Role of the decrement in intraislet insulin for the glucagon response to hypoglycemia in humans

OBJECTIVE: Animal and in vitro studies indicate that a decrease in beta-cell insulin secretion, and thus a decrease in tonic alpha-cell inhibition by intraislet insulin, may be an important factor for the increase in glucagon secretion during hypoglycemia. However, in humans this role of decreased intraislet insulin is still unclear. RESEARCH DESIGN AND METHODS: We studied glucagon responses to hypoglycemia in 14 nondiabetic subjects on two separate occasions. On both occasions, insulin was infused from 0 to 120 min to induce hypoglycemia. On one occasion, somatostatin was infused from -60 to 60 min to suppress insulin secretion, so that the decrement in intraislet insulin during the final 60 min of hypoglycemia would be reduced. On the other occasion, subjects received an infusion of normal saline instead of the somatostatin. RESULTS: During the 2nd h of the insulin infusion, when somatostatin or saline was no longer being infused, plasma glucose ( approximately 2.6 mmol/l) and insulin levels ( approximately 570 pmol/l) were comparable in both sets of experiments (both P > 0.4). In the saline experiments, insulin secretion remained unchanged from baseline (-90 to -60 min) before insulin infusion and decreased from 1.20 +/- 0.12 to 0.16 +/- 0.04 pmol . kg(-1) . min(-1) during insulin infusion (P < 0.001). However, in the somatostatin experiments, insulin secretion decreased from 1.18 +/- 0.12 pmol . kg(-1) . min(-1) at baseline to 0.25 +/- 0.09 pmol . kg(-1) . min(-1) before insulin infusion so that it did not decrease further during insulin infusion (-0.12 +/- 0.10 pmol . kg(-1) . min(-1), P = 0.26) indicating the complete lack of a decrement in intraislet insulin during hypoglycemia. This was associated with approximately 30% lower plasma glucagon concentrations (109 +/- 7 vs. 136 +/- 9 pg/ml, P < 0.006) and increments in plasma glucagon above baseline (41 +/- 8 vs. 67 +/- 11 pg/ml, P < 0.008) during the last 15 min of the hypoglycemic clamp. In contrast, increases in plasma growth hormone were approximately 70% greater during hypoglycemia after somatostatin infusion (P < 0.007), suggesting that to some extent the increases in plasma glucagon might have reflected a rebound in glucagon secretion. CONCLUSIONS: These results provide direct support for the intraislet insulin hypothesis in humans. However, the exact extent to which a decrement in intraislet insulin accounts for the glucagon responses to hypoglycemia remains to be established.     

Increasing the decrement in insulin secretion improves glucagon responses to hypoglycemia in advanced type 2 diabetes

OBJECTIVE: In advanced beta-cell failure, counterregulatory glucagon responses may be impaired due to a reduced decrement in insulin secretion during the development of hypoglycemia. The present studies were therefore undertaken to test the hypothesis that these may be improved by increasing this decrement in insulin secretion. RESEARCH DESIGN AND METHODS: Twelve subjects with type 2 diabetes who have been insulin requiring were studied as a model of advanced beta-cell failure. Glucagon responses were examined during a 90-min hypoglycemic clamp (approximately 2.8 mmol/l) on two separate occasions. On one occasion, tolbutamide was infused for 2 h before the clamp so that the decrement in insulin secretion during the induction of hypoglycemia would be increased. On the other occasion, normal saline was infused as a control. RESULTS: Before the hypoglycemic clamp, infusion of tolbutamide increased insulin secretion approximately 1.9-fold (P < 0.001). However, during hypoglycemia, insulin secretion decreased to similar rates on both occasions (P = 0.31) so that its decrement was approximately twofold greater following the tolbutamide infusion (1.63 +/- 0.20 vs. 0.81 +/- 0.17 pmol x kg(-1) x min(-1), P < 0.001). This was associated with more than twofold-greater glucagon responses (42 +/- 11 vs. 19 +/- 8 ng/l, P < 0.002) during the hypoglycemic clamp but unaltered glucagon responses to intravenous arginine immediately thereafter (449 +/- 50 vs. 453 +/- 50 ng/l, P = 0.78). CONCLUSIONS: Increasing the decrement in insulin secretion during the development of hypoglycemia improves counterregulatory glucagon responses in advanced beta-cell failure. These findings further support the concept that the impaired counterregulatory glucagon responses in advanced beta-cell failure may at least partially be due to a reduced decrement in insulin secretion.     

Defective suppression by insulin of leucine-carbon appearance and oxidation in type 1, insulin-dependent diabetes mellitus. Evidence for insulin resistance involving glucose and amino acid metabolism.

To determine whether a resistance to insulin in type 1, insulin-dependent diabetes mellitus (IDDM) is extended to both glucose and amino acid metabolism, six normal subjects and five patients with IDDM, maintained in euglycemia with intravenous insulin administration, were infused with L-[4,5-3H]leucine (Leu) and [1-14C]alpha ketoisocaproate (KIC). Steady-state rates of leucine-carbon appearance derived from protein breakdown (Leu + KIC Ra) and KIC (approximately leucine) oxidation were determined at basal and during sequential euglycemic, hyperinsulinemic (approximately 40, approximately 90 and approximately 1,300 microU/ml) clamps. In the euglycemic postabsorptive diabetic patients, despite basal hyperinsulinemia (24 +/- 6 microU/ml vs. 9 +/- 1 microU/ml in normals, P less than 0.05), Leu + KIC Ra (2.90 +/- 0.18 mumol/kg X min), and KIC oxidation (0.22 +/- 0.03 mumol/kg X min) were similar to normal values (Leu + KIC Ra = 2.74 +/- 0.25 mumol/kg X min) (oxidation = 0.20 +/- 0.02 mumol/kg X min). During stepwise hyperinsulinemia, Leu + KIC Ra in normals decreased to 2.08 +/- 0.19, to 2.00 +/- 0.17, and to 1.81 +/- 0.16 mumol/kg X min, but only to 2.77 +/- 0.16, to 2.63 +/- 0.16, and to 2.39 +/- 0.08 mumol/kg X min in the diabetic patients (P less than 0.05 or less vs. normals at each clamp step). KIC oxidation decreased in normal subjects to a larger extent than in the diabetic subjects. Glucose disposal was reduced at all insulin levels in the patients. In summary, in IDDM: (a) Peripheral hyperinsulinemia is required to normalize both fasting leucine metabolism and blood glucose concentrations. (b) At euglycemic hyperinsulinemic clamps, lower glucose disposal rates and a defective suppression of leucine-carbon appearance and oxidation were observed. We conclude that in type 1 diabetes a resistance to the metabolic effects of insulin on both glucose and amino acid metabolism is present.     

Regulation of glucose homeostasis in humans with denervated livers.

The liver plays a major role in regulating glucose metabolism, and since its function is influenced by sympathetic/ parasympathetic innervation, we used liver graft as a model of denervation to study the role of CNS in modulating hepatic glucose metabolism in humans. 22 liver transplant subjects were randomly studied by means of the hyperglycemic/ hyperinsulinemic (study 1), hyperglycemic/isoinsulinemic (study 2), euglycemic/hyperinsulinemic (study 3) as well as insulin-induced hypoglycemic (study 4) clamp, combined with bolus-continuous infusion of [3-3H]glucose and indirect calorimetry to determine the effect of different glycemic/insulinemic levels on endogenous glucose production and on peripheral glucose uptake. In addition, postabsorptive glucose homeostasis was cross-sectionally related to the transplant age (range = 40 d-35 mo) in 4 subgroups of patients 2, 6, 15, and 28 mo after transplantation. 22 subjects with chronic uveitis (CU) undergoing a similar immunosuppressive therapy and 35 normal healthy subjects served as controls. The results showed that successful transplantation was associated with fasting glucose concentration and endogenous glucose production in the lower physiological range within a few weeks after transplantation, and this pattern was maintained throughout the 28-mo follow-up period. Fasting glucose (4. 55+/-0.06 vs. 4.75+/-0.06 mM; P = 0.038) and endogenous glucose production (11.3+/-0.4 vs. 12.9+/-0.5 micromol/[kg.min]; P = 0.029) were lower when compared to CU and normal patients. At different combinations of glycemic/insulinemic levels, liver transplant (LTx) patients showed a comparable inhibition of endogenous glucose production. In contrast, in hypoglycemia, after a temporary fall endogenous glucose production rose to values comparable to those of the basal condition in CU and normal subjects (83+/-5 and 92+/-5% of basal), but it did not in LTx subjects (66+/-7%; P < 0.05 vs. CU and normal subjects). Fasting insulin and C-peptide levels were increased up to 6 mo after transplantation, indicating insulin resistance partially induced by prednisone. In addition, greater C-peptide but similar insulin levels during the hyperglycemic clamp (study 1) suggested an increased hepatic insulin clearance in LTx as compared to normal subjects. Fasting glucagon concentration was higher 6 mo after transplantation and thereafter. During euglycemia/hyperinsulinemia (study 3), the insulin-induced glucagon suppression detectable in CU and normal subjects was lacking in LTx subjects; furthermore, the counterregulatory response during hypoglycemia was blunted. In summary, liver transplant subjects have normal postabsorptive glucose metabolism, and glucose and insulin challenge elicit normal response at both hepatic and peripheral sites. Nevertheless, (a) minimal alteration of endogenous glucose production, (b) increased concentration of insulin and glucagon, and (c) defective counterregulation during hypoglycemia may reflect an alteration of the liver-CNS-islet circuit which is due to denervation of the transplanted graft.     

Defective glucose counterregulation after subcutaneous insulin in noninsulin-dependent diabetes mellitus. Paradoxical suppression of glucose utilization and lack of compensatory increase in glucose production, roles of insulin resistance, abnormal neuroendocrine responses, and islet paracrine interactions.

To characterize glucose counterregulatory mechanisms in patients with noninsulin-dependent diabetes mellitus (NIDDM) and to test the hypothesis that the increase in glucagon secretion during hypoglycemia occurs primarily via a paracrine islet A-B cell interaction, we examined the effects of a subcutaneously injected therapeutic dose of insulin (0.15 U/kg) on plasma glucose kinetics, rates of glucose production and utilization, and their relationships to changes in the circulating concentrations of neuroendocrine glucoregulatory factors (glucagon, epinephrine, norepinephrine, growth hormone, and cortisol), as well as to changes in endogenous insulin secretion in 13 nonobese NIDDM patients with no clinical evidence of autonomic neuropathy. Compared with 11 age-weight matched nondiabetic volunteers in whom euglycemia was restored primarily by a compensatory increase in glucose production, in the diabetics there was no compensatory increase in glucose production (basal 2.08 +/- 0.04----1.79 +/- 0.07 mg/kg per min at 21/2 h in diabetics vs. basal 2.06 +/- 0.04----2.32 +/- 0.11 mg/kg per min at 21/2 h in nondiabetics, P less than 0.01) despite the fact that plasma insulin concentrations were similar in both groups (peak values 22 +/- 2 vs. 23 +/- 2 microU/ml in diabetics and nondiabetics, respectively). This abnormality in glucose production was nearly completely compensated for by a paradoxical decrease in glucose utilization after injection of insulin (basal 2.11 +/- 0.03----1.86 +/- 0.06 mg/kg per min at 21/2 h in diabetics vs. basal 2.08 +/- 0.04----2.39 +/- 0.11 mg/kg per min at 21/2 h nondiabetics, P less than 0.01), which could not be accounted for by differences in plasma glucose concentrations; the net result was a modest prolongation of hypoglycemia. Plasma glucagon (area under the curve [AUC] above base line, 12 +/- 3 vs. 23 +/- 3 mg/ml X 12 h in nondiabetics, P less than 0.05), cortisol (AUC 2.2 +/- 0.5 vs. 4.0 +/- 0.7 mg/dl X 12 h in nondiabetics, P less than 0.05), and growth hormone (AUC 1.6 +/- 0.4 vs. 2.9 +/- 0.4 micrograms/ml X 12 h in nondiabetics, P less than 0.05) responses in the diabetics were decreased 50% while their plasma norepinephrine responses (AUC 49 +/- 12 vs. 21 +/- 5 ng/ml X 12 h in nondiabetics, P less than 0.05) were increased twofold (P less than 0.05) and their plasma epinephrine responses were similar to those of the nondiabetics (AUC 106 +/- 17 vs. 112 +/- 10 ng/ml X 12 h in nondiabetics). In both groups of subjects, increases in plasma glucagon were inversely correlated with plasma glucose concentrations (r = -0.80 in both groups, P less than 0.01) and suppression of endogenous insulin secretion (r = -0.57 in nondiabe     

Plasma catecholamine responses to hypoglycemia in children and adolescents with IDDM

OBJECTIVE: The goal of this study was to assess whether children and adolescents with insulin-dependent diabetes mellitus (IDDM) have decreased catecholamine responses to insulin-induced hypoglycemia as has been reported in adults and to explore the pathogenesis of the decreased response in terms of possible relationships to autonomic neuropathy or hyperinsulinism. RESEARCH DESIGN AND METHODS: A before-and-after trial on the effects of 3 days of intensive insulin therapy was conducted on 60 subjects with IDDM (age 15.4 +/- 2.6 yr, duration of diabetes 7.8 +/- 3.5 yr). The control group consisted of 5 children with non-growth hormone-deficient short stature (age 14.8 +/- 3.2 yr). Hypoglycemia was induced with an intravenous insulin bolus (0.15-0.75 U/kg) after insulin withdrawal and 3 days of intensive insulin therapy in diabetic subjects on an inpatient basis to assess the role of hyperinsulinism in suppressing the catecholamine response to hypoglycemia. Control subjects were studied once and received an insulin bolus of 0.1 microU/kg. Autonomic neuropathy was assessed by computerized assessment of the basal R-R variation during inspiration and expiration and the pancreatic polypeptide response to hypoglycemia. RESULTS: Basal plasma catecholamine levels were lower in diabetic subjects after intensive insulin therapy than in control subjects (P = 0.008). The peak and incremental catecholamine responses after insulin withdrawal and intensive insulin therapy in IDDM subjects were significantly decreased compared with control subjects (P less than 0.001). Peak catecholamine responses to hypoglycemia in IDDM were decreased after intensive insulin therapy (P = 0.002). This was particularly true in those with plasma glucose nadir levels of less than 2.2 mmol (P less than 0.001). The diminished catecholamine responses were primarily due to decreased peak epinephrine responses after intensive insulin therapy compared with insulin withdrawal (P = 0.011). There were no significant correlations between the catecholamine response to hypoglycemia and age, duration of diabetes, pancreatic polypeptide, or R-R interval. CONCLUSIONS: These results suggest that children and adolescents with IDDM after insulin withdrawal have diminished catecholamine response to hypoglycemia compared with control subjects and indicate that short-term intensive insulin therapy diminishes this response further. Thus, hyperinsulinism may play a role in suppressing the catecholamine response to hypoglycemia. There is no evidence for a clinical or subclinical role of autonomic neuropathy to explain the altered catecholamine responses.     

Role of Glucagon, Catecholamines, and Growth Hormone in Human Glucose Counterregulation: EFFECTS OF SOMATOSTATIN AND COMBINED α- AND β-ADRENERGIC BLOCKADE ON PLASMA GLUCOSE RECOVERY AND GLUCOSE FLUX RATES AFTER INSULIN-INDUCED HYPOGLYCEMIA

To further characterize mechanisms of glucose counterregulation in man, the effects of pharmacologically inducd deficiencies of glucagon, growth hormone, and catecholamines (alone and in combination) on recovery of plasma glucose from insulin-induced hypoglycemia and attendant changes in isotopically ([3-(3)H]glucose) determined glucose fluxes were studied in 13 normal subjects. In control studies, recovery of plasma glucose from hypoglycemia was primarily due to a compensatory increase in glucose production; the temporal relationship of glucagon, epinephrine, cortisol, and growth hormone responses with the compensatory increase in glucose appearance was compatible with potential participation of all these hormones in acute glucose counterregulation. Infusion of somatostatin (combined deficiency of glucagon and growth hormone) accentuated insulin-induced hypoglycemia (plasma glucose nadir: 36+/-2 ng/dl during infusion of somatostatin vs. 47+/-2 mg/dl in control studies, P < 0.01) and impaired restoration of normoglycemia (plasma glucose at min 90: 73+/-3 mg/dl at end of somatostatin infusion vs. 92+/-3 mg/dl in control studies, P<0.01). This impaired recovery of plasma glucose was due to blunting of the compensatory increase in glucose appearance since glucose disappearance was not augmented, and was attributable to suppression of glucagon secretion rather than growth hormone secretion since these effects of somatostatin were not observed during simultaneous infusion of somatostatin and glucagon whereas infusion of growth hormone along with somatostatin did not prevent the effect of somatostatin. The attenuated recovery of plasma glucose from hypoglycemia observed during somatostatin-induced glucagon deficiency was associated with plasma epinephrine levels twice those observed in control studies. Infusion of phentolamine plus propranolol (combined alpha-and beta-adrenergic blockade) had no effect on plasma glucose or glucose fluxes after insulin administration. However, infusion of somatostatin along with both phentolamine and propranolol further impaired recovery of plasma glucose from hypoglycemia compared to that observed with somatostatin alone (plasma glucose at end of infusions: 52+/-6 mg/dl for somatostatin-phentolamine-propranolol vs. 72+/-5 mg/dl for somatostatin alone, P < 0.01); this was due to further suppression of the compensatory increase in glucose appearance (maximal values: 1.93+/-0.41 mg/kg per min for somatostatin-phentolamine-propranolol vs. 2.86+/-0.32 mg/kg per min for somatostatin alone, P < 0.05). These results indicate that in man (a) restoration of normoglycemia after insulin-induced hypoglycemia is primarily due to a compensatory increase in glucose production; (b) intact glucagon secretion, but not growth hormone secretion, is necessary for normal glucose counterregulation, and (c) adrenergic mechanisms do not normally play an essential role in this process but become critical to recovery from hypoglycemia when glucagon secretion is impaired.     

Enhanced glycemic responsiveness to epinephrine in insulin-dependent diabetes mellitus is the result of the inability to secrete insulin. Augmented insulin secretion normally limits the glycemic, but not the lipolytic or ketogenic, response to epinephrine in humans.

To determine if the enhanced glycemic response to epinephrine in patients with insulin-dependent diabetes mellitus (IDDM) is the result of increased adrenergic sensitivity per se, increased glucagon secretion, decreased insulin secretion, or a combination of these, plasma epinephrine concentration-response curves were determined in insulin-infused (initially euglycemic) patients with IDDM and nondiabetic subjects on two occasions: once when insulin and glucagon were free to change (control study), and again when insulin and glucagon were held constant (islet clamp study). During the control study, plasma C-peptide doubled, and glucagon did not change in the nondiabetic subjects, whereas plasma C-peptide did not change but glucagon increased in the patients. The patients with IDDM exhibited threefold greater increments in plasma glucose, largely the result of greater increments in glucose production. This enhanced glycemic response was apparent with 30-min increments in epinephrine to plasma concentrations as low as 100-200 pg/ml, levels that occur commonly under physiologic conditions. During the islet clamp study (somatostatin infusion with insulin and glucagon replacement at fixed rates), the heightened glycemic response was unaltered in the patients with IDDM, but the nondiabetic subjects exhibited an enhanced glycemic response to epinephrine indistinguishable from that of patients with IDDM. In contrast, the FFA, glycerol, and beta-hydroxybutyrate responses were unaltered. Thus, we conclude the following: Short, physiologic increments in plasma epinephrine cause greater increments in plasma glucose in patients with IDDM than in nondiabetic subjects, a finding likely to be relevant to glycemic control during the daily lives of such patients as well as during the stress of intercurrent illness. Enhanced glycemic responsiveness of patients with IDDM to epinephrine is not the result of increased sensitivity of adrenergic receptor-effector mechanisms per se nor of their increased glucagon secretory response; rather, it is the result of their inability to augment insulin secretion. Augmented insulin secretion, albeit restrained, normally limits the glycemic response, but not the lipolytic or ketogenic responses, to epinephrine in humans.     

Glucagon, catecholamine and pancreatic polypeptide secretion in type I diabetic recipients of pancreas allografts.

Successful pancreas transplantation in type I diabetic patients restores normal fasting glucose levels and biphasic insulin responses to glucose. However, virtually no data from pancreas recipients are available relative to other islet hormonal responses or hormonal counterregulation of hypoglycemia. Consequently, glucose, glucagon, catecholamine, and pancreatic polypeptide responses to insulin-induced hypoglycemia and to stimulation with arginine and secretin were examined in 38 diabetic pancreas recipients, 54 type I diabetic nonrecipients, and 26 nondiabetic normal control subjects. Glucose recovery after insulin-induced hypoglycemia in pancreas recipients was significantly improved. Basal glucagon levels were significantly higher in recipients compared with nonrecipients and normal subjects. Glucagon responses to insulin-induced hypoglycemia were significantly greater in the pancreas recipients compared with nonrecipients and similar to that observed in control subjects. Glucagon responses to intravenous arginine were significantly greater in pancreas recipients than that observed in both the nonrecipients and normal subjects. No differences were observed in epinephrine responses during insulin-induced hypoglycemia. No differences in pancreatic polypeptide responses to hypoglycemia were observed when comparing the recipient and nonrecipient groups, both of which were less than that observed in the control subjects. Our data demonstrate significant improvement in glucose recovery after hypoglycemia which was associated with improved glucagon secretion in type I diabetic recipients of pancreas transplantation.     

Impaired hormonal responses to hypoglycemia in spontaneously diabetic and recurrently hypoglycemic rats. Reversibility and stimulus specificity of the deficits.

To evaluate the roles of iatrogenic hypoglycemia and diabetes per se in the pathogenesis of defective hormonal counterregulation against hypoglycemia in insulin-dependent diabetes mellitus (IDDM), nondiabetic, and spontaneously diabetic BB/Wor rats were studied using a euglycemic/hypoglycemic clamp. In nondiabetic rats, recurrent (4 wk) insulin-induced hypoglycemia (mean daily glucose, MDG, 59 mg/dl) dramatically reduced glucagon and epinephrine responses by 84 and 94%, respectively, to a standardized glucose fall from 110 to 50 mg/dl. These deficits persisted for > 4 d after restoring normoglycemia, and were specific for hypoglycemia, with normal glucagon and epinephrine responses to arginine and hypovolemia, respectively. After 4 wk of normoglycemia, hormonal counterregulation increased, with the epinephrine, but not the glucagon response reaching control values. In diabetic BB rats (MDG 245 mg/dl with intermittent hypoglycemia), glucagon and epinephrine counterregulation were reduced by 86 and 90%, respectively. Chronic iatrogenic hypoglycemia (MDG 52 mg/dl) further suppressed counterregulation. Prospective elimination of hypoglycemia (MDG 432 mg/dl) improved, but did not normalize hormonal counterregulation. In diabetic rats, the glucagon defect appeared to be specific for hypoglycemia, whereas deficient epinephrine secretion also occurred during hypovolemia. We concluded that both recurrent hypoglycemia and the diabetic state independently lead to defective hormonal counterregulation. These data suggest that in IDDM iatrogenic hypoglycemia magnifies preexisting counterregulatory defects, thereby increasing the risk of severe hypoglycemia.     

The role of adrenergic mechanisms in the substrate and hormonal response to insulin-induced hypoglycemia in man.

Sequential determinations of glucose outflow and inflow, and rates of gluconeogenesis from alanine, before, during and after insulin-induced hypoglycemia were obtained in relation to alterations in circulating epinephrine, norepinephrine, glucagon, cortisol, and growth hormone in six normal subjects. Insulin decreased the mean (+/-SEM) plasma glucose from 89+/-3 to 39+/-2 mg/dl 25 min after injection, but this decline ceased despite serum insulin levels of 153+/-22 mul/ml. Before insulin, glucose inflow and outflow were constant averaging 125.3+/-7.1 mg/kg per h. 15 min after insulin, mean glucose outflow increased threefold, but then decreased at 25 min, reaching a rate 15% less than the preinsulin rate. Glucose inflow decreased 80% 15 min after insulin, but increased at 25 min, reaching a maximum of twice the basal rate. Gluconeogenesis from alanine decreased 68% 15 min after insulin, but returned to preinsulin rates at 25 min, and remained constant for the next 25 min, after which it increased linearly. A fourfold increase in mean plasma epinephrine was found 20 min after insulin, with maximal levels 50 times basal. Plasma norepinephrine concentrations first increased significantly at 25 min after insulin, whereas significantly increased levels of cortisol and glucagon occurred at 30 min, and growth hormone at 40 min after insulin. Thus, insulin-induced hypoglycemia in man results from both a decrease in glucose production and an increase in glucose utilization. Accelerated glycogenolysis produced much of the initial, posthypoglycemic increment in glucose production. The contribution of glycogenolysis decreased with time, while that of gluconeogenesis from alanine increased. Of the hormones studied, only the increments in plasma catecholamines preceded or coincided with the measured increase in glucose production after hypoglycemia. It therefore seems probable that adrenergic mechanisms play a major role in the initiation of counter-regulatory responses to insulin-induced hypoglycemia in man.     

Role of changes in insulin and glucagon in glucose homeostasis in exercise.

This experiment was performed to determine if plasma glucose homeostasis is maintained in normal human volunteers during light exercise (40% maximal oxygen consumption [VO2 max]) when changes in insulin and glucagon are prevented. Hormonal control was achieved by the infusion of somatostatin, insulin, and glucagon. Glucose kinetics and oxidation rates were determined with stable isotopic tracers of glucose, and by indirect calorimetry. Two different rates of replacement of insulin and glucagon were used; in one group, insulin was clamped at 19.8 +/- 2.6 microU/ml (high-insulin group), and in the other group insulin was clamped at 9.2 +/- 1.3 microU/ml (low-insulin group). Glucagon was maintained at 261 +/- 16.2 and 124 +/- 6.4 pg/ml, respectively, in the high-insulin and low-insulin groups. Without hormonal control, plasma glucose homeostasis was maintained during exercise because the increase in glucose uptake was balanced by a corresponding increase in glucose production. When changes in insulin and glucagon were prevented, plasma glucose concentration fell, particularly in the high-insulin group. Glucose uptake increased to a greater extent than when hormones were not controlled, and glucose production did not increase sufficiently to compensate. The increase in glucose uptake in the hormonal control groups was associated with an increased rate of glucose oxidation. When euglycemia was maintained by glucose infusion in the hormonal control subjects, the modest increase in glucose production that otherwise occurred was prevented. It is concluded that during light exercise there must be a reduction in insulin concentration and/or an increase in glucagon concentration if plasma glucose homeostasis is to be maintained. If such changes do not occur, hypoglycemia, and hence exhaustion, may occur.     

Maintenance of basal insulin secretion in severe non-insulin-dependent diabetes

It has been postulated that glucose regulation is secondary to maintenance of normal basal insulin secretion. Serum glucose, insulin, and C-peptide levels were measured at fasting in 209 consecutive non-insulin-dependent diabetic patients and after glucose stimulation in 193 patients. The basal serum insulin C-peptide levels were not significantly different in control subjects (mean 22 +/- 8.8 microU/ml) and in patients with varying severity of diabetes (mean 24 +/- 9.6 microU/ml) except in the most severely diabetic group [fasting serum glucose greater than 350 mg/dl (19.4 mmol/L), mean 19 +/- 7 microU/ml]. In 39 patients who developed ketonuria without acidosis during follow-up, the mean basal serum insulin was 22 microU/ml during the episode of ketonuria, 21 microU/ml during the glucose tolerance test, and 25 microU/ml after glucose stimulation (statistically nonsignificant differences). Our data suggest that hyperglycemia compensates for beta-cell impairment so that basal insulin secretion usually stays above the threshold for ketoacidosis unless there is marked beta-cell impairment. Patients who fail to increase insulin in response to nutrient challenge are at risk of developing ketosis.     

Glucagon response to hypoglycemia in sympathectomized man.

Hypoglycemia stimulates immunoreactive glucagon (IRG) secretion and increases the activity of the sympathetic nervous system. To ascertain if the augmented alpha cell activity evoked by glucopenia is mediated by the adrenergic nervous system, the glucagon response to insulin-induced hypoglycemia of five subjects with neurologically complete cervical transections resulting from trauma, thereby disrupting their hypothalamic sympathetic outflow, was compared to six healthy volunteers. In addition to clinical neurological evaluation, completeness of sympathectomy was verified by failure to raise plasma norepinephrine levels during hypoglycemia compared to the two- and threefold increase observed in controls. Total IRG response (IRG area above basal 0-90 min) and peak IRG levels achieved were the same in the quadriplegics and the controls. Although the glucagon rise tended to be slower, and the peak levels attained occurred later in the quadriplegic patients than in the controls, this response was appropriate for their sugar decline, which was slower and reached the nadir later than in the control subjects. These observations that the glucagon release during insulin-induced hypoglycemia is normal in subjects whose hypothalamic sympathetic outflow has been interrupted provide strong evidence that the sympathetic nervous system does not mediate the glucagon response to hypoglycemia.     

Effect of Intermittent Endogenous Hyperglucagonemia on Glucose Homeostasis in Normal and Diabetic Man

Infusion of glucagon causes only a transient increase in glucose production in normal and diabetic man. To assess the effect of intermittent endogenous hyperglucagonemia that might more closely reflect physiologic conditions, arginine (10 g over 30 min) was infused four times to 8 normal subjects and 13 insulin-dependent diabetic subjects (4 of whom were infused concomitantly with somatostatin to examine effects of arginine during prevention of hyperglucagonemia). Each arginine infusion was separated by 60 min. Diabetic subjects were infused throughout the experiments with insulin at rates (0.07-0.48 mU/kg per min) that had normalized base-line plasma glucose and rates of glucose appearance (Ra) and disappearance (Rd). Basal plasma glucagon and arginine-induced hyperglucagonemia were similar in both groups; basal serum insulin in the diabetics (16+/-1 muU/ml, P < 0.05) exceeded those of the normal subjects (10+/-1 muU/ml, P < 0.05) but did not increase with arginine. Serum insulin in normal subjects increased 15-20 muU/ml with each arginine infusion. In both groups each arginine infusion increased plasma glucose and Ra. Increments of Ra in the diabetics exceeded those of normal subjects, (P < 0.02); Rd was similar in both groups. In normal subjects, plasma glucose returned to basal levels after each arginine infusion, whereas in the diabetics hyperglycemia persisted reaching 151+/-15 mg/dl after the last arginine infusion. When glucagon responses were prevented by somatostatin, arginine infusions did not alter plasma glucose or Ra.CONCLUSIONS: Infusion of arginine acutely increases plasma glucose and glucose production in man solely by stimulating glucagon secretion; physiologic increments in plasma glucagon (100-150 pg/ml) can result in sustained hyperglycemia when pancreatic beta cell function is limited.     

C-peptide suppression test for insulinoma.

During hypoglycemia induced by an infusion of porcine insulin, impaired suppression of endogenous insulin secretion as measured by C-peptide was demonstrated in 11 of 12 patients with insulinoma. During hypoglycemia (plasma glucose less than or equal to 40 mg/dl) the mean C-peptide immunoreactivity (CPR) of normal subjects was less than or equal to 1.2 ng/ml, whereas 11 of 12 insulinoma patients had a mean CPR of larger than or equal to 1.9 ng/ml. One patient showed normal CPR suppression by these criteria but may have shown impaired CPR suppression for glucose less than or equal to 30 mg/dl. Impaired CPR suppression during insulin-induced hypoglycemia may prove to be a useful test for insulinoma.     

Fenfluramine increases insulin action in patients with NIDDM

These studies examined the effect of fenfluramine on insulin action and insulin secretion in healthy subjects and patients with non-insulin-dependent diabetes mellitus (NIDDM). In the first study, a double-blind crossover design was used in healthy subjects to compare the effect of short-term fenfluramine therapy (60 mg orally for 3 days) with placebo. Insulin secretion and whole-body insulin sensitivity (determined by frequently sampled intravenous glucose tolerance tests with analysis by the minimal-model method) were unchanged by fenfluramine. In the second study, involving patients with NIDDM inadequately controlled on submaximal to maximal doses of oral hypoglycemic agents, a double-blind crossover strategy was used to compare baseline studies (conducted after a run-in period) with fenfluramine (60 mg orally) or placebo for 4 wk. There was a significant fall in fasting blood glucose after therapy with fenfluramine compared with the baseline study period (13.0 +/- 1.2 vs. 8.4 +/- 0.89 mM, mean +/- SE, P less than .01) with no significant fall in fasting serum insulin (20 +/- 2 vs. 24 +/- 3 microU/ml) or C-peptide (1.3 +/- 0.2 vs. 1.3 +/- 0.1 nM). During euglycemic-hyperinsulinemic (1 mU.kg-1.min-1) clamp studies there was a significant increase in insulin action from 12.7 +/- 2.3 to 17.3 +/- 1.8 min-1.10(3) microU.ml-1 (P less than .05), although clamp insulin levels were lower after fenfluramine treatment (136 +/- 14 vs. 96 +/- 9 microU/ml, P less than .02), reflecting an enhanced metabolic clearance rate for insulin (12.7 +/- 1.5 vs. 20.1 +/- 2.1 ml.kg-1.min-1, P less than .025).(ABSTRACT TRUNCATED AT 250 WORDS)     

Retrograde perfusion as a model for testing the relative effects of glucose versus insulin on the A cell.

In order to determine whether the A cell may be directly suppressed by glucose in the absence of insulin, canine pancreata were perfused in vitro, both antegrade, through the arterial system and retrograde, through the venous system. Studies of the islet microvasculature have suggested that blood flows from the B cell core to the mantle; thus, the A cell may be tonically inhibited by intra-islet insulin. Retrograde perfusion may then be expected to prevent insulin from reaching the A cell, releasing it from inhibition. Retrograde perfusion with 88 mg/dl glucose markedly increased both insulin and glucagon secretion relative to antegrade levels. In a series of experiments, glucose concentrations were changed from 88 to 200 mg/dl. An antegrade glucose change resulted in increased insulin (134+/-21%; P less than 0.0025) and decreased glucagon (-26+/-9%, P less than 0.025) secretion. A retrograde glucose increase resulted in increased secretion of both insulin (91+/-15%; P less than 0.0005) and glucagon (23+/-9%; P less than 0.0125). To confirm that retrograde perfusion deprived the A cell of endogenous core derived, vascularly delivered insulin, possibly resulting in increased insulin sensitivity, 0.3 mU/ml exogenous porcine insulin was infused. Antegrade, 0.3 mU/ml insulin, had no effect on glucagon secretion (P less than 0.250), while retrograde infusion of 0.3 mU/ml insulin significantly inhibited glucagon secretion (-31 + 8%; P less than 0.0005). The results of our study support the concept that the direction of blood flow and of flow-dependent intra-islet hormone interactions are from the islet B cell core to the mantle. It was further concluded that the normal A cell may not be suppressed by glucose in the absence of insulin.     

The effect of administration of human growth hormone on the plasma growth hormone, cortisol, glucose, and free fatty acid response to insulin: evidence for growth hormone autoregulation in man

The effect of administration of human growth hormone (HGH) (3 mg every 6 hr for 6 days) on the endogenous GH response to insulin-induced hypoglycemia at 8, 12, 24, and 48 hr posttreatment was studied in 11 healthy male adults. Free fatty acid, cortisol, and glucose responses pre- and posttreatment with HGH were evaluated concurrently. Control subjects received saline injections to evaluate relationship of GH responses to the periodicity of insulin tolerance tests. The data were compared for each subject pre- and posttreatment with HGH as well as by comparison of the results of the saline-treated group with those of the HGH-treated group.The mean maximal GH concentration in response to insulin-induced hypoglycemia for all the subjects (n = 16) was 31.1 +/-3.6 ng/ml (+/-SEM) on day 1 of the control period and 23.4 +/-3.1 (SEM) on day 2, not statistically significant.A significant decrease in the maximal peak GH response (n = 8) after insulin-induced hypoglycemia was observed at 8 and 12 hr after HGH administration was terminated with mean peak values for GH of 4.6 +/-1.3 ng/ml and 10.4 +/-1.9 ng/ml, respectively (P < 0.01). A progressive return to control values was noted between 12 and 24 hr. The GH responsiveness of the saline-treated group (n = 5) was unchanged from that observed during the control period.The fasting glucose values were unchanged in the GH-treated group from those of the control period or of the saline-treated controls. Insulin resistance was apparent at 8 hr posttreatment with HGH. No differences in FFA response after insulin-induced hypoglycemia were observed in GH-treated or saline-treated subjects. The rise in plasma cortisol after insulin-induced hypoglycemia was comparable in the GH-treated and saline-treated group. Diurnal variation in plasma cortisol was maintained during the period of GH suppression.These observations support the concept that GH can modulate its secretion by means of an auto-feedback mechanism.     

Plasma insulin response to various secretagogues in insulinoma

Since the development of radioimmunoassay for insulin, the diagnosis of insulinoma has been made easily. However, it has been assumed that insulinoma is heterogenous in the histological structure as well as in clinical findings. Therefore, the present study was performed to investigate the insulin response to various stimuli and to evaluate the various insulin response tests in 19 patients with insulinoma. The fasting blood glucose was 19 to 90 mg/100 ml in insulinoma and 81 +/- 5 (mean +/- S.D.) mg/100 ml in normal controls. Plasma insulin (IRI) in insulinoma ranged from 10 to 255 microU/ml, while in the control it was 14 +/- 9 microU/ml. However, insulin/blood glucose ratio increased in insulinoma (0.2-11.2) compared with the normal control (0.18 +/- 0.11). In oral glucose tolerance tests, plasma IRI increased and reached peak levels of 48-244 microU/ml, remaining elevated in most cases. In the intravenous tolbutamide test, plasma IRI increased conspicuously to 82-1,330 microU/ml and hypoglycemic coma was provoked in 54%. Plasma IRI was elevated in the intravenous glucagon test and reached the peak levels of 85-400 microU/ml, which exceeded those of the control group. Plasma IRI increased to more than 100 microU/ml after arginine infusion and formed bizarre curves. There were no correlations between plasma IRI response to various stimuli and malignancy, type of B-granule or insulin content of insulinoma tumors. It is concluded that fasting plasma IRI, insulin/glucose ratio, tolbutamide test and glucagon test are highly valuable for the diagnosis of insulinoma.