The artificial pancreas: evaluating risk of hypoglycaemia following errors that can be expected with prolonged at-home use.

AIMS: Artificial pancreas systems show benefit in closely monitored at-home studies, but may not have sufficient power to assess safety during infrequent, but expected, system or user errors. The aim of this study was to assess the safety of an artificial pancreas system emulating the ß-cell when the glucose value used for control is improperly calibrated and participants forget to administer pre-meal insulin boluses. METHODS: Artificial pancreas control was performed in a clinic research centre on three separate occasions each lasting from 10 p.m. to 2 p.m. Sensor glucose values normally used for artificial pancreas control were replaced with scaled blood glucose values calculated to be 20% lower than, equal to or 33% higher than the true blood glucose. Safe control was defined as blood glucose between 3.9 and 8.3 mmol/l. RESULTS: Artificial pancreas control resulted in fasting scaled blood glucose values not different from target (6.67 mmol/l) at any scaling factor. Meal control with scaled blood glucose 33% higher than blood glucose resulted in supplemental carbohydrate to prevent hypoglycaemia in four of six participants during breakfast, and one participant during the night. In all instances, scaled blood glucose reported blood glucose as safe. CONCLUSIONS: Outpatient trials evaluating artificial pancreas performance based on sensor glucose may not detect hypoglycaemia when sensor glucose reads higher than blood glucose. Because these errors are expected to occur, in-hospital artificial pancreas studies using supplemental carbohydrate in anticipation of hypoglycaemia, which allow safety to be assessed in a controlled non-significant environment should be considered as an alternative. Inpatient studies provide a definitive alternative to model-based computer simulations and can be conducted in parallel with closely monitored outpatient artificial pancreas studies used to assess benefit.

Interstitial fluid glucose dynamics during insulin-induced hypoglycaemia.

AIMS/HYPOTHESIS: Glucose sensors often measure s.c. interstitial fluid (ISF) glucose rather than blood or plasma glucose. Putative differences between plasma and ISF glucose include a protracted delay during the recovery from hypoglycaemia and an increased gradient during hyperinsulinaemia. These have often been investigated using sensor systems that have delays due to signal smoothing, or require long equilibration times. The aim of the present study was to define these relationships during hypoglycaemia in a well-equilibrated system with no smoothing. METHODS: Hypoglycaemia was induced by i.v. insulin infusion (360 pmol.m(-2).min(-1)) in ten non-diabetic subjects. Glucose was sequentially clamped at approximately 5, 4.2 and 3.1 mmol/l and allowed to return to normoglycaemia. Subjects wore two s.c. glucose sensors (Medtronic MiniMed, Northridge, CA, USA) that had been inserted for more than 12 h. A two-compartment model was used to quantify the delay and gradient. RESULTS: The delay during the fall in plasma glucose was not different from the delay during recovery (8.3+/-0.67 vs 6.3+/-1.1 min; p=0.27) and no differences were observed in the ratio of sensor current to plasma glucose at basal insulin (2.7+/-0.25 nA.mmol(-1).l) compared with any of the hyperinsulinaemic clamp phases (2.8+/-0.18, 2.7+/-0.021, 2.9+/-0.21; p=NS). The ratio was significantly elevated following recovery to normoglycaemia (3.1+/-0.2 nA.mmol(-1).l; p<0.001). CONCLUSIONS/INTERPRETATION: The elevated ratio suggests that the plasma to ISF glucose gradient was decreased following hypoglycaemia, possibly due to increased skin blood flow. Recovery from hypoglycaemia is not accompanied by a protracted delay and insulin does not increase the plasma to s.c. ISF glucose gradient.

Modeling insulin action for development of a closed-loop artificial pancreas.

Three models of glucose homeostasis are compared in terms of their steady-state dose-response characteristics, how they characterize glucose distribution kinetics, and how they characterize the dynamics of insulin action. The three models [minimal model, AIDA (Automated Insulin Dosage Advisor), and a model by Sorensen] are used to discuss a wider variety of questions related to metabolic modeling. Simulations are performed comparing each model's response to an intravenous glucose tolerance test, with and without incremental insulin responses, to existing data in individuals with type 1 diabetes mellitus. Predicted changes in blood glucose following a subcutaneous bolus of insulin or an incremental increase in basal insulin delivery are simulated. From these results, the models are evaluated as potential candidates for simulating changes in treatment and developing a closed-loop insulin delivery algorithm. While no consensus model is proposed, relevant issues needing to be addressed are highlighted.

Closed-loop insulin delivery-the path to physiological glucose control.

The development of an artificial pancreas for the treatment of insulin-dependent diabetes is a highly desired endeavor for patients, physicians, scientists, and engineers. Historical algorithms and recent progress in research and technology are reviewed in the present article, together with aspects of beta-cell physiology that lead to normal glucose tolerance. Algorithms are evaluated for their ability to deliver insulin as to recreate, as closely as possible, glucose and insulin profiles observed in healthy individuals. Emphasis is placed upon how the algorithms compare to the beta-cell's secretory response, specifically first-phase and second-phase insulin secretion. Experimental closed-loop data employing intravenous and subcutaneous glucose sensors and implanted and external insulin pumps (Medtronic MiniMed, Northridge CA) are presented.

High glucose stimulates early response gene c-Myc expression in rat pancreatic beta cells.

Glucose-induced insulin secretion from hyperglycemic 90% pancreatectomized rats is markedly impaired, possibly because of loss of beta cell differentiation. Association of these changes with beta cell hypertrophy, increased mRNA levels of the transcription factor c-Myc, and their complete normalization by phlorizin treatment suggested a link between chronic hyperglycemia, increased c-Myc expression, and altered beta cell function. In this study, we tested the effect of hyperglycemia on rat pancreatic islet c-Myc expression both in vivo and in vitro. Elevation of plasma glucose for 1-4 days (glucose infusion/clamp) was followed by parallel increases in islet mRNA levels (relative to TATA-binding protein) of c-Myc and two of its target genes, ornithine decarboxylase and lactate dehydrogenase A. Similar changes were observed in vitro upon stimulation of cultured islets or purified beta cells with 20 and 30 mmol.liter(-1) glucose for 18 h. These effects of high glucose were reproduced by high potassium-induced depolarization or dibutyryl-cAMP and were inhibited by agents decreasing cytosolic Ca(2+) or cAMP concentrations. In conclusion, the expression of the early response gene c-Myc in rat pancreatic beta cells is stimulated by high glucose in a Ca(2+)-dependent manner and by cAMP. c-Myc could therefore participate to the regulation of beta cell growth, apoptosis, and differentiation under physiological or pathophysiological conditions.

Gene expression of VEGF and its receptors Flk-1/KDR and Flt-1 in cultured and transplanted rat islets.

BACKGROUND: Vascular endothelial growth factor (VEGF) and its two receptor tyrosine kinases, Flk-1/KDR and Flt-1, may play an important role in mediating the revascularization of transplanted pancreatic islets. METHODS: Using semiquantitative multiplex reverse-transcribed polymerase chain reaction we determined the gene expression of VEGF and its receptors in cultured and transplanted rat islets. RESULTS: After exposure of islet cells to hypoxia in vitro, increases were found in the gene expression of the VEGF120 and VEGF164 isoforms, with simultaneous increases in VE-cadherin, Flk-1/KDR, and Flt-1. In vivo studies consisted of analysis of islet grafts transplanted into both normal and diabetic recipients. Expression of both VEGF120 and VEGF164 in grafts was up-regulated for the first 2-3 days after transplantation, with the response being more prolonged in the diabetic rats. These increases were followed by reduced expression of VEGF on days 5, 7, and 9. Increases in the expression of VE-cadherin in islet grafts in normal and diabetic recipients tended to parallel VEGF expression, with the increases in both probably being caused by hypoxia. The early increases of VEGF expression were followed by a rise in the expression of VEGF receptors, which probably represents the early stages of angiogenesis. Graft expression of Flk-1/KDR and Flt-1 was enhanced at 3 and 5 days in the normoglycemic recipients, while in the diabetic recipients increases were found later on days 5, 7, and 14. CONCLUSIONS: The delayed expression of VEGF receptors in the diabetic recipients could reflect impaired angiogenesis caused by the diabetic milieu; this delay could contribute to the less outcomes of grafts transplanted into a hyperglycemic environment.

Adaptation of beta-cell mass to substrate oversupply: enhanced function with normal gene expression.

Although type 2 diabetes mellitus is associated with insulin resistance, many individuals compensate by increasing insulin secretion. Putative mechanisms underlying this compensation were assessed in the present study by use of 4-day glucose (GLC; 35% Glc, 2 ml/h) and lipid (LIH; 10% Intralipid + 20 U/ml heparin; 2 ml/h) infusions to rats. Within 2 days of beginning the infusion of either lipid or glucose, plasma glucose profiles were normalized (relative to saline-infused control rats; SAL; 0.45% 2 ml/h). During glucose infusion, plasma glucose was maintained in the normal range by an approximately twofold increase in plasma insulin and an approximately 80% increase in beta-cell mass. During LIH infusion, glucose profiles were also maintained in the normal range. Plasma insulin responses during feeding were doubled, and beta-cell mass increased 54%. For both groups, the increase in beta-cell mass was associated with increased beta-cell proliferation (98% increase during GLC and 125% increase during LIH). At the end of the 4-day infusions, no significant changes were observed in islet-specific gene transcription (i.e., the expression of islet hormone genes, glucose metabolism genes, and insulin transcription factors were unaffected). Two days after termination of the infusions, the glucose-stimulated plasma insulin response was increased approximately 67% in glucose-infused animals. No sustained effect on insulin secretory capacity was observed in the LIH animals. The increase in plasma insulin response after glucose infusion was achieved in the absence of any change in insulin clearance. We conclude that, in rats, an increase in insulin demand after an increase in glucose appearance or free fatty acid leads to an increase in beta-cell mass, mediated in part by an increase in beta-cell proliferation, and that these compensatory changes lead to increased insulin secretion, normal plasma glucose levels, and the maintenance of normal islet gene expression.

Islets in alginate macrobeads reverse diabetes despite minimal acute insulin secretory responses.

BACKGROUND: Encapsulation of islets has been widely investigated as a treatment for diabetes. The characteristics and dynamics of insulin secretion by encapsulated islets in response to glucose and other secretagogues are not well understood. METHODS: In our study, macroencapsulated syngeneic islets at 3-4 wk after transplantation were studied for insulin release in response to i.v. glucose (hyperglycemic clamps at 250 or 350 mg/dl plasma glucose), arginine (i.v. bolus, 100 mg/kg), glucagon-like peptide-1 (i.v. infusion for 20 min, 2.2 pmol/kg/min), and meal challenge. Syngeneic islets (6000 islets) were encapsulated in alginate macrobeads (2-3 mm diameter) with or without poly-L-lysine coating and transplanted into the peritoneal cavity of STZ-diabetic Lewis rats. Normal (nontransplanted) and diabetic Lewis rats transplanted with "naked" islets under the kidney capsule served as controls. RESULTS: Animals transplanted with macrobeads displayed subnormal insulin responses to glucose, arginine, and glucagon-like peptide-1 despite achieving normoglycemia faster than animals with renal subcapsular islet transplants. Plasma insulin responses to meal challenges were blunted in animals with macrobeads resulting in increased plasma glucose excursions. CONCLUSIONS: We conclude that, after transplantation into diabetic Lewis rats, macroencapsulated islets have significantly impaired insulin secretion despite achieving normal fed glycemic levels.

Beta-cell dysfunction in 48-hour glucose-infused rats is not a consequence of elevated plasma lipid or islet triglyceride levels.

The abnormal insulin secretion found in human diabetics and animal models of diabetes has been attributed to the deleterious effects of chronic hyperglycemia and/or elevated circulating levels of nonesterified fatty acids (NEFAs). In this study, abnormal glucose-induced insulin secretion (GIIS) was generated by a 48-hour infusion of glucose and assessed by the isolated perfused pancreas technique. In these hyperglycemic animals, abnormal GIIS is accompanied by a decrease in plasma NEFAs, while plasma and, more importantly, islet triglycerides remain at levels comparable to those in the controls. It is concluded that the abnormal insulin secretion in this glucose infusion model was likely caused by 48 hours of hyperglycemia and not by changes in circulating or islet lipids.

Improved vascularization of planar membrane diffusion devices following continuous infusion of vascular endothelial growth factor.

Improving blood vessel formation around an immunobarrier device should improve the survival of the encapsulated tissue. In the present study we investigated the formation of new blood vessels around a planar membrane diffusion device (the Baxter Theracyte System) undergoing a continuous infusion of vascular endothelial growth factor through the membranes and into the surrounding tissue. Each device (20 microl) had both an inner immunoisolation membrane and an outer vascularizing membrane. Human recombinant vascular endothelial growth factor-165 was infused at 100 ng/day (low dose: n = 6) and 500 ng/day (high dose: n = 7) for 10 days into devices implanted s.c. in Sprague-Dawley rats; noninfused devices transplanted for an identical period were used as controls (n = 5). Two days following the termination of VEGF infusion, devices were loaded with 20 microl of Lispro insulin (1 U/kg) and the kinetics of insulin release from the lumen of the device was assessed. Devices were then explanted and the number of blood vessels (capillary and noncapillary) was quantified using morphometry. High-dose vascular endothelial growth factor infusion resulted in two- to threefold more blood vessels around the device than that obtained with the noninfused devices and devices infused with low-dose vascular endothelial growth factor. This increase in the number of blood vessels was accompanied by a modest increase in insulin diffusion from the device in the high-dose vascular endothelial growth factor infusion group. We conclude that vascular endothelial growth factor can be used to improve blood vessel formation adjacent to planar membrane diffusion devices.

Can interstitial glucose assessment replace blood glucose measurements?

Current treatment regiments for individuals depending on exogenous insulin are based on measurements of blood glucose obtained through painful finger sticks. The shift to minimal or noninvasive continuous glucose monitoring primarily involves a shift from blood glucose measurements to devices measuring subcutaneous interstitial fluid (ISF) glucose. As the development of these devices progresses, details of the dynamic relationship between blood glucose and interstitial glucose dynamics need to be firmly established. This is a challenging task insofar as direct measures of ISF glucose are not readily available. The current article investigated the dynamic relationship between plasma and ISF glucose using a model-based approach. A two-compartment model system previously validated on data obtained with the MiniMed Continuous Glucose Monitoring System (CGMS) is reviewed and predictions from the original two-compartment model were confirmed using new data analysis of glucose dynamics in plasma and hindlimb lymph (lymph is derived from ISF) in the anesthetized dog. From these data sets, the time delay between plasma and ISF glucose in dogs was established (5-12 minutes) and a simulation study was performed to estimate the errors introduced if ISF is taken as a surrogate for blood. From the simulation study, the error component resulting from the differences in plasma and ISF glucose was estimated to be < 6% during normal day-to-day use in an individual with diabetes (error component calculated as the standard deviation of the ISF/plasma glucose differences under conditions where the maximal time delay was used). This difference is most likely within the variance between arterial and venous blood glucose. We conclude that the differences between plasma and ISF glucose will not be a significant obstacle in advancing the use of ISF as an alternative to blood glucose measurements.

Subcutaneous glucose predicts plasma glucose independent of insulin: implications for continuous monitoring.

The present study investigated the relationship between blood and subcutaneous interstitial fluid (ISF) glucose by employing an amperometric glucose sensor specifically developed for 3-day continuous glucose monitoring. The apparent sensor sensitivity and ISF glucose equilibration delay were estimated on separate days during hyperglycemic clamps in four dogs in which insulin was either suppressed with somatostatin, allowed to change, or increased with an exogenous infusion. A 2-h sensor "settling-in" period was allowed before the clamps. During insulin deficiency, the sensor sensitivity and ISF glucose delay were 0.23 +/- 0.03 nA per mg/dl and 4.4 +/- 0. 8 min. Sensitivity was not affected by increases in endogenous (0.30 +/- 0.04 vs. 0.28 +/- 0.04 nA per mg/dl) or exogenous insulin (0.18 +/- 0.01 vs. 0.16 +/- 0.01 nA per mg/dl) nor was the delay (3.3 +/- 1.2 vs. 5.7 +/- 1.1 and 9.2 +/- 2.6 vs. 12.3 +/- 1.7 min; P > 0.05 for all). Sensor glucose accurately predicted plasma glucose without correcting for delays <10 min (r > 0.9 for all), whereas for longer delays a digital corrective filter was used (r = 0.91 with filter). We conclude that the relationship between blood and ISF glucose is not affected by insulin and that delays in ISF glucose equilibration can be corrected with digital filters.

Physiological insulinemia acutely modulates plasma leptin.

Whether insulin acutely regulates plasma leptin in humans is controversial. We examined the dosage-response and time-course characteristics of the effect of insulin on leptin in 10 men (age 42+/-2 years [mean+/-SE]; BMI 29.3+/-2.0 kg/m2). Each individual underwent four 9-h euglycemic clamps (insulin at 20, 40, 80, and 400 mU x m[-2] x min[-1) and a control saline infusion. Although plasma glucose and insulin levels remained constant, leptin diminished from 9.1+/-3.0 to 5.9+/-2.1 ng/ml (P < 0.001) by the end of the control experiment. Conversely, plasma leptin showed a dosage-dependent increase during the insulin infusions that was evident within 30-60 min. The insulin-induced increase in leptin was proportionately lower in obese insulin-resistant men. Free fatty acids (FFAs) decreased during insulin and did not change during saline infusions. ED50 (the dose producing half-maximal effect) for insulin's effect on leptin and FFA was similar (138+/-36 vs. 102+/-24 pmol/l, respectively; P=0.11). To further define the role of physiological insulinemia, we compared the effect of a very low dosage insulin infusion (10 mU x m[-2] x min[-1]) with that of a control saline infusion in another group of 10 men (mean age 39+/-3 years; BMI 27.1+/-1.0 kg/m2). Plasma leptin remained stable during that insulin infusion, but fell by 37+/-2% in the control experiment. Thus physiological insulinemia can acutely regulate plasma leptin. Insulin could mediate the effect of caloric intake on leptin and could be a determinant of its plasma concentration. Inadequate insulin-induced leptin production in obese and insulin-resistant subjects may contribute to the development or worsening of obesity.

Role of portal insulin delivery in the disappearance of intravenous glucose and assessment of insulin sensitivity.

The contribution of portal insulin delivery to the disappearance of glucose administered intravenously was assessed in the present study. Paired insulin-modified intravenous glucose tolerance tests (IVGTTs) were performed in dogs in which insulin was administered into the portal vein or into a peripheral vein. Peripheral insulin levels were matched in the paired IVGTTs by adjusting the portal insulin dose in proportion to first-pass hepatic insulin extraction. Two sets of IVGTTs were performed. In the first set, hepatic insulin extraction was assumed to be 50% (insulin doses of 0.03 U/kg portal and 0.015 U/kg peripheral; n = 6); in the second set, the assumed extraction rate was reduced to 33% (0.0225 U/kg portal and 0.015 U/kg peripheral; n = 8). In the second set of experiments, a control "zero" dose (no insulin injection) was also performed. For these latter three IVGTTs, the exogenous glucose bolus was labeled with 3-[3H]glucose (25 microCi) to separately assess insulin's effects on the rate of glucose disappearance (Rd) and endogenous glucose production (EGP). For the paired IVGTT based on 33% extraction, the area under the insulin curves after the portal insulin injection was within 2% of that observed with peripheral insulin injection (1,820 +/- 711 vs. 1,791 +/- 661 microU/ml min; P = 0.79). For these conditions, neither the glucose profiles nor the minimal model estimate of insulin sensitivity (S(I)) was significantly influenced by the higher portal insulin delivery (S(I): 3.69 +/- 0.56 vs. 3.35 +/- 0.60 10(-4) min(-1) per microU/ml; portal vs. peripheral; P > 0.05). Analysis of the 3-[3H]glucose tracer dynamics failed to reveal any differences in the portal versus peripheral insulin effect on glucose disappearance or production. We conclude that portal insulin delivery per se does not significantly affect insulin's ability to normalize plasma glucose during acute glucose challenges.

Diurnal and ultradian rhythmicity of plasma leptin: effects of gender and adiposity.

Plasma leptin shows a nocturnal rise and a pulsatile pattern. This work was undertaken to determine the effects of gender and obesity on this pattern. Twenty-four-hour leptin profiles were evaluated in 31 subjects [17 male, 14 female; age: 36 +/- 2 yr (mean +/- SEM); body mass index: 27.5 +/- 1.0 kg/m2]. Plasma leptin profiles were higher in obese (body mass index > 27 kg/m2) than in lean subjects and higher in women than in men, regardless of fat mass. Leptin showed diurnal rhythmicity with peaks between 2200-0300 (median: 0120) and nadirs between 0800 and 1740 (median: 1033). Spectral analysis revealed 2 components (periodicities: 24 and 12 h) with higher relative amplitudes in lean than in obese subjects. The relative diurnal amplitude also was higher in men than in women, controlling for adiposity. Insulinemia, female sex, and age were negative determinants of diurnal rhythm relative amplitude. Pulse analysis revealed 3.6 +/- 0.3 pulses/24 h, occurring mostly 2-3 h after meals. Pulse frequency correlated negatively with fat mass and insulinemia (Spearman's r = -0.54 and -0.37, respectively; P < 0.05 for each). Thus, obesity is associated not only with higher leptin levels but also with blunted diurnal excursions and dampened pulsatility. This abnormal rhythmicity may contribute to leptin resistance in obesity. The significance of the sexual dimorphism in the diurnal amplitude is unclear, but it may be related to leptin's putative role as a metabolic signal to the reproductive axis.

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.

Method of insulin administration has no effect on insulin sensitivity estimates from the insulin-modified minimal model protocol.

The effect of the method of insulin administration on insulin sensitivity estimates from the insulin modified minimal model (MINMOD) protocol was evaluated using the tolbutamide-boosted protocol as a reference. The study included 21 nondiabetic men ages 40 +/- 2 years (mean +/- SE) with a BMI of 26.6 +/- 1.1 kg/m2. Each subject underwent four frequently sampled intravenous glucose tolerance tests (FSIGTT), one with tolbutamide and three with the same insulin dosage (0.03 U/kg) given as a bolus or infusion over 5 or 10 min. The insulin sensitivity index (SI) of each subject was calculated from each FSIGTT with MINMOD. Insulin sensitivity indexes from the four FSIGTTs were highly correlated (r > 0.85, P < 0.001). SI(insulin) from the bolus and the 5- and 10-min infusion protocols were similar, but were 21 +/- 5, 29 +/- 5, and 23 +/- 4% lower than SI(tolbutamide), respectively. SG(tolbutamide) and SG(insulin) were not different among the four protocols and were significantly correlated (r > 0.55, P < 0.01). Thus the tolbutamide and insulin protocols must not be used interchangeably in any single cross-sectional or longitudinal study. When the same insulin dosage is used, the method of its administration has no bearing on insulin sensitivity estimates from the insulin-modified FSIGTT. The same method of insulin administration should be used, however, in any single study for purpose of standardization.

Differences between the tolbutamide-boosted and the insulin-modified minimal model protocols.

The insulin-modified frequently sampled intravenous glucose tolerance test (FSIGTT) with minimal model analysis (MINMOD) was compared with the tolbutamide protocol and the glucose clamp in 35 nondiabetic subjects (age 38 +/- 2 years [mean +/- SE], BMI 27.2 +/- 0.9 kg/m2). Each subject underwent two FSIGTTs, one with tolbutamide (300 mg) and the other with insulin (0.03 U/kg) and a euglycemic hyperinsulinemic clamp (40 mU x m(-2) x min(-1)). Insulin sensitivity was determined from each FSIGTT with MINMOD and from the clamp. Insulin sensitivity indexes (S(I)) from the two FSIGTTs were significantly correlated (r = 0.77, P < 0.001), but S(I(insulin)) was 29 +/- 4% lower than S(I(tolbutamide)). Both S(I(insulin)) and S(I(tolbutamide)) correlated significantly with S(I(clamp)) (r = 0.70 and 0.71, P < 0.001 for each). Expressed in the same units (dl/min per pU/ml), S(I(tolbutamide)) was on average 13 +/- 6% lower than S(I(clamp)) (4.51 +/- 0.40 vs. 5.36 +/- 0.36 x 10(-2), P = 0.009), whereas S(I(insulin)) was 44 +/- 4% lower. S(G(tolbutamide)) and S(G(insulin)) were not different (1.88 +/- 0.10 vs. 2.01 +/- 0.09 x 10(-2) min(-1), P = 0.167) and were significantly correlated (r = 0.50, P = 0.002). Thus, insulin sensitivity estimates from both protocols correlate significantly with each other and with the clamp. They are quantitatively discrepant, however, possibly due to differences in the route of insulin delivery, saturation of insulin action, and/or tolbutamide-induced proinsulin release. Data obtained from these two MINMOD protocols are not directly comparable, and the same protocol must be used in any single cross-sectional or longitudinal study.

Indirect effect of insulin to suppress endogenous glucose production is dominant, even with hyperglucagonemia.

Suppression of endogenous glucose production (EGP) is one of insulin's primary metabolic effects and failure of this action is a major contributor to fasting hyperglycemia of type 2 diabetes mellitus. Classically, insulin was thought to suppress the liver directly, via hyperinsulinemia in the portal vein. Recently, however, we and others have demonstrated that at least part, and possibly most of insulin's action to suppress EGP is normally mediated via an extrahepatic (i.e., indirect) mechanism. We have suggested that this mechanism involves insulin suppression of adipocyte lipolysis, leading to lowered FFA and reduced EGP ("Single Gateway Hypothesis"). Previous studies of the indirect insulin effect from this laboratory were done under conditions of lowered portal glucagon. Because of the possibility that the direct (i.e., portal) effect of insulin may have been underestimated with hypoglucagonemia, these studies examined the relative importance of portal insulin, versus peripheral insulin (administered at one-half the dose to equalize peripheral insulin levels) at four rates of portal glucagon infusion: 0, 0.65 (under-), 1.5 (basal-), and 3.0 ng/kg per min (over-replacement). Portal versus peripheral insulin suppressed steady-state EGP to the same extent (52%), confirming that the primary effect of insulin to suppress EGP is via the peripheral mechanism. This conclusion was maintained regardless of portal glucagonemia, although there was some evidence for an increase in the direct insulin effect at hyperglucagonemia. The indirect effect of insulin is the primary mechanism of steady-state EGP suppression under normal conditions. The direct effect increases with hyperglucagonemia; however, the indirect effect remains predominant even under those conditions.