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C Cobelli

Publications and source records attributed to C Cobelli.

At least 163 records · Page 9Linked to original sources
PubMed1982

Model of the kinetics of ketone bodies in humans.

The kinetics of ketone bodies was studied in normal humans by giving a combined bolus intravenous injection of labeled acetoacetate ([14C]AcAc) and D(--)-beta-hydroxybutyrate (beta-[14C]-OHB) to seven subjects after an overnight fast, on two different occasions, and by collecting frequent blood samples for 100 min. Kinetic data were analyzed with both noncompartmental and compartmental modeling techniques. A four-compartment model, representing AcAc and beta-OHB in blood and two equilibrating ketone body compartments, inside the liver and extrahepatic tissues, was chosen as the most reliable mathematical representation; it is physiologically plausible and was able to accurately fit the data. The model permitted evaluation of the in vivo rate of ketone body production in the liver, the individual plasma clearance rates of AcAc and beta-OHB, their initial volumes of distribution, and the transfer rate parameters among the four ketone body compartments. Moreover, the model provided estimates of the components of the rates of appearance of AcAc and beta-OHB in plasma due to newly synthesized ketone body from acetyl-CoA in the liver, and to interconversion and recycling in the liver and extrahepatic tissues. The model also was used to evaluate other methodologies currently employed in the analysis of ketone body turnover data: the conventional approach based on use of the combined specific activity of AcAc and beta-OHB required assumptions not satisfied in vivo, leading to substantial errors in key parameter estimates.

3-Hydroxybutyric Acid↗
PubMed1982

Insulin sensitivity, binding, and kinetics in pancreatogenic and type I diabetes.

Pancreatogenic diabetes (PD), secondary either to chronic calcific pancreatitis or to pancreatectomy, is characterized by higher frequency of hypoglycemic events during insulin therapy in comparison with type I insulin-dependent diabetes (IDD). Not only glucagon deficiency, but an enhanced peripheral tissue sensitivity to insulin could account for this metabolic behavior. We investigated several facets of insulin action, e.g., tissue sensitivity to insulin, insulin binding to red cells, and insulin kinetics in seven patients with PD in comparison with type I. Tissue sensitivity to insulin was evaluated by means of the glucose-insulin clamp technique as M/I x 100 ratio (mg . kg .-1 min-1/muU . ml-1), where M is the amount of glucose infused by Biostator GCIIS to clamp BG at basal level and I is the free insulin plateau concentration achieved by a primed-constant insulin infusion. At high BG 15 h after the last injection of regular insulin M/I x 100 was 7.79 (range 4.25-9.75) in PD and 4.20 (range 1.20-6.91) in D (P less than 0.05). At low and equal BG M/I x 100 was 8.55 (range 6.35-9.72) in PD and 3.42 (range 1.19-6.75) in D (P less than 0.01). The rate of endogenous glucose production was nearly totally suppressed in both groups of patients. Just before the two clamps, 125I-insulin specific binding to red cells was studied. The maximum specific binding was significantly higher in PD than in D at high BG (10.7 +/- 1.7 vs. 7.4 +/- 0.8/10(9) red cells) and at low and equal BG (12.4 +/- 1.2 vs. 6.8 +/- 0.8). Receptor concentration also was significantly higher in PD thant in D (P less than 0.02) while no significant differences were found in high affinity (Ke). Insulin kinetic data were analysed by using both "Model independent" (or noncompartmental) method and compartmental modeling. Patients with PD had significantly higher (P less than 0.05) plasma clearance of insulin.

Adult↗
PubMed1981

Modeling and identification of metabolic systems.

Introductory principles of physiological systems analysis by computer simulation or computation are introduced. The problems of model formulation, identification, and validation are examined. Selected guidelines for successful modeling are suggested, and examples of such applications in the field of endocrinology and metabolism are given.

Animals↗
PubMed1981

A modeling study of the effect of fasting on bilirubin kinetics in Gilbert's syndrome.

The mechanism of fasting hyperbilirubinemia (FH) is not fully understood. We investigated basal bilirubin kinetics in 20 Gilbert's patients and in 7 healthy volunteers. The study was repeated in seven of these Gilbert's patients after 48-h fasting. A two-compartment model proved to be adequate for interpreting crystalline bilirubin kinetics in these individuals. The parameters of bilirubin kinetics were estimated by employing a maximum likelihood parameter estimation technique. Consistency of the model and uniqueness of the estimated parameter values (from the covariance matrix) were shown. Our results confirmed previous observations regarding impaired bilirubin kinetics in Gilbert's patients as compared to controls. The main results obtained from kinetic studies in Gilbert's patients after fasting were i) no modification in the bilirubin clearance, and ii) a more than twice increase of bilirubin turnover. These data indicate that FH is related to an increased bilirubin production (mainly intrahepatic). Furthermore, evidence arises from this study that the bilirubin tolerance test is a useful diagnostic test for Gilbert's syndrome.

Adolescent↗
PubMed1981

Physiologic evaluation of factors controlling glucose tolerance in man: measurement of insulin sensitivity and beta-cell glucose sensitivity from the response to intravenous glucose.

The quantitative contributions of pancreatic responsiveness and insulin sensitivity to glucose tolerance were measured using the "minimal modeling technique" in 18 lean and obese subjects (88-206% ideal body wt). The individual contributions of insulin secretion and action were measured by interpreting the dynamics of plasma glucose and insulin during the intravenous glucose tolerance test in terms of two mathematical models. One, the insulin kinetics model, yields parameters of first-phase (phi 1) and second-phase (phi 2) responsivity of the beta-cells to glucose. The other glucose kinetics model yields the insulin sensitivity parameters, SI. Lean and obese subjects were subdivided into good (KG greater than 1.5) and lower (KG less than 1.5) glucose tolerance groups. The etiology of lower glucose tolerance was entirely different in lean and obese subjects. Lean, lower tolerance was related to pancreatic insufficiency (phi 2 77% lower than in good tolerance controls [P less than 0.03]), but insulin sensitivity was normal (P greater than 0.5). In contrast, obese lower tolerance was entirely due to insulin resistance (SI diminished 60% [P less than 0.01]); pancreatic responsiveness was not different from lean, good tolerance controls (phi 1: P greater than 0.06; phi 2: P greater than 0.40). Subjects (regardless of weight) could be segregated into good and lower tolerance by the product of second-phase beta-cell responsivity and insulin sensitivity (phi 2 . SI). Thus, these two factors were primarily responsible for overall determination of glucose tolerance. The effect of phi 1 was to modulate the KG value within those groups whose overall tolerance was determined by phi 2 . SI. This phi 1 modulating influence was more pronounced among insulin sensitive (phi 1 vs. KG, r = 0.79) than insulin resistant (obese, low tolerance; phi 1 vs. KG, r = 0.91) subjects. This study demonstrates the feasibility of the minimal model technique to determine the etiology of impaired glucose tolerance.

Body Weight↗
PubMed1980

Transfer function matrix of a compartmental model. A program for the rapid writing of its symbolic expression.

A method for the fast writing of the symbolic expression of the transfer function matrix of a compartmental model has been implemented on a FORTRAN G program. Special care has been devoted to the reduction of computations and memory by means of original subroutines. The program input is very simple and the output very clear to general users. The program may be employed for the study of all input-output relations of he considered compartmental model. Details on the computational methods adopted for its implementation are given. Application examples are reported.

Computers↗
PubMed1980

Parameter and structural identifiability concepts and ambiguities: a critical review and analysis.

The notion of identifiability addresses the question of whether it is at all possible to obtain unique solutions for unknown parameters of interest in a mathematical model, from data collected in well-defined stimulus-response experiments performed on a dynamic system represented by the model. This critical aspect of the modeling and data reduction problem is reviewed, analyzed, and unified, with emphasis on applications in biology. Several physiological system models are examined in detail. They illustrate the importance of identifiability analysis prior to performing a parameter estimation experiment, the algebraic difficulties, possible pitfalls, and not-so-apparent ambiguities that may be encountered--even for simple models--and the utility of the concept in experiment design. Methods for testing for identifiability also are reviewed and compared, with emphasis on applicability and limitations. The usefulness of a given model for predicting the time course of system variables (not parameters) of interest normally inaccessible to direct measurement (e.g., tissue concentrations), a common use for quantitative models, is shown to depend intimately on the identifiability properties of the model, in a manner not easy but essential to assess: state variable predictions made in this manner can be inherently ambiguous.

Kinetics↗
PubMed1980

Quantitative estimation of beta cell sensitivity to glucose in the intact organism: a minimal model of insulin kinetics in the dog.

We propose an approach to quantifying the sensitivity of B cells to glucose in the intact organism, whereby we interpret the complex dynamic plasma insulin response to glucose injection in terms of a minimal mathematical model of posthepatic insulin delivery and insulin clearance. The best model for this purpose was chosen by comparing the ability of a series of proposed models to account precisely for plasma insulin dynamics. Intravenous glucose tolerance tests (IVGTT) (300 mg/kg) were performed on conscious dogs, and blood was sampled frequently until the basal steady state was reestablished. Glucose injection produced variable plasma insulin responses, which were characterized by an early peak (76 microU/ml above basal), a plateau with occasional additional peaks, and by an abrupt return of plasma insulin to basal by 37 min. A set of eight models was examined; one emerged as superior, in that it was able to account for insulin dynamics with the smallest number of physiologically meaningful parameters (N = 4). The chosen (minimal) model assumes that (1) clearance of insulin is of the first order, (2) the initial peak represents a bolus of insulin loaded into the plasma after the glucose injection, and (3) the rate of the secondary rise in insulin is determined by the concentration of glucose in plasma above a specific threshold value. The sensitivity of first phase insulin delivery to glucose (phi 1; 1.28 +/- 0.15 microU/ml per min per mg/dl), the sensitivity of the secondary phase to glucose concentration [phi 2; 0.038 +/- 0.005 (microU/mg) . min-2], and the threshold for glucose stimulation of second phase secretion (h; 125 +/- 8 mg/100 ml) were all precisely estimated from the dynamic insulin responses. These three parameters of insulin kinetics (phi 1, phi 2, and h) can be calculated from a single IVGTT, and they characterize the insulin responsiveness of a single individual. Estimating these characteristic parameters of insulin kinetics from IVGTT data has potential for quantitating the individual factors contributing to glucose-stimulated insulin secretion in intact animal models, and it may be applicable to man.

Animals↗
PubMed1980

Identifiability in tracer experiments.

The paper deals with structural identifiability of compartmental systems via tracer experiments, i.e., with the problem of stating a priori whether a planned input-output identification experiment allows the estimation of all unknown parameters of a compartmental system. Compartmental models and tracer experiments are briefly reviewed. A formal definition of structural identifiability is given on the basis of system theory concepts. A structural identifiability test is presented and necessary conditions are given which can be tested on the compartmental diagram, without writing the symbolic expressions of the input-output relationships. Three examples are presented; the first two show how to practically employ the suggested rules, while the third one refers to a pharmacokinetic model.

Cell Compartmentation↗
PubMed1979

Quantitative estimation of insulin sensitivity.

We have evaluated the feasibility of using a mathematical model of glucose disappearance to estimate insulin sensitivity. Glucose was injected into conscious dogs at 100, 200, or 300 mg/kg. The measured time course of insulin was regarded as the "input," and the falling glucose concentration as the "output" of the physiological system storing and using glucose. Seven mathematical models of glucose uptake were compared to identify the representation most capable of simulating glucose disappearance. One specific nonlinear model was superior in that it 1) predicted the time course of glucose after glucose injection, 2) had four parameters that could be precisely estimated, and 3) described individual experiments with similar parameter values. Insulin sensitivity index (SI), defined as the dependence of fractional glucose disappearance on plasma insulin, was the ratio of two parameters of the chosen model and could be estimated with good reproducibility from the 300 mg/kg injection experiments (SI = 7.00 X 10(-4) +/- 24% (coefficient of variation) min-1/(microU/ml) (n = 8)). Thus, from a single glucose injection it is possible to obtain a quantitative index of insulin sensitivity that may have clinical applicability.

Animals↗
PubMed1977

A computer program for the analysis of controllability, observability and structural identifiability of biological compartmental systems.

A computer program to check structural identifiability of biological compartmental systems, that is the priori possibility of estamating all unknown system parameters through a multi input-multi output tracer experiment is presented. The procedure, as based only on the adopted compartmental structure and the chosen input-output experiment, is independent of the numerical values of the parameters: therefore the program can be usefully employed before parameter estimation algorithms, to assure that all the unknown parameters evidenced in the model can be estimated from the experimental data. After a short review on compartmental models, controllability observability and structural identifiability are defined and techniques to check them are provided. The digital computer implementation of the whole procedure is discussed in detail. Some typical program runs regarding the application to biological systems are given.

Computers↗