Kinetics Of Insulin Secretion Research Articles

A lumped parameter mathematical model of a hollow fiber membrane device for the controlled insulin release

The purpose of this paper is to analyze the performances of a hollow fiber membrane bioreactor with Langerhans islets entrapped in the shell. A number of experiments have been performed in order to characterize the device with respect to the fluid dynamics and mass transport. A theoretical analysis of the bioreactor has been carried out, leading to the development of a lumped parameter mathematical model for the description of glucose and insulin transport. Actually a number of more sophisticated transport models have been proposed in the literature for similar devices. The purpose of this paper was, however, the presentation of a simpler approach, aiming at a quick description of the system behavior. The model is based on the mass transfer equations, accounting for the radial diffusion of species, their axial and radial convection—the latter due to Starling fluxes—and insulin generation. The kinetics of insulin secretion has been modelled in terms of a linear two-parameter rate equation, accounting for the glucose concentration level and the insulin negative bio-feedback. Diffusive mass transfer across the membrane has been described according to the series resistance's model. The resulting equations have been solved numerically in terms of glucose and insulin concentration distributions, under different operating conditions, with reference to a range of values for the characteristic dimensionless parameters of the model.

Differences in insulin secretion between the rat and mouse: role of cAMP

Although information regarding insulin secretion usually is considered equivalent when generated in the mouse or the rat, it is established that the kinetics of insulin secretion from mouse and rat pancreatic beta cells differ. The mechanisms underlining these differences are not understood. The in vitro perfused pancreas and isolated islets of the mouse or rat were employed in this study to investigate the role of cyclic adenosine monophosphate (cAMP), a major positive modulator of beta-cell function, as one differentiating signal for the uniquely different insulin release from the beta cells of these commonly used rodents. Glucose-stimulated first-phase insulin release from the perfused pancreas of the rat was higher than the mouse when calculated per gram of pancreas or as fractional secretion, but this phase was identical in the two species when results were adjusted for total body weight. Whether related to insulin content, pancreatic weight or body weight, the rat pancreas responded to glucose with a progressively increasing second-phase insulin release compared to the mouse pancreas, which secreted a flat second-phase of lesser magnitude. Isolated islets from rat and mouse were comparable in insulin content whereas the basal cAMP level of mouse islets was less than half that of the rat. At submaximal stimulation with glucose or glucose + IBMX or forskolin, mouse islets exhibited lower cAMP levels to a given stimulus than the rat. In rat islets cAMP levels increased to approximately 1000 fmol per islet, although insulin secretion maximized by 100-150 fmol.(ABSTRACT TRUNCATED AT 250 WORDS)

A microperifusion system with environmental control for studying insulin secretion by pancreatic tissue.

A continuous flow reactor (perifusion system) was fabricated and tested for measuring the kinetics of insulin secretion from isolated pancreatic islets of Langerhans in response to step changes in the glucose concentration and oxygen partial pressure in the perfusate flowing around the islets. The system was capable of making rapid changes in perfusate glucose concentration and pO2, had rapid dynamic response for measuring the change in insulin secretion rate as a result of these changes in perfusate, and was suitable for studying very small volumes of tissue. Initial experiments with this system demonstrated that (1) the response of isolated rat islets to glucose stimulation was very fast, with the first phase peak occurring in as little as about 10 s, (2) bulk perfusate oxygen partial pressure levels of 30 mmHg or less reduced the second-phase insulin secretion rate in graded fashion, (3) the reduction in secretion rate began within 1 min following an oxygen partial pressure decrease, and (4) the reduction in secretion rate was reversible, with a burst of insulin secretion occurring during the first minute after partial pressure restoration.

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)

Regulation of energy metabolism in pancreatic islets by glucose and tolbutamide.

The kinetics of insulin secretion and oxygen uptake in response to D-glucose and tolbutamide were compared in mouse pancreatic islets. In addition, the role of decreased ATP as a driving force for secretagogue-induced oxygen consumption was examined. D-glucose (10-30 mmol/l) triggered a biphasic insulin release which always coincided with a monophasic increase in islet oxygen uptake. In the presence of D-glucose (5-30 mmol/l), tolbutamide (3-500 mumol/l) consistently elicited an initial peak of insulin secretion which was followed by a continued decline. Tolbutamide-induced secretory profiles were accompanied by similar respiratory profiles. Oxygen consumption per ng of insulin released during the test phase was higher after elevation of the glucose concentration than after addition of tolbutamide. In conjunction with 5 or 10 mmol/l D-glucose, but not with 15 or 30 mmol/l D-glucose, tolbutamide (30-100 mumol/l) lowered islet ATP content significantly (p less than 0.02). Phosphocreatine was not found in isolated islets, although they contained substantial creatine kinase activity. It is concluded that the driving force for tolbutamide-induced oxygen uptake is a decrease in the phosphorylation potential caused by the work load imposed by stimulation of the secretion process. However, a major proportion of the respiratory response to glucose also results from enhancement of biosynthesis.

Novel secretory granule morphology in physically fixed pancreatic islets.

Protein A-gold immunocytochemistry has been applied to physically fixed beta cells from rat islets of Langerhans. The punctate nature of the gold particles permits improved resolution of the antigenic sites without obscuring the fine ultrastructural preservation obtained by physical fixation. There is a filamentous material within the halo of the secretory granules that is not preserved by aqueous, chemical fixation. When viewed in stereo the filaments appear as an annular cobweb or a series of wheel spokes attached to a centrally located hub (the dense core of the granule). The filaments demonstrate insulin-like immunoreactivity using the protein A-gold technique. The immunoreactivity appears to be restricted to the filaments and the surface of the dense cores. This may be a consequence of the preservation of a solid, insolubilized core state that resists penetration by the antibody and/or the protein A-gold complex. However, the evidence that there is a halo pool of insulin which is separate from the massive core aggregate suggests that i) correspondingly massive exocytotic pits may not be as mandatory for insulin release as has been assumed and ii) the complex kinetics of insulin secretion may be, in part, a reflection of multiple insulin compartments within secretory granules.

Regulation of insulin secretion by energy metabolism in pancreatic B-cell mitochondria. Studies with a non-metabolizable leucine analogue

In mouse pancreatic islets the kinetics of insulin secretion and O2 uptake in response to the non-metabolizable leucine analogue (+/-)-BCH (2-endo- aminonorbornane -2-carboxylic acid) were compared. In addition, the fuel-mobilizing effect of (+/-)-BCH was studied with a mitochondrial fraction from islets. (1) Within 2 min 20 mM-(+/-)-BCH markedly enhanced insulin release or O2 consumption by islets respiring in the absence of exogenous fuels. During prolonged exposure to 20 mM-(+/-)-BCH secretion declined more rapidly than O2 uptake. (2) L-Glutamine (10 mM) prevented the decrease of both insulin release and O2 uptake of islets exposed to 20mM-(+/-)-BCH. During the second phase of insulin release in response to 20 mM-(+/-)-BCH + 10 mM-L-glutamine, kinetics of secretion and respiration correlated closely. (3) Initial peaks were consistently seen in the (+/-)-BCH-induced secretory profiles, but never in the respiratory profiles. (4) In contrast with L-glycerol 3-phosphate, L-malate or pyruvate, L-glutamine or L-glutamate maintained low rates of oxidative phosphorylation in B-cell mitochondria. The effects of L-glutamine or L-glutamate were potentiated severalfold by (+/-)-BCH. (5) The effects of other branched-chain amino acids on oxidative phosphorylation resembled their effects on insulin release, redox state of nicotinamide nucleotides and glutamate dehydrogenase activity. (6) The results support the view that (+/-)-BCH stimulates insulin secretion via a primary enhancement of hydrogen supply to the respiratory chain of B-cell mitochondria.

Beta-Cell function relative to islet volume and hormone content in the isolated perfused mouse pancreas.

The beta-cell function, total islet volume, and number were studied in 1- to 18-month-old mice, together with the extractable pancreatic insulin and glucagon. The beta-cell function, determined as the total amount of insulin released in response to glucose from the in vitro perfused pancreas showed an age-related increase, without any differences in the kinetics of insulin secretion between young and old mice. The total islet number and area in each individual pancreas was determined planimetrically after selective staining of the islets by perfusing the pancreas with dithizone. The islet area increased from 5.4 +/- 1.7 mm2 at 1 month to 16.3 +/- 2.1 mm2 at 18 months, whereas the number of islets remained virtually unchanged (1072 +/- 51). Pancreatic insulin increased with age by nearly 500%, in contrast to a 35% reduction in pancreatic glucagon. There was a strong relationship between body weight and total pancreatic DNA (P = 4.7 X 10(-8)), islet area (P = 3.2 X 10(-7)), insulin secretory capacity (P = 7 X 10(-4)), and total pancreatic insulin (P = 1.9 X 10(-5)), but no relationship between body weight and islet number. The insulin secretory capacity increased proportionally to the increase in islet area (P = 9.9 X 10(-3)). The islet area and total pancreatic insulin were closely related (P = 2.8 X 10(-12)), as were pancreatic insulin and the insulin secretory capacity (P = 3.3 X 10(-11)). There was a negative relationship between pancreatic glucagon and islet area (P = 0.005) and between pancreatic glucagon and insulin (P = 0.01). The close relationship between pancreatic insulin and islet area, shown to be an expression of islet volume, makes it possible to estimate the volume of the endocrine pancreas after standard RIA of pancreatic insulin. The combined morphometric and physiological analysis is unique in studying islet cell function relative to the volume of the endocrine pancreas.

Abnormal insulin secretion in adult canines.

Insulin secretion after intravenous administration of glucose was studied in fourteen dogs, twelve of which were four years of age or older. Based on several characteristics of insulin secretion kinetics, normal and impaired insulin responses were defined in the experimental subjects. Among those, five exhibited a marked depression of the initial stage of insulin release which was followed by a stage of relative hyperinsulinemia. Similarities between canine and human insulin secretion patterns were noted, and the potential usefulness of the procedure for the early detection of canine diabetes mellitus discussed.

Insulin Release from the Isolated Perfused Rat Pancreas Containing Insulinomata Induced by Streptozotocin and Nicotinamide: Effects of Glucose and Responses to Tumor Removal

The kinetics of insulin secretion in response to glucose were studied in the in vitro perfused rat pancreas before and after removal of islet cell tumors induced by streptozotocin and nicotinamide. In addition, insulin secretion before and after tumor removal was compared with that from normal pancreata before and after sham operations, respectively. Thus, the two pancreas preparations were subjected to repeated perfusions with glucose. Perfusion of pancreata containing tumors with 8.4 mM glucose resulted in biphasic release in a pattern similar to that of normal pancreas. However, both basal and stimulated insulin secretion of tumor-bearing pancreata were greater than either those of pancreata from which tumors had been excluded by ligature or those of normal pancreata before sham operation. A second increase in the concentration of glucose from 2.8 to 8.4 mM also produced a biphasic release of insulin from extratumoral pancreata as well as from sham-operated normal pancreata. However, the insulin secretory response to glucose of extratumoral pancreatic tissue was less than that of control pancreatic tissue. Our findings indicate that pancreatic islet cell tumors induced by streptozotocin and nicotinamide respond to glucose with typical biphasic insulin release. Thus, chemically induced rat insulinomata may provide a readily available and valuable model of insulin-secreting tissue, analogous to normal islets. Furthermore, our study suggests that the B cell function of pancreata containing tumors is inhibited by the preexisting tumor-induced hyperinsulinism or by its metabolic consequences.

Ethanol inhibition of insulin secretion by perifused rat islets.

In vivo and in vitro effects of ethanol on the kinetics of insulin secretion in response to glucose and tolbutamide were studied in perifused rat islets. Phases I and II insulin response to 16.7 mM glucose was decreased 46% and 48%, respectively, in islets of rats given ethanol intragastrically 1 g/kg 1 h prior to sacrifice. Mean blood ethanol levels at the time of animal sacrifice were 19.4 mmol/l. The magnitude of insulin suppression was not significantly enhanced with higher ethanol doses, 2 or 3 g/kg, although mean blood ethanol levels increased to 25.9 and 60.3 mmol/l, respectively. Similarly, significant inhibition of both phases of insulin response to glucose occurred when ethanol 1 or 3 g/kg was given intraperitoneally instead of orally. Ethanol had no effect on insulin secretion when given orally 4 h instead of 1 h prior to islet isolation. Ethanol, 65 mmol/l, added directly to rat islets perifusate simultaneously with 16.7 mM glucose decreased both phases I and II insulin response nearly half; whereas addition of 21.7 instead of 65 mmol/l ethanol had no effect. Pre-treatment of islets with 21.7 or 65 mmol/l ethanol during 30 min basal islets perifusion period had no effect on subsequent insulin response to 16.7 mM glucose. Insulin response to 10 mM tolbutamide was decreased nearly 81% by the simultaneous presence of 65 mmol/l ethanol in islets perifusate.

Kinetics of insulin secretion following glucose infusion in patients with atherosclerotic peripheral vascular disease.

Kinetics of insulin secretion following an i.v. glucose infusion, according to the protocol described by Cerasi and Luft, were studied in 19 patients with angiographically documented atherosclerotic peripheral vascular disease and in a group of appropriate controls without clinical signs of disease. No significant differences were noted between patients and controls in plasma cholesterol levels and in the K value following a standard i.v. glucose tolerance test. Blood glucose levels were significantly lower in the patients, whereas mean plasma insulin and triglyceride levels were significantly higher. Analysis of the glucose and insulin responses to the glucose infusion test indicated that 31.6% of the patients had a delayed and sluggish insulin response to the glucose load, fitting the criteria suggested for the diagnosis of prediabetes, versus 10% of the appropriate controls. In particular, simulation of the plasma insulin responses by a square-wave glucose stimulus, confirmed that in a significantly higher number of patients the early insulin peak was below normal limits. The results of this study suggest that increased insulin secretion is not present in patients with atherosclerotic vascular disease, in contrast to reports by other authors, and that inefficient insulin secretory mechanisms may be observed in these patients, thus possibly contributing to the development of the atherosclerotic disease.

Kinetics of Insulin Secretion in Chronic Pancreatitis and Mild Maturity Onset Diabetes (Evidence for “Gut Hormone” Action Beyond Glucoreceptor and Cyclic Adenosine Monophosphate Mediated Insulin Release)

We studied insulin responses to glucose with and without cholecystokinin-pancreozymin and aminophyllin infusions in normal, chronic pancreatitic and genetic (maturity-onset) diabetic subjects. Glucose was given alone as separate 5 and 10 g boluses followed by infusion at 250 mg/min. and 500 mg/min., respectively. Chronic pancreatitis patients and genetic diabetic patients had decreased Imax values, indicating a decreased insulin reserve. Sensitivity to glucose was normal in pancreatitic subjects, but the diabetic subjects had a raised G50 value, compatible with glucoreceptor dysfunction. Infusions of aminophyllin enhanced insulin responses (Imax) to glucose injection in normal subjects and to a lesser degree in pancreatitic subjects, but decreased sensitivity to glucose (increase in G50) in both groups. Although the Imax value in pancreatitic subjects was significantly lower than in the control subjects during the glucose plus aminophyllin infusion, the blood glucose concentration in the pancreatitic subjects was nonetheless decreased. This suggests that pancreatitic subjects have increased endogenous insulin sensitivity. Aminophyllin had no effect in diabetic subjects. Crude cholecystokinin-pancreozymin changed the shape of the glucose/insulin dose response curve in normal, pancreatitic and diabetic subjects. These findings further suggest that the defect in insulin secretion in pancreatitic subjects is partly situated at the cyclic adenosine monophosphate stage of insulin release. Crude cholecystokin-pancreozymin seems to affect insulin release at a point beyond the cyclic adenosine monophosphate stage.