Biosynthetic Human C-peptide Research Articles

C-peptide: a redundant relative of insulin?

Historically, focus on C-peptide has been relegated to the exploitation of its peculiar pharmacokinetics (i.e. distribution and elimination) to derive accurate assessment of beta cell secretion, whereas, more recently, greater interest has been placed on its pharmacodynamics (i.e. the biological actions it exerts). In the present paper we will address the latter subject, but to put things in perspective—and because it is well known that kinetics and the time course of dynamics are intimately linked—we will begin with a brief overview of the kinetic properties of C-peptide. C-peptide is stored together with insulin in the mature granules within the beta cell. It is secreted intraportally with insulin, on an equimolar basis, and shows the characteristic biphasic shape in response to i.v. challenges [1, 2]. Whereas newly secreted insulin undergoes a large (∼50%) hepatic extraction [3], the rate of which probably varies, all of the released C-peptide reaches the peripheral circulation without appreciable extraction by the liver. For this reason, C-peptide has proven to be a fundamental tool for accurate quantification of beta cell secretion. In fact, with the aid of kinetic modelling and deconvolution (the so-called Eaton– Polonsky approach [4, 5]), C-peptide levels can be used to provide a reliable measure of the insulin secretory rate. To estimate C-peptide secretion by deconvolution, knowledge of C-peptide kinetics is required. C-peptide kinetics are usually determined by analysing the C-peptide decay curve following a bolus injection of biosynthetic C-peptide. Polonsky described the kinetics of peripherally administered biosynthetic human C-peptide in terms of a linear, two-compartment model. The half-life of C-peptide has been shown to be relatively slow (approximately 35 min), and glomerular filtration is the major route of its removal. The slow metabolism of C-peptide is consistent with the hypothesis that C-peptide diffuses into the extracellular space, but is not integrated into cell membranes. In contrast, insulin is rapidly removed from the circulation (half-life approximately 5 min) via insulin receptor-mediated uptake followed by lysosomal degradation. In terms of its biological action, C-peptide has been long believed to be biologically inert, a sort of useless byproduct of insulin. In recent years, several intracellular effects of C-peptide have been described in vitro, and some in vivo studies have shown that C-peptide has an effect on clinical manifestations of diabetes complications (mainly in the short term). Such biological effects have been reviewed in the companion paper by Wahren et al. [6]. The key point of our argument is not whether C-peptide does or does not have a biological effect (this has been already proven beyond any reasonable doubt), but whether such a biological effect is actually relevant and can be profitably exploited to improve the health of diabetic patients by fending off the complications associated with diabetes. The crucial question is: Are the in vitro and in vivo data sufficient to consider C-peptide an endogenous molecule with a distinct physiology that makes it a suitable addition to the arsenal of therapeutic agents available to the clinician? In our opinion, this is not the case. We believe that C-peptide is neither the ‘shavings of the carpenter’s Diabetologia (2007) 50:500–502 DOI 10.1007/s00125-006-0576-x

Measuring pre-hepatic insulin secretion using a population model of C-peptide kinetics: accuracy and required sampling schedule.

The accuracy of calculations of pre-hepatic insulin secretion were investigated, to provide independent validation of a population model of C-peptide kinetics. The effects of sampling frequency were also assessed. Five normal subjects (aged 28 to 43 years; BMI (kg/m2) 20.5 to 24.5) and five subjects with non-insulin-dependent diabetes mellitus (NIDDM) treated by diet alone (aged 34 to 57 years; BMI 22.6 to 25.6) were given a variable intravenous infusion of biosynthetic human C-peptide (BHCP) (t=-60 to 240 min) mimicking meal stimulated C-peptide secretion, with short-term oscillations (peak approximately every 12 min) superimposed on the infusion profile. Plasma C-peptide was measured every 5 min (t=0 to 240 min). The BHCP infusion was reconstructed from C-peptide measurements using a population model of C-peptide kinetics and a deconvolution method. Bias, defined as the percentage difference between the total amount of calculated BHCP and the total amount of infused BHCP (t=0 to 240 min), indicated that overall C-peptide secretion can be measured with 14% [95% confidence interval (CI) -11 to 39%] and 21% (95% CI -3 to 45%) accuracy in normal subjects and subjects with NIDDM respectively. Accuracy was not reduced by reducing the sampling frequency to every 30 min. The root mean square error, measuring the average deviation between the infused and normalised calculated BHCP profiles, was also independent of the sampling frequency [mean (95% CI) 0.9 (0.3 to 1.6) pmol/kg per min in normal subjects; 1.0 (0.9 to 1.1) pmol/kg per min in subjects with NIDDM]. Deconvolution employing a population model of C-peptide kinetics can be used to estimate postprandial total C-peptide secretion with biases of 14% and 22% respectively in normal subjects and subjects with NIDDM. Plasma C-peptide samples need only be drawn every 30 minutes.

Effect of C-peptide administration on whole body glucose utilization in STZ-induced diabetic rats.

Recent studies suggest that C-peptide stimulates glucose transport in isolated skeletal muscle. In order to determine the effect of C-peptide on whole body glucose utilization, streptozotocin (60 mg kg-1) (STZ)-induced diabetic and normal rats were studied using the euglycaemic clamp procedure and continuous infusion of somatostatin (1.0 micrograms kg-1 min-1) in pentobarbital-anaesthetized rats. Plasma insulin levels during the 6.0- and 30.0-mU kg-1 min-1 insulin infusions rose to 70-90 microU mL-1 and 500-700 microU mL-1, respectively. Blood glucose concentrations were clamped at 7.5-7.9 mmol L-1 in the diabetic rats and at basal levels or 7.7 mmol L-1 in the non-diabetic (normal) rats. Biosynthetic human C-peptide (0.5 nmol kg-1 min-1) was infused in 12 diabetic and 11 normal rats, resulting in concentrations of 26-41 nmol L-1. The metabolic clearance rate of glucose (MCR) for the diabetic rats receiving C-peptide (12.0 +/- 1.0 mL kg-1 min-1) was significantly (P < 0.01) higher than that in the diabetic rats given saline (6.3 +/- 0.7 mL kg-1 min-1) or a randomly scrambled C-peptide (7.8 +/- 1.3 mL kg-1 min-1) at low-dose insulin infusion but not at the high-dose insulin infusion. In normal rats C-peptide did not significantly increase the MCR for glucose. These results thus demonstrate that C-peptide has the capacity to increase glucose utilization in STZ-induced diabetic rats.

Hyperglycemia facilitates urinary excretion of C-peptide by increasing glomerular filtration rate in non-insulin-dependent diabetes mellitus

We have evaluated the feasibility of monitoring the 24-hour urinary excretion rate of C-peptide (U-CPR) as a measure of integrated β-cell function in patients with non-insulin-dependent diabetes mellitus (NIDDM). In 37 normoalbuminuric patients, U-CPR of 117.9 ± 9.1 μg/d (mean ± SEM) during the poorly controlled glycemic phase (fasting plasma glucose [FPG], 171 ± 7 mg/dL; hemoglobin A 1C [HbA 1c], 8.8% ± 0.4%) was significantly higher than the value of 83.3 ± 13.7 μg/d ( P < .001) during the well-controlled phase (FPG, 135. ± 6 mg/dL; HbA 1c, 7.0% ± 0.2%), although the plasma insulin response to meals was lower during the former phase (53.3 ± 6.3 μU/mL) versus the latter phase (65.7 ± 6.6, P < .005). Endogenous creatinine clearance (Ccr) was significantly elevated during the poorly controlled phase (105.4 ± 7.3 v 88.7 ± 4.7 mL/min, P < .005). In 26 microalbuminuric patients, the plasma insulin response was greater during good glycemic control, but U-CPR did not differ between the two phases. Ccr was comparable at two phases in this group (92.7 ± 7.4 v 91.1 ± 5.9 mL/min, NS). U-CPR correlated positively with Ccr in both groups ( r = .593, P < .001 in normoalbuminuria; r = .585, P < .001 in microalbuminuria). In addition, when biosynthetic human C-peptide was infused intravenously at an identical rate in two healthy subjects, resulting steady-state plasma levels of CPR were lower, and fractional U-CPR was higher during the moderately hyperglycemic phase versus the euglycemic phase. The calculated metabolic clearance rate (MCR) of C-peptide was found to be increased in the presence of hyperglycemia. These results suggest that hyperglycemia enhances U-CPR through an increase of glomerular filtration rate (GFR), and therefore U-CPR as an index of residual β-cell function in diabetes should be used cautiously.

The β-Cell in Diabetes: From Molecular Genetics to Clinical Research

Pancreatic insulin secretion rates can be accurately derived by mathematical deconvolution of peripheral C-peptide concentrations either by using individual C-peptide kinetic parameters obtained by analysis of the decay curve of biosynthetic human C-peptide or by using published group parameters with appropriate adjustments for age and degree of obesity. Since the cross-reactivity of proinsulin and related peptides is low (< 10%) in many C-peptide assays, this experimental approach avoids the spurious increase in insulin immunoreactivity resulting from cross-reactivity with proinsulin and related peptides in the insulin assay. Application of this technique has demonstrated that the phenotypic expression of beta-cell dysfunction differs in subjects with different genetic mechanisms of non-insulin-dependent diabetes mellitus (NIDDM). Subjects who have maturity-onset diabetes of the young (MODY) due to mutations in the glucokinase gene demonstrate different patterns of altered insulin secretion when compared with subjects who have mutations in the MODY1 gene on chromosome 20. Glucokinase mutations affect the ability of the beta-cell to detect and respond to small increases in glucose above the basal level. However, compensatory mechanisms operative in vivo, which include a priming effect of glucose on insulin secretion, limit the severity of the observed insulin secretory defect, resulting in a generally mild clinical course in these subjects. In contrast, mutations in the MODY1 gene are associated with an inability to increase insulin secretion as the plasma glucose concentration increases above 7-8 mmol/l and the normal priming effect of glucose on insulin secretion is lost. These characteristics of the dose-response relationships between glucose and insulin secretion result in a more severe degree of hyperglycemia than observed in subjects with glucokinase mutations, and these subjects more frequently need insulin treatment. These alterations are evident in prediabetic subjects with normal glucose levels who carry the MODY1 mutation, suggesting that defective beta-cell function is the primary pathogenetic defect in the diabetic syndrome in these subjects. Studies performed in the classic form of NIDDM demonstrate that subjects with mild glucose intolerance and normal fasting glucose concentrations and glycosylated hemoglobin levels consistently demonstrate defective beta-cell function. These results are consistent with studies in the Zucker diabetic fatty rat, an animal model of NIDDM in which prediabetic animals demonstrate extensive alterations in expression of multiple genes involved in the regulation of insulin secretion. It thus appears that abnormal beta-cell function is present at a relatively early stage in the evolution of NIDDM, even before the onset of overt hyperglycemia.

Improving insulin therapy: achievements and challenges.

Microvascular complications of diabetes can be forestalled by effective glycemic control. However, the inherent limitations of standard subcutaneous insulins reduce their ability to control glycemia without risk of significant hypoglycemia and hyperinsulinemia. Hypoglycemia is unacceptable for most patients and may be dangerous. Hyperinsulinemia is undesirable because it causes weight gain and it has a putative association with atherosclerosis. This paper summarizes the major historical improvements in insulin therapy, and calls attention to the fact that none of the presently available commercial preparations in any combination is capable of simulating the profile of normal insulin secretion--the latter being regarded as the most effective means of normalizing glycemia. For this reason, a variety of new approaches to simulating the pharmacokinetics or glucodynamics of insulin secretion are under investigation. Fast-acting insulin analogues suitable for subcutaneous injection have been developed and appear to mimic the physiological insulin response more closely than standard insulins. Less progress has been made with basal insulins. Intravenous insulin has pharmacodynamic advantages but practical disadvantages of administration. Nasal insulin would be an attractive treatment modality only if its bioavailability could be significantly increased and its safety assured. Other interventions which improve glucose metabolism without necessarily simulating normal insulin secretion are under investigation. These include biosynthetic human C-peptide, insulin-like growth factor-1 and glucagon-like peptide 1 (7-36 amide).

Insulin secretory abnormalities in subjects with hyperglycemia due to glucokinase mutations.

Pancreatic beta-cell function was studied in six subjects with mutations in the enzyme glucokinase (GCK) who were found to have elevated fasting and postprandial glucose levels in comparison to six normoglycemic controls. Insulin secretion rates (ISRs) were estimated by deconvolution of peripheral C-peptide values using a two-compartment model and individual C-peptide kinetics obtained after bolus intravenous injections of biosynthetic human C-peptide. First-phase insulin secretory responses to intravenous glucose and insulin secretion rates over a 24-h period on a weight maintenance diet were not different in subjects with GCK mutations and controls. However, the dose-response curve relating glucose and ISR obtained during graded intravenous glucose infusions was shifted to the right in the subjects with GCK mutations and average ISRs over a glucose range between 5 and 9 mM were 61% lower than those in controls. In the controls, the beta cell was most sensitive to an increase in glucose at concentrations between 5.5 and 6.0 mM, whereas in the patients with GCK mutations the point of maximal responsiveness was increased to between 6.5 and 7.5 mM. Even mutations that resulted in mild impairment of in vitro enzyme activity were associated with a > 50% reduction in ISR. The responsiveness of the beta cell to glucose was increased by 45% in the subjects with mutations after a 42-h intravenous glucose infusion at a rate of 4-6 mg/kg per min. During oscillatory glucose infusion with a period of 144 min, profiles from the subjects with mutations revealed reduced spectral power at 144 min for glucose and ISR compared with controls, indicating decreased ability to entrain the beta cell with exogenous glucose. In conclusion, subjects with mutations in GCK demonstrate decreased responsiveness of the beta cell to glucose manifest by a shift in the glucose ISR dose-response curve to the right and reduced ability to entrain the ultradian oscillations of insulin secretion with exogenous glucose. These results support a key role for the enzyme GCK in determining the in vivo glucose/ISR dose-response relationships and define the alterations in beta-cell responsiveness that occur in subjects with GCK mutations.

Influence of combined C-peptide and insulin administration on renal function and metabolic control in diabetes type 1.

The possible influence of C-peptide on renal function and metabolic control in patients with type 1 diabetes was examined in a double blind, randomized study. Nine patients received insulin and equimolar amounts of biosynthetic human C-peptide for 1 month (group 1), and nine were given insulin only (group 2). C-Peptide levels in plasma ranged from 0.3-2.6 nmol/L in group 1 during the study, whereas group 2 had undetectable levels. The urinary excretion of albumin in group 1 was 21 +/- 6 micrograms/min before the study and decreased by 40% and 55% after 2 and 4 weeks, respectively (P < 0.05). No change was seen in group 2. The glomerular filtration rate fell by 6% after 2 and 4 weeks (P < 0.05) in group 1, whereas no change was observed in group 2. Fluorescein leakage across the blood-retinal barrier decreased by 30% in group 1 (P < 0.05) and was unaltered in group 2. Hemoglobin-A1c and fructosamine values decreased by 9-16% in group 1 (P < 0.05), but not in group 2. The findings suggest that administration of C-peptide plus insulin, compared to insulin alone, to type 1 diabetic patients may reduce glomerular permeability and improve metabolic control.

Quantitative evaluation of the effect of low-intensity exercise on insulin secretion in man

We studied insulin secretion rates (ISR) during low-intensity exercise (40% peak aerobic capacity [Vo 2]) in 12 normal subjects to assess the contribution of altered insulin secretion to the reduction in peripheral insulin concentrations associated with exercise. ISR were calculated by a previously validated method of two-compartment analysis of peripheral C-peptide concentrations using individual parameters derived following a bolus injection of biosynthetic human C-peptide. In addition, the effect of low-intensity exercise on kinetic parameters of C-peptide was evaluated. The results showed that low-intensity exercise did not significantly affect C-peptide kinetics. Peripheral insulin concentrations and ISR decreased to a similar degree throughout exercise. There was a mean maximum decrease in serum insulin concentrations from 42 ± 5.4 pmol/L basally to 24 ± 2.6 pmol/L, constituting a 51% ± 5.9% decrease ( P < .001), and ISR decreased from 85.7 ± 11.9 pmol/min to a nadir of 45.6 ± 10.6 pmol/min ( P < .001), a 48% ± 8.4% decline. Plasma glucose and glucagon concentrations did not change significantly either during or after exercise, although there was a matched twofold increase in glucose utilization and disposal rates. We suggest that the reduction in peripheral insulin concentrations during exercise is due to reduced insulin secretion.

Relationship of insulin secretory pulses to sex hormone-binding globulin in normal men.

Insulin has emerged as a regulator of sex hormone-binding globulin (SHBG) production in vitro and in vivo. A role for insulin in regulating SHBG exists in insulin resistant states such as obesity and polycystic ovary syndrome. The relationship of in vivo insulin secretion rates to SHBG levels in healthy normal men is less well documented. Hepatic synthesis of SHBG may be influenced by quantitative insulin exposure as well as qualitative characteristics such as frequency and amplitude of insulin secretory pulses. The present study was undertaken to assess these relationships in 10 normal men. Adiposity was determined by the body mass index and fat distribution by the waist hip ratio. Peripheral insulin sensitivity was determined by the euglycemic clamp technique at an insulin infusion rate of 287 pmol/min.m2. SHBG levels were determined in the fasting state by RIA. Arterialized venous samples for C-peptide were obtained every 2 min for 90 min in the basal state. Individual C-peptide kinetics were derived after a bolus injection of biosynthetic human C-peptide and a previously validated two compartmental model. Insulin secretion rates at each time point were calculated using the plasma C-peptide values and the C-peptide kinetics. Insulin secretion rates were unrelated to SHBG concentrations (r = -0.29, P > 0.05). The insulin secretory pulse interval had a significant positive association with SHBG levels (r = 0.86, P < 0.05). Insulin secretory pulse amplitude, body mass index, waist hip ratio, and peripheral insulin sensitivity were not associated with SHBG concentrations in a regression analysis. We postulate that insulin secretory pulse frequency may be an important determinant of SHBG synthesis in normal man.

Renal and splanchnic exchange of human biosynthetic C-peptide in Type 1 (insulin-dependent) diabetes mellitus

Biosynthetic human C-peptide or NaCl (154 mmol.l-1) was given intravenously to 13 Type 1 (insulin-dependent) diabetic patients to determine the renal and splanchnic exchange of C-peptide. Catheters were inserted percutaneously into an artery and a renal and hepatic vein. Infusions of C-peptide were given for 60 min at two dose levels (5 and 30 pmol.kg-1.min-1). Insulin was infused throughout the study (0.5 mU.kg-1.min-1) and plasma glucose was kept constant by a variable glucose infusion. The regional blood flows were measured by indicator dilution techniques. In 11 of the 13 patients basal C-peptide levels were not detectable. The arterial steady-state C-peptide concentration was 0.81 +/- 0.10 nmol.l-1 and 2.33 +/- 0.30 nmol.l-1 at the low and high rate infusions, respectively. Renal uptake was 124 +/- 18 pmol.min-1 at the low infusion corresponding to 39% of the infused amount. At the higher dose C-peptide infusion renal uptake increased to 155 +/- 21 pmol.min-1 (p less than 0.05). Urinary excretion of C-peptide was 7 +/- 2 pmol.min-1 at the low dose infusion and increased to 34 +/- 6 pmol.min-1 at the high dose infusion (p less than 0.01). The proportions of infused amount excreted were fairly constant and between 2% and 3%. No net exchange of C-peptide was found across the splanchnic vascular bed. The rate of glucose infusion had to be increased by 35% during the low dose C-peptide, but not during NaCl infusion in order to maintain a constant plasma glucose concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of effect of high-dose biosynthetic human C-peptide on pancreatic hormone release in normal subjects

We studied the effect of high doses of biosynthetic human C-peptide on pancreatic hormone secretion in response to oral (75g) and intravenous ([IV] 0.33 g/kg of D50%) glucose on normal volunteers. The infusion of human C-peptide at a rate of 360 ng/kg/min body weight, increased the plasma C-peptide concentration from a basal level of 0.32 ± 0.04 pmol/mL to 38.5 ± 1.8 pmol/ml. Overall, C-peptide had no significant effect on the serum levels of glucose, insulin, proinsulin, glucagon, and pancreatic polypeptide, either under basal conditions or following IV and oral glucose administration. However, small decreases in glucose and insulin concentrations that were not statistically significant were seen during the first hour after C-peptide infusion. The results of the present studies are therefore consistent with the conclusion that even supraphysiologic plasma concentrations of infused C-peptide do not affect basal insulin secretion or overall insulin secretory responses to oral or IV glucose. However, we cannot definitively exclude a small reduction in insulin secretion in the first hour after oral glucose ingestion.

Effects of Weight Loss and Reduced Hyperglycemia on the Kinetics of Insulin Secretion in Obese Non-Insulin Dependent Diabetes Mellitus*

Impairment in pancreatic production of insulin, a cardinal feature of noninsulin dependent diabetes mellitus (NIDDM), was quantified and the kinetics of insulin secretion characterized in six obese individuals with NIDDM before and after weight loss (18.0 +/- 3.0 kg, mean +/- SEM) using a validated mathematical model that employs C-peptide as a marker of the in vivo rate of insulin secretion. The metabolic clearance of C-peptide, assessed by decay analysis after bolus injection of biosynthetic human C-peptide, was not changed by weight loss (0.143 +/- 0.009 L/min.m2 vs. 0.137 +/- 0.010 L/min.m2). Kinetic parameters from each individual's decay curve before and after weight loss were used to derive accurate rates of secretion during the basal (postabsorptive) state, an oral glucose tolerance test and two hyperglycemic clamps. Basal rates of insulin secretion declined 20 +/- 5 pmol/min.m2 (96 +/- 15 to 76 +/- 15 pmol/min.m2, P less than 0.05) concomitant with decreases of 6.9 +/- 0.9 mmol/L in fasting serum glucose (13.7 +/- 1.0 to 6.8 +/- 0.7 mmol/L, P less than 0.05), 60 +/- 14 pmol/L in serum insulin (134 +/- 30 to 74 +/- 15 pmol/L, P less than 0.05), and 0.15 +/- 0.03 pmol/ml in plasma C-peptide (0.67 +/- 0.11 to 0.52 +/- 0.08 pmol/ml, P less than 0.05) concentrations. As expected, weight loss resulted in improved glucose tolerance as measured by the glycemic profiles during the oral glucose tolerance test (P less than 0.05 analysis of variance). The insulin secretory response before weight loss showed a markedly reduced ability to respond appropriately to an increase in the ambient serum glucose. After weight loss, the pancreatic response was more dynamic (P less than 0.05, analysis of variance) and parralleled the moment-to-moment changes in glycemia. Insulin production above basal doubled (11.2 +/- 3.2 to 24.5 +/- 5.8 nmol/6h.m2, P less than 0.05) and peak rates of insulin secretion above basal tripled (55 +/- 16 to 157 +/- 32 pmol/min/m2, P less than 0.05). To assess the beta-cell response to glucose per se and the changes associated with weight reduction, two hyperglycemic clamps were performed at steady state glucose levels in the range characteristic of individuals with severe NIDDM. At a fixed glycemia of 20 mmol/L, average rates of insulin secretion increased almost 2-fold with treatment (161 +/- 41 to 277 +/- 60 pmol/min.m2, P less than 0.05). At an increment of 6 mmol/L glucose above prevailing fasting glucose levels, the average rate of insulin secretion increased 53% (120 +/- 21 to 183 +/- 39 pmol/min.m2, P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Glyburide Enhances the Responsiveness of the β-Cell to Glucose but Does not Correct the Abnormal Patterns of Insulin Secretion in Noninsulin-Dependent Diabetes Mellitus*

Eleven patients with noninsulin-dependent diabetes mellitus were studied before and after 6-10 weeks of glyburide therapy. Patients were studied during a 24-h period on a mixed diet comprising 30 Cal/kg divided into three meals. The following day a hyperglycemic clamp study was performed, with glucose levels clamped at 300 mg/dL (16.7 mmol/L) for a 3-h period. Insulin secretion rates were calculated by deconvolution of peripheral C-peptide concentrations using individual C-peptide clearance kinetics derived after bolus injection of biosynthetic human C-peptide. After 6-10 weeks on glyburide, the identical studies were repeated. In response to glyburide, the fasting plasma glucose level decreased from 12.3 +/- 1.2 to 6.8 +/- 0.9 mmol/L. Although the mean glucose over the 24 h of the meal study decreased from 12.7 +/- 1.4 to 10.8 +/- 1.2 mmol/L, postprandial hyperglycemia persisted on therapy, and after breakfast, glucose levels exceeded 10 mmol/L and did not return to fasting levels for the remainder of the day. Fasting serum insulin, plasma C-peptide, and the insulin secretion rate were not different before (152 +/- 48 pmol/L, 0.82 +/- 0.16 pmol/mL, and 196 +/- 34 pmol/min, respectively) and after (186 +/- 28 pmol/L, 0.91 +/- 0.11 pmol/mL, and 216 +/- 23 pmol/min, respectively) glyburide treatment despite lowering of the glucose level. However, average insulin and C-peptide concentrations over the 24-h period increased from 366 +/- 97 pmol/L and 1.35 +/- 0.19 pmol/mL to 434 +/- 76 pmol/L and 1.65 +/- 0.15 pmol/mL, respectively. The total amount of insulin secreted over the 24-h period rose from 447 +/- 58 nmol before therapy to 561 +/- 55 nmol while receiving glyburide. Insulin secretion was demonstrated to be pulsatile in all subjects, with periodicity ranging from 2-2.5 h. The number of insulin secretory pulses was not altered by glyburide, whereas pulse amplitude was enhanced after lunch and dinner, suggesting that the increased insulin secretion is characterized by increased amplitude of the individual pulses. In response to a hyperglycemic clamp at 300 mg/dL (16.7 mmol/L), insulin secretion rose more than 2-fold, from 47 +/- 9 nmol over the 3-h period before treatment to 103 +/- 21 nmol after glyburide therapy. We conclude that the predominant mechanism of action of glyburide in patients receiving therapy for 6-10 weeks is to increase the responsiveness of the beta-cell to glucose.(ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral insulin parallels changes in insulin secretion more closely than C-peptide after bolus intravenous glucose administration.

The changes in peripheral serum insulin and plasma C-peptide levels and in the insulin secretory rate in response to iv glucose (0.5 g/kg BW) administration were studied in seven normal subjects. Insulin secretory rates were calculated according to a two-compartment model of distribution for C-peptide, using individual C-peptide kinetics calculated from iv bolus injections of biosynthetic human C-peptide. The mean plasma glucose level increased from a fasting level of 5.1 +/- 0.1 (+/- SE) to a peak of 24.0 +/- 1.0 mmol/L at 3 min and reached basal levels 101 +/- 6 min after glucose administration. The mean serum insulin value increased from 50 +/- 12 to a peak of 405 +/- 58 pmol/L at 3 min and then declined to fasting levels 139 +/- 14 min after the stimulus. In contrast, the mean plasma C-peptide level increased from 390 +/- 50 to a peak of 1460 +/- 210 pmol/L at 3 min and only began declining 45 min after glucose administration, reaching fasting levels 191 +/- 15 min after the stimulus. The mean insulin secretory rate increased from 69.8 +/- 19.9 to a peak of 1412.7 +/- 159.1 pmol/min at 3 min (15.3 +/- 2.5-fold elevation over baseline) and reached basal levels 135 +/- 12 min after the stimulus. The clearance of endogenous insulin during the basal period (2.505 +/- 0.365 L/min) and that during the 4 h after the stimulus (2.319 +/- 0.230 L/min) were similar. In conclusion, after bolus iv glucose administration: 1) the insulin secretory rate is more closely represented by changes in peripheral serum insulin than in plasma C-peptide levels; and 2) no change in endogenous insulin clearance occurs.

Reduction of insulin clearance during hyperglycemic clamp. Dose-response study in normal humans.

Insulin secretion and clearance were studied in eight normal subjects who underwent hyperglycemic clamp studies at plasma glucose levels of 120, 225, and 300 mg/dl on three occasions. Insulin secretion rates were calculated during a 1-h baseline period and during 3 h of glucose clamping from a two-compartmental analysis of peripheral C-peptide concentrations with individual kinetic parameters derived after intravenous bolus injections of biosynthetic human C-peptide. At the 300-mg/dl clamp level, the insulin secretion rate increased to a value 9.9 +/- 0.7 times that of basal at the end of the clamp (mean +/- SE), whereas over the same period, the peripheral insulin concentrations increased to a greater extent, reaching a value 15.4 +/- 1.2 times that of basal (P = .002). This greater relative increase in the insulin concentration in comparison with the corresponding insulin secretion rate suggests a reduction in the clearance of endogenous insulin. A similar trend was seen at the 225-mg/dl clamp level, but the relative increase in the insulin concentration (9.9 +/- 1.5 times that of basal) was not significantly higher than the relative increase in the insulin secretion rate (8.1 +/- 0.5 times that of basal, P = .17). At the 120-mg/dl clamp level, the relative increases in the insulin secretion rate (2.7 +/- 0.2 times that of basal) and the insulin concentration (2.4 +/- 0.2 times that of basal) were similar (P = .26), indicating no reduction in endogenous insulin clearance during moderate stimulation of insulin secretion. In conclusion, a reduction in endogenous insulin clearance occurs during greater stimulation of insulin secretion at higher glucose-clamp levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-dependent effects of oral and intravenous glucose on insulin secretion and clearance in normal humans.

Insulin secretion and clearance were studied in 2 groups of 7 normal subjects who each received 25, 50, and 100 g of glucose either orally or intravenously (iv) on separate occasions. Insulin secretion rates were calculated during a 1-h base line and for 5 h after glucose administration from a two-compartmental analysis of peripheral C-peptide concentrations using individual kinetic parameters derived after iv bolus injections of biosynthetic human C-peptide. Incremental glucose areas after oral or iv glucose increased as a function of the glucose dose (P = 0.0001). Incremental insulin secretion increased with increasing doses of both oral and iv glucose (P = 0.0001). The metabolic clearance rate (MCR) of endogenous insulin was calculated as the ratio of the total area under the insulin secretion rate curve and the simultaneous peripheral insulin concentration curve. The basal MCR was 1,879.5 +/- 110.5 ml/min (mean +/- SE). The poststimulatory MCR decreased with increasing doses of both oral and iv glucose concomitant with the greater insulin secretory response (P = 0.0014). This decrease in insulin clearance was not significantly different between oral and iv administration of glucose (P = 0.495). In conclusion diminished insulin clearance may be seen after marked stimulation of insulin secretion with larger doses of oral and iv glucose.

Insulin secretion and hepatic extraction in humans by minimal modeling of C-peptide and insulin kinetics.

Methods for measuring insulin secretion and hepatic insulin extraction in vivo, e.g., hepatic vein catheterization, are invasive, and can be applied during steady state only. We introduce a noninvasive method for measuring in vivo insulin secretion and its extraction by the liver during an intravenous glucose tolerance test (IVGTT). This method is based on a minimal model of C-peptide secretion and kinetics that is used for interpreting plasma C-peptide concentration data during an IVGTT in normal humans. The model allows the reconstruction of the time course of insulin secretion and, used in conjunction with a minimal model of insulin delivery and kinetics (described in a previous study), provides a noninvasive measure of the time course of hepatic insulin extraction [H(t)]. The C-peptide model also provides a direct prehepatic measure of beta-cell sensitivity to glucose, expressed by two parameters related to first (phi IC)- and second (phi IIC)-phase insulin secretion. In the 11 healthy volunteers we studied, these parameters were 61 +/- 11 pM.min-1.mg-1.dl and 0.0154 +/- 0.0034 pM.min-2.mg-1.dl, respectively. H(t) showed an initial decrement for approximately 30-50 min (from a fasting value of 63 +/- 8% to a nadir of 53 +/- 9%) after the glucose stimulus, then a steady value of approximately 62% was reestablished and maintained throughout the experiment. The validity of the C-peptide model was further assessed by comparing its estimate of the fractional plasma clearance rate (k01) with that obtained in experiments in which biosynthetic human C-peptide was administered.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative study of insulin secretion and clearance in normal and obese subjects.

The secretion and hepatic extraction of insulin were compared in 14 normal volunteers and 15 obese subjects using a previously validated mathematical model of insulin secretion and rate constants for C-peptide derived from analysis of individual decay curves after intravenous bolus injections of biosynthetic human C-peptide. Insulin secretion rates were substantially higher than normal in the obese subjects after an overnight fast (86.7 +/- 7.1 vs. 50.9 +/- 4.8 pmol/m2 per min, P less than 0.001, mean +/- SEM), over a 24-h period on a mixed diet (279.6 +/- 24.2 vs. 145.8 +/- 8.8 nmol/m2 per 24 h, P less than 0.001), and during a hyperglycemic intravenous glucose infusion (102.2 +/- 10.8 vs. 57.2 +/- 2.8 nmol/m2 per 180 min, P less than 0.001). Linear regression analysis revealed a highly significant relationship between insulin secretion and body mass index. Basal hepatic insulin extraction was not significantly different in the normal and obese subjects (53.1 +/- 3.8 vs. 51.6 +/- 4.0%). In the normal subjects, fasting insulin did not correlate with basal hepatic insulin extraction, but a significant negative correlation between fasting insulin and hepatic insulin extraction was seen in obesity (r = -0.63, P less than 0.02). This finding reflected a higher extraction in the six obese subjects with fasting insulin levels within the range of the normal subjects than in the nine subjects with elevated fasting insulin concentrations (61 +/- 3 vs. 45 +/- 6%, P less than 0.05). During the hyperglycemic clamp, the insulin secretion rate increased to an average maximum of 6.2-fold over baseline in the normal subjects and 5.8-fold in the obese subjects. Over the same time, the peripheral insulin concentration increased 14.1-fold over baseline in the normals and 16.6-fold over baseline in the obese, indicating a reduction in the clearance of endogenously secreted insulin. Although the fall in insulin clearance tended to be greater in the obese subjects, the differences between the two groups were not statistically significant. Thus, under basal, fasting conditions and during ingestion of a mixed diet, the hyperinsulinemia of obesity results predominantly from increased insulin secretion. In patients with more marked basal hyperinsulinemia and during intense stimulation of insulin secretion, a reduction in insulin clearance may contribute to the greater increase in peripheral insulin concentrations that are characteristic of the obese state.+

Basal and 24-h C-peptide and insulin secretion rate in normal man.

An understanding of the metabolic abnormalities rising from inappropriate insulin delivery in diabetic patients demands a knowledge of 24-h and basal insulin secretion rates in normal man. We have used biosynthetic human C-peptide to determine its kinetic parameters in 10 normal subjects and applied these to measurements of plasma concentrations in the same subjects to determine pancreatic secretion rate. Metabolic clearance rate measured by stepped primed infusion of biosynthetic human C-peptide at rates of 10, 19 and 26 nmol/h was 4.7 +/- 0.7 (+/- SD) ml X kg-1 X min-1, and was independent of infusion rate. Fractional clearance (T1/2, 26 +/- 3 min) and distribution volume (0.178 +/- 0.039 l/kg) were calculated from the decline in concentration after cessation of the highest rate infusion. Basal insulin secretion calculated from the C-peptide metabolic clearance rate and plasma concentrations for the period 02.00 to 07.00 hours was 1.3 +/- 0.4 U/h. Over 24 h total insulin secretion on a standard high carbohydrate diet was 63 +/- 15 U, calculated from the area under the C-peptide concentration curve. Basal insulin secretion, therefore, accounted for 50 +/- 8% of total insulin secretion. Although only 5.6 +/- 1.1% of C-peptide was detected in 24-h urine collections, urinary C-peptide excretion was significantly related to 24-h C-peptide secretion (r = 0.74, p less than 0.02).