Extracellular glucose distribution is not altered by insulin: analysis of plasma and interstitial L-glucose kinetics

Estimating in vivo potassium distribution and fluxes with stable potassium isotopes

Extracellular potassium (K⁺) homeostasis is achieved by a concerted effort of multiple organs and tissues. A limitation in studies of K⁺ homeostasis is inadequate techniques to quantify K⁺ fluxes into and out of organs and tissues in vivo. The goal of the present study was to test the feasibility of a novel approach to estimate K⁺ distribution and fluxes in vivo using stable K⁺ isotopes. ⁴¹K was infused as KCl into rats consuming control or K⁺-deficient chow (n = 4 each), ⁴¹K-to-³⁹K ratios in plasma and red blood cells (RBCs) were measured by inductively coupled plasma mass spectrometry, and results were subjected to compartmental modeling. The plasma ⁴¹K/³⁹K increased during ⁴¹K infusion and decreased upon infusion cessation, without altering plasma total K⁺ concentration ([K⁺], i.e., ⁴¹K + ³⁹K). The time course of changes was analyzed with a two-compartmental model of K⁺ distribution and elimination. Model parameters, representing transport into and out of the intracellular pool and renal excretion, were identified in each rat, accurately predicting decreased renal K⁺ excretion in rats fed K⁺-deficient vs. control diet (P < 0.05). To estimate rate constants of K⁺ transport into and out of RBCs, ⁴¹K/³⁹K were subjected to a simple model, indicating no effects of the K⁺-deficient diet. The findings support the feasibility of the novel stable isotope approach to quantify K⁺ fluxes in vivo and sets a foundation for experimental protocols using more complex models to identify heterogeneous intracellular K⁺ pools and to answer questions pertaining to K⁺ homeostatic mechanisms in vivo.

Interstitium versus Blood Equilibrium in Glucose Concentration and its Impact on Subcutaneous Continuous Glucose Monitoring Systems

The relationship between both interstitial and blood glucose remains a debated topic, on which there is still no consensus. The experimental evidence suggests that blood and interstitial fluid glucose levels are correlated by a kinetic equilibrium, which as a consequence has a time and magnitude gradient in glucose concentration between blood and interstitium. Furthermore, this equilibrium can be perturbed by several physiological effects (such as foreign body response, wound-healing effect, etc.), with a consequent reduction of interstitial fluid glucose versus blood glucose correlation. In the present study, the impact of operating in the interstitium on continuous glucose monitoring systems (CGMs) will be discussed in depth, both for the application of CGMs in the management of diabetes and in other critical areas, such as tight glycaemic control in critically ill patients.

Identification of intraday metabolic profiles during closed-loop glucose control in individuals with type 1 diabetes

BACKGROUND

Algorithms for closed-loop insulin delivery can be designed and tuned empirically; however, a metabolic model that is predictive of clinical study results can potentially accelerate the process.

METHODS

Using data from a previously conducted closed-loop insulin delivery study, existing models of meal carbohydrate appearance, insulin pharmacokinetics, and the effect on glucose metabolism were identified for each of the 10 subjects studied. Insulin's effects to increase glucose uptake and decrease endogenous glucose production were described by the Bergman minimal model, and compartmental models were used to describe the pharmacokinetics of subcutaneous insulin absorption and glucose appearance following meals. The composite model, comprised of only five equations and eight parameters, was identified with and without intraday variance in insulin sensitivity (S(I)), glucose effectiveness at zero insulin (GEZI), and endogenous glucose production (EGP) at zero insulin.

RESULTS

Substantial intraday variation in SI, GEZI and EGP was observed in 7 of 10 subjects (root mean square error in model fit greater than 25 mg/dl with fixed parameters and nadir and/or peak glucose levels differing more than 25 mg/dl from model predictions). With intraday variation in these three parameters, plasma glucose and insulin were well fit by the model (R(2) = 0.933 +/- 0.00971 [mean +/- standard error of the mean] ranging from 0.879-0.974 for glucose; R(2) = 0.879 +/- 0.0151, range 0.819-0.972 for insulin). Once subject parameters were identified, the original study could be reconstructed using only the initial glucose value and basal insulin rate at the time closed loop was initiated together with meal carbohydrate information (glucose, R(2) = 0.900 +/- 0.015; insulin delivery, R(2) = 0.640 +/- 0.034; and insulin concentration, R(2) = 0.717 +/- 0.041).

CONCLUSION

Metabolic models used in developing and comparing closed-loop insulin delivery algorithms will need to explicitly describe intraday variation in metabolic parameters, but the model itself need not be comprised by a large number of compartments or differential equations.

Modeling glucose and water dynamics in human skin

BACKGROUND

Glucose is heterogeneously distributed in the different physiological compartments in the human skin. Therefore, for the development of a noninvasive measurement method, both a good quantification of the different compartments of human skin and an understanding of glucose transport processes are important.

METHODS

The composition of human skin was quantified by histology research. Based on this information a mathematical model was developed to simulate glucose dynamics in human skin.

RESULTS

The model predicts dynamically glucose concentrations in the different layers of the skin as a result of changes in blood glucose concentration. The model was validated with published time course data of blood and interstitial fluid glucose during a clamp study with three different set points for blood glucose, and model outcomes were compared to measurements for the lag time and gradient. According to the model, glucose in the interstitial fluid of the dermis best matches the amplitude and dynamics of blood glucose.

CONCLUSIONS

The new data obtained from quantitative histology appeared crucial for the model. The proposed model was successfully validated. This result was obtained without tuning or fitting of any parameter. It was shown how the model can be used to set standards for measurements and to define the best measurement depth for noninvasive glucose monitoring.

Physiological hyperinsulinemia has no detectable effect on access of macromolecules to insulin-sensitive tissues in healthy humans

OBJECTIVE

Physiologically elevated insulin concentrations promote access of macromolecules to skeletal muscle in dogs. We investigated whether insulin has a stimulating effect on the access of macromolecules to insulin-sensitive tissues in humans as well.

RESEARCH DESIGN AND METHODS

In a randomized, controlled trial, euglycemic-hyperinsulinemic clamp (1.2 mU x kg(-1) x min(-1) insulin) and saline control experiments were performed in 10 healthy volunteers (aged 27.5 +/- 4 years, BMI 22.6 +/- 1.6 kg/m(2)). Distribution and clearance parameters of inulin were determined in a whole-body approach, combining primed intravenous infusion of inulin with compartment modeling. Inulin kinetics were measured in serum using open-flow microperfusion in interstitial fluid of femoral skeletal muscle and subcutaneous adipose tissue.

RESULTS

Inulin kinetics in serum were best described using a three-compartment model incorporating a serum and a fast and a slow equilibrating compartment. Inulin kinetics in interstitial fluid of peripheral insulin-sensitive tissues were best represented by the slow equilibrating compartment. Serum and interstitial fluid inulin kinetics were comparable between the insulin and saline groups. Qualitative analysis of inulin kinetics was confirmed by model-derived distribution and clearance parameters of inulin. Physiological hyperinsulinemia (473 +/- 6 vs. 18 +/- 2 pmol/l for the insulin and saline group, respectively; P < 0.001) indicated no effect on distribution volume (98.2 +/- 6.2 vs. 102.5 +/- 5.7 ml/kg; NS) or exchange parameter (217.6 +/- 34.2 vs. 243.1 +/- 28.6 ml/min; NS) of inulin to peripheral insulin-sensitive tissues. All other parameters identified by the model were also comparable between the groups.

CONCLUSIONS

Our data suggest that in contrast to studies performed in dogs, insulin at physiological concentrations does not augment recruitment of insulin-sensitive tissues in healthy humans.

Development of a highly responsive needle-type glucose sensor using polyimide for a wearable artificial endocrine pancreas

To produce a long-life, stable, miniature glucose sensor for a wearable artificial endocrine pancreas (WAEP), we developed a novel microneedle-type glucose sensor using polyimide, designated the PI sensor (outer diameter, 0.3 mm; length, 16 mm), and investigated its characteristics in vitro and in vivo. In the in vitro study, we tested the sensor in 0.9% NaCl solution with varying glucose concentrations and observed an excellent linear relationship between the sensor output and glucose concentration (range: 0-500 mg/100 ml). In in vivo experiments, the PI sensor was inserted into the abdominal subcutaneous tissue of beagle dogs (n = 5), and interstitial fluid glucose concentrations were monitored after sensor calibration. Simultaneously, blood glucose concentrations were also monitored continuously with another PI sensor placed intravenously. The correlation and time delay between subcutaneous tissue glucose (Y) and blood glucose concentrations (X: 30-350 mg/100 ml) were Y = 1.03X + 7.98 (r = 0.969) and 6.6 +/- 1.2 min, respectively. We applied the new WAEP system/PI sensor and an intravenous insulin infusion algorithm developed previously for glycemic control in diabetic dogs. The use of the WAEP system resulted in excellent glycemic control after an oral glucose challenge of 1.5 g/kg (post-challenge blood glucose levels: 176 +/- 18 mg/100 ml at 65 min and 93 +/- 23 mg/100 ml at 240 min), without any hypoglycemia. Thus, we confirmed that our new PI sensor has excellent sensor characteristics in vitro and in vivo. The new WAEP using this sensor is potentially suitable for clinical application.

Physiological hyperinsulinemia in dogs augments access of macromolecules to insulin-sensitive tissues

Pharmacological doses of insulin increase limb blood flow and enhance tissue recruitment for small solutes such as glucose. We investigated whether elevating insulin within the physiological range (68 +/- 6 vs. 425 +/- 27 pmol/l) can influence tissue recruitment of [(14)C]inulin, an inert diffusionary marker of molecular weight similar to that of insulin itself. During hyperinsulinemic-euglycemic clamps, transport parameters and distribution volumes of [(14)C]inulin were determined in conscious dogs by applying a three-compartment model to the plasma clearance data of intravenously injected [(14)C]inulin (0.8 microCi/kg). In a second set of experiments in anesthetized dogs with direct cannulation of the hindlimb skeletal muscle lymphatics, we measured a possible effect of physiological hyperinsulinemia on the response of the interstitial fluid of skeletal muscle to intravenously injected [(14)C]inulin and compared this response with the model prediction from plasma data. Physiological hyperinsulinemia caused a 48 +/- 10% (P < 0.005) and a 35 +/- 15% (P < 0.05) increase of peripheral and splanchnic interstitial distribution volumes for [(14)C]inulin. Hindlimb lymph measurements directly confirmed the ability of insulin to enhance the access of macromolecules to the peripheral interstitial fluid compartment. The present results show that physiological hyperinsulinemia will enhance the delivery of a substance of similar molecular size to insulin to previously less intensively perfused regions of insulin-sensitive tissues. Our data suggest that the delivery of insulin itself to insulin-sensitive tissues could be a mechanism of insulin action on cellular glucose uptake independent of and possibly synergistic with either enhanced blood flow distribution or GLUT4 transporter recruitment to enhance glucose utilization. Because of the differences between inulin and insulin itself, whether delivery of the bioactive hormone is increased remains speculative.

L-glucose absorption in house sparrows (Passer domesticus) is nonmediated

We previously demonstrated in intact house sparrows substantial absorption in vivo of L-glucose, the stereoisomer of D-glucose that is assumed not to interact with the intestine's D-glucose transporter. Results of some studies challenge this assumption for other species. Therefore, we tested it in vitro and in vivo, based on the principle that if absorption of a compound (L-glucose) is mediated, then absorption of its tracer will be competitively inhibited by high concentrations of either the compound itself or other compounds (e.g., D-glucose) whose absorption is mediated by the same mechanism. An alternative hypothesis that L-glucose absorption is primarily paracellular predicts that its absorption in vivo will be increased (not decreased) in the presence of D-glucose, because the permeability of this pathway is supposedly enhanced when Na(+)-coupled glucose absorption occurs. First, using intact tissue in vitro, we found that uptake of tracer-radiolabeled L-glucose was not significantly inhibited by high concentrations (100 mM) of either L-glucose or 3-O-methyl-D-glucose, a non-metabolizable but actively transported D-glucose analogue. Second, using intact house sparrows, we found that fractional absorption of the L-glucose tracer was significantly increased, not reduced, when gavaged along with 200 mM 3-O-methyl-D-glucose. This result was confirmed in another experiment where L-glucose fractional absorption was significantly higher in the presence vs. absence of food in the gut. The greater absorption was apparently not due simply to longer retention time of digesta, because no significant difference was found among retention times. Our results are consistent with the idea that L-glucose is absorbed in a non-mediated fashion, largely via the paracellular pathway in vivo.

Timing of changes in interstitial and venous blood glucose measured with a continuous subcutaneous glucose sensor

The objective of this study was to use a subcutaneous continuous glucose sensor to determine time differences in the dynamics of blood glucose and interstitial glucose. A total of 14 patients with type 1 diabetes each had two sensors (Medtronic/MiniMed CGMS) placed subcutaneously in the abdomen, acquiring data every 5 min. Blood glucose was sampled every 5 min for 8 h, and two liquid meals were given. A smoothing algorithm was applied to the blood glucose and interstitial glucose curves. The first derivatives of the glucose traces defined and quantified the timing of rises, peaks, falls, and nadirs. Altogether, 24 datasets were used for the analysis of time differences between interstitial and blood glucose and between sensors in each patient. Time differences between blood and interstitial glucose ranged from 4 to 10 min, with the interstitial glucose lagging behind blood glucose in 81% of cases (95% CIs 72.5 and 89.5%). The mean (+/-SD) difference between the two sensors in each patient was 6.7 +/- 5.1 min, representing random variation in sensor response. In conclusion, there is a time lag of interstitial glucose behind blood glucose, regardless of whether glycemia is rising or falling, but intersensor variability is considerable in this sensor system. Comparisons of interstitial and blood glucose kinetics must take statistical account of variability between sensors.

Assessment of transcapillary glucose exchange in human skeletal muscle and adipose tissue

We studied the kinetics of glucose exchange between plasma and interstitial fluid (ISF) in human skeletal muscle and adipose tissue under fasting conditions. Five normal human subjects received an intravenous [6,6-2H2]glucose infusion in a prime-continuous fashion. During the tracer infusion, the open-flow microperfusion technique was employed to frequently sample ISF from quadriceps muscle and subcutaneous adipose tissue. The tracer glucose kinetics observed in muscle and adipose tissue ISF were found to be well described by a capillary-tissue exchange model. As a measure of transcapillary glucose exchange efficiency, the 95% equilibrium time was calculated from the identified model parameters. This time constant was similar for skeletal muscle and adipose tissue (28.6 +/- 3.2 vs. 26.8 +/- 3.6 min; P = 0.60). Furthermore, we found that the (total) interstitial glucose concentration was significantly lower (P < 0.01) in muscle (3.32 +/- 0.46 mmol/l) and adipose tissue (3.51 +/- 0.17 mmol/l) compared with arterialized plasma levels (5.56 +/- 0.13 mmol/l). Thus the observed gradients and dynamic relationships between plasma and ISF glucose in muscle and adipose tissue provide evidence that transcapillary exchange of glucose is limited in these two tissues under fasting conditions.

Partitioning glucose distribution/transport, disposal, and endogenous production during IVGTT

We have separated the effect of insulin on glucose distribution/transport, glucose disposal, and endogenous production (EGP) during an intravenous glucose tolerance test (IVGTT) by use of a dual-tracer dilution methodology. Six healthy lean male subjects (age 33 +/- 3 yr, body mass index 22.7 +/- 0.6 kg/m(2)) underwent a 4-h IVGTT (0.3 g/kg glucose enriched with 3-6% D-[U-(13)C]glucose and 5-10% 3-O-methyl-D-glucose) preceded by a 2-h investigation under basal conditions (5 mg/kg of D-[U-(13)C]glucose and 8 mg/kg of 3-O-methyl-D-glucose). A new model described the kinetics of the two glucose tracers and native glucose with the use of a two-compartment structure for glucose and a one-compartment structure for insulin effects. Insulin sensitivities of distribution/transport, disposal, and EGP were similar (11.5 +/- 3.8 vs. 10.4 +/- 3.9 vs. 11.1 +/- 2.7 x 10(-2) ml small middle dot kg(-1) small middle dot min(-1) per mU/l; P = nonsignificant, ANOVA). When expressed in terms of ability to lower glucose concentration, stimulation of disposal and stimulation of distribution/transport accounted each independently for 25 and 30%, respectively, of the overall effect. Suppression of EGP was more effective (P < 0.01, ANOVA) and accounted for 50% of the overall effect. EGP was suppressed by 70% (52-82%) (95% confidence interval relative to basal) within 60 min of the IVGTT; glucose distribution/transport was least responsive to insulin and was maximally activated by 62% (34-96%) above basal at 80 min compared with maximum 279% (116-565%) activation of glucose disposal at 20 min. The deactivation of glucose distribution/transport was slower than that of glucose disposal and EGP (P < 0.02) with half-times of 207 (84-510), 12 (7-22), and 29 (16-54) min, respectively. The minimal-model insulin sensitivity was tightly correlated with and linearly related to sensitivity of EGP (r = 0.96, P < 0.005) and correlated positively but nonsignificantly with distribution/transport sensitivity (r = 0.73, P = 0.10) and disposal sensitivity (r = 0.55, P = 0.26). We conclude that, in healthy subjects during an IVGTT, the two peripheral insulin effects account jointly for approximately one-half of the overall insulin-stimulated glucose lowering, each effect contributing equally. Suppression of EGP matches the effect in the periphery.