C-peptide and insulin secretion. Relationship between peripheral concentrations of C-peptide and insulin and their secretion rates in the dog

Critical evaluation of the combined model approach for estimation of prehepatic insulin secretion

The combined model approach uses kinetic analysis of both plasma insulin and C-peptide dynamics to estimate prehepatic insulin secretion rates and parameters of insulin and C-peptide kinetics. The original model used single-compartment kinetics to describe both insulin and C-peptide despite knowledge that C-peptide follows two-compartment kinetics. The performance of the model under rapidly changing secretory conditions has come into question. Thus a more complex combined model is introduced, incorporating two-compartmental C-peptide disappearance. The addition of two-compartment C-peptide kinetics required a novel numerical approach to allow estimation of model parameters. This simulation study was undertaken to 1) compare the performance of the original combined model and 2) examine the numerical method used to identify parameters for the extended combined model with two-compartment C-peptide kinetics under simulated conditions of rapidly changing insulin and C-peptide. Monte Carlo simulation revealed that the original combined model does not provide accurate estimates of prehepatic insulin secretion under rapid kinetics. However, the extended combined model provides accurate reconstruction of prehepatic insulin secretory profile without separate quantification of C-peptide kinetics.

Mechanisms of hyperinsulinemia and hyperglucagonemia after liver transplantation

These studies were undertaken to evaluate the mechanisms for changes in plasma insulin and glucagon levels observed post-liver transplantation. Two groups of pigs were studied: a control group (n = 8) underwent laparotomy and catheter placement in the carotid artery and portal and hepatic veins. Hepatic blood flow was measured by ultrasonic flow probes placed around the hepatic artery and portal vein. An experimental group (n = 8) underwent orthotopic liver transplantation and similar instrumentation. On Day 1 after surgery, an estimate of insulin and glucagon secretion and hepatic extraction was determined using arteriovenous difference techniques. Serum assays were performed for markers of hepatic and renal function. Plasma insulin levels of the transplanted pigs were higher in the carotid artery (4 +/- 1 microU/ml vs 7 +/- 1 microU/ml), but not in the hepatic vein (5 +/- 1 microU/ml vs 7 +/- 1 microU/ml) and in the portal vein (10 +/- 2 microU/ml vs 12 +/- 2 microU/ml). Arterial plasma C-peptide was significantly greater in the transplanted group (0.23 +/- 0.02 ng/ml vs 0.42 +/- 0.03 ng/ml); however, the molar ratio of C-peptide and insulin was not different between the two groups (3.6 +/- 0.9 vs 3.4 +/- 0.4). Plasma glucagon levels of the transplanted pigs were significantly elevated in the carotid artery (111 +/- 11 pg/ml vs 323 +/- 65 pg/ml), portal vein (221 +/- 27 pg/ml vs 495 +/- 69 pg/ml), and hepatic vein (142 +/- 15 pg/ml vs 395 +/- 58 pg/ml). The estimate of pancreatic secretion of insulin (115 +/- 28 microU/kg.min) vs 71 +/- 21 microU/kg.min) and glucagon (2.0 +/- 0.4 ng/kg.min vs 2.7 +/- 0.7 ng/kg.min) and the fractional hepatic extraction rate of insulin (35 +/- 8% vs 32 +/- 5%) were not different between the two groups. However, the hepatic fractional extraction rate of glucagon was significantly decreased in the transplanted group (25 +/- 5% vs 11 +/- 3%). Therefore, the hyperglucagonemia observed 24 hr following liver transplantation is partly due to reduced hepatic fractional extraction of glucagon while the hyperinsulinemia is mainly due to the nonhepatic clearance of insulin. We speculate that decreased renal function may contribute to the hyperinsulinemia, elevated C-peptide concentrations, and hyperglucagonemia.

Protein targeting via the "constitutive-like" secretory pathway in isolated pancreatic islets: passive sorting in the immature granule compartment

We have suggested the existence of a novel "constitutive-like" secretory pathway in pancreatic islets, which preferentially conveys a fraction of newly synthesized C-peptide, insulin, and proinsulin, and is related to the presence of immature secretory granules (IGs). Regulated exocytosis of IGs results in an equimolar secretion of C-peptide and insulin; however an assay of the constitutive-like secretory pathway recently demonstrated that this route conveys newly synthesized C-peptide in molar excess of insulin (Arvan, P., R. Kuliawat, D. Prabakaran, A.-M. Zavacki, D. Elahi, S. Wang, and D. Pilkey. J. Biol. Chem. 266:14171-14174). We now use this assay to examine the kinetics of constitutive-like secretion. Though its duration is much shorter than the life of mature granules under physiologic conditions, constitutive-like secretion appears comparatively slow (t1/2 approximately equal to 1.5 h) compared with the rate of proinsulin traffic through the ER and Golgi stacks. We have examined whether this slow rate is coupled to the rate of IG exit from the trans-Golgi network (TGN). Escape from the 20 degrees C temperature block reveals a t1/2 less than or equal to 12 min from TGN exit to stimulated release of IGs; the time required for IG formation is too rapid to be rate limiting for constitutive-like secretion. Further, conditions are described in which constitutive-like secretion is blocked yet regulated discharge of IGs remains completely intact. Thus, constitutive-like secretion appears to represent an independent secretory pathway that is kinetically restricted to a specific granule maturation period. The data support a model in which passive sorting due to insulin crystallization results in enrichment of C-peptide in membrane vesicles that bud from IGs to initiate the constitutive-like secretory pathway.

Studies on the mechanism of action of sulphonylureas in type II diabetic subjects: gliquidone

The mechanism of action of sulphonylureas is not completely understood. In the present study we evaluated the effects of gliquidone, a second-generation compound, on several metabolic parameters in 22 patients with untreated newly-diagnosed type II (noninsulin-dependent) diabetes mellitus. After either 1 or 6 months of treatment with gliquidone plus isocaloric diet we observed: 1) a significant decrease in fasting plasma glucose and glycemic profile after oral glucose load; 2) unchanged fasting and postglucose plasma insulin levels; 3) no change in fasting C-peptide levels but a significant increase in C-peptide concentrations after glucose challenge; 4) a significant increase in glucose disappearance rate from plasma following iv insulin injection; 5) an increase in the insulin-induced reduction of plasma levels of free-fatty acids; 6) no change in plasma C-peptide levels following iv insulin injection; 7) a significant increase in specific insulin binding to monocytes. After 6 but not 1 month of gliquidone therapy we also found an increase in the activity of hexokinase in circulating mononuclear leukocytes. These results suggest that the hypoglycemic effect of gliquidone occurs through either an increased beta cell response to glucose stimulus or an enhanced insulin sensitivity. The latter effect seems to depend on both receptor and postreceptor mechanisms.

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)

Plasma C-peptide response to oral glucose load in hyperthyroidism

Aim of the present study was to evaluate the pancreatic beta cell response to oral glucose load in a group of patients with hyperthyroidism. For this purpose plasma C-peptide at fasting and after a 100 g oral glucose load was measured in 8 newly-diagnosed untreated hyperthyroid patients with fasting normoglycemia, and 8 sex-, age-, and weight-matched healthy controls. As compared to healthy subjects, patients with hyperthyroidism showed higher plasma glucose levels (incremental area 5405 +/- 742 vs 2729 +/- 539 mg/dl x 180 min, p less than 0.05), and slightly reduced plasma C-peptide concentrations (incremental area 166 +/- 12 vs 182 +/- 36 pmol/ml x 180 min, p = NS) following oral glucose load. The ratios between plasma C-peptide and plasma glucose incremental areas were lower in hyperthyroid patients than in controls (3.66 +/- 0.85 vs 10.41 +/- 3.08, p less than 0.05). These data suggest that hyperthyroidism is characterized by a decreased pancreatic beta cell response to oral glucose load.

First-phase insulin response to glucose in nonobese or obese subjects with glucose intolerance: analysis by C-peptide secretion rate

This study was proposed to clarify the impairment of first-phase insulin response to glucose in subjects with glucose intolerance by analysis of C-peptide secretion rate after glucose or glucagon injection. The rate was calculated from kinetic analysis of peripheral C-peptide behavior. The rate reached the peak two minutes after glucose injection and then rapidly declined (first-phase secretion) in control subjects. In nonobese subjects with impaired glucose tolerance (IGT) or non-insulin-dependent diabetes mellitus (NIDDM), the rate promptly increased in response to glucose and was followed by a second phase increase. The time course of the rate in the subjects was slightly different from that in control subjects. There was a progressively greater deficit in the first-phase increase with increasing severity of glucose intolerance. The time course of the rate in the obese subjects with NIDDM was different from that in control subjects. The first-phase increase was reduced in the obese subjects with NIDDM. The glucose disappearance rate was correlated with the first-phase increase. Since the time course of the rate after glucagon injection in all subjects did correspond well with that in the control subjects, variation of metabolic clearance rate of endogenous C-peptide among the subjects may be negligible for this study. This study provides the precise time course of first- and second-phase insulin response to glucose injection in nonobese and obese subjects with IGT or NIDDM as well as convincing evidence of the progressive reduction of first-phase insulin response with increasing severity of glucose intolerance. First-phase insulin response to glucose might be slightly delayed in some obese subjects with NIDDM.

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.

Effects of the combination of insulin and glibenclamide in type 2 (non-insulin-dependent) diabetic patients with secondary failure to oral hypoglycaemic agents

The effects of combined insulin and sulfonylurea therapy on glycaemic control and B-cell function was studied in 15 Type 2 (non-insulin-dependent) diabetic patients who had failed on treatment with oral hypoglycaemic agents. The patients were first treated with insulin alone for four months. Five patients were given two daily insulin doses and ten patients one dose. During insulin treatment the fasting plasma glucose fell from 14.5 +/- 0.8 to 8.8 +/- 0.4 mmol/l and the HbA1 concentration from 12.6 +/- 0.4 to 9.2 +/- 0.2%. This improvement of glycaemic control was associated with a suppression of basal (from 0.31 +/- 0.04 to 0.10 +/- 0.02 nmol/l) and glucagon-stimulated (from 0.50 +/- 0.08 to 0.19 +/- 0.04 nmol/l) C-peptide concentrations. Four months after starting insulin therapy the patients were randomised to a four-month double-blind cross-over treatment with insulin combined with either 15 mg glibenclamide per day or with placebo. Addition of glibenclamide to insulin resulted in a further reduction of the fasting plasma glucose (7.9 +/- 0.5 mmol/l) and HbA1 (8.3 +/- 0.2%) concentration whereas the basal (0.21 +/- 0.03 nmol/l) and glucagon-stimulated C-peptide concentrations (0.34 +/- 0.06 nmol/l) increased again. Addition of placebo to insulin had no effect. The daily insulin dose could be reduced by 25% after addition of glibenclamide to insulin, while it remained unchanged when insulin was combined with placebo. The fasting free insulin concentration did not differ between the glibenclamide and placebo periods (28 +/- 6 vs 30 +/- 5 mmol/l).(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.

Prolonged insulin resistance following insulin-induced hypoglycaemia

Nineteen normal male volunteers underwent a 10-h glucose clamp study to examine the duration and mechanism of insulin resistance after hypoglycaemia. Dextrose delivery by the Biostator to maintain the target blood glucose level fell below baseline 2 h after induction of hypoglycaemia and remained suppressed for at least 7 h after insulin hypoglycaemia. Insulin secretion as manifested by C-peptide levels remained suppressed for 3-4 h after insulin hypoglycaemia despite return of blood glucose to baseline by 90 min. Glucose kinetic data (3-3H-glucose) performed in six of the subjects indicated that the prolonged insulin resistance was due to significantly increased hepatic glucose production and to suppressed glucose utilisation, persisting for at least 4 h after counterregulatory hormone levels had returned to normal. Post-hypoglycaemic insulin resistance as determined by dextrose delivery was markedly attenuated and the rise in hepatic glucose output totally eliminated in five hypopituitary subjects without growth hormone or cortisol responses to hypoglycaemia. We conclude that post-hypoglycaemic insulin resistance occurs in non-diabetic subjects and persists for at least 7 h following hypoglycaemia. This prolonged insulin resistance is largely related to release of growth hormone and cortisol.

Metabolism and placental transfer of 125I-proinsulin and 125I-tyrosylated C-peptide in the pregnant rhesus monkey

125I-Proinsulin or 125I-tyrosylated-C-peptide (125I-tyr-CP) was administered to pregnant Rhesus monkeys by bolus followed by constant infusion to examine placental transfer of these peptides. At the end of each infusion, fetuses were exsanguinated in situ via the umbilical vein. The bolus-constant infusion technique produced a steady state in maternal plasma of immunoprecipitable label, measured using excess insulin or C-peptide antiserum. In animals infused with 125I-proinsulin, analysis of umbilical venous plasma revealed no apparent transfer to the fetus of immunoprecipitable label. In animals infused with 125I-tyr-CP, 3-13% of the umbilical venous plasma radioactivity was immunoprecipitable, representing 1.4-5.8% of the immunoprecipitable radioactivity in maternal plasma at delivery. Gel filtration chromatography of umbilical venous plasma revealed that the immunoprecipitated moiety was a fragment of 125I-tyr-CP. Analysis of maternal plasma showed that the predominant peak of radioactivity represented intact C-peptide. A peak corresponding to the fetal immunoprecipitable peak was also present. Analysis of simultaneous maternal arterial and uterine vein plasma samples showed that degradation of 125I-tyr-CP occurred across the uterus. Studies in one nonpregnant and three postpartum animals indicated that pregnancy increased the rate of metabolism of 125I-tyr-CP. When 125I-tyr-CP was incubated with trophoblastic cells in culture, degradation to a species corresponding on gel filtration to the immunoprecipitable fetal metabolite was found. We conclude that proinsulin, like insulin, does not traverse the placenta. Immunoreactive fragments of C-peptide do cross, however, and pregnancy alters the metabolism of 125I-tyr-CP, probably owing to placental degradation.

Fractional hepatic extraction of insulin in man: is it constant?

The present study was designed to compare insulin extraction by the liver following oral glucose administrations of different size, in order to evaluate insulin removal by the liver in relation to the insulin exposure, and to the amount of ingested glucose. Insulin secretion by the pancreas was estimated by the measurement of peripheral C-peptide levels, and insulin extraction by the liver by the analysis of peripheral C-peptide to insulin ratios and relations. Ten healthy subjects (5 males and 5 females), aged 16 to 66 yr, with normal bw, and without family history of diabetes mellitus were investigated by means of the administration, on alternate days, of 50 and 150 g oral glucose loads. After the 150 g oral glucose load plasma glucose levels were significantly higher than after the 50 g oral glucose administration: glucose incremental areas of 1.45 +/- 0.12 vs. 0.55 +/- 0.04 mmol/l X min, respectively (p less than 0.001). Similarly, insulin concentrations were significantly higher following 150 g than after 50 g glucose ingestion: insulin incremental areas of 0.52 +/- 0.09 vs. 0.20 +/- 0.04 nmol/l X min (p less than 0.001). Also C-peptide levels were higher after 150 vs. 50 g oral glucose load: C-peptide incremental areas of 1.85 +/- 0.41 vs. 0.64 +/- 0.13 nmol/l X min (p less than 0.01). C-peptide to insulin molar ratios were similar during the two glucose challenge, and averaged 5.25 +/- 0.42 vs. 5.08 +/- 0.50 after 50 and 150 g oral glucose loads, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Splanchnic insulin metabolism in obesity. Influence of body fat distribution

The effects of obesity and body fat distribution on splanchnic insulin metabolism and the relationship to peripheral insulin sensitivity were assessed in 6 nonobese and 16 obese premenopausal women. When compared with the nonobese women, obese women had significantly greater prehepatic production and portal vein levels of insulin both basally and following glucose stimulation. This increase correlated with the degree of adiposity but not with waist-to-hip girth ratio (WHR). WHR, however, correlated inversely with the hepatic extraction fraction and directly with the posthepatic delivery of insulin. The latter correlated with the degree of peripheral insulinemia. The decline in hepatic insulin extraction with increasing WHR also correlated with the accompanying diminution in peripheral insulin sensitivity. Increasing adiposity is thus associated with insulin hypersecretion. The pronounced hyperinsulinemia of upper body fat localization, however, is due to an additional defect in hepatic insulin extraction. This defect is closely allied with the decline in peripheral insulin sensitivity.

Further evidence that insulin metabolism is a major determinant of peripheral insulin response to oral glucose in subjects with mild glucose intolerance

In mild glucose intolerance plasma concentration of C-peptide seems to give an estimate of pancreatic B cell secretion more reliable than plasma insulin itself. In the present study we measured the plasma levels of insulin and C-peptide after oral glucose load in 100 mildly glucose intolerant subjects, focusing our attention on high and low insulin responders. According to an insulin incremental area after oral glucose higher or lower than the mean +/- SD of the mean, 16 subjects were classified as "high insulin responders", and 17 as "low insulin responders". The two groups were similar for sex, age and bw. Mean insulin incremental area was almost 9-fold greater in high insulin responders than in low insulin responders (0.88 +/- 0.03 vs 0.10 +/- 0.01 pmol/ml min, p less than 0.001). Also mean C-peptide incremental area was significantly greater in high insulin responders than in low insulin responders, but the differences between the two groups were smaller. Indeed, mean C-peptide area was approximately 2.5-fold greater in high insulin responders than in low insulin responders (1.58 +/- 0.12 vs 0.66 +/- 0.07 pmol/ml min, p less than 0.001). These results give further support to the concept that in mild glucose intolerance insulin metabolism is a major determinant of peripheral insulin response to oral glucose load.

The measurement and validation of the nonsteady-state rates of C-peptide appearance in the dog

In order to verify the calculation of nonsteady rates of secretion of C-peptide, dog C-peptide was infused into 5 normal conscious dogs at varying rates. Using the decay curve obtained following a preliminary injection of C-peptide in each animal, concentrations during the infusion, and mathematical deconvolution, the rate of appearance of the C-peptide was calculated. This rate was within 12% of the infusion rates, with 94% of the C-peptide infused recovered in the calculation. The metabolic clearance of C-peptide was calculated to be 10.1 +/- 1.0 ml/min following both its injection and constant infusion. In conclusion, within the limits of the errors determined, C-peptide and therefore insulin secretion can be calculated on a continuous basis under nonsteady-state conditions.

C-peptide: an index of insulin secretion

Insight into the natural history of beta cell function in IDDM patients obtained by C-peptide measurements is reviewed. It is argued that residual insulin secretion of metabolic importance is present in all IDDM patients during the initial course of the disease. After some months, beta cell function reaches its maximum; thereafter it declines at different rates dependent on the age at onset of diabetes and, possibly, on the presence of ICA and HLA-antigens. As many as 15% of IDDM patients retain life-long beta cell function that persists at approximately 10% of that observed in nondiabetic individuals. The residual endogenous insulin secretion is characterized by reduced capacity, as well as abnormal insulin secretory kinetics; these defects in residual insulin secretion can be modulated by changes in metabolic regulation as well as by immunosuppression during the initial course of the disease.

Use of biosynthetic human C-peptide in the measurement of insulin secretion rates in normal volunteers and type I diabetic patients

We undertook this study to examine the accuracy of plasma C-peptide as a marker of insulin secretion. The peripheral kinetics of biosynthetic human C-peptide (BHCP) were studied in 10 normal volunteers and 7 insulin-dependent diabetic patients. Each subject received intravenous bolus injections of BHCP as well as constant and variable rate infusions. After intravenous bolus injections the metabolic clearance rate of BHCP (3.8 +/- 0.1 ml/kg per min, mean +/- SEM) was not significantly different from the value obtained during its constant intravenous infusion (3.9 +/- 0.1 ml/kg per min). The metabolic clearance rate of C-peptide measured during steady state intravenous infusions was constant over a wide concentration range. During experiments in which BHCP was infused at a variable rate, the peripheral concentration of C-peptide did not change in proportion to the infusion rate. Thus, the infusion rate of BHCP could not be calculated accurately as the product of the C-peptide concentration and metabolic clearance rate. However, the non-steady infusion rate of BHCP could be accurately calculated from peripheral C-peptide concentrations using a two-compartment mathematical model when model parameters were derived from the C-peptide decay curve in each subject. Application of this model to predict constant infusions of C-peptide from peripheral C-peptide concentrations resulted in model generated estimates of the C-peptide infusion rate that were 101.5 +/- 3.4% and 100.4 +/- 2.8% of low and high dose rates, respectively. Estimates of the total quantity of C-peptide infused at a variable rate over 240 min based on the two-compartment model represented 104.6 +/- 2.4% of the amount actually infused. Application of this approach to clinical studies will allow the secretion rate of insulin to be estimated with considerable accuracy. The insulin secretion rate in normal subjects after an overnight fast was 89.1 pmol/min, which corresponds with a basal 24-h secretion of 18.6 U.

Current approaches to measurement of insulin secretion

The studies reviewed in this article have indicated that C-peptide and insulin are cosecreted from the beta cell in equimolar concentration, that C-peptide does not undergo significant hepatic extraction, and that its MCR remains constant over the physiologic range of concentrations. Furthermore, the peripheral kinetics of distribution of C-peptide can be described by a two-compartment mathematical model. If model parameters are derived in individual subjects by analysis of C-peptide decay curves, this information can subsequently be used to derive insulin secretion rates with a great degree of accuracy from endogenously secreted C-peptide concentrations. The application of this approach to clinical research studies should greatly enhance our understanding of insulin secretion.