms/hr). The steady-state insulin levels achieved were 85 +/- 6 microU/ml and 74 +/- 8 microU/ml with and without SRIF, respectively (difference not significant). Glucose disposal rates between 120 and 180 min of the clamp were 7.11 +/- 0.10 and 7.35 +/- 0.10 mg X kg-1 X min-1 with and without SRIF, respectively (difference not significant). We concluded that in doses commonly used in human investigation, SRIF does not increase glucose disposal.     

Sulfonylurea compounds uncouple the glucose dependence of the insulinotropic effect of glucagon-like peptide 1

Glucagon-like peptide (GLP)-1 mimetics have been reported to cause hypoglycemia when combined with sulfonylureas. This study investigated the impact of tolbutamide on the glucose dependence of the GLP-1-mediated effects on insulin, glucagon, and somatostatin secretion in the in situ perfused rat pancreas. At 3 mmol/l glucose, GLP-1 alone did not augment insulin secretion, whereas tolbutamide alone caused a rapid increase in insulin secretion. However, when GLP-1 and tolbutamide were administered simultaneously, insulin secretion increased significantly to 43.7 +/- 6.2 pmol/min (means +/- SE), exceeding the sum of the responses to GLP-1 (2.0 +/- 0.6 pmol/min; P = 0.019) and tolbutamide (11.3 +/- 3.8; P = 0.005) alone by a factor of 3.3. At 11 mmol/l glucose, co-infusion of GLP-1 and tolbutamide augmented insulin secretion to 141.7 +/- 10.3 vs. 115.36 +/- 14.1 (GLP-1) and 42.5 +/- 7.3 pmol/min (tolbutamide). Interestingly, increases in somatostatin secretion, both by glucose and GLP-1, were consistently paralleled by suppression of glucagon release. In conclusion, we demonstrate uncoupling of GLP-1 from its glucose dependence by tolbutamide. This uncoupling probably explains the tendency of GLP-1 to provoke hypoglycemia in combination with sulfonylureas. The results suggest that closure of ATP-sensitive K(+) channels by glucose might be involved in the glucose dependence of GLP-1's insulinotropic effect and that somatostatin acts as a paracrine regulator of glucagon release.     

Evidence for an intrahepatic contribution to the waning effect of glucagon on glucose production in the conscious dog

We previously showed that infusion of glucagon at four times the basal rate into conscious dogs given somatostatin and basal replacement amounts of insulin caused hyperglycemia (217 15 mg/dl) for at least 3 h and an initial increment in hepatic glucose production of 5.5 0.8 mg/kg . min. After 3 h, however, the effect of hyperglucagonemia on glucose production had declined by 85%. The aim of the present study was to determine the importance of hyperglycemia in the "downregulation" of the action of glucagon. Six overnight-fasted conscious dogs were given somatostatin (0.8 microgram/kg . min) plus basal intraportal replacement amounts of insulin (263 microunits/kg . min) and glucagon (0.65 ng/kg . min). Hyperglycemia (276 12 md/dl) was established after 2 h by the infusion of exogenous glucose. The glucagon infusion rate was then increased fourfold 1 h later, so that the plasma glucagon level rose from 95 16 to 227 35 pg/ml. The glucose concentration was maintained at a fixed value despite the increase in glucagon by decreasing the glucose infusion rate by an amount equal to the increase in endogenous glucose production induced by the hormone. Glucose production was measured using a primed infusion of 3H-glucose. With the insulin (11 2 microunits/ml) and glucose levels fixed, the elevation in glucagon caused an initial increment of 5.1 0.7 mg/kg . min in glucose production which was followed by a fall of only 2.5 0.4 mg/kg . min (50%) over the next 3 hr. Thus, when the plasma glucagon level is raised fourfold under conditions in which insulin mobilization cannot occur, the effect of hyperglucagonemia on glucose production will be partially offset by the resulting hyperglycemia and partly inhibited by an hepatic factor(s).     

Similar dose responsiveness of hepatic glycogenolysis and gluconeogenesis to glucagon in vivo

This study was undertaken to determine whether the dose-dependent effect of glucagon on gluconeogenesis parallels its effect on hepatic glycogenolysis in conscious overnight-fasted dogs. Endogenous insulin and glucagon secretion were inhibited by somatostatin (0.8 micrograms X kg-1 X min-1), and intraportal replacement infusions of insulin (213 +/- 28 microU X kg-1 X min-1) and glucagon (0.65 ng X kg-1 X min-1) were given to maintain basal hormone concentrations for 2 h (12 +/- 2 microU/ml and 108 +/- 23 pg/ml, respectively). The glucagon infusion was then increased 2-, 4-, 8-, or 12-fold for 3 h, whereas the rate of insulin infusion was left unchanged. Glucose production (GP) was determined with 3-[3H]glucose, and gluconeogenesis (GNG) was assessed with tracer (U-[14C]alanine conversion to [14C]glucose) and arteriovenous difference (hepatic fractional extraction of alanine, FEA) techniques. Increases in plasma glucagon of 53 +/- 8, 199 +/- 48, 402 +/- 28, and 697 +/- 149 pg/ml resulted in initial (15-30 min) increases in GP of 1.1 +/- 0.4 (N = 4), 4.9 +/- 0.5 (N = 4), 6.5 +/- 0.6 (N = 6), and 7.7 +/- 1.4 (N = 4) mg X kg-1 X min-1, respectively; increases in GNG (approximately 3 h) of 48 +/- 19, 151 +/- 50, 161 +/- 25, and 157 +/- 7%, respectively; and increases in FEA (3 h) of 0.14 +/- 0.07, 0.37 +/- 0.05, 0.42 +/- 0.04, and 0.40 +/- 0.17, respectively. In conclusion, GNG and glycogenolysis were similarly sensitive to stimulation by glucagon in vivo, and the dose-response curves were markedly parallel.     

Studies on growth hormone secretion. VII. Effects of somatostatin on plasma GH, insulin, and glucagon in sheep.

To determine whether synthetic somatostatin originally isolated from sheep hypothalamus can inhibit hormone secretion in the same species, we measured plasma levels of GH, insulin, glucagon, and glucose of normal sheep under a variety of experimental conditions in the presence and absence of somatostatin infusion. An oral dose of 2.5 mg./kg. 3,5-dimethypyrazole increase plasma GH from 10.9 to 376.9 ng. per milliliter, which was suppressed by 50 per cent and 80 per cent with 0.5 and 1 mg. synthetic cyclic somatostatin, respectively. Linear somatostatin (0.5 mg.) was without effect in two animals tested. Propionate (0.5 mmole per kilogram) and arginine (10 gm.) induced a rise in plasma insulin and GH, and glucagon was effectively blocked by cyclic somatostatin (0.5 mg.). Similarly, somatostatin inhibited glucose, and glucagon provoked GH and insulin secretory responses without affecting glucose or FFA levels. Somatostatin had no effect on the disappearance of injected glucagon. Finally, addition of somatostatin to incubation media prevented PGE promoted GH release, and suppressed cyclic AMP accumulation, although to a lesser extent, in sheep anterior pituitary pieces. In view of the large amounts required to suppress stimulated hormone release and the general lack of specificity of somatostatin, it is suggested that this peptide may have a functional role only in the release of hormones of the pituitary, where it could occur in relatively high local concentrations. Its inhibition of extrapituitary hormone secretion may be purely a pharmacologic effect that, nevertheless, suggests an interference with a step common to the secretory process of hormones.