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G Toffolo

Publications and source records attributed to G Toffolo.

50 records · Page 3Linked to original sources
PubMed1987

Models to interpret kinetic data in stable isotope tracer studies.

In contrast to "weightless" radioactive tracers, stable isotope tracers have nonnegligible mass and are naturally present in the system, and the measured variable is a ratio of two isotopic species. These features do not allow stable isotopic tracer data analysis using straightforward analogy with radioactive tracer approaches, even though this practice is common. In this study, we present kinetic variables, models, and measurements for the analysis and interpretation of stable isotope tracer data. Assumptions and mathematical techniques for modeling the data when perturbation is both nonnegligible and negligible are discussed. Emphasis is placed on the rich information content of the dynamic portion of a stable isotope tracer curve and on the role of compartmental and noncompartmental modeling approaches for its interpretation. A presumed and commonly used analogy between the radioactive specific activity and stable isotopic enrichment is shown to be incorrect. We show that the proper analogue of specific activity is the tracer-to-tracee molar ratio. This variable is not a directly measurable one, but a formula is derived that allows its computation from the data. A method for reconstructing the time course in blood of the concentration component due to endogenous synthesis is presented. This allows measurement of the extent of the perturbation in the case where a nonweightless tracer is used. Special attention is given to data analysis originating from a multiple tracer experiment, a configuration necessary for studying more complex systems, e.g., the kinetics of interacting substrates.

3-Hydroxybutyric Acid↗
PubMed1986

Estimation of insulin sensitivity and glucose clearance from minimal model: new insights from labeled IVGTT.

The "minimal model" of glucose disappearance provides noninvasive estimates of insulin sensitivity and glucose effectiveness from an intravenous glucose tolerance test (IVGTT). However, this model does not allow the separation of glucose production from utilization. To overcome this limitation, labeled glucose was injected along with cold glucose in six normal dogs, and both cold and labeled glucose time courses were monitored along with insulin concentration. A revised minimal model was fitted to tracer data to obtain new measures of insulin sensitivity (SI* = 6.41 +/- 0.91 10(-4) min-1 X microU-1 X ml-1) and fractional glucose clearance (SG* = 0.0092 +/- 0.0009 min-1). SG* was compared with a direct measure obtained by a hepatic arterial-venous difference technique, which yielded a value of 0.0097 +/- 0.0002, virtually identical to SG*, thereby validating the model estimate. When the original minimal model was identified from cold data, we obtained S1 = 4.52 +/- 1.39 and SG = 0.042 +/- 0.009. SI* and SG* were different from SI and SG, respectively. In particular SG overestimates fractional glucose clearance by approximately five times. The revised minimal model yields glucose disposal parameters SI* and SG* that are not affected by the confounding effect of insulin and glucose inhibition of glucose production. Limitations inherent in cold IVGTT and original minimal model are overcome by labeled IVGTT and the revised minimal model, while test simplicity remains.

Animals↗
PubMed1985

Ketone body metabolism in normal and diabetic human skeletal muscle.

Although the liver is considered the major source of ketone bodies (KB) in humans, these compounds may also be formed by nonhepatic tissues. To study this aspect further, 3-[14C]hydroxybutyrate (BOH) or [3-14C]acetoacetate (AcAc) were constantly infused after a priming dose and contemporaneous arterial and venous samples were taken at splanchnic, heart, kidney, and leg sites in eight normal subjects (N) undergoing diagnostic catheterization and at the forearm site in five normal and six ketotic diabetic (D) subjects. After 70 min of infusion, tracer and tracee levels of AcAc and BOH reached a steady state in the artery and vein in both normal and diabetic subjects. The venous-arterial (V-A) difference at the forearm step for cold KB was negligible both in normal and diabetic subjects, whereas for labeled KB it was approximately 10-fold higher in diabetic subjects (V-A AcAc, -31 +/- 7 and -270 +/- 34 dpm/ml in N and D, respectively; V-A BOH, -38 +/- 6 and -344 +/- 126 dpm/ml in N and D, respectively). We assumed that the V-A difference in tracer concentration was consistent with dilution of the tracer by newly synthesized tracee inside the muscle and calculated that the forearm muscle produces KB at a rate of 16.2 +/- 3.3 mumol/min in D and 0.9 +/- 0.9 mumol/min in N. These findings can be accounted for by the hypothesis that the disappearance flux of KB from circulation was replaced by an equivalent flux of KB entering the vein at the muscle step in D but not in N. Moreover, in N KB were not only produced but also utilized by the splanchnic area (39 +/- 9 mumol/min).(ABSTRACT TRUNCATED AT 250 WORDS)

3-Hydroxybutyric Acid↗
PubMed1985

Acetoacetate and 3-hydroxybutyrate kinetics in obese and insulin-dependent diabetic humans.

[3-14C]acetoacetate (AcAc) and beta-[3-14C]hydroxybutyrate (beta-OHB) administration, measurements of labeled AcAc and beta-OHB in blood, and kinetic modeling have been used to investigate ketone body (KB) metabolism in five normal, five obese, and eight insulin-withdrawn diabetic subjects. Diabetic subjects were divided in mildly ketotic (MKD) and highly ketotic (HKD) patients according to beta-OHB blood level. A four-compartmental model successfully described the tracer kinetic data in obese and normal subjects, whereas in diabetic patients a five-compartmental model was necessary. Obese subjects showed a significantly lower (P less than 0.05) KB de novo synthesis (R30 = 159 +/- 54 (SD) mumol X min-1 X m-2) in comparison with normal subjects (282 +/- 93), but the clearance rates of AcAc (PCR1) and beta-OHB (PCR2) were similar in the two groups. R30 was 596 +/- 534 in MKD and 1,278 +/- 445 (P less than 0.01) in HKD. PCR1 was not significantly different both in MKD and HKD in comparison with normal subjects. In contrast PCR2 was markedly reduced in HKD (0 +/- 0 ml X min-1 X m-2) in comparison with MKD (1,031 +/- 615) and normal subjects (782 +/- 278). The percentage distribution of KB among various tissues inside the organism of diabetic subjects is abnormal. Both AcAc and beta-OHB recycling and mean residence time are not normal in HKD. A significant correlation was found between C-peptide and KB production in diabetes. These results suggest that a selective defect of beta-OHB peripheral utilization is important in determining and maintaining severe diabetic ketoacidosis.

3-Hydroxybutyric Acid↗
PubMed1985

Effect of insulin on the distribution and disposition of glucose in man.

Understanding the influence of insulin on glucose turnover is the key to interpreting a great number of metabolic situations. Little is known, however, about insulin's effect on the distribution and exchange of glucose in body pools. We developed a physiological compartmental model to describe the kinetics of plasma glucose in normal man in the basal state and under steady-state conditions of euglycemic hyperinsulinemia. A bolus of [3-3H]glucose was rapidly injected into a peripheral vein in six healthy volunteers, and the time-course of plasma radioactivity was monitored at very short time intervals for 150 min. A 1-mU/min kg insulin clamp was then started, thereby raising plasma insulin levels to a high physiological plateau (approximately 100 microU/ml). After 90 min of stable euglycemic hyperinsulinemia, a second bolus of [3-3H]glucose was given, and plasma radioactivity was again sampled frequently for 90 min more while the clamp was continued. Three exponential components were clearly identified in the plasma disappearance curves of tracer glucose of each subject studied, both before and after insulin. Based on stringent statistical criteria, the data in the basal state were fitted to a three-compartment model. The compartment of initial distribution was identical to the plasma pool (40 +/- 3 mg/kg); the other two compartments had similar size (91 +/- 12 and 96 +/- 9 mg/kg), but the former was in rapid exchange with plasma (at an average rate of 1.09 +/- 0.15 min-1), whereas the latter exchanged 10 times more slowly (0.12 +/- 0.01 min-1). The basal rate of glucose turnover averaged 2.15 +/- 0.12 mg/min kg, and the total distribution volume of glucose in the postabsorptive state was 26 +/- 1% of body weight. In view of current physiological information, it was assumed that the more rapidly exchanging pool represented the insulin-independent tissues of the body, while the slowly exchanging pool was assimilated to the insulin-dependent tissues. Insulin-independent glucose uptake was estimated (from published data) at 75% of basal glucose uptake, and was constrained not to change with euglycemic hyperinsulinemia. When the kinetic data obtained during insulin administration were fitted to this model, neither the size nor the exchange rates of the plasma or the rapid pool were appreciably changed. In contrast, the slow pool was markedly expanded (from 96 +/- 9 to 190 +/- 30 mg/kg, P less than 0.02) at the same time as total glucose disposal rose fourfold above basal (to 7.96 +/- 0.85 mg/min kg, P less than 0.001). Furthermore, a significant direct correlation was found to exist between the change in size of the slow pool and the insulin-stimulated rate of total glucose turnover (r=0.92, P<0.01). We conclude that hyperinsulinemia, independent of hyperglycemia, markedly increases the exchangeable mass of glucose in the body, presumably reflecting the accumulation of free, intracellular glucose in insulin-dependent tissues.

Adult↗
PubMed1984

Compartmental vs. noncompartmental modeling for two accessible pools.

We examine the limitations of noncompartmental vs. compartmental modeling when two accessible pools are available in kinetic experiments. Focus is on the estimation of mean residence time, whole-body mass, and steady-state equivalent distribution volume. Examples illustrate these points.

Kinetics↗
PubMed1983

Is the "pool-fraction" paradigm a valid model for assessment of in vivo turnover in non-steady state?

Quantification of in vivo turnover of endogenous substances in nonsteady state is of fundamental importance for understanding a variety of physiological and clinical metabolic situations. Toward this end, a pool-fraction model has become a paradigm in the glucose and ketone body areas. We discuss the basic assumptions on which the pool-fraction model is based and the criteria on which it has been validated. Specific comments are then made on its current and potential use for quantifying the non-steady-state turnover of glucose, ketone bodies, and insulin. We conclude that the quantitative reliability of predictions provided by the pool-fraction model is quite poor and that new developments are needed for quantifying the non-steady-state situation.

Animals↗
PubMed1983

Insulin-mediated glucose disposal in type I diabetes: evidence for insulin resistance.

To clarify whether type I diabetes is characterized by insulin resistance, insulin-mediated glucose metabolism (M; milligrams per kg/min) was estimated by means of the glucose clamp technique in five insulin-dependent diabetic patients and six normal subjects. Three glucose clamps were carried out under different metabolic conditions. Free insulin plateaux were similar during each clamp in both groups. The first clamp was performed in normal subjects after an overnight fast [blood glucose, 80 +/- 3 mg/dl (mean +/- SEM)] and in diabetic patients 18 h after insulin withdrawal (blood glucose, 366 +/- 47 mg/dl). Diabetic patients had a M value (4.25 +/- 0.74) not different from normals (5.38 +/- 0.63; P = NS). The second clamp was done with the same glycemic values (approximately 125 mg/dl) in both groups. M increased to 8.07 +/- 1.06 (P less than 0.01) in the normal subjects and decreased to 2.87 +/- 0.50 (P = NS) in the diabetic patients. The M value in the diabetic patients was lower than that in the normal subjects (P less than 0.05). The third clamp was performed in three diabetic patients after 1 month of treatment with continuous sc insulin infusion. The mean blood glucose level was 88 +/- 6 mg/dl, and M was 3.23 +/- 0.38, significantly lower than that of the normal subjects in the basal state (P less than 0.05). No differences were found in insulin binding to erythrocytes. The mean plasma clearance rate (milliliters per m2/min) of free insulin was the same in both groups (428 +/- 113 in normal subjects and 354 +/- 83 in diabetic patients). Basal endogenous glucose production was higher in the diabetics (3.13 +/- 0.48 mg/kg X min) than in the normal subjects (1.71 +/- 0.57). During the clamp, however, endogenous glucose production was similarly inhibited (approximately 95%) in both groups. Multiple glucose clamp studies were also performed at three different insulin infusion rates (21, 73, and 760 mU/m2 X min, respectively) to generate an insulin-dose response curve for glucose disposal in six diabetic patients treated with continuous sc insulin infusion for at least 6 months. This allowed investigation of the effect of chronic strict insulin therapy leading to normal glucose and intermediary metabolite levels and identification of the cellular mechanism of insulin resistance. A significant reduction of the maximal glucose disposal rate (10.7 +/- 0.5 mg/kg X min) was found in these diabetic patients compared to that in normal subjects (14.9 +/- 1.0; P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
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↗
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↗
PubMedDate not supplied

Ketone body kinetics in vivo using simultaneous administration of acetoacetate and 3-hydroxybutyrate labelled with stable isotopes.

Isotope dilution studies of ketone body (KB) turnover have usually been performed using a single 14C tracer and the so called 'combined KB specific activity'. By definition, this approach does not allow to evaluate the individual kinetics of acetoacetate (AcAc) and 3-hydroxybutyrate (R-BHB) which is feasible only using the separate administration of 14C tracer AcAc and R-BHB. In the present study we followed a different approach using the simultaneous administration in vivo of [1,2,13C2] AcAc and m [1,2,3,4(13)C4] R-BHB which allows to evaluate the individual kinetics of the two KB in the some study, thus minimizing the magnitude of blood sampling and the potential changes in the metabolic conditions of each subject. The four isotopic 13C/12C KB ratios of AcAc and R-BHB tracer and tracee blood concentrations along with the fluorimetric measurement of 12C concentrations were determined in each blood sample. Using compartmental analysis following single dose bolus injection the production rate of KB was 206 +/- 57 mumol/min/1.73 m2 (mean +/- SD). The turnover rate of KB using noncompartmental analysis, during continuous infusion in a separate study was 294 +/- 41. The plasma clearance rates of AcAc and R-BHB were 1966 +/- 502 and 1443 +/- ml/min/1.73 m2, respectively. The mean residence time was 17 +/- 3 min and the total distribution volume 20 +/- 9.7 l/m2. We conclude that: (1) stable isotope tracer infusion allows the contemporary in vivo administration of the two KB and the simultaneous assessment of individual AcAc and R-BHB kinetics; (2) the estimated compartmental and noncompartmental parameters of KB turnover were similar to those observed in normal overnight fasting subjects following separate radioactive tracer injections.

3-Hydroxybutyric Acid↗