Specific and direct radioimmunoassay for human proinsulin in serum

Summary A routine radioimmunoassay for human proinsulin in serum has been developed. The reagents used were: antibodies against the C-peptide part of the proinsulin molecule, human proinsulin as the standard and¹²⁵I-labelled synthetic human Tyr-C-peptide as the tracer. The first step in this assay comprises the binding of proinsulin to insulin antibodies covalently coupled to Sepharose (S-AIS). Although bound to the solid-phase S-AIS, the proinsulin retains its second immunogenic site, viz., the C-peptide part of the molecule, accessible. Hence a surplus of C-peptide antibodies is added to the S-AIS-bound proinsulin, and the residual amount of C-peptide antibody is determined by addition of¹²⁵I-Tyr-C-peptide. The detection limit is approximately 0.01 pmol/ml. The advantages of this method are: (1) its high specificity (proinsulin is determined as a molecule having both an insulin and a C-peptide moiety), (2) its simplicity and rapid performance, and (3) the low detection limit of the assay. Fasting sera from 24 nondiabetics, 9 maturityonset diabetics and 10 newly diagnosed insulin requiring diabetics showed the following concentrations of proinsulin: 0.009±0.005, 0.022±0.23 and 0.010±0.009 pmol/ml (mean±SD). One hour after 1.75 g/kg oral glucose, the values increased to 0.052±0.023, 0.046±0.022 and 0.032±0.022 pmol/ml. The fasting proinsulin constituted 19, 23 and 17% of the IRI, respectively, whereas 1 h post glucose these values changed to 8, 9 and 31% of IRI. Serum from 10 insulin-treated diabetics containing insulin antibodies contained from 0–1.80 pmol/ml, whereas the C-peptide levels in the same patients were 0–0.35 pmol/ml. It is suggested that insulin requiring diabetics hypersecrete proinsulin due to the inability of their B-cell to arrange proinsulin in secretory granules for adequate proinsulin/insulin conversion.     

Insulin and C-peptide plasma levels in patients with severe chronic pancreatitis and fasting normoglycemia

The aim of the present study was to evaluate insulin secretion by the pancreatic B cell in a group of patients with severe chronic pancreatitis and without overt diabetes. For this purpose we have measured plasma insulin and C-peptide peripheral levels in the fasting state and after a 100-g oral glucose load in 10 patients with severe chronic pancreatitis and fasting normoglycemia, and in 10 sex-, age-, and weight-matched healthy controls. As compared to normal subjects, patients with chronic pancreatitis showed: (1) significantly higher plasma glucose levels after oral glucose load (area under the plasma glucose curve 1708 +/- 142 vs 1208 +/- 47 mmol/liter X 240 min, P less than 0.005); (2) plasma insulin levels significantly higher at fasting (0.11 +/- 0.008 vs 0.08 +/- 0.005 nmol/liter, P less than 0.01) but not after oral glucose administration (area under the plasma insulin curve 79 +/- 12 vs 88 +/- 16 nmol/liter X 240 min); (3) significantly lower plasma C-peptide concentrations both in the fasting state (0.15 +/- 0.01 vs 0.54 +/- 0.05 nmol/liter, P less than 0.001) and after oral glucose load (area under the plasma C-peptide curve 211 +/- 30 vs 325 +/- 37 nmol/liter X 240 min, P less than 0.05). The finding of diminished plasma C-peptide levels suggests that chronic pancreatitis is associated with an impaired B-cell function even in the absence of overt diabetes. The increased or unchanged plasma insulin levels in spite of decreased plasma C-peptide concentrations indicate that in chronic pancreatitis insulin metabolism is reduced, most likely within the liver.     

C-peptide in diabetic children after stimulation with clucagon compared with fasting C-peptide levels in non-diabetic children

Fasting serum C-peptide and total immunoreactive insulin (IRI) were determined in 38 non-diabetic children and adolescents 6-22 years old. C-peptide varied between 0.22-0.73 pmol/ml (mean +/- SD, 0.45 +/- 0.11). There was a tendency to higher values during puberty. No difference was found between subjects with or without a family history for diabetes. IRI varied between 0-31 millimcron/m1 (mean +/- SD, 11.3 +/- 6.5). The C-peptide response to glucagon was studied in 10 insulin dependent juvenile diabetics 11-19 years old, who had had measurable amounts of fasting C-peptide on some occasions during the previous years. Duration of diabetes varied between 4-12 years. A slight but significant rise in C-peptide level occurred in 3 patients. Their metabolic control estimated on the basis of daily urinalysis was "excellent" or "good". The results support the hypothesis that even trace remnants of the beta cell function may be of importance for the metabolic control in juvenile diabetes.     

Radioimmunological determination of human C-peptide in serum

A routing radioimmunoassay for human C-peptide in serum is described. Antibodies against human C-peptide were raised by immunizing guinea pigs with human b-component. Nine out of 12 animals produced useful antibodies within 6 months. Insulin antibodies coupled to Sepharose were used to bind human proinsulin and insulin in the serum and after centrifugation C-peptide was determined in the supernatant. The detection limit of the assay (calculated as 2 SD from zero) was about 0.003 pmole of C-peptide (in 100 mul). The main sources of error were: (1) Normal and diabetic sera devoid of C-peptide gave a displacement of 125I-Tyr-C-peptide varying from 0 to 0.16 nM (6 different antisera). Only one antiserum (M 1181) showed no displacement, and the values of C-peptide determined with this antiserum in normal and diabetic sera were lower than the values determined with another antiserum, which gave a value of 0.07 nM in the sera free of C-peptide. It is suggested that displacement found with most antisera is due to substances in serum that are not related to C-peptide or proinsulin. (2) Serial dilutions of pancreatic extracts and sera may yield dilution curves slightly different to those of the synthetic standard. Possible explanations are discussed. These sources of error can be eliminated or reduced by the proper selection of antisera. Fasting sera from 15 normals, 8 maturity-onset diabetics and 10 insulin-requiring diabetics showed the following concentrations of C-peptide: (M 1181) 0.35 +/- 0.09, 0.74 +/- 0.51 and 0.21 +/- 0.14 (nM, mean +/- SD). One hour after 1.75 g/kg oral glucose the values increased to 2.24 +/- 0.71, 2.34 +/- 0.24 nM.     

Insulin and C-peptide responses to 75 g oral glucose load in the healthy man

Plasma insulin and C-peptide levels in the fasting state and after a 2-h 75 g oral glucose tolerance test (OGTT) in a large number of healthy subjects are reported. 247 volunteers (134 males, 113 females), aged 13-69 years, who had a negative history of diabetes, no history of significant disease, normal physical examination, normal body weight, normal glucose tolerance, normal blood tests, and who were taking no drugs were studied. Results, mean +/- SEM (range): fasting glucose concentration = 4.64 +/- 0.03 mmol/l (3.10 - 6.10), 1-h glucose concentration = 5.23 +/- 0.10 mmol/l (2.20 - 9.90), 2-h glucose concentration = 4.11 +/- 0.06 mmol/l (2.00 - 6.80); fasting insulin level = 0.088 +/- 0.002 nmol/l (0.03 - 0.28), 1-h insulin level = 0.45 +/- 0.01 nmol/l (0.06 - 1.63), 2-h insulin level = 0.24 +/- 0.01 nmol/l (0.05 - 1.12); fasting C-peptide concentration = 0.60 +/- 0.01 nmol/l (0.14 - 1.34), 1-h C-peptide concentration = 2.17 +/- 0.05 (0.63 - 8.56), 2-h C-peptide concentration = 1.77 +/- 0.04 nmol/(0.35 - 5.74). Fasting insulin and fasting C-peptide concentrations correlated to post-glucose insulin and C-peptide concentrations, respectively. At each sampling-point insulin concentration correlated to C-peptide concentration. After glucose ingestion, both insulin and C-peptide plasma levels correlated significantly with the corresponding glucose levels. During fasting, C-peptide but no insulin level correlated to glucose level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of porcine gastric inhibitory polypeptide on beta-cell function in type I and type II diabetes mellitus

The effect of highly purified natural porcine GIP on C-peptide release was examined in six type I (insulin-dependent) diabetics (IDD) with residual beta-cell function, six type II non-insulin-dependent) diabetics (NIDD), and six normal subjects. All subjects were normal weight. From -120 minutes to 180 minutes glucose or insulin was infused IV to achieve a constant plasma glucose level of 8 mmol/L. On two separate days GIP (2 pmol/kg/min) or isotonic NaCl at random were infused from 0 to 30 minutes. After 10 minutes of GIP infusion plasma IR-GIP concentrations were in the physiologic postprandial range. At 30 minutes a further increase in IR-GIP to supraphysiologic levels occurred. In all subjects plasma, C-peptide increased more after 10 minutes of GIP infusion (IDD, 0.48 +/- 0.05; NIDD, 0.79 +/- 0.11; normal subjects, 2.27 +/- 0.29 nmol/L) than on the corresponding day with NaCl infusion (IDD, 0.35 +/- 0.03; NIDD, 0.62 +/- 0.08; normal subjects, 1.22 +/- 0.13 nmol/L, P less than .05 for all). The responses of the diabetics were significantly lower than that of the normal subjects (P less than .001 for both groups). No further increase in C-peptide occurred during the remaining 20 minutes of the GIP infusion in the diabetic subjects (IDD, 0.49 +/- 0.05; NIDD, 0.83 +/- 0.10 nmol/L). In the presence of a plasma glucose concentration of 8 mmol/L, physiologic concentrations of porcine GIP caused an immediate but impaired beta-cell response in IDD and NIDD patients.     

Glycemic control with an artificial pancreas improves insulin responses to both oral and I.V. Glucose in nonobese noninsulin-dependent diabetic subjects

Summary Insulin secretory responses to both oral and intravenous glucose were investigated in 12 nonobese noninsulin-dependent diabetic subjects before and after strict metabolic control of blood glucose levels without weight loss. Glycemic control was achieved by applying an artificial pancreas to all diabetics for 2 or 3 days, which led to restoration of normal fasting blood glucose levels and to significant reduction of fasting plasma insulin (p<0.01) and C-peptide (p<0.05) levels. Initially, the insulin response to oral glucose was weak and delayed, but increased significantly after treatment (p<0.01), although none of the diabetic subjects achieved completely normal glucose tolerance. The i.v. glucose tolerance test (0.33 g/kg) revealed that all diabetics lacked acute insulin response in the basal state with low glucose disappearance rates (0.37±0.07 %/min). After 48h of normoglycemia, these figures did not change significantly, although the insulinogenic index (insulin area/glucose area) was significantly increased (p<0.05). A marked increase in both phases of insulin secretion was evident when a larger intravenous glucose pulse (0.66 g/kg) was used in some diabetics in order to raise the blood glucose concentrations of the post-treatment test to those of the pre-treatment test. In absolute terms, the insulin responses of the post-treatment tests were not significantly different from those of sex, age- and weightmatched control subjects, but were signficantly lower if related to the corresponding plasma glucose responses (insulinogenic index lower than that of controls). These studies in nonobese noninsulin-dependent diabetic subjects indicate that glycemic control with an artificial pancreas improves insulin response to glucose, suggesting that chronic hyperglycemia may stress the impaired B-cell secretory capacity of diabetes.     

Plasma C-peptide levels identify insulin-treated diabetic patients suitable for oral hypoglycaemic therapy

Plasma C-peptide levels were measured fasting and 2 h after an oral glucose load in 37 insulin-treated diabetic patients to assess their clinical value in identifying any noninsulin-dependent diabetic patients. All subjects were changed from insulin to oral hypoglycaemic therapy and followed for 3 months. Twenty patients (group A) completed the trial without requiring insulin and 17 (group B) required restabilization on insulin due to deteriorating metabolic control. Fasting and 2 h C-peptide levels were significantly higher in group A (0.11 +/- 0.09 and 0.17 +/- 0.12 nmol/l; mean +/- S.D.) compared with group B (0.02 +/- 0.03 and 0.02 +/- 0.03 nmol/l) (p less than 0.002 and p less than 0.002). The fasting C-peptide levels at 3 months (0.28 +/- 0.14 nmol/l) were also significantly higher than the fasting levels at the beginning of the study (p less than 0.002). Fasting and 2-h glucose levels were lower in group A (11.0 +/- 3.7 and 17.6 +/- 5.2 mmol/l) than in group B (14.4 +/- 6.2 and 23.1 +/- 5.9 mmol/l; p less than 0.05 and 0.02, respectively). The differences in glycosylated haemoglobin and fasting glucose levels at the start of the study and after 3 months of oral therapy were not statistically significant. Although C-peptide values overlapped in groups A and B, they were of greater value in identifying patients suitable for oral therapy than any single clinical criterion, and thus may help in identifying insulin-treated diabetic patients who may be treated with oral therapy without deterioration in metabolic control.     

Relationship between insulin sensitivity, insulin secretion and glucose tolerance in cirrhosis

Hepatic insulin extraction is difficult to measure in humans; as a result, the interrelationship between defective insulin secretion and insulin insensitivity in the pathogenesis of glucose intolerance in cirrhosis remains unclear. To reassess this we used recombinant human C-peptide to measure C-peptide clearance in cirrhotic patients and controls and thus derive C-peptide and insulin secretion rates after a 75-gm oral glucose load and during a 10 mmol/L hyperglycemic clamp. Cirrhotic patients were confirmed as insulin-insensitive during a euglycemic clamp (glucose requirement: 4.1 +/- 0.1 mg/kg/min vs. 8.1 +/- 0.5 mg/kg/min; p less than 0.001), which also demonstrated a low insulin metabolic clearance rate (p less than 0.001). Although intolerant after oral glucose, the cirrhotic patients had glucose requirements identical to those of controls during the hyperglycemic clamp (cirrhotic patients: 6.1 +/- 1.0 mg/kg/min; controls: 6.3 +/- 0.7 mg/kg/min), suggesting normal intravenous glucose tolerance. C-peptide MCR was identical in cirrhotic patients (2.93 +/- 0.16 ml/min/kg) and controls (2.96 +/- 0.24 ml/min/kg). Insulin secretion was higher in cirrhotic patients, both fasting (2.13 +/- 0.26 U/hr vs. 1.09 +/- 0.10 U/hr; p less than 0.001) and from min 30 to 90 of the hyperglycemic clamp (5.22 +/- 0.70 U/hr vs. 2.85 +/- 0.22 U/hr; p less than 0.001). However, with oral glucose the rise in serum C-peptide concentration was relatively delayed, and the insulin secretion index (secretion/area under 3-hr glucose curve) was not elevated. Hepatic insulin extraction was reduced both in fasting and during the hyperglycemic clamp (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered C-peptide/insulin molar ratios and glucose turnover rates after stimulation in nondiabetic offsprings of type II diabetic patients

We evaluated the serum glucose/insulin/C-peptide dynamics and C-peptide/insulin molar ratios during sequential standard meal and intravenous (IV) glucagon testing for 240 minutes in eight genetically predisposed but nondiabetic female offsprings of type II diabetic patients and seven weight-matched, normal female controls. Glucose turnover rates and metabolic clearance rates of glucose (MCRG) were also determined isotopically by the D-[3-3H]glucose infusion technique. All the subjects had normal fasting serum glucose and glycosylated hemoglobin (HbA1) values. After meal ingestion, mean serum glucose concentrations were not different except for 120 to 180 minute values, which were significantly higher in the offsprings v controls. After intravenous glucagon infusion, percent maximum increments of glucose were no different. Mean basal immunoreactive insulin (IRI) levels were significantly (P less than 0.02) higher in the nondiabetic offsprings v controls. Following meal ingestion, mean IRI rose to a peak at 40 minutes in both groups, but values were significantly (P less than 0.001) higher in the offsprings v controls. After glucagon administration, the percent maximum increment was significantly (P less than 0.05) lower in the offsprings v controls. Despite exaggerated IRI levels in the offsprings, the mean fasting and stimulated C-peptide levels were identical in both groups throughout the study period. Basal and stimulated C-peptide/IRI molar ratios were quantitatively lower but qualitatively no different in the nondiabetic offsprings v controls throughout the study period. Mean basal hepatic glucose output (HGO) was higher but not statistically different in the offsprings compared with the controls (2.10 +/- 0.28 v 1.65 +/- 0.15 mg/kg X min).(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhancement of B-cell secretion by blood glucose normalization in type 2 diabetes is associated with fasting C-peptide levels

OBJECTIVES: To investigate (i) the variability of beneficial effects achieved by short-term near-normalization of blood glucose in type 2 diabetes patients, and (ii) the relationship of beneficial effects to individual characteristics of diabetes. DESIGN: Arginine-induced insulin and glucagon release tested at two glucose levels before and after 3 days of intensive insulin treatment. SETTING: The Department of Endocrinology and Diabetology, Karolinska Hospital, Stockholm, Sweden. SUBJECTS: Type 2 diabetes patients with poor metabolic control sampled from an area-based population of diabetes patients. RESULTS: Levels of fasting blood glucose declined from 15.0 +/- 0.9 to 8.5 +/- 0.7 mmol L-1, C-peptide from 0.81 +/- 0.06 to 0.49 +/- 0.05 nmol L-1 and percent proinsulin (of total IRI) from 7.8 +/- 1.0 to 3.2 +/- 0.6%. At comparable glucose levels arginine-induced insulin secretion was enhanced 46.3 +/- 19.5% (range -36 to 220%). Enhancement correlated with extent of blood glucose normalization and also with fasting C-peptide levels and with overweight. Arginine-induced glucagon secretion was nonsignificantly depressed (17.2 +/- 7.4%, range -59 to 29%). Insulin sensitivity assessed by M:I ratio was increased by a median of 95%. CONCLUSIONS: In type 2 diabetes patients reversibility of the effects of poor metabolic control on B-cell function is variable. Variability is related to B-cell mass in individual patients with type 2 diabetes.     

Effects of dexamethasone on glucose-induced insulin and proinsulin release in low and high insulin responders

We compared the effects of dexamethasone-induced insulin resistance on B-cell secretory performance in 12 low insulin responders (LIR) and in eight high insulin responders (HIR). A hyperglycemic clamp (120 minutes) was performed before and after the subjects had ingested dexamethasone 3 mg x 2 for 2 1/2 days. Fasting levels of blood glucose increased from 4.60 +/- 0.13 to 5.74 +/- 0.23 mmol/L after dexamethasone in LIR and from 4.37 +/- 0.18 to 5.26 +/- 0.13 mmol/L in HIR. Dexamethasone treatment increased fasting levels of total immunoreactive insulin (IRI), C-peptide, and proinsulin, as well as the proinsulin to IRI ratio to a similar degree in LIR and HIR. The amount of glucose infused to uphold hyperglycemia during the clamp decreased by 54% after dexamethasone in LIR and by 46% in HIR. Mean level of stimulated IRI during the clamp increased after dexamethasone by 43% in LIR and by 53% in HIR. Mean level of stimulated C-peptide increased by 11% (not significant) in LIR and by 24% in HIR. Mean level of stimulated proinsulin increased by 86% in LIR and by 93% in HIR. The effects of dexamethasone on insulin secretion varied among individuals, since steroid treatment failed to affect IRI responses to glucose in two LIR and two HIR. The magnitude of dexamethasone effects on secretion was not correlated to pre-dexamethasone insulin sensitivity as assessed by a somatostatin-insulin-glucose infusion test (SIGIT) or by M/I (glucose infused/insulin level) ratios of the control clamp.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of acetylsalicylic acid on plasma glucose and the response of glucose regulatory hormones to intravenous glucose and arginine in insulin treated diabetics and normal subjects

The effects of oral acetylsalicylic acid (ASA) 3.2 g a day for 5 days on glucose and glucose regulatory hormones were examined in 4 normal subjects, and in 14 insulin requiring diabetic subjects, 9 of whom had significant residual Beta cell function as assessed by C-peptide secretion and 5 of whom did not. In all subjects plasma samples were assayed for glucose, C-peptide, glucagon, growth hormone, and cortisol during sequential intravenous glucose and arginine infusions while the subject was receiving ASA or placebo. The plasma samples from the normal subjects were assayed for insulin.ASA significantly increased early insulin release (p < 0.02) and decreased plasma glucose (p < 0.02) in response to intravenous glucose in the normal subjects. ASA had no effect on glucose or hormone responses to intravenous arginine.The C-peptide positive diabetics had a significantly lower basal plasma glucose while receiving ASA, and this difference persisted throughout the test (p < 0.02). C-peptide levels were similar to control during ASA ingestion. ASA had no significant effect on plasma glucose in the C-peptide negative diabetics.ASA significantly increased fasting plasma glucagon in the normal subjects (p < 0.05) and C-peptide negative diabetics (p < 0.05). There was a significant positive correlation between the changes in plasma glucose and plasma glucagon both basally and 10 min after the commencement of the glucose infusion for the C-peptide negative diabetics but not for the C-peptide positive diabetics or the normal subjects. ASA had no effect on plasma growth hormone or cortisol levels.The reduction of plasma glucose by ASA in man is dependent on continuing β cell function. In the absence of β cell function the stimulatory effect of ASA on glucagon production becomes the major determinant of ASA's effect on plasma glucose. The effects of ASA on pancreatic islet function appears to be the major determinant of its effect on glucose handling.     

The effect of various blood glucose levels on post-glucagon C-peptide secretion in type 2 (non insulin-dependent) diabetes.

We investigated how different plasma glucose concentrations could significantly modify the C-peptide response to glucagon. Twenty poorly-controlled (HbA1c 10.2 +/- 1.5%) non insulin-dependent (NIDDM) subjects (body mass index 27 +/- 1.8), 2 treated with diet alone and 18 with oral hypoglycemic agents were studied. The first day glucagon (1 mg iv) was injected, patients being fasting and untreated. Mean plasma glucose levels were 11.4 +/- 1.2 mM. On a second non consecutive day, after an overnight fast, the same patients were connected to a closed-loop insulin infusion system (Betalike, Genoa), their blood glucose concentrations were stabilized within a normoglycemic range (5-5.5 mM) for 2 h and insulin infusion was stopped. The glucagon test was repeated 30 min later. Blood samples were taken 0, 6, 10, 20 min after glucagon injection. In the second test, basal, and 6, 10 and 20 min post-glucagon glucose levels were significantly lower (p less than 0.001); similarly C-peptide concentrations were significantly reduced both in basal conditions (0.55 +/- 0.04 vs 0.37 +/- 0.04 nM; p less than 0.001) and 6 (0.92 +/- 0.06 vs 0.6 +/- 0.06; p less than 0.001), 10 (0.79 +/- 0.06 vs 0.56 +/- 0.06; p less than 0.001) and 20 min (0.64 +/- 0.05 vs 0.44 +/- 0.04; p less than 0.001) after stimulation. The C-peptide secretion area showed the same trend (49.5 +/- 4.8 vs 32.1 +/- 5.8; p less than 0.001). In conclusion, our data confirms that blood glucose levels modulate the pancreatic insulin secretion; glycemic normalization significantly reduced both basal and post-glucagon C-peptide release.     

Studies of the Entero-insular Axis Following Pancreas Transplantation in Man: Neural or Hormonal Control?

To study the role of hormonal and neural factors in the control of the entero-insular axis the insulin, C-peptide, and glucose-dependent insulinotropic peptide (GIP) responses to oral and intravenous glucose were investigated in 5 patients who had received a combined kidney and paratopic pancreas transplant, with physiological portal venous drainage. The incremental areas under the insulin and C-peptide responses to oral glucose were significantly greater than the responses to intravenous glucose (insulin: patients 7983 +/- 1937 (+/- SE) vs 3513 +/- 2188 mU l-1 min, p less than 0.002, control subjects 5505 +/- 1035 vs 1066 +/- 484 mU l-1 min, p less than 0.004; C peptide: patients 440 +/- 80 vs 144 +/- 61 nmol l-1 min, p less than 0.01, control subjects 200 +/- 38 vs 63 +/- 16 nmol l-1 min, P less than 0.01). The incretin effects for insulin (patients 4.4 +/- 1.4, control subjects 7.7 +/- 1.8) and C-peptide (patients 4.4 +/- 0.9, control subjects 3.7 +/- 0.9) and the GIP responses to oral and intravenous glucose were not significantly different between transplant patients and control subjects. As the incretin effect was preserved, despite a denervated pancreas, hormonal rather than neural factors may be more important in mediating increased insulin secretion after oral carbohydrate. The normal GIP response is compatible with its proposed role as an insulinotropic hormone.     

Differences in basal and poststimulation glucose homeostasis in nondiabetic first degree relatives of black and white patients with type 2 diabetes mellitus.

Black Americans (blacks) have a higher prevalence and earlier onset of type 2 diabetes than white Americans (whites). To examine metabolic differences in both races, we measured the basal glucose turnover rates (D-]3-3H]glucose technique) and plasma glucose, insulin, and C-peptide levels before and after an oral glucose load in 24 glucose-tolerant black and 14 white female relatives of patients with type 2 diabetes. Eight black and 8 white female subjects with no family history of diabetes served as controls. Mean fasting and postglucose plasma glucose levels were not significantly different between the black and white relatives and the control subgroups. Mean fasting plasma insulin and C-peptide levels were slightly greater but not significantly different between the relatives. After oral glucose ingestion, mean incremental integrated plasma insulin areas were significantly (P less than 0.02) greater in the black than the white relatives (70 +/- 14 vs. 29 +/- 6 nM.min). In addition, incremental integrated C-peptide areas were greater in the black than the white relatives (303 +/- 55 vs. 115 +/- 33; P less than 0.005). Similarly, we found significantly greater integrated incremental insulin (61 +/- 11 vs. 22 +/- 3 nM.min; P less than 0.02) and C-peptide (248 +/- 58 vs. 47 +/- 16; P less than 0.005) areas in the black than the white controls, respectively. The estimated basal and postglucose hepatic insulin extraction values, expressed as molar ratios of C-peptide and insulin, were not significantly different between the relatives. While basal hepatic insulin extraction was significantly (P less than 0.05) lower in the black controls, the postprandial insulin clearance was not different between the black and white controls. Mean basal hepatic glucose production was greater (P less than 0.02) in the black than the white relatives (2.49 +/- 0.13 vs. 2.02 +/- 0.12 mg/kg.min). Similarly, the black controls had greater hepatic glucose production than the white controls (2.36 +/- 0.15 vs. 1.81 +/- 0.08 mg/kg.min; P less than 0.001). We conclude that basal and poststimulation glucose homeostatic regulation appear to be different in black and white females, regardless of family history of type 2 diabetes.     

Reduced insulin secretion in normoglycaemic patients with beta-thalassaemia major.

AIMS: To assess insulin sensitivity and secretion in the fasting state in regularly transfused patients with beta-thalassaemia major with normal glucose response during an oral glucose tolerance test and to estimate its possible relation to iron overload. METHODS: We measured fasting glucose, insulin and C-peptide levels in 24 patients with beta-thalassaemia major and 18 control subjects matched for age and body mass index. Insulin sensitivity and insulin release index were calculated according to the homeostasis model assessment (HOMA). Correlations with age, body mass index and serum ferritin were also calculated. RESULTS: Fasting glucose levels in patients were increased compared with control subjects (5.5 +/- 0.12 vs. 4.7 +/- 0.13 mmol/l, mean +/- SEM, P < 0.001). Pancreatic B-cell insulin secretion in the fasting state (estimated by SC(HOMA)) was lower in thalassaemic patients (SC(HOMA) 88.5 +/- 11.11 vs. 184.3 +/- 23.72 in control subjects, P < 0.001). Patients were then divided into those with impaired (IFG) and normal (NFG) fasting glucose. SC(HOMA) was higher in the patients with NFG compared with those with IFG patients (110.6 +/- 17.63 vs. 66.3 +/- 10.88, respectively, P < 0.05) but estimated insulin sensitivity (ISI(HOMA)) was similar. Plasma values of C-peptide correlated positively with ferritin (r = 0.42, P = 0.04) and SC(HOMA) (r = 0.45, P = 0.02) and negatively with ISI(HOMA) (r = -0.43, P = 0.03). CONCLUSIONS: These results support the concept that impaired B-cell function, as reflected by a reduction in the insulin secretion index, is present in beta-thalassaemic patients with normoglycaemia before changes in oral glucose tolerance tests are apparent.     

Effects of the combination of insulin and gliclazide compared with insulin alone in type 2 diabetic patients with secondary failure to oral hypoglycemic agents.

Twenty non-insulin-dependent diabetic patients on insulin therapy for more than 2 months due to secondary failure to oral hypoglycemic agents (OHA) were additionally treated with gliclazide, 80 mg b.i.d., for 1 month and 160 mg b.i.d. for a further 2 months, while reducing insulin dose gradually according to glycemic control. At the end of the first month, fasting blood glucose had decreased from 12.8 +/- 0.7 to 9 +/- 0.8 mM (mean +/- standard error; P < 0.005) and thereafter remained stable. Insulin requirements decreased from 34.2 +/- 2.5 to 18.3 +/- 3.2 U/day (P < 0.001). Three patients were able to cease insulin treatment altogether. A direct correlation was found between final insulin dose and previous duration of infusion monotherapy (r = 0.52; P < 0.05). C-peptide/glucose score (fasting C-peptide/fasting BG x 100) increased from 0.11 +/- 0.03 to 0.21 +/- 0.05 (P < 0.05). We conclude that combined therapy reduces insulin requirement by increasing endogenous secretion, which may mainly affect hepatic glucose production as indicated by greater improvement in fasting vs. post-prandial blood glucose. This therapy could avoid hyperinsulinemia, which has been reported to be involved in macrovascular complications, and the additional haemovascular properties of gliclazide could make it more effective in such a combination.     

C-peptide reserve in insulin-dependent diabetes

Summary Residual beta cell secretory capacity was assessed in short term (2 months to 2 years) and long term (5 to 8 years) insulin-dependent diabetics by measurement of serum C-peptide immunoreactivity during three provocative tests: glucose, tolbutamide, and glucagon. Minimal C-peptide secretion could be detected in only one out of seven long term diabetics by the stimulatory tests. All seven short-term diabetics responded to at least one provocative test of beta cell reserve, although these responses were blunted. The greatest C-peptide responses occurred after glucagon administration (mean increase 0.62 pmol/ml) in short-term responders. Patients who responded to one test did not necessarily respond to another stimulus. There was no correlation between basal C-peptide levels and the ability to provoke further C-peptide secretion by any of the three tests. C-peptide responses did not correlate with % Haemoglobin A_1c, mean fasting blood glucose levels, or mean blood glucose concentrations during an oral glucose tolerance test. The data indicate that stimulation tests are only useful in assessing endogenous beta cell reserve in patients with diabetes of less than 5 years duration. In diabetics of longer duration there is little insulin reserve above basal levels.     

Infusion of synthetic human C-peptide does not affect plasma glucose, serum insulin, or plasma glucagon in healthy subjects

We studied six healthy male subjects to determine whether a four-hour infusion of synthetic human C-peptide sufficient to achieve mean (+/- SD) peripheral plasma concentrations of 1.3 +/- 0.7 pmol/mL affected plasma glucose, serum insulin, or plasma glucagon. Subjects were studied in a fasting state and following an oral glucose load during four-hour 0.9% NaCl (control) and C-peptide (mean dose: 70 nmol) infusions. No differences were observed between saline and C-peptide infusions for mean values of fasting plasma glucose (94 +/- 6 v 87 +/- 5 mg/dL), serum insulin (3 +/- 1 v 2 +/- 1 microU/mL), or plasma glucagon (124 +/- 65 v 112 +/- 70 pg/dL). Following oral glucose ingestion no differences were detected between saline and C-peptide infusions for mean peak values of plasma glucose (168 +/- 18 v 168 +/- 31) and serum insulin (59 +/- 6 v 57 +/- 21) or mean nadir values of plasma glucagon (80 +/- 73 v 75 +/- 70). There was a slight delay in the insulin rise following oral glucose on the C-peptide infusion day, but differences between mean values for individual sampling times were not statistically significantly different.