The insulin-like growth factor binding protein-1 in low and high insulin responders before and during dexamethasone treatment.

To investigate the influence of normal insulin levels on levels of the insulin-like growth factor binding protein-1 (IGFBP-1) we measured this peptide postabsorptively and during hyperglycemic clamp in 17 healthy subjects, nine with low insulin response (LIR) and eight with high insulin response (HIR). The study was performed before and after 60 hours of treatment with dexamethasone 6 mg/d. The fasting levels of IGFBP-1 were significantly higher in LIR, 36 +/- 2.5 micrograms/L, than in HIR, 22 +/- 2.6 micrograms/L (P less than .01), while no differences in glucose, insulin, and C-peptide concentrations were found. Dexamethasone induced an increase in basal concentrations of insulin, while IGFBP-1 levels decreased to 18.8 +/- 2 micrograms/L in LIR (P less than .01) and to 14.0 +/- 0.9 micrograms/L in HIR (P less than .05). There was no correlation between the individual basal IGFBP-1 concentrations and basal insulin levels. In contrast, basal levels of IGFBP-1 were inversely correlated to the integrated insulin or C-peptide concentrations during the hyperglycemic clamp both before (r = -.67, P less than .01) and during dexamethasone (r = -.79, P less than .001). Dexamethasone, which increased the insulin resistance, did not change the relationship between basal IGFBP-1 and the glucose-induced insulin release. In conclusion, the morning levels of IGFBP-1 in healthy subjects reflect the acute beta-cell responsiveness to glucose, which may correspond to integrated diurnal insulin levels. The inhibitory effects of dexamethasone on the morning levels of IGFBP-1 can be explained by attendant hyperinsulinemia.     

Ninhydrin inhibition of glucose-induced insulin release

Ninhydrin, a compound which shares chemical properties strikingly similar to alloxan was found to mimic basically the inhibitory effect of alloxan on glucose-induced insulin release. Exposure of pancreatic islets for five minutes to 85 mumol/l ninhydrin produced approximately ninety percent inhibition of subsequent glucose-induced insulin release without altering basal secretion. Both D-glucose and D-mannose provided substantial protection against the inhibitory effect of ninhydrin, and the alpha anomer of D-glucose was more effective than the beta anomer in preventing ninhydrin inhibition of insulin release. Evidence for a common site of inhibition by ninhydrin and alloxan in the insulin release process is discussed.     

Studies of high and low insulin responders with the hyperglycemic clamp technique

We have investigated insulin responsiveness in relation to insulin sensitivity during sequential hyperglycemic clamping in low insulin responders (LIR), high insulin responders (HIR) and in women with a history of gestational diabetes (GD). Designation of HIR and LIR was done on the basis of mathematical modeling of the insulin response to a glucose infusion test. Insulin sensitivity was determined by a somatostatin-insulin-glucose infusion test (SIGIT) according to which LIR were subdivided into groups with higher or lesser sensitivity. Hyperglycemic clamping (60 min, 11 mmol/L of glucose) induced diphasic insulin and C-peptide responses in all groups. Insulin and C-peptide responses were significantly higher in HIR than in other groups. The ratio of first phase to total insulin response was higher in HIR v GD but did not differ between other groups. A second identical clamp was performed after a 60-minute rest period. Except in HIR, insulin levels attained were then moderately but significantly higher than during the first clamp. Conversely, the glucose utilization (mg/kg/min) to insulin (mU/L) = M/L ratio was markedly increased in LIR with high insulin sensitivity but not in other groups. We conclude that (1) large and consistent differences exist in glucose-induced insulin secretion from the pancreas between nondiabetic subjects; (2) time dynamics of insulin secretion and priming effects of glucose are similar in LIR with lesser and higher sensitivity; and (3) in the latter group a glucose stress affects insulin sensitivity more markedly than insulin responsiveness.     

Effect of bombesin on glucose-induced insulin release in humans

The effect of bombesin on basal and glucose-stimulated insulin release was studied in male healthy volunteers. Glucose was administered by oral, intravenous or intraduodenal route during saline or bombesin infusion (5 ng/kg/min for 60 min). The peptide had no significant effect on basal levels of glucose and insulin. However, during its administration, the insulin response and the expected rise in blood glucose after oral glucose load (50 g) were strongly inhibited, and the gastric emptying of liquids was significantly delayed. On the contrary, the insulin response to intravenous glucose (20 g) was significantly increased by bombesin without changes in plasma glucose levels. Finally, when glucose was infused into the duodenum, thus bypassing the stomach, the insulin response was significantly increased by the peptide. In this case, too, plasma glucose levels after glucose load were virtually identical during either bombesin or saline infusion. These data clearly demonstrate that the direct effect of bombesin on insulin release is stimulatory and suggest that the inhibitory effect observed after oral glucose is connected with the action of the peptide on gastric emptying, the delay of which slows the entry of glucose into the small bowel.     

The stimulus-secretion coupling of glucose-induced insulin release

Summary When isolated islets of Langerhans are suddenly exposed to glucose, the entry of the hexose into islet cells first occurs at a high rate resulting in rapid equilibration of free glucose across the cell membrane; thereafter, the rate of net glucose uptake depends on its metabolism. More than 95% of the glucose taken up by the islets is converted to triosephosphate. The fractional contribution of the sorbitol and pentose-phosphate pathways to such a process does not exceed 10%. The output of lactate from the islets accounts for approximately half of the glycolytic flux. At increasing glucose concentrations up to 4.3 mM, the rate of glycolysis increases towards a first asymptotic value; at higher glucose levels (up to 27.8 mM), a sigmoidal pattern is seen tending towards a second saturation value. The total ATP content of the islets does not correlate with their insulin-secretory activity. It is suggested that, in the process of glucose-induced insulin release, glycolysis may regulate physiological processes possibly located in the micro-environment of the cell boundary.     

Inhibition of glucose-induced insulin release by xylazine

The effects of xylazine, a sedative that is extensively used in veterinary medicine, on glucose-stimulated insulin release have been studied using isolated perifused rat pancreatic islets. Xylazine had no effect on insulin release under basal conditions. Glucose-stimulated insulin release was inhibited in a concentration-dependent manner over the range of 10(-9)-10(-6) M. Maximum inhibition even with 10(-5) M xylazine was around 80%, which suggested that xylazine is only a partial agonist at its receptor. The inhibitory effect could be completely eliminated by yohimbine, an alpha 2-adrenergic antagonist, and was unaffected by prazosin, an alpha 1-adrenergic antagonist. That xylazine is a partial agonist was verified by the fact that when present in excess, it could stimulate insulin release which had already been inhibited to 100% by epinephrine. It is concluded that xylazine is a potent partial alpha 2-adrenergic agonist and that this mechanism is responsible, at least in part, for the hyperglycemia and insulinopenia associated with its use.     

The stimulus-secretion coupling of glucose-induced insulin release

Summary When isolated rat islets were exposed to glucose, the concentrations of NADH and NADPH, and the NADH/NAD⁺ and NADPH/NADP⁺ ratios were increased. The dose-response curve resembled that characterising the glucose-induced secondary rise in⁴⁵Ca efflux, displaying a sigmoidal pattern with a half-maximal value at glucose 7.5 mmol/l. The glucose-induced increase in NAD(P)H was detectable within 1 min of exposure to the sugar. Except for the fall in ATP concentration and ATP/ADP ratio found at very low glucose concentrations (zero to 1.7 mmol/l) no effect of glucose (2.8–27.8 mmol/l) upon the steady-state concentration of adenine nucleotides was observed. However, a stepwise increase in glucose concentration provoked a dramatic and transient fall in the ATP concentration, followed by a sustained increase in both O₂ consumption and oxidation of exogenous + endogenous nutrients. This may be essential to meet the energy requirements in the stimulated B-cell. Although no significant effect of glucose upon intracellular pH was detected by the 5,5-dimethyloxazolidine-2,4-dione method, the net release of H⁺ was markedly increased by glucose, with a hyperbolic dose-response curve (half-maximal response at glucose 2.9 mmol/l) similar to that characterising the glucose-induced initial fall in⁴⁵Ca efflux. It is proposed that the generation of both NAD(P)H and H⁺ participates in the coupling of glucose metabolism to distal events in the secretory sequence, especially the ionophoretic process of Ca²⁺ inward and outward transport, and that changes in these parameters occur in concert with an increased turn-over rate of high-energy phosphate intermediates.Recipient of a Pfizer Traval Award through the European Association for the Study of DiabetesOn leave from the Fundacion Jimenez Diaz, Madrid, Spain     

The stimulus-secretion coupling of glucose-induced insulin release

Summary Above a threshold of 3.0–4.2 mmol/l, D-glucose provoked a transient increase in ³²P fractional outflow rate from rat pancreatic islets prelabelled with ³²P-orthophosphate. Nutrients which stimulate insulin release in the absence of glucose, -ketoisocaproate and L-leucine, also provoked a phosphate flush. No flush occurred in islets exposed to non-insulinotropic nutrients (L-glutamine and L-lactate) or non-nutrient secretagogues (arginine, tolbutamide, theophylline). A late increase in ³²P fractional outflow rate was observed in Ca²⁺ deprived islets stimulated with BaCl₂ and theophylline. The occurrence of a phosphate flush did not appear to be attributable to changes in insulin release, cyclic AMP content, membrane polarisation, K⁺ conductance, or reduced pyridine nucleotide content. The ³²P response to glucose was slightly decreased in the absence of extracellular Ca²⁺ or HCO₃ ^-, markedly impaired in the absence of K⁺, and virtually abolished in the presence of menadione (10 mol/l). It is proposed that the occurrence of a phosphate flush is linked to the metabolism of nutrient secretagogues, possibly via an increase in O₂ uptake and the production rate of NAD(P)H and ATP.     

Oral glucose tolerance test and insulin sensitivity in low insulin responders

Oral and iv glucose tolerance, insulin response to iv and oral glucose load as well as insulin sensitivity were evaluated in 58 'low insulin responders'. They were selected from a group of 226 healthy subjects with normal fasting blood glucose and normal iv glucose tolerance test on the basis of a low insulin response during a standardized glucose infusion test (GIT). The insulin response to GIT was analysed by parameter identification in a mathematical model (parameter KI). Insulin sensitivity was also measured by computer analysis of GIT (parameter KG) and, in a limited group of subjects, by a somatostatin infusion test. Thirty-three low insulin responders had normal OGTT, whereas 5 demonstrated borderline-1, 16 borderline-2, and 4 decreased OGTT. The first group of subjects demonstrated normal or enhanced insulin sensitivity. Borderline and decreased OGTT, in most instances, was accompanied by decreased insulin sensitivity, implying that a subgroup of low insulin responders exhibited signs of both impaired insulin response to glucose and insulin resistance. Since these defects characterize manifest type-2 diabetes, these subjects possibly may run a high risk to develop this type of diabetes. On the other hand, low insulin response in combination with increased insulin sensitivity may reflect adaptation of the secretory capacity of B-cells to the need of insulin.     

C肽和胰岛素对高糖诱导的内皮细胞凋亡的影响

高糖可导致人脐静脉内皮细胞凋亡,不同浓度的C肽和胰岛素对高糖诱导的内皮细胞凋亡影响程度不同。合理应用C肽可能对糖尿病大血管病变的防治起到有益的作用。     

Proportional inhibition of glucose-induced insulin release by somatostatin

To test the hypothesis that somatostatin in a proportional inhibitor of glucose-induced insulin release, we examined the effect of somatostatin on the release of insulin stimulated either by endogenous signals (basal), by glucose infusion, or by glucose injection. Somatostatin infusions (1.7 μg/min × 30 min) produced a decrement of basal insulin output from the right lobe of the canine pancreas that was proportional to the initial rate of basal insulin secretion (r = −0.87, p < 0.001, n = 16) Glucose infusions of 1–6 mg/kg/min produced much higher rates of insulin output; again, the decrement of insulin output produced by somatostatin correlated with the initial rate of insulin secretion (r = −0.68, p < 0.01, n = 16). Rapid intravenous injection of either 2 or 20 g of glucose produced a wide range of acute insulin responses (AIR). Somatostatin produced a decrement that was proportional to the original magnitude of the AIR (r = −0.70, p < 0.005, n = 16). Thus, the absolute amount of inhibition produced by somatostatin increases, not decreases, with the magnitude of the stimulus. These data suggest that the inhibitory effect of somatostatin cannot be overcome by increasing the size of glucose stimulus. Thus, exogenous somatostatin, and presumably endogenous somatostatin as well, will produce an inhibitory effect at any physiologic level of glucose stimulation that the β-cell receives.     

Suppression by insulin treatment of glucose-induced inhibition of insulin release in non-insulin-dependent diabetics

Although restoration of normoglycemia in non-insulin-dependent diabetic subjects improves insulin release evoked by several secretagogues, conflicting data were reported concerning the effect of intensive insulin therapy on the first-phase response of the B-cell to an intravenous glucose challenge. In the present study, 14 non-insulin-dependent diabetics underwent an intravenous glucose test performed before and after 20 h of glycemic normalization. Before insulin treatment, glucose failed, as a rule, to provoke an early positive secretory response. On the contrary, a paradoxical inhibition of insulin release was observed in most patients. This phenomenon was reproducible when a second test was performed 120 min after the first one. The paradoxical inhibition was not observed any more after glycemic normalization. As judged from the paired difference (delta) between the early increment in insulin release before and after insulin treatment, normoglycemia resulted in an improved secretory response (delta greater than 5.0 microU/ml) in seven patients, whilst the first-phase response remained little affected (delta less than 3.0 microU/ml) in the other seven subjects. These findings suggest that an impaired first-phase response to glucose does not always represent an irreversible primary defect of the pancreatic B-cell in diabetic subjects.     

Effect of oleic and octanoic acids on glucose-induced insulin release in vitro.

The isolated perfused rat pancreas was utilized to characterize the effect of 1.5 mM oleic acid and 1.5 mM octanoic acid on insulin release in response to glucose (4.4, 8.3, 11.1, 16.7 and 27.7 mM). Both oleic and octanoic acids potentiated glucose-induced insulin release without modification of the pattern of secretion. At 4.4 mM glucose, the potentiation of insulin release by fatty acids was clearcut for the first phase but not significant with respect to the total amount of insulin released during the stimulation period. Maximal potentiation was observed at 11.1 mM glucose both with oleic and octanoic acids. At all glucose concentrations, octanoic acid was more effective than oleic acid in the potentiation of the insulin secretion.     

Kinetics of 5-hydroxytryptophan potentiation of glucose-induced insulin release

We have previously shown that L-5-hydroxytryptophan (L-5-HTP) potentiates glucose-induced insulin release in islets isolated from ob/ob-mice. In the present study, the kinetics of this effect were further studied. The combined effect of glucose and L-5-HTP was dependent on the concentration of both compounds. The threshold concentration for the potentiating effect of 5-HTP was on the order of 0.01-0.1 mmol/l, and the maximal effect was reached at 1-4 mmol/l extracellular concentration. L-5-HTP enhanced glucose-induced insulin release without changing the apparent Km for glucose (9.5-10 mmol/l). The effect of L-5-HTP increased with time for about 40 to 60 min and was reversibly reduced when either L-5-HTP was omitted or the glucose concentration was lowered. These findings support the view that L-5-HTP is a strong potentiator of glucose-induced insulin release.     

Preferential release of proinsulin relative to insulin in non-insulin-dependent diabetes mellitus

A radioimmunoassay, using an antiserum that is specific for human proinsulin, has been used to study the response of serum proinsulin to low (25 g) and high (75 g) oral glucose loads in non-obese patients with non-insulin-dependent diabetes mellitus (NIDDM). Diabetic patients were treated by diet only (N = 8) or were receiving oral anti-hyperglycemic agents (N = 8) and therapy was not interrupted during the study. In the fasted state, proinsulin concentrations were higher (P less than 0.05) in the drug-treated patients (31 +/- 3 pmol/l (SEM)) compared with age- and weight-matched healthy subjects (22 +/- 2 pmol/l; N = 10), but concentrations in the diet-treated patients 25 +/- 3 pmol/l) were not significantly different. Following 25 g and 75 g glucose loads, the rises in serum immunoreactive insulin and C-peptide concentrations in both groups of diabetic patients were impaired and delayed relative to those in the control subjects. The responses of serum proinsulin, however, were not significantly different in the NIDDM patients compared with controls at any time point up to 180 min except in the case of drug-treated patients receiving 25 g of glucose who had elevated (P less than 0.05) proinsulin concentrations at 150 min and 180 min after ingestion. It is concluded that NIDDM is not associated with an exaggerated release of proinsulin in response to glucose compared with healthy subjects, but the islets have maintained the ability to release proinsulin better than the ability to release insulin.     

Diazoxide attenuates glucose-induced defects in first-phase insulin release and pulsatile insulin secretion in human islets

Humans with type-2 diabetes mellitus (TTDM) have hyperglycemia ( approximately 11 mM) and impaired glucose-mediated insulin secretion characterized by impaired first-phase insulin release (FPIR) and pulsatile insulin release. Culture of islets from nondiabetic humans in very high glucose concentrations ( approximately 20-30 mM) for 96 h causes impaired FPIR. We sought to determine 1). whether human islets cultured at a glucose concentration of approximately 11 mM (comparable to TTDM) recapitulates impaired insulin secretion in TTDM, specifically impaired FPIR and insulin pulse mass with an increased proinsulin/insulin (PI/I) secretion ratio; and 2). whether these changes can be attenuated by addition of diazoxide to islets cultured with 11 mM glucose. Islets cultured with 11 mM glucose for 96 h had 75% depleted insulin stores (P < 0.05), decreased FPIR and insulin pulse mass (P < 0.05), and an approximately 3-fold increase in the ratio of PI/I islet content and in secretion ratio (P < 0.05). Addition of diazoxide to islets cultured with 11 mM glucose decreased insulin secretion during static incubation, leading to relative preservation of insulin stores and enhanced insulin secretion during subsequent perifusion; FPIR increased by 162% (P < 0.05) and insulin pulse mass by 150% (P < 0.05) vs. no diazoxide. The mean islet PI/I content and islet PI/I secretion ratio were also decreased by approximately 70% (P < 0.05) by prior addition of diazoxide to islets during culture with 11 mM glucose. FPIR and insulin pulse mass were related to islet insulin stores (P < 0.001 for FPIR and P < 0.001 for pulse amplitude). In conclusion, the pattern of defects of insulin secretion present in TTDM (impaired FPIR and pulsatile insulin secretion, increased PI/I ratio) can be recapitulated in human islets cultured with 11 mM glucose for 96 h. These defects can be at least partially offset by concurrent inhibition of insulin secretion by diazoxide, which also preserves insulin stores. Defective insulin secretion in TTDM may be, at least in part, due to depletion of available insulin stores secondary to chronic increased demand (insulin resistance and hyperglycemia) in the setting of a decreased beta-cell mass.     

Amino acid modulation of glucose-induced insulin and glucagon release in diabetic patients

In order to determine whether amino acids have a beneficial effect on glucose tolerance in diabetes, the effect of intravenous infusion of mixed amino acids on plasma insulin, glucagon, and blood glucose responses to oral glucose loading was studied in patients with mild to moderate diabetes. Intravenous infusion of mixed amino acids over a period of 30 min which was started 30 min or immediately before oral glucose loading significantly augmented the insulin response but did not improve the blood glucose curve, probably due to excessive glucagon response. However, amino acid infusion over a period of 60 min started immediately before oral glucose loading evoked a sustained rise of plasma insulin associated with a lesser degree of glucagon secretion, thus causing significant improvement of the blood glucose curve.