Dube, Simmi

Background Hypoglycemia results in a reduction in spontaneous cardiac baroreflex sensitivity (scBRS) and a shift in the baroreflex working range to higher heart rates; this effect may be mediated by the carotid chemoreceptors. We hypothesized changes in baroreflex control of heart rate with hypoglycemia would be blunted in carotid body resected patients when compared with healthy controls. Methods Five patients (3M/2F) with bilateral carotid body resection for glomus tumors and ten healthy controls (7M/3F) completed a 180‐min hyperinsulinemic, hypoglycemic (~3.3 µmol/mL) clamp. Changes in heart rate, blood pressure, and scBRS were assessed. Results scBRS was reduced from baseline during hypoglycemia (p<0.01) and was lower in carotid body resected patients when compared with controls (p=0.01). Hypoglycemia‐mediated reductions in systolic and mean blood pressure were significantly greater in patients when compared with controls (p<0.05). Despite lower blood pressures, increases in heart rate with hypoglycemia were blunted in patients (p<0.01). Conclusion Reductions in scBRS with hypoglycemia cannot be exclusively attributed to the carotid chemoreceptors; however, the chemoreceptors play a role in resetting the baroreflex working range to higher heart rates during hypoglycemia such that a rise in heart rate is not observed in carotid body resected patients. These data support a contribution of the carotid chemoreceptors to baroreflex control of blood pressure during hypoglycemia and highlight potential widespread effects of carotid body resection in humans. Funding DK090541, HL120570

Carotid bodies (CB) are desensitized by hyperoxia/hyperglycemia and sensitized by hypoxia/hypoglycemia. We have reported in healthy humans that CB chemoreceptors secrete counterregulatory hormones in response to normoxia and hypoglycemia. While the role of central nervous system in glucose counterregulation is known, it is unknown whether alteration in CB response to glucose contributes to the pathogenesis of hypoglycema in T1D. We hypothesized that glucose counterregulation is impaired in T1D due to reduced sensitization of CB. 10 T1D and 12 healthy subjects (ND), matched for BMI (23 ± 3 vs 26 ± 5 kg/m 2 ) underwent a 180 min hyperinsulinemic (2 mU/kg TBW/min), hypoglycemic clamp (~3.3mM) under normoxic conditions (SpO2 98 ± 1%). T1D subjects did not report autonomic symptoms. [3‐ 3 H] glucose was infused to measure glucose turnover. Results were compared using means of last forty minutes of clamp data. Statistical analyses were done using unpaired t‐tests and p<0.05 was considered significant. In the presence of matched glucose (3.3±0.1 vs.3.3±0.1 mM) and insulin concentrations (783±106 vs. 812±185 pM;p=0.66), counterregulatory hormonal responses epinephrine (274 ± 165 vs. 742 ± 279 pg/ml) and glucagon (30 ± 11 vs 105 ± 31pg/ml) to hypoglycemia were significantly lower (p<0.001) in T1D vs. ND respectively. The glucose infusion rate required to maintain hypoglycemia was significantly higher (36 ± 1 vs.28 ± 0 µmol/kg/min, p <0.001) and endogenous glucose production was significantly lower (10 ± 3 vs.14 ± 4 µmol/kg/min, p<0.05) in T1D vs. ND with unchanged insulin stimulated glucose uptake. We conclude that in T1D the CB sensitivity to hypoglycemia is impaired and may contribute to increased risk of hypoglycemia in these individuals. Future studies are required to evaluate CB response in hypoglycemia unaware individuals with T1D. Grant Funding Source : DK90541

Background: Quantitative assessment of the dynamic relationship between plasma and interstitial fluid (ISF) glucose and the estimation of the plasma-to-ISF delay are of major importance to determine the accuracy of subcutaneous glucose sensors, an essential component of open- and closed-loop therapeutic systems for type 1 diabetes mellitus (T1DM). The goal of this work is to develop a model of plasma-to-ISF glucose kinetics from multitracer plasma and interstitium data, obtained by microdialysis, in healthy and T1DM subjects, under fasting conditions. Materials and Methods: A specific experimental design, combining administration of multiple tracers with the microdialysis technique, was used to simultaneously frequently collect plasma and ISF data. Linear time-invariant compartmental modeling was used to describe glucose kinetics from the tracer data because the system is in steady state. Results: A two-compartment model was shown accurate and was identified from both plasma and ISF data. An “equilibration time” between plasma and ISF of 9.1 and 11.0 min (median) in healthy and T1DM subjects, respectively, was calculated. Conclusions: We have demonstrated that, in steady-state condition, the glucose plasma-to-ISF kinetics can be modeled with a linear two-compartment model and that the “equilibration time” between the two compartments can be estimated with precision. Future studies will assess plasma-to-interstitium glucose kinetics during glucose and insulin perturbations in both healthy and T1DM subjects.

Hypoglycemia results in a reduction in cardiac baroreflex sensitivity and a shift in the baroreflex working range to higher heart rates. This effect is mediated, in part, by the carotid chemoreceptors. Therefore, we hypothesized hypoglycemia-mediated changes in baroreflex control of heart rate would be blunted in carotid body–resected patients when compared with healthy controls. Five patients with bilateral carotid body resection for glomus tumors and 10 healthy controls completed a 180-minute hyperinsulinemic, hypoglycemic (≈3.3 mmol/L) clamp. Changes in heart rate, blood pressure, and spontaneous cardiac baroreflex sensitivity were assessed. Baseline baroreflex sensitivity was not different between groups ( P >0.05). Hypoglycemia resulted in a reduction in baroreflex sensitivity in both the groups (main effect of time, P <0.01) and responses were lower in resected patients when compared with controls (main effect of group, P <0.05). Hypoglycemia resulted in large reductions in systolic (−17±7 mm Hg) and mean (−14±5 mm Hg) blood pressure in resected patients that were not observed in controls (interaction of group and time, P <0.05). Despite lower blood pressures, increases in heart rate with hypoglycemia were blunted in resected patients (interaction of group and time, P <0.01). Major novel findings from this study demonstrate that intact carotid chemoreceptors are essential for increasing heart rate and maintaining arterial blood pressure during hypoglycemia in humans. These data support a contribution of the carotid chemoreceptors to blood pressure control and highlight the potential widespread effects of carotid body resection in humans.

New Findings What is the central question of this study? Hyperoxia blunts hypoglycaemia counterregulation in healthy adults. We hypothesized that this effect is mediated by the carotid bodies and that: (i) hyperoxia would have no effect on hypoglycaemia counterregulation in carotid body‐resected patients; and (ii) carotid body‐resected patients would exhibit an impaired counterregulatory response to hypoglycaemia. What is the main finding and its importance? Our data indicate that the effect of hyperoxia on hypoglycaemic counterregulation is mediated by the carotid bodies. However, a relatively normal counterregulatory response to hypoglycaemia in carotid body‐resected patients highlights: (i) the potential for long‐term adaptations after carotid body resection; and (ii) the importance of redundant mechanisms in mediating hypoglycaemia counterregulation. Hyperoxia reduces hypoglycaemia counterregulation in healthy adults. We hypothesized that this effect is mediated by the carotid bodies and that: (i) hyperoxia would have no effect on hypoglycaemia counterregulation in patients with bilateral carotid body resection; and (ii) carotid body‐resected patients would exhibit an impaired counterregulatory response to hypoglycaemia. Five patients (three male and two female) with bilateral carotid body resection for glomus tumours underwent two 180 min hyperinsulinaemic, hypoglycaemic (∼3.3 mmol l−1) clamps separated by a minimum of 1 week and randomized to either normoxia (21% fractional inspired O2) or hyperoxia (100% fractional inspired O2). Ten healthy adults (seven male and three female) served as control subjects. Hypoglycaemia counterregulation in carotid body‐resected patients was not significantly altered by hyperoxia (area under the curve expressed as a percentage of the normoxic response: glucose infusion rate, 111 ± 10%; cortisol, 94 ± 6%; glucagon, 107 ± 7%; growth hormone, 92 ± 10%; adrenaline, 89 ± 26%; noradrenaline, 79 ± 15%; main effect of condition, P > 0.05). This is in contrast to previously published results from healthy adults. However, the counterregulatory responses to hypoglycaemia during normoxia were not impaired in carotid body‐resected patients when compared with control subjects (main effect of group, P > 0.05). Our data provide further corroborative evidence that the effect of hyperoxia on hypoglycaemic counterregulation is mediated by the carotid bodies. However, relatively normal counterregulatory responses to hypoglycaemia in carotid body‐resected patients highlight the importance of redundant mechanisms in mediating hypoglycaemia counterregulation.

Background and Objective: The premise of effective closed-loop insulin therapy for type 1 diabetes (T1D) relies on the accuracy of continuous interstitial fluid glucose sensing that represents the crucial afferent arm of such a system. An important determinant of sensor accuracy is the physiological time lag of glucose transport from the vascular to the interstitial space. The purpose of current studies was to determine the physiological time lag of glucose transport from the vascular to the abdominal subcutaneous interstitial space in T1D. Method: Four microdialysis catheters were inserted into the abdominal subcutaneous space in 6 T1D subjects under overnight fasted conditions. Plasma glucose was maintained at 113.7 ± 6.3 mg/dl using a continuous intravenous insulin infusion. After sequential intravenous bolus administrations of glucose isotopes, timed plasma and interstitial fluid samples were collected chronologically and analyzed for tracer enrichments. Results: We observed a median (range) time lag of tracer appearance (time to detection) into the interstitial space after intravenous bolus of 6.8 (4.8-9.8) minutes, with all participants having detectable values by 9.8 minutes. Conclusions: We conclude that in the overnight fasted state in T1D adults, the delay of glucose appearance from the vascular to the interstitial space is less than 10 minutes, thereby implying that this minimal physiological time lag should not be a major impediment to the development of an effective closed-loop control system for T1D.

New Findings What is the central question of this study? Activation of the carotid body chemoreceptors with hypoxia alters baroreceptor‐mediated responses in humans. We aimed to examine whether this relationship can be translated to other chemoreceptor stimuli (i.e. hypoglycaemia). What is the main finding and its importance? We show that hypoglycaemia‐mediated changes in heart rate variability and baroreflex sensitivity cannot be attributed exclusively to the carotid chemoreceptors; however, the chemoreceptors play a role in resetting the baroreflex working range during hypoglycaemia. These results provide a potential mechanism for impaired glycaemic control and increased risk of cardiac arrhythmias in patients with carotid chemoreceptor overactivity (i.e. sleep apnoea). Activation of the carotid body chemoreceptors with hypoxia alters baroreceptor‐mediated responses. We aimed to examine whether this relationship can be translated to other chemoreceptor stimuli (i.e. hypoglycaemia) by testing the following hypotheses: (i) activation of the carotid body chemoreceptors with hypoglycaemia would reduce spontaneous cardiac baroreflex sensitivity (sCBRS) in healthy humans; and (ii) desensitization of the carotid chemoreceptors with hyperoxia would restore sCBRS to baseline levels during hypoglycaemia. Ten young healthy adults completed two 180 min hyperinsulinaemic [2 mU (kg fat‐free mass)−1 min−1], hypoglycaemic (∼3.2 μmol ml−1) clamps, separated by at least 1 week and randomized to normoxia (arterial partial pressure of O2, 122 ± 10 mmHg) or hyperoxia (arterial partial pressure of O2, 424 ± 123 mmHg; to blunt activation of the carotid body glomus cells). Changes in heart rate, blood pressure, plasma catecholamines, heart rate variability (HRV) and sCBRS were assessed. During hypoglycaemia, HRV and sCBRS were reduced (P < 0.05) and the baroreflex working range was shifted to higher heart rates. When hyperoxia was superimposed on hypoglycaemia, there was a greater reduction in blood pressure and a blunted rise in heart rate when compared with normoxic conditions (P < 0.05); however, there was no detectable effect of hyperoxia on sCBRS or HRV during hypoglycaemia (P > 0.05). In summary, hypoglycaemia‐mediated changes in HRV and sCBRS cannot be attributed exclusively to the carotid chemoreceptors; however, the chemoreceptors appear to play a role in resetting the baroreflex working range during hypoglycaemia.

The accuracy of continuous interstitial fluid (ISF) glucose sensing is an essential component of current and emerging open- and closed-loop systems for type 1 diabetes. An important determinant of sensor accuracy is the physiological time lag of glucose transport from the vascular to the interstitial space. We performed the first direct measurement of this phenomenon to our knowledge in eight healthy subjects under an overnight fasted condition. Microdialysis catheters were inserted into the abdominal subcutaneous space. After intravenous bolus administrations of glucose tracers, timed samples of plasma and ISF were collected sequentially and analyzed for tracer enrichments. After accounting for catheter dead space and assay noise, the mean time lag of tracer appearance in the interstitial space was 5.3–6.2 min. We conclude that in the overnight fasted state in healthy adults, the physiological delay of glucose transport from the vascular to the interstitial space is 5–6 min. Physiological delay between blood glucose and ISF glucose, therefore, should not be an obstacle to sensor accuracy in overnight or fasting-state closed-loop systems of insulin delivery or open-loop therapy assessment for type 1 diabetes.

Transaldolase (TA) exchange overestimates gluconeogenesis measured with deuterated water (2H2O). However, it is unknown whether TA differs in people with type 2 diabetes (T2DM).2H2O was ingested, and [1-13C]acetate and [3-3H]glucose were infused in T2DM ( n = 10) and healthy nondiabetic (ND; n = 8) subjects. TA was assessed from the ratio of13C3 to13C4 glucose enrichment (13C3/13C4) measured by13C NMR. Glucose turnover was measured before (∼16-h fast) and during hyperglycemic (∼10 mM) moderate-dose insulin (∼0.35 mU·kg−1·min−1) clamp.13C3/13C4 in T2DM vs. ND was <1.0 and not different at baseline and clamp, indicating equivalent TA. To determine whether incomplete triose phosphate isomerase (TPI) exchange contributed to asymmetric13C3/13C4, [U-13C]glycerol was infused in lieu of [1-13C]acetate during a separate visit in a subset of ND ( n = 7) subjects. Ratio of13C3/13C4 obtained following either tracer was <1.0 at baseline and during clamp, indicating that TPI exchange was essentially complete and did not contribute to asymmetric glucose enrichment. Uncorrected and corrected rates of gluconeogenesis were no different ( P = not significant) in T2DM vs. ND both at baseline and during clamp. TA correction resulted in equivalent estimates of corrected gluconeogenesis in T2DM and ND that were ∼25–35% lower than uncorrected gluconeogenesis both at baseline and during the clamp. The asymmetric enrichment of glucose from13C-gluconeogenic tracers is attributable to TA exchange and can be utilized to correct for TA exchange. In conclusion, TA exchange does not differ between T2DM and ND under fasting or hyperglycemic clamp conditions, and the2H2O method continues to provide an accurate estimation of gluconeogenesis.

Abstract Carbohydrate metabolism in humans is regulated by insulin secretion from pancreatic β‐cells and glucose disposal by insulin‐sensitive tissues. Insulin facilitates glucose utilization in peripheral tissues and suppresses hepatic glucose production. Any defects in insulin action predispose an individual to glucose intolerance and Type 2 diabetes mellitus. Early detection of defects in insulin action could provide opportunities to prevent or delay progression of the disease state. There are different approaches to assess insulin action. Initial methods, such as peripheral insulin concentration and simple indices, have several limitations. Subsequently, researchers developed methodologies using intravenous glucose infusion to determine glucose fluxes. However, these methodologies are limited by being non‐physiological. Newer, innovative techniques that have been developed are more sophisticated and physiological. By modelling glucose kinetics using isotope dilution techniques, several robust parameters can be obtained that are physiologically relevant and sound. This brief review summarizes most of the non‐physiological and physiological methodologies used to measure the variables of insulin action.