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K Polonsky

Publications and source records attributed to K Polonsky.

At least 19 recordsLinked to original sources
PubMed2001

Dysregulation of insulin secretion in children with congenital hyperinsulinism due to sulfonylurea receptor mutations.

Mutations in the high-affinity sulfonylurea receptor (SUR)-1 cause one of the severe recessively inherited diffuse forms of congenital hyperinsulinism or, when associated with loss of heterozygosity, focal adenomatosis. We hypothesized that SUR1 mutations would render the beta-cell insensitive to sulfonylureas and to glucose. Stimulated insulin responses were compared among eight patients with diffuse hyperinsulinism (two mutations), six carrier parents, and ten normal adults. In the patients with diffuse hyperinsulinism, the acute insulin response to intravenous tolbutamide was absent and did not overlap with the responses seen in either adult group. There was positive, albeit significantly blunted, acute insulin response to intravenous dextrose in the patients with diffuse hyperinsulinism. Graded infusions of glucose, to raise and then lower plasma glucose concentrations over 4 h, caused similar rises in blood glucose but lower peak insulin levels in the hyperinsulinemic patients. Loss of acute insulin response to tolbutamide can identify children with diffuse SUR1 defects. The greater response to glucose than to tolbutamide indicates that ATP-sensitive potassium (KATP) channel-independent pathways are involved in glucose-mediated insulin release in patients with diffuse SUR1 defects. The diminished glucose responsiveness suggests that SUR1 mutations and lack of KATP channel activity may contribute to the late development of diabetes in patients with hyperinsulinism independently of subtotal pancreatectomy.

ATP-Binding Cassette Transporters↗
PubMed1999

Free fatty acids impair hepatic insulin extraction in vivo.

Hyperinsulinemia is a common finding in obesity and results from insulin hypersecretion and impaired hepatic insulin extraction. In vitro studies have shown that free fatty acids (FFAs), which are often elevated in obesity, can impair insulin binding and degradation in isolated rat hepatocytes. To investigate whether FFAs impair hepatic insulin extraction (E(H)) in vivo, either saline (SAL) or 10% Intralipid (0.03 ml x kg(-1) x min(-1)) plus heparin (0.44 U x kg(-1) x min(-1)) (IH) was infused into normal dogs to elevate FFA levels. Insulin was infused intraportally at 18 pmol x kg(-1) x min(-1) for 150 min (period A, high insulin dose), and then at 2.4 pmol x kg(-1) x min(-1) for another 150 min (period B, low insulin dose). After the low portal insulin dose, additional insulin was infused peripherally at 8.4 pmol x kg(-1) x min(-1) for 120 min (period C) to assess the clearance of insulin from the peripheral plasma. In 16 paired experiments, FFA levels were 1,085 +/- 167, 1,491 +/- 240, 1,159 +/- 221 micromol/l (IH) and 221 +/- 44, 329 +/- 72, 176 +/- 44 micromol/l (SAL) in periods A, B, and C, respectively. Peripheral insulin levels were greater with IH (P < 0.001) than with SAL in all periods (1,620 +/- 114, 126 +/- 12, 1,050 +/- 72 pmol/l for IH vs. 1,344 +/- 168, 96 +/- 4.2, 882 +/- 60 pmol/l for SAL). Glucose clearance was impaired by IH in all periods (P < 0.05), whereas glucose production was slightly increased by IH during period B. Peripheral insulin clearance (Cl) and E(H) were calculated from the insulin infusion rate and insulin concentration data in each period by taking into account the nonlinearity of insulin kinetics. Cl was lower (P < 0.01) with IH (9.6 +/- 0.6, 12.0 +/- 0.9, 10.2 +/- 0.6 ml x kg(-1) x min(-1)) than with SAL (11.2 +/- 1, 13.6 +/- 0.7, 11.9 +/- 0.9 ml x kg(-1) x min(-1)) in periods A, B, and C. E(H) was also lower (P < 0.05) with IH (25 +/- 4, 40 +/- 5, 32 +/- 5%) than with SAL (30 +/- 2.8, 47 +/- 3, 38 +/- 3%). We conclude that FFAs can impair hepatic insulin extraction in vivo at high and low insulin levels, an effect that may contribute to the peripheral hyperinsulinemia of obesity.

Animals↗
PubMed1998

Insulin acutely suppresses glucose production by both peripheral and hepatic effects in normal dogs.

To determine whether the predominant effect of insulin in suppressing tracer-determined glucose production (Ra) is hepatic or peripheral, we infused insulin peripherally (PER) and portally (POR) at both low (0.75 pmol.kg-1.min-1) and high physiological rates (2.7 pmol.kg-1.min-1) during euglycemic clamps in normal dogs. We also infused insulin peripherally at one-half these rates (1/2 PER) to match the peripheral insulin levels in POR and thus obtain a selective POR vs. 1/2 PER difference in hepatic insulin levels. At the high-rate insulin infusion, peripheral insulin levels were greatest with PER (PER = 212 +/- 10 pM, n = 5; POR = 119 +/- 5 pM, n = 6; 1/2 PER = 122 +/- 5 pM, n = 6). Calculated hepatic insulin levels were greatest with POR (POR = 227 +/- 13 pM, PER = 206 +/- 19 pM, 1/2 PER = 123 +/- 8 pM). High-dose PER yielded a greater suppression of Ra than POR (79 +/- 18 vs. 56 +/- 6%, P < .001). Ra was only suppressed by 45 +/- 6% with 1/2 PER (P < 0.01 vs. POR on 6 paired experiments). Free fatty acid (FFA) was suppressed by 57 +/- 8% with PER and only by 33 +/- 5 and 37 +/- 2% with POR and 1/2 PER, respectively. The low-dose PER and POR yielded an equal Ra suppression (PER = 46 +/- 9%, POR = 43 +/- 4%). Only 1/2 PER was associated with a lower suppression of Ra (36 +/- 8, P < 0.05 vs. POR). FFA showed similar suppression in all three groups (approximately 25%). Using both insulin infusion rates, the percent Ra suppression per unit difference in peripheral insulin was approximately twofold greater than that per unit difference in hepatic insulin. These results suggest that, during euglycemic clamps without somatostatin in normal dogs, Ra suppression is mediated by both peripheral and hepatic effects of insulin and that peripheral insulin, at least at high physiological infusion rates, is more potent than hepatic insulin in suppressing Ra.

Animals↗
PubMed1997

Transgenic studies with a keratin promoter-driven growth hormone transgene: prospects for gene therapy.

Keratinocytes are potentially appealing vehicles for the delivery of secreted gene products because they can be transferred to human skin by the relatively simple procedure of grafting. Adult human keratinocytes can be efficiently propagated in culture with sufficient proliferative capacity to produce enough epidermis to cover the body surface of an average adult. However, the feasibility of delivering secreted proteins through skin grafting rests upon (i) the strength of the promoter in keratinocytes and (ii) the efficiency of protein transport through the basement membrane of the stratified epithelium and into the bloodstream. In this paper, we use transgenic technology to demonstrate that the activity of the human keratin 14 promoter remains high in adult skin and that keratinocyte-derived human growth hormone (hGH) can be produced, secreted, and transported to the bloodstream of mice with efficiency that is sufficient to exceed by an order of magnitude the circulating hGH concentration in growing children. Transgenic skin grafts from these adults continue to produce and secrete hGH stably, at approximately 1/10 physiological levels in the bloodstream of nontransgenic recipient mice. These studies underscore the utility of the keratin 14 promoter for expressing foreign transgenes in keratinocytes and demonstrate that keratinocytes can be used as effective vehicles for transporting factors to the bloodstream and for eliciting metabolic changes. These findings have important implications for considering the keratinocyte as a possible vehicle for gene therapy.

Adult↗
PubMed1997

A novel point mutation in the insulin gene giving rise to hyperproinsulinemia.

A 58-yr-old obese white Caucasian male type 2 diabetic, entered into the UK Prospective Diabetes Study, was found to have raised fasting total proinsulin levels 708 pmol/L(-1) (normal range, 3-16 pmol/L(-1)) and normal specific plasma insulin level 29 pmol/L(-1) (normal range, 21-75 pmol/L(-1)). Immunoreactive plasma insulin, measured by RIA, was 503 pmol/L(-1). DNA was extracted, the insulin gene amplified by the PCR, and by direct sequencing, a novel point mutation, G1552C, was identified, which resulted in the substitution of proline (CCT) for arginine (CGT) at position 65. This prevented cleavage of the C-peptide A-chain dibasic cleavage site (lys-arg) by the processing protease in the pancreatic beta-cells. The plasma proinsulin and insulin levels were in accord with expression of both the wild-type and the mutant alleles. The G1552C mutation was not linked with diabetes, because it was present in a 37-yr-old nondiabetic daughter and not in a 35-yr-old daughter who had had gestational diabetes.

Arginine↗
PubMed1997

Late post-prandial hypoglycaemia as the sole presenting feature of secreting pancreatic beta-cell adenoma in a subtotally gastrectomized patient.

In this paper we describe for the first time late post-prandial hypoglycaemia as the sole presenting feature of an insulinoma in a patient who had previously undergone subtotal gastrectomy. The symptoms of hypoglycaemia always occurred 1-3 h after meals, not in the fasting state. Because of the history of gastrectomy and because post-prandial hypoglycaemia was reproduced by an oral glucose tolerance test, the diagnosis of reactive hypoglycaemia was made. Eighteen months later a fasting test was performed: venous plasma glucose decreased from 3.8 mmol/l to 2.7 mmol/l between 14 and 20 h of fast while plasma immunoreactive insulin did not decrease and plateaued at 185 pmol/l. Plasma C-peptide (0.9 nmol/l) and proinsulin (70 pmol/l, split 64, 65) were also elevated. All islet hormones increased in response to i.v. glucose and were suppressed after diazoxide. Although pre-operative procedures were negative in localizing an insulinoma, the patient underwent an operation and an insulinoma was detected at the body level of the pancreas. Thus, insulinoma should be considered in the differential diagnosis of reactive hypoglycaemia in gastrectomized patients. Response of islet hormones to glucose and their suppression by diazoxide are evidence of a secreting insulinoma even in the absence of preoperative localization of the pancreatic adenoma.

Blood Glucose↗
PubMed1996

Report of the American Diabetes Association's Task Force on standardization of the insulin assay.

Recent large-scale epidemiological studies demonstrate that blood concentrations of immunoreactive insulin predict the development of NIDDM and IDDM and are associated with the risk of several degenerative diseases, such as coronary and peripheral vessel atherosclerosis, hypertension, and dyslipidemia. The reliability of these measurements is dependent on a biological assay that has not been well standardized between laboratories. Recognizing this, the American Diabetes Association organized a task force to assess comparability of blood insulin measurements between laboratories and to suggest techniques to improve comparability. The task force found that identical serum and plasma samples measured in different laboratories produced widely disparate values that were unacceptable for population comparisons. Use of a single reference standard did little to improve comparability. Assay characteristics such as linearity, recovery, accuracy, and cross-reactivity to proinsulin and its primary conversion intermediates varied among the laboratories, and they did not readily explain differences in the measurements made from assay to assay. Use of the same assay kit in different laboratories did not always ensure comparable measurements. Linear regression of assay results from one laboratory to an arbitrarily chosen reference assay greatly improved comparability and demonstrated the potential value in comparing each assay to a reference method. The task force report defines acceptable assay characteristics and proposes a three-step process of insulin assay proficiency and comparability. A central reference assay and ongoing sample exchange will be needed to allow reliable comparisons of insulin measurements made in different laboratories. Rigorous quality control and continuous quality improvement are needed to maintain reliability of the insulin measurement.

Diabetes Mellitus↗
PubMed1995

Measurement of proinsulin and intermediates. Validation of immunoassay methods by high-performance liquid chromatography.

Human proinsulin and 32-33 split proinsulin have been measured in the peripheral circulation by immunoradiometric assays (IRMAs) and have been shown to be elevated in impaired glucose tolerance and non-insulin-dependent diabetes mellitus (NIDDM). The IRMA for 32-33 split proinsulin did not discriminate between this molecule and des-32 or des-31,32 split proinsulin. We describe the comparison of IRMA for human plasma proinsulin and 32-33 split proinsulins with assays combined with high-performance liquid chromatography (HPLC), which can discriminate between 32-33 split, des-32 split, and des-31,32 split proinsulin. Subjects were those with normal glucose tolerance (n = 8) and those with NIDDM (n = 17), who were studied while fasting and 30 min after a glucose load. After collection, blood was centrifuged promptly, and the serum/plasma was stored frozen until assay. Both IRMA and HPLC methods were calibrated against synthetic peptides. Interassay coefficients of variation for the IRMA for proinsulin and 32-33 split proinsulin were < 13% over the ranges 3.8-65 pmol/l and 6.4-65 pmol/l, respectively. The following regression lines were obtained: proinsulin IRMA = -0.143 + 1.066 HPLC, r = 0.860; 32-33 split proinsulin IRMA = 0.048 + 1.051 HPLC; and des-31,32 split proinsulin, r = 0.814. For both analytes, there was no significant difference in the relationship of IRMA to HPLC results between the various subject groups and various time points. Thus, the IRMA for proinsulin has been validated by an independent method.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Relative effects of calorie restriction and weight loss in noninsulin-dependent diabetes mellitus.

In obese patients with noninsulin-dependent diabetes mellitus (NIDDM), reducing calorie intake improves glycemic control, often more rapidly than weight loss. Conversely, after weight loss has been achieved, metabolic control can deteriorate once calorie intake is increased, even if there is no regaining of weight. The current study, therefore, tested the hypothesis that restricting calorie consumption has an important role, independent of weight loss, in metabolic regulation of NIDDM patients. Isotopic determinations of hepatic glucose production (HGP), post-absorptively and after ingestion of 75 g glucose (dual glucose isotope method), were made in conjunction with measurement of insulin secretion and insulin sensitivity in seven obese NIDDM volunteers after four periods of controlled calorie intake: 1) 7 days of a baseline weight maintenance diet, 2) followed immediately by 7 days of calorie restriction (800 Cal/day); 3) followed by a weight loss program that consisted of 2 months of a very low calorie diet (400 Cal/day) and then 4 weeks of gradual refeeding and 7 days on a weight maintenance diet; and 4) a final week of calorie restriction (800 Cal/day). The initial brief interval of calorie restriction produced substantial decreases in fasting plasma glucose, HGP, and fasting plasma triglyceride and increases in insulin sensitivity and secretion. After a substantial weight loss (12.7 +/- 2 kg), each parameter improved further, with the effect of weight loss approximately equal to that obtained with initial calorie restriction. Reimposing calorie restriction after weight loss had little effect, except that fasting plasma glucose and HGP improved slightly further. In obese NIDDM subjects, a 7-day period of calorie restriction produces approximately half of the overall improvement in HGP, insulin sensitivity, and insulin secretion that is obtained after a substantial loss of weight. These findings indicate that calorie restriction has an important regulatory effect on the metabolism of obese patients with NIDDM that is independent of weight loss.

Adult↗
PubMed1990

Pattern of postprandial carbohydrate metabolism and effects of portal and peripheral insulin delivery.

The importance of portal insulin delivery in the regulation of postprandial carbohydrate metabolism is uncertain. To address this question, three groups of dogs were studied: one group in which pancreatic venous drainage was transected and reanastomosed (portal insulin delivery), one in which the pancreatic drainage was transected and anastomosed to the inferior vena cava (peripheral insulin delivery), and one that received only a sham operation. Plasma insulin was greater (P less than 0.05) during peripheral insulin delivery than in either the portal or sham groups, respectively, before and after meal ingestion. On the other hand, C-peptide concentrations did not differ between groups, resulting in a higher (P less than 0.001) insulin to C-peptide ratio in the peripheral group. This indicated that the hyperinsulinemia in the peripheral group was due to decreased insulin clearance rather than increased insulin secretion. Isotopically determined splanchnic uptake of ingested glucose, postprandial suppression of hepatic glucose release, incorporation of CO2 into glucose (a qualitative measure of gluconeogenesis), and total-body glucose uptake were virtually identical in all groups. Similarly, plasma lipid, beta-hydroxybutyrate, and lactate concentrations did not differ between groups. Our data indicate that, despite differences in systemic insulin concentration, portal and peripheral insulin delivery comparably regulate hepatic and extrahepatic carbohydrate metabolism after meal ingestion.

Animals↗
PubMed1986

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.

Animals↗
PubMed1986

Lack of direct inhibition of insulin secretion by exogenous insulin in the canine pancreas.

To test whether insulin secretion is self-regulatory, canine pancreata were isolated and perfused in vitro and were infused with 0.3, 0.6, or 1.2 mU/ml exogenous insulin. Basal and arginine-stimulated concentrations of C-peptide, glucagon, and somatostatin were measured. There were no significant differences between basal secretion nor the increment of arginine-stimulated secretion for each respective hormone at each exogenous insulin concentration. The second preparation studied was a vascularly isolated, yet innervated, in situ perfused pancreas. Exogenous insulin (1 mU/kg per min) was infused "systemically"; the pancreas received no insulin. Endogenous pancreatic insulin and C-peptide secretion was suppressed, while pancreatic glucagon secretion increased during systemic insulin infusion. No changes in pancreatic hormone secretion occurred after the sympathetic nerves were sectioned. These results suggest that exogenous insulin does not directly suppress the B cell, but can suppress insulin secretion through an indirect neurally mediated, insulin-dependent nerve mechanism.

Animals↗
PubMed1986

The limitations to and valid use of C-peptide as a marker of the secretion of insulin.

The accuracy with which the secretion rate of insulin can be calculated from peripheral concentrations of C-peptide was investigated in conscious mongrel dogs. Biosynthetic human C-peptide and insulin were infused intraportally and their concentrations measured in the femoral artery. During steady-state infusions of C-peptide, the peripheral concentration changed in proportion to the infusion rate and the metabolic clearance rate (5.2 +/- 0.3 ml/kg/min) remained constant over a wide range of plasma concentrations. Application of a two-compartment mathematical model, in which the model parameters were estimated from analysis of C-peptide decay curves after intravenous bolus injections, allowed the intraportal infusion rate of C-peptide to be derived from peripheral C-peptide concentrations, even under non-steady-state conditions. Estimates of the intraportal infusion rate based on this model were 102.4 +/- 2.6% of the actual infusion rate as it was increasing and 102.3 +/- 5.5% of this rate as it was falling. The peripheral C-peptide: insulin molar ratio was influenced by the rate at which equimolar intraportal infusions of C-peptide and insulin were changed. The baseline C-peptide: insulin molar ratio (4.1 +/- 0.9) increased to peak values of 8.2 +/- 0.6, 10.3 +/- 2.0, and 14.9 +/- 1.3 when the infusion rate was increased and then decreased rapidly. Peak values of only 5.7 +/- 1.2 were found if the intraportal infusion rate was changed slowly.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Long-term follow-up of canine segmental pancreatic autografts.

The functional and morphologic characteristics of free-draining, pancreatic segmental autografts (FDPS) were studied in 8 beagle dogs that had survived longer than 4 yr. After pancreatectomy, animals received FDPS autografts of the left pancreatic limb, representing approximately one-third of the total pancreas with iliac vessel anastamoses. The grafted recipients were given pancreatic enzyme supplements (Viokase). After transplantation (tx), all 8 animals sustained fasting euglycemia with no evidence of microvascular complications. After 4 yr, IVGTT revealed K-values (%/min) that were not significantly different from age-matched controls (2.9 +/- 0.5 versus 3.7 +/- 0.6, P greater than 0.05). Mean fasting serum insulin levels were significantly greater in the tx animals (49 +/- 5 microU/ml versus 12.2 microU/ml, P less than 0.001), although the incremental response to i.v. glucose (0.5 g/kg) was less than in controls (P less than 0.05). Mean fasting plasma C-peptide levels (0.09 +/- 0.01 pmol/ml versus 0.21 +/- 0.5 pmol/ml) and peak C-peptide responses to i.v. glucose were both significantly less than in controls. Sequential pancreatic biopsies up to 2.5 yr post-tx showed atrophy of the exocrine pancreas with coalescence of islets and mild fibrosis that did not progress with time. Immunoperoxidase stains confirmed the presence of insulin, glucagon, and somatostatin within nests of islet cells. Four years after transplantation of FDPS autografts in pancreatectomized dogs, excellent function is retained. The consequences of peripheral hyperinsulinemia remain to be determined.

Animals↗
PubMed1984

The metabolic effects of biosynthetic human proinsulin in individuals with type I diabetes.

To assess the significance of deficiency of circulating proinsulin in patients with type I diabetes mellitus, we studied the metabolic effects of biosynthetic human proinsulin in 24 patients. After withdrawing insulin, an infusion of proinsulin to physiological plasma levels did not prevent elevations of plasma glucose or beta-hydroxybutyrate. During steady state infusions of insulin and proinsulin, 13.7 times the steady state plasma level of proinsulin compared to insulin was required to maintain euglycemia. This finding indicates that proinsulin is approximately 7.3% as biologically active as insulin on a molar basis in maintaining glucose control. The MCRs of insulin and proinsulin during these steady state infusions were 12.5 +/- 2.2 (+/- SEM) and 2.62 +/- 0.33 ml/kg X min, respectively. After maintaining euglycemia overnight with an infusion of insulin or proinsulin and then acutely stopping these infusions, the rise in plasma glucose after proinsulin was delayed significantly compared to insulin, consistent with proinsulin's slower clearance. Further studies are necessary to determine whether biosynthetic human proinsulin has a specific role in the treatment of diabetes.

Blood Glucose↗
PubMed1984

The effects of biosynthetic human proinsulin on carbohydrate metabolism.

Large quantities of biosynthetic human proinsulin have recently become available through recombinant DNA technology. Since the in vivo effects of human proinsulin have not been studied in man, we compared the dose-response relationship for stimulation of glucose disposal and suppression of hepatic glucose output by proinsulin and insulin. Ten normal subjects were studied using the euglycemic glucose clamp technique. The human proinsulin and insulin infusion rates were chosen to achieve steady-state proinsulin levels 10-fold higher than insulin levels on a molar basis, based on previous observations that porcine proinsulin has approximately 10% the potency of insulin. Proinsulin infusion rates of 2.75, 7.5, 22.5, and 45 micrograms/m2/min were compared with insulin infusion rates of 0.63, 1.67, 5, and 10 micrograms/m2/min. Primed, continuous infusions of insulin yielded steady-state levels within 25 min, whereas proinsulin levels did not reach a steady state for 120-180 min. The metabolic clearance rate of insulin was 11-12 ml/kg/min at the lower infusion rates but fell to 8.4 ml/kg/min at the highest infusion rate. The metabolic clearance rate of proinsulin was 3.0-3.5 ml/kg/min at all infusion rates. Dose-response analysis demonstrated that proinsulin-mediated glucose disposal was approximately 8% that of insulin. In contrast, proinsulin-mediated suppression of hepatic glucose output was approximately 12% that seen with insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1983

Metabolism of C-peptide in the dog. In vivo demonstration of the absence of hepatic extraction.

The in vivo hepatic metabolism of connecting peptide (C-peptide) in relation to that of insulin has not been adequately characterized. A radioimmunoassay for dog C-peptide was therefore developed and its metabolism studied in conscious mongrel dogs, with sampling catheters chronically implanted in their portal and hepatic veins and femoral artery. The hepatic extraction of endogenous C-peptide under basal conditions was negligible (4.3 +/- 4.5%) and was similar to the hepatic extraction of C-peptide measured during the constant exogenous infusion of C-peptide isolated from dog pancreas. Simultaneously measured hepatic extraction of endogenous and exogenously infused insulin were 43.8 +/- 7.6 and 47.5 +/- 4.4%, respectively. The metabolic clearance rate of infused C-peptide was 11.5 +/- 0.8 ml/kg per min and was constant over the concentration range usually encountered under physiological conditions. In additional experiments, the effect of parenteral glucose administration on the hepatic extraction of C-peptide and insulin was investigated. The hepatic extraction of C-peptide (6.2 +/- 4.0%) was again negligible in comparison with that of insulin (46.7 +/- 3.4%). Parenteral glucose administration did not affect the hepatic extraction of either peptide irrespective of whether it was infused peripherally, intraportally, or together with an intraportal infusion of gastrointestinal inhibitory polypeptide. The fasting C-peptide insulin molar ratio in both the portal vein (1.2 +/- 0.1) and femoral artery (2.1 +/- 0.3) was also unaffected by the glucose stimulus. These results therefore indicate that, since the hepatic extraction of C-peptide is negligible and its clearance kinetics linear, the peripheral C-peptide concentration should accurately reflect the rate of insulin secretion. New approaches to the quantitation of hepatic extraction and secretion of insulin by noninvasive techniques are now feasible.

Animals↗