Intravenous Tolbutamide Research Articles

P-115: Sequence variants in the sulfonylurea receptor gene are associated with non-insulin dependent diabetes mellitus and with decreased insulin secretion after an intravenous tolbutamide challenge

P-115: Sequence variants in the sulfonylurea receptor gene are associated with non-insulin dependent diabetes mellitus and with decreased insulin secretion after an intravenous tolbutamide challenge

Studies of Association of Variants Near the HHEX, CDKN2A/B, and IGF2BP2 Genes With Type 2 Diabetes and Impaired Insulin Release in 10,705 Danish Subjects

In the present study, we aimed to validate the type 2 diabetes susceptibility alleles identified in six recent genome-wide association studies in the HHEX/KIF11/IDE (rs1111875), CDKN2A/B (rs10811661), and IGF2BP2 (rs4402960) loci, as well as the intergenic rs9300039 variant. Furthermore, we aimed to characterize quantitative metabolic risk phenotypes of the four variants. The variants were genotyped in the population-based Inter99 cohort (n = 5,970), the ADDITION Study (n = 1,626), a population-based sample of young healthy subjects (n = 377), and in additional type 2 diabetic case (n = 2,111) and glucose-tolerant (n = 521) subjects. The case-control studies involved a total of 4,089 type 2 diabetic patients and 5,043 glucose-tolerant control subjects. We validated association of variants near HHEX/KIF11/IDE, CDKN2A/B, and IGF2BP2 with type 2 diabetes. Interestingly, in middle-aged people, the rs1111875 C-allele of HHEX/KIF11/IDE strongly associated with lower acute insulin response during an oral glucose tolerance test (P = 6 x 10(-7)). In addition, decreased insulin release following intravenous tolbutamide injection was observed in young healthy subjects (P = 0.02). Also, a reduced insulin release was observed for the CDKN2A/B rs10811661 T-allele after both oral and intravenous glucose challenges (P = 0.001 and P = 0.009, respectively). We validate that variants in the proximity of the HHEX/KIF11/IDE, CDKN2A/B, and IFG2BP2 loci associate with type 2 diabetes. Importantly, variations within the HHEX/KIF11/IDE and CDKN2A/B loci confer impaired glucose- and tolbutamide-induced insulin release in middle-aged and young healthy subjects, suggesting a role for these variants in the pathogenesis of pancreatic beta-cell dysfunction.

Insulin secretion, peripheral insulin sensitivity and insulin-receptor binding in subjects with different degrees of obesity

The aim of the present study was to investigate insulin secretion, insulin-receptor binding and peripheral insulin sensitivity in subjects with different degrees of obesity. 36 obese subjects with normal glucose tolerance and different degrees of obesity and 40 healthy normal-weight subjects participated in the study. Peripheral insulin sensitivity was measured by using the euglycaemic hyperinsulinaemic clamp technique, and insulin-receptor binding-on circulating mononuclear blood cells. Insulin secretion was studied during intravenous tolbutamide test. The subjects with I degree of obesity demonstrated a significant decrease in the number of total (p<0.0001) and high-affinity (p<0.01) insulin receptors per cell, as well as significantly higher insulin receptor affinity (p<0.01) as compared to the normal-weight subjects. The subjects with II degree of obesity also demonstrated a significant decrease in the number of total (p<0.0001) and high-affinity receptors (p<0.001) per cell as well as an increase (p<0.001) in insulin-receptor affinity as compared to the controls. The significantly decreased receptor number in the subjects with I and II degree of obesity was accompanied by an increase in insulin receptor affinity; thus their insulin-receptor binding being maintained similar to the controls. The subjects with III degree obesity presented a significant decrease (p<0.0001) in the number of both the total and high-affinity insulin receptors as well as a reduction in insulin receptor affinity as compared to the controls. Therefore the percentage of specifically bound insulin was significantly lower (p<0.01) as compared to that of the control group. Insulin resistance in the obese subjects is associated with secondary hyperinsulinaemia, which is present in subjects with I and II degree of obesity; while in severely obese subjects exhaustion of beta-cell secretory capacity is observed. We consider that III degree of obesity appears to be a risk factor for type 2 diabetes mellitus as the alterations in insulin sensitivity, insulin-receptor binding and beta-cell secretion are quite similar to the reported in diabetic patients.

Does the aspartic acid to asparagine substitution at position 76 in the pancreas duodenum homeobox gene (PDX1) cause late-onset type 2 diabetes?

Considerable data support an inherited defect in insulin secretion as one component of type 2 diabetes. Coding variants of the pancreas duodenum homeobox gene (PDX1) were proposed to predispose late-onset type 2 diabetes and to decrease transactivation in vitro. We tested the hypothesis that the Asp76Asn (D76N) variant that was identified in several populations predisposed type 2 diabetes and reduced insulin secretion. We performed a case-control study in 191 control subjects and 190 individuals with type 2 diabetes, all of European descent, then characterized the D76N variant in 704 members of 68 families. We compared the phenotypic characteristics of those with and without the variant by diagnostic status and determined the insulin secretory response to intravenous glucose and tolbutamide among nondiabetic family members. D76N was not associated with type 2 diabetes, either in our population or when all reported studies in Caucasians were combined. D76N did not segregate with diabetes among the families examined. Among D76N carriers, nondiabetic individuals had a lower waist-to-hip ratio and a trend to lower BMI than their diabetic counterparts. Diabetic carriers of D76N were significantly leaner by BMI (P = 0.012) and tended to be younger than diabetic individuals with the D/D genotype. However, insulin secretion in response to oral and intravenous glucose challenge and to intravenous tolbutamide was not reduced in D76N carriers. The D76N variant of PDX1 does not significantly alter insulin secretion or act as a high-risk susceptibility allele for late-onset type 2 diabetes as proposed previously, although we cannot exclude a minor role in increasing risk of diabetes.

Genetic cause of hyperglycaemia and response to treatment in diabetes

Type 2 diabetes shows evidence of underlying heterogeneity. No studies have assessed whether different causes for diabetes change the response to oral hypoglycaemic therapy. In a few cases, patients with diabetes caused by mutations in the hepatocyte nuclear factor 1alpha (HNF-1alpha) gene have been described as sensitive to the hypoglycaemic effects of sulphonylureas. We aimed to see whether the glycaemic response to the sulphonylurea gliclazide and the biguanide metformin differed in HNF-1alpha diabetes and type 2 diabetes, and to investigate the mechanism for differences in sulphonylurea sensitivity. We did a randomised crossover trial of glicazide and metformin in 36 patients, either with diabetes caused by HNF-1alpha mutations or type 2 diabetes, who were matched for body-mass index and fasting plasma glucose. The primary outcome was reduction in fasting plasma glucose. Analysis was by intention to treat. We assessed possible mechanisms for sulphonylurea sensitivity through insulin sensitivity, insulin secretory response to glucose and tolbutamide, and tolbutamide clearance. Patients with HNF-1alpha diabetes had a 5.2-fold greater response to gliclazide than to metformin (fasting plasma glucose reduction 4.7 vs 0.9 mmol/L, p=0.0007) and 3.9-fold greater response to gliclazide than those with type 2 diabetes (p=0.002). Patients with HNF-1alpha diabetes had a strong insulin secretory response to intravenous tolbutamide despite a small response to intravenous glucose, and were more insulin sensitive than those with type 2 diabetes. Sulphonylurea metabolism was similar in both patient groups. The cause of hyperglycaemia changes the response to hypoglycaemic drugs; HNF-1alpha diabetes has marked sulphonylurea sensitivity. This pharmacogenetic effect is consistent with models of HNF-1alpha deficiency, which show that the beta-cell defect is upstream of the sulphonylurea receptor. Definition of the genetic basis of hyperglycaemia has implications for patient management.

Nesidioblastosis in adults: a challenging cause of organic hyperinsulinism

Nesidioblastosis in adults has been reintroduced into the differential diagnosis of organic hyperinsulinism by the description of 'noninsulinoma pancreatogenous hypoglycaemia syndrome (NIPHS)'. Pathologic specimens of all adult patients (n = 66) operated on for organic hyperinsulinism were re-examined. Five patients fulfilled the histomorphological criteria of nesidioblastosis. Retrospective review of clinical presentation, results of 72-h fasts, intravenous tolbutamide tolerance tests, pre- and intraoperative localization studies and surgical therapy was performed. In contrast to NIPHS, fasting tests became positive after 8-14 h. Tolbutamide tests were positive and preoperative imaging showed negative results in all patients. At first operation distal pancreatic resections were performed in three patients, resection of the pancreatic body in one patient and biopsy of the pancreatic tail in one patient. Two of three patients with recurrent disease had to be reoperated. One patient showed a coexistence of nesidioblastosis and multiple small insulinomas and is part of a kindred with autosomal dominantly inherited 'familial islet-cell adenomatosis'. Surgical exploration is indicated only after thorough biochemical diagnosis. An aggressive strategy for preoperative localization including selective arterial calcium stimulation testing seems justified. There may be a combination of nesidioblastosis and islet cell tumours. A link between beta-cell hyperplasia and progression to insulinoma based on not yet known genetic causes can be suspected.

Carriers of an inactivating beta-cell ATP-sensitive K(+) channel mutation have normal glucose tolerance and insulin sensitivity and appropriate insulin secretion.

Insulin release from the pancreatic beta-cells is controlled by ATP-sensitive K(+) (K(ATP)) channels, which consist of a hetero-octameric complex of four sulfonylurea receptor 1 (SUR1) and four Kir6.2 proteins. Mutations in the SUR1 gene are the major cause of congenital hyperinsulinemia (CHI). Despite the hereditary nature of CHI, studies of glucose homeostasis in heterozygous relatives of CHI patients are lacking. Theoretically, in the heterozygous state of the SUR1 gene mutation, only 1 of 16 K(ATP) channels consists of entirely normal subunits. The aim of our study was to investigate in vivo the glucose homeostasis of heterozygous SUR1 mutation carriers. We studied 8 parents of CHI patients, all 8 of whom were heterozygous for the inactivating SUR1 mutation V187D, and 10 matched control subjects. We evaluated glucose tolerance and insulin secretory capacity with oral and intravenous glucose tests, rates of whole-body glucose uptake with hyperinsulinemic-euglycemic clamps, C-peptide response to hypoglycemia during hyperinsulinemic-hypoglycemic clamp, and function of the K(ATP) channels with intravenous tolbutamide test. Carriers of the V187D substitution had normal glucose tolerance, normal tissue sensitivity to insulin, and no signs of inappropriate insulin secretion. The normal insulin response to tolbutamide indicated that heterozygosity for the V187D mutation did not impair K(ATP) channel function. We conclude that the heterozygous carriers of the SUR1 mutation had normal glucose metabolism and insulin secretion, indicating that carriers of recessive K(ATP) channel mutations are unlikely to be at an increased risk of hypoglycemia or other disturbances in glucose metabolism.

Genetic variability of the SUR1 promoter in relation to beta-cell function and Type II diabetes mellitus.

We aimed to examine the promoter of SUR1 for genetic variation and to determine if variants were associated with Type II (non-insulin-dependent) diabetes mellitus or measures of beta-cell function. We examined 465 bp upstream of the ATG site in 46 Type II diabetic patients and 15 glucose tolerant control subjects by SSCP-heteroduplex analysis. We identified an a --> t substitution 437 bp upstream of the ATG site. The allelic frequency was similar in 455 unrelated Type II diabetic patients and in 203 glucose tolerant control subjects matched for age (0.036, [95 % CI 0.019-0.053] vs 0.034 [95 % CI 0.009-0.059]; p = 0.92). Among the glucose tolerant subjects there were no differences between non-carriers (n = 189) and carriers (n = 14) of the variant in fasting values or 30 min values of plasma glucose and serum insulin during an oral glucose tolerance test. In a study of 233 glucose tolerant offspring of and spouses to Danish Caucasian Type II diabetic patients, non-carriers (n = 193) and carriers (n = 37) of the -437 a/t polymorphism did not differ in glucose or tolbutamide stimulated insulin response during an intravenous glucose tolerance test with intravenous tolbutamide injection [AUCs-insulin (0-8) min, 2290 +/- 1660 vs 2308 +/- 1935 pmol/l x min and AUCs-insulin(20-30 min), 3113 +/- 2033 vs. 3393 +/- 2830 pmol/l x min, respectively]. We have identified a novel a/t polymorphism of the SUR1 gene promoter which is not associated with Type II diabetes mellitus or measures of beta-cell function. Previous reported non-functional variants of SUR1 associated with Type II diabetes mellitus still need to be accounted for.

American Diabetes Association 60th Scientific Sessions, 2000: thiazolidinediones, obesity, and related topics.

This is the second of four reports on the American Diabetes Association^M(ADA) 60th Scientific Sessions held in San Antonio, TX, in June 2000. It^Mcovers topics related to thiazolidinediones, insulin resistance, and^Mobesity. At a debate at the meeting, Thomas A. Buchanan, Los Angeles, CA, spoke in favor of the use of thiazolidinediones (TZDs) for prevention of diabetes. He described a prevention trial in a mainly Hispanic group of patients who had previously had gestational diabetes mellitus (GDM) at ∼30 years of age. Approximately 10% of such women have type 2 diabetes after pregnancy, and their ultimate diabetes risk approximates 50%. These women have decreased β-cell function and insulin resistance, both during and after pregnancy. Buchanan hypothesized that the β-cell defect is either caused or worsened by insulin resistance. In 1994, given the hyperbolic relationship between insulin secretion and insulin resistance, his group adopted the strategy of increasing insulin sensitivity with troglitazone. The initial short-term study was of women who had been diagnosed with GDM within the past 4 years and impaired glucose tolerance (IGT), whose risk of developing type 2 diabetes is ∼80%. Insulin sensitivity increased with treatment, and the insulin responses to oral and intravenous glucose and to intravenous tolbutamide, suggesting “afterload reduction for the β-cell.” Based on these data, the Troglitazone in Prevention of Diabetes (TRIPOD) study was started in a group of women who had had GDM and who had an estimated 70% 5-year risk of developing diabetes; 121 received placebo and 114 troglitazone (400 mg daily), with 27 and 29 months of follow-up. There was no significant difference in weight gain or pregnancy rates. Intention-to-treat analysis showed a 60% reduction in diabetes, with a 12.4% annual risk in patients receiving placebo and 5.7% annual risk in those treated with troglitazone. Buchanan noted that approximately one-third of the …

Resistance to insulin's acute direct hepatic effect in suppressing steady-state glucose production in individuals with type 2 diabetes.

We and others have shown that insulin acutely suppresses glucose production in fasting nondiabetic humans and dogs, by both a direct hepatic effect and an indirect (extrahepatic) effect, and in diabetic dogs by an indirect effect alone. In type 2 diabetes, there is resistance to insulin's ability to suppress hepatic glucose production, but it has not previously been determined whether the resistance is primarily at the level of the hepatocyte or the peripheral tissues. To determine whether the diabetic state reduces the direct effect of insulin in humans, we studied nine patients with untreated type 2 diabetes who underwent three studies each, 4-6 weeks apart. 1) Portal study (POR): intravenous tolbutamide was infused for 3 h with calculation of pancreatic insulin secretion from peripheral plasma C-peptide. 2) Peripheral study (PER): equidose insulin was infused by peripheral vein. 3) Half-dose peripheral insulin study (1/2 PER): matched peripheral insulin levels with study 1. In all studies, glucose was clamped at euglycemia, glucose turnover was measured with the constant specific activity method, and 3-[3H]glucose was purified by high-performance liquid chromatography. Peripheral insulin was lower in POR versus PER but slightly higher in POR versus 1/2 PER, although most of the difference could be accounted for by higher proinsulin levels in POR (stimulated by tolbutamide). Calculated portal insulin was approximately 1.3-fold higher in POR versus PER and approximately 2.2-fold higher in POR versus 1/2 PER. In the final 30 min of the clamp, glucose production reached a lower steady-state level in PER than in POR (4.0 +/- 0.4 vs. 5.3 +/- 0.5 pmol(-1) x kg(-1) x min(-1), P < 0.05), despite the higher hepatic insulin level in POR. In contrast with our studies in nondiabetic individuals, glucose production was not more suppressed at steady state in POR versus 1/2 PER (5.3 +/- 0.4 micromol x kg(-1) x min(-1)), despite much higher hepatic insulin levels in POR. In conclusion, this is the first study in patients with type 2 diabetes to characterize insulin resistance to the acute direct suppressive effect of insulin on hepatic glucose production.

Decreased glucose tolerance, not decreased insulin sensitivity, is a maturational abnormality in the male offspring of a parent with early coronary artery disease

We investigated whether the male offspring of a parent with early coronary artery disease (before the age of 60; n = 61) exhibit decreased insulin sensitivity compared with controls matched for age and body mass index (BMI) (n = 39). The insulin sensitivity index (S I) was determined by the minimal modeling method of Bergman from a frequently sampled intravenous glucose tolerance test with intravenous tolbutamide. Offspring and controls had a similar S I, insulin-independent glucose utilization (S G), first-phase insulin response (AIR G), and area under the glucose curve. When subjects were separated into two age groups, younger subjects aged 15 to 30 years and older subjects aged 31 to 45 years, important differences were seen. S G was significantly increased in younger offspring compared with controls (22.8 ± 2.3 v 16.8 ± 2.3 × 10 −3 · min −1, P < .05). Older offspring had a significantly increased area under the glucose curve compared with controls (18,250 ± 322 v 17,225 ± 347 mg/dL · min −1, P < .05). Older offspring also had decreased S I compared with younger offspring (5.0 ± 0.4 v 6.6 ± 0.9 × 10 −4 · min −1, · μ U/mL, P < .05), but this difference was eliminated after adjusting for BMI and waist to hip ratio (5.5 ± 0.4 v 5.8 ± 0.9 × 10 −3 · min −1, nonsignificant). This study does not support the concept that insulin resistance is an early atherogenic risk factor in offspring at risk for coronary disease because of their family history. However, it does point to the importance of maturational changes in glucose homeostasis in these offspring.

Novel MODY3 mutations in the hepatocyte nuclear factor-1alpha gene: evidence for a hyperexcitability of pancreatic beta-cells to intravenous secretagogues in a glucose-tolerant carrier of a P447L mutation.

One form of maturity-onset diabetes of the young (MODY3) results from mutations in the hepatocyte nuclear factor (HNF)-1alpha gene, located on chromosome 12q24.2. The primary objective of the present study was to search for genetic variation in the HNF-1alpha gene in nine nonrelated Danish Caucasian subjects with MODY. Direct sequencing of the coding region and intron-exon boundaries of the HNF-1alpha gene revealed 2 novel and 1 previously reported missense mutations and 2 novel frameshift mutations in five of nine MODY subjects. These five mutations were found in neither 84 NIDDM patients nor 84 control subjects. One glucose-tolerant lean male with a P447L missense mutation, which in his relatives caused MODY, underwent an oral glucose tolerance test (OGTT), a tolbutamide modified frequently sampled intravenous glucose tolerance test, and a glucagon test to examine for a possible early beta-cell abnormality. He had a low insulin secretion rate during an OGTT, but a twofold increase in pancreatic beta-cell response after intravenous glucose and a 2.5- to 4-fold increase in beta-cell response after either intravenous tolbutamide or intravenous glucagon loads. In conclusion, 1) mutations in the HNF-1alpha gene are common in Danish Caucasian MODY patients, and 2) early stages in the pathogenesis of MODY3 caused by the P447L mutation may be characterized by a hyperexcitability of beta-cells to intravenous secretagogues.

A drug interaction study between cicletanine and tolbutamide in healthy volunteers.

To determine the possible interaction between the antihypertensive agent cicletanine and the hypoglycaemic drug tolbutamide. Time-courses of glycaemia and serum immunoreactive insulin (IRI) were followed in 10 healthy subjects after two tolbutamide infusions in each volunteer; initially alone and 9 days later concomitantly with repeated oral cicletanine. Any drug interaction was quantified on the basis of a decrease or increase in the AUC with time of glycaemia and IRI by subtraction of baseline concentration (AUC0(240)-GLY and AUC0(60)-IRI). Peak glycaemia and peak IRI, and the corresponding time to peaks, were also assessed. Following tolbutamide, mean AUC0(240)-GLY values were 97.7 and 98.8 mmol.l-1.min, without or with cicletanine, respectively; the corresponding AUC0(60)-IRI were 485 and 321 mU.l-1.min. Mean peak glycaemia values were 0.996 and 1.071 mmol.l-1. Regarding the peak IRI, a decrease was observed after tolbutamide and cicletanine: median values were 29.2 and 17.4 mU.l-1. The corresponding median time to peak glycaemia and IRI values were 30 and 30 min and 5 (all subjects) and 5 min. No clinically relevant interaction was shown after the concomitant administration of repeated oral doses of cicletanine and acute intravenous tolbutamide to healthy volunteers.

Calcium Reverses Octreotide Inhibition of Insulin and Glucagon Levels in Patients with Insulinoma and Glucagonoma

Excessive secretion of peptides causes the clinical syndromes associated with functional gastro-intestinal tumours. The somatostatin analogue octreotide acetate inhibits peptide release from a variety of tumours. This study investigated the interactions of calcium and somatostatin analogues on peptide release in two patients, one with a glucagonoma (patient A) and one with an insulinoma (patient B). Peptide responses were evaluated before (fasting levels) and after provocative tests (a 4-hour calcium infusion, an intravenous tolbutamide infusion, a secretin bolus and a standard test meal) in the absence and presence of octreotide acetate treatment (100 micrograms subcutaneously every 8 h). Patients A and B had elevated fasting plasma levels of glucagon and insulin, respectively, which were reduced by octreotide therapy by 73 and 50%, respectively. The peak provoked levels and calculated values for peptide synthesis were lower after octreotide therapy. In both patients, tolbutamide provoked most peptide release, and calcium infusion was the least susceptible to the effects of octreotide therapy. Calcium appears to inhibit octreotide suppression of glucagon and insulin secretion in patients with glucagonoma and insulinoma, respectively. Calcium may stimulate peptide release from endocrine tumours by suppressing the inhibitory effects of endogenous somatostatin. Normalisation of serum calcium, either surgically or pharmacologically, may improve the effectiveness of somatostatin analogue therapy.

Tolbutamide causes a modest increase in insulin secretion in cystic fibrosis patients with impaired glucose tolerance

We examined the effect of intravenous (IV) tolbutamide administration on glucose and hormone levels in cystic fibrosis (CF) patients with impaired first-phase insulin secretion and oral glucose tolerance (oral glucose tolerance test [OGTT]) and compared them with CF patients with only an impaired first-phase insulin secretion and healthy control subjects. Five CF patients with an impaired OGTT, ie, a serum glucose value of 7.8 mmol/L or greater 120 minutes after an oral glucose load (group I), five CF patients with a normal OGTT, ie, a serum glucose not exceeding 7.8 mmol/L 120 minutes after oral glucose (group II), and five healthy control (CON) subjects underwent IV glucose tolerance tests with glucose alone (IVGTT) and glucose administered in conjunction with tolbutamide ([IVTTT] 25 mg/kg; maximum dose, 1 g). Serum glucose levels were measured using the glucose oxidase method; insulin, C-peptide, and glucagon levels were measured by the double-antibody radioimmunoassay (RIA) technique. Serum immunoreactive trypsin (IRT) and hemoglobin A 1 (HbA 1) levels and height and weight were measured for each subject, and in addition, pulmonary function was assessed in those with CF. There were no significant differences in the area under the curve (AUC) for glucose or glucagon levels or the serum glucose disappearance rate ( k value) between group I, group II, or CON subjects during the IVGTT. First-phase insulin and C-peptide secretion was abnormal during IVGTT and IVTTT in the CF groups: in group I it was severely impaired, whereas in group II it was between group I and CON values. During the IVTTT serum glucose levels and glucose k values were not significantly altered in any of the three groups as compared with the IVGTT. Tolbutamide administration significantly increased the AUC for serum insulin in group II and CON subjects as compared with IVGTT values (67% and 135%, respectively, P < .05), with a modest ( P > .05) increase in group I levels (28%). Likewise, there were significant increases in the AUC for C-peptide with IVTTT in group II (49%, P < .05) and CON subjects (58%, P < .01), with a lesser increase in group I (21%). The glucagon AUC was not significantly altered in IVTTT as compared with IVGTT in any group. These observations suggest that endocrine function in CF subjects is a continuous spectrum from those with diabetes mellitus to normals. Only long-term studies will determine if the residual secretion of insulin in response to oral sulfonylurea is of clinical importance.

VLDL production is decreased to a similar extent by acute portal vs. peripheral venous insulin

Acute changes in very low-density lipoprotein (VLDL) triglyceride (TG) and VLDL apolipoprotein (apo) B production were examined in 11 healthy young males in response to insulin delivered either by the peripheral venous route or secreted directly by the pancreas. Steady rates of pancreatic insulin secretion were achieved for 5 h by a programmed intravenous tolbutamide infusion, while euglycemia was maintained with a dextrose infusion. Insulin secretory rate was calculated from peripheral C-peptide levels by deconvolution, and, in a subsequent study, exogenous insulin was infused peripherally to match this pancreatic insulin secretory rate in each subject. Changes in VLDL TG and VLDL apo B production were determined semiquantitatively on each occasion by examining the change in slope of the specific activity (SA) of 3H-labeled triglyceride glycerol ([3H]TGG) and 131I-VLDL apo B vs. time curves, respectively, occurring with acute hyperinsulinemia. Plasma-free fatty acids (FFA), TG, apo B, and VLDL TG/VLDL apo B ratio decreased to a similar extent in both studies after the onset of hyperinsulinemia. VLDL TG production decreased significantly in both the tolbutamide (-47.1 +/- 7.3%, P < 0.002) and the exogenous insulin infusion study (-52.8 +/- 12.4%, P < 0.005). VLDL apo B production also decreased significantly in both studies (-58.9 +/- 7.5%, P = 0.0007 and -52.1 +/- 6.8%, P < 0.006, respectively), and there were no significant differences between studies. Tolbutamide was shown to have no independent effect on VLDL TG or VLDL apo B production in four insulin-deficient diabetic subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

A new method for comparing portal and peripheral venous insulin delivery in humans: tolbutamide versus insulin infusion.

We describe a new noninvasive method for comparing insulin secreted acutely by the pancreas vs. a matched peripheral venous insulin infusion in humans. An intravenous tolbutamide infusion algorithm was developed that produced sustained steady rates of portal insulin secretion over 5 h in 11 healthy young men. Plasma glucose levels were maintained in the euglycemic range by adjusting the rate of an iv dextrose (20%) infusion. The pancreatic insulin secretory rate was calculated from peripheral C-peptide levels by deconvolution using standard parameters for a two-compartment mathematical model for C-peptide distribution and metabolism. On a subsequent occasion in the same subject, exogenous insulin was infused peripherally at a rate that matched the earlier tolbutamide-induced pancreatic insulin secretory rate, and euglycemia was maintained with a variable 20% dextrose infusion. The assumption that tolbutamide, when used in this fashion, has no independent insulin-like or insulin-potentiating effect at either low or high levels of peripheral insulinemia, does not affect insulin clearance, and does not suppress peripheral glucagon levels was validated in four patients with insulin-dependent diabetes mellitus. Mean peripheral immunoreactive insulin was significantly higher in the insulin infusion study than in the tolbutamide study (286 +/- 31 vs. 156 +/- 21 pmol/L; P = 0.0001). The dextrose infusion rates required to maintain euglycemia were also higher in the insulin infusion study (0.44 +/- 0.03 vs. 0.32 +/- 0.03 mmol/kg.min; P = 0.003). The MCR of insulin was greater in the tolbutamide infusion vs. the exogenous insulin infusion study (32.6 +/- 2.9 vs. 19.8 +/- 2.2 mL/kg.min; P = 0.0003), probably due to the hepatic first pass effect on insulin clearance when insulin is delivered by the portal route. This noninvasive method can be used in future studies to examine the relative importance of direct hepatic vs. peripheral effects of insulin in controlling hepatic glucose and lipid production.

Prolonged Maximal Stimulation of Insulin Secretion in Healthy Subjects Does Not Provoke Preferential Release of Proinsulin

Release of immature secretory granules rich in incompletely processed proinsulin has been proposed to explain the relative hyperproinsulinemia in type 2 diabetic and insulinoma patients because of a constant secretory drive resulting from hyperglycemia and autonomous secretion, respectively. To test this hypothesis, insulin secretion was stimulated by a combination of hyperglycemia (11 mmol/L clamp), intravenous (i.v.) tolbutamide (1 g), and i.v. glucagon (initial bolus 10 micrograms/kg body weight, maintenance infusion 2 micrograms/kg body weight per hour) for 3 h. Circulating IR-insulin and IR-C-peptide concentrations increased 89-fold and 14-fold over basal values, respectively, but IR-proinsulin concentrations increased only ninefold over basal values. Estimation of the amount of insulin secreted (based on deconvolution analysis of plasma C-peptide values) showed that approximately 76 +/- 21 U were secreted during the stimulation period. This amount is a significant proportion of pancreatic insulin content in normal humans. In molar terms, IR-proinsulin (integrated incremental response multiplied by metabolic clearance rate of proinsulin) relative to IR-C-peptide (= insulin) secretion (deconvolution analysis) was estimated to be equal or even lower than the known proportion in islets (0.22 +/- 0.05%). Thus, using a near-maximal stimulation of insulin secretion maintained long enough to cause release of amounts of insulin approaching the estimated pancreatic content, no preferential release of proinsulin was observed in normal humans. Therefore, the hyperproinsulinemia of type 2 diabetes and in insulinoma patients may be caused by additional defects in the proinsulin to insulin conversion process.

Androgen Response to Endogenous Insulin Secretion during the Frequently Sampled Intravenous Glucose Tolerance Test in Normal and Hyperandrogenic Women*

Women with ovarian hyperandrogenism frequently have insulin resistance, whose underlying mechanism remains to be determined. In the present study we have investigated the relationship between insulin sensitivity and the acute effect of endogenous insulin secretion on circulating androgen levels. Insulin sensitivity, glucose-mediated insulin release, and glucose/insulin-stimulated androgen responses were determined during a frequently sampled iv glucose tolerance test in a group of 19 women with clinical evidence of polycystic ovary syndrome (PCOS) and 9 age- and weight-matched controls. Insulin (I), glucose, androstenedione, testosterone (T), free T, and dehydroepiandrosterone (DHEA) levels were measured before and during the 3 h following iv administration of glucose (300 mg/kg). Intravenous tolbutamide (300-500 mg) was injected 20 min after the glucose injection. Insulin sensitivity (SI) was calculated by application of the minimal model of glucose kinetics. Fasting androstenedione, T, free T, and I concentrations were significantly higher in the women with PCOS than in controls (P less than 0.02). In PCOS subjects, fasting I was correlated with both T (r = 0.51; P less than 0.05) and DHEA (r = 0.706; P less than 0.01). SI was significantly lower in PCOS subjects [SI, 68.35 +/- 8.34 min-1/(nmol/mL] than in control subjects (SI, 133.36 +/- 21.7 min-1/(nmol/mL)]. A significant decline in DHEA levels was observed in control subjects 3 h after glucose administration (from 28.4 +/- 3.0; final, 16.2 +/- 2.4; P less than 0.02). PCOS women with normal insulin sensitivity [SI, greater than 75.0 min-1/(nmol/mL)] showed a similar fall in DHEA (from 20.3 +/- 2.5 to 12.8 +/- 1.8 nmol/L; P less than 0.02). No significant change occurred in insulin-resistant PCOS subjects [SI, less than 75.0 min-1/(nmol/mL)]. Other androgen levels showed a modest nonsignificant decline during the study in PCOS and control groups. These findings confirm the weight-independent insulin resistance of some hyperandrogenic women. The failure of glucose-stimulated endogenous insulin secretion to significantly depress DHEA levels in insulin-resistant women with PCOS may account in part for their androgen excess.

Diagnostic Interpretation of the Intravenous Tolbutamide Test for Insulinoma

The plasma glucose and insulin responses to intravenous administration of tolbutamide in patients with insulinoma and healthy control subjects were compared to determine the sensitivity and specificity of the intravenous tolbutamide test for the diagnosis of insulinoma. The records of 406 healthy persons without concurrent disease known to affect glucose homeostasis or insulin concentrations and 41 patients with histologically confirmed insulinoma who underwent standard intravenous tolbutamide testing during the period from 1976 to 1986 were reviewed. The 5th percentile of the mean of plasma glucose levels at the 120-, 150-, and 180-minute points (G120-180) after injection of tolbutamide was 55 mg/dl for lean and 62 mg/dl for obese control subjects. With 95% specificity, the sensitivity of these criteria was 95% for lean and 100% for obese patients with insulinoma. Minimal differences were observed between men and women. At 95% specificity, the sensitivities were less for the ratio of the 180-minute plasma glucose to fasting plasma glucose level (64% in lean and 75% in obese patients), for mean of plasma insulin at the 120-, 150-, and 180-minute points (IRI120-180) (53% in lean and 36% in obese patients), for maximal insulin concentration (27% in lean and 31% in obese patients), for increase in insulin concentration above basal (18% in lean and 23% in obese patients), and for the combined criteria of the 5th percentile of G120-180 and the 95th percentile of IRI120-180 (47% in lean and 73% in obese patients).(ABSTRACT TRUNCATED AT 250 WORDS)