Impact of Free Fatty Acids on Vascular Insulin Responses Across the Arterial Tree: A Randomized Crossover Study

CONTEXT

Vascular insulin resistance is commonly observed in obesity and diabetes; yet, insulin action across the vascular tree and the relationship between insulin responses at different vascular locations remains incompletely defined.

OBJECTIVE

To elucidate the impact of elevated free fatty acids (FFAs) on insulin action across the arterial tree and define the relationship among insulin actions in the different arterial segments.

METHODS

This randomized crossover study assigned healthy lean adults to 2 separate admissions with euglycemic insulin clamp superimposed for the final 120 minutes of 5-hour lipid or matched-volume saline infusion. Vascular measures including peripheral and central arterial blood pressure, brachial artery flow-mediated dilation (FMD), carotid femoral pulse wave velocity (cfPWV), augmentation index (AIx), pulse wave separation analysis, subendocardial viability ratio (SEVR), and skeletal and cardiac muscle microvascular perfusion were determined before and after insulin clamp. Insulin-mediated whole body glucose disposal was calculated.

RESULTS

Insulin enhanced FMD, AIx, reflection magnitude, and cardiac and skeletal muscle microvascular perfusion. Elevation of plasma FFA concentrations to the levels seen in the postabsorptive state in people with insulin resistance suppressed SEVR, blunted insulin-induced increases in FMD and cardiac and skeletal muscle microvascular blood volume, and lowered insulin's ability to reduce AIx and reflection magnitude. In multivariate regression, insulin-mediated muscle microvascular perfusion was independently associated with insulin-mediated FMD and cfPWV.

CONCLUSION

Clinically relevant elevation of plasma FFA concentrations induces pan-arterial insulin resistance, the vascular insulin resistance outcomes are interconnected, and insulin-mediated muscle microvascular perfusion associates with cardiovascular disease predictors. Our data provide biologic plausibility whereby a causative relationship between FFAs and cardiovascular disease could exist, and suggest that further attention to interventions that block FFA-mediated vascular insulin resistance may be warranted.

Empagliflozin improves vascular insulin sensitivity and muscle perfusion in persons with type 2 diabetes

Sodium glucose cotransporter 2 inhibitors (SGLT2is) improved major adverse cardiovascular events (MACE), heart failure, and renal outcomes in large trials; however, a thorough understanding of the vascular physiological changes contributing to these responses is lacking. We hypothesized that SGLT2i therapy would diminish vascular insulin resistance and improve hemodynamic function, which could improve clinical outcomes. To test this, we treated 11 persons with type 2 diabetes for 12 wk with 10 mg/day empagliflozin and measured vascular stiffness, endothelial function, peripheral and central arterial pressures, skeletal and cardiac muscle perfusion, and vascular biomarkers before and at 120 min of a euglycemic hyperinsulinemic clamp at weeks 0 and 12. We found that before empagliflozin treatment, insulin infusion lowered peripheral and central aortic systolic pressure (P < 0.05) and muscle microvascular blood flow (P < 0.01), but showed no effect on other vascular measures. Following empagliflozin, insulin infusion improved endothelial function (P = 0.02), lowered peripheral and aortic systolic (each P < 0.01), diastolic (each P < 0.05), mean arterial (each P < 0.01), and pulse pressures (each P < 0.02), altered endothelial biomarker expression, and decreased radial artery forward and backward pressure amplitude (each P = 0.02). Empagliflozin also improved insulin-mediated skeletal and cardiac muscle microvascular perfusion (each P < 0.05). We conclude that empagliflozin enhances insulin's vascular actions, which could contribute to the improved cardiorenal outcomes seen with SGLT2i therapy.NEW & NOTEWORTHY The physiological underpinnings of the cardiovascular benefits of SGLT2 inhibitors remain uncertain. We tested whether empagliflozin mitigates vascular insulin resistance in patients with type 2 diabetes. Aortic and peripheral systolic, diastolic, mean and pulse pressures, endothelial function, vascular stiffness, and heart and muscle microvascular perfusion were measured before and during an insulin infusion at baseline and after 12 wk of empagliflozin. After empagliflozin, vascular responses to insulin improved dramatically.

Insulin-induced vasoconstriction is prevalent in muscle microvasculature of otherwise healthy persons with type 1 diabetes

Insulin's microvascular actions and their relationship to insulin's metabolic actions have not been well studied in adults with type 1 diabetes mellitus (T1DM). We compared the metabolic and selected micro- and macrovascular responses to insulin by healthy adult control (n = 16) and subjects with T1DM (n = 15) without clinical microvascular disease. We measured insulin's effect on 1) skeletal muscle microvascular perfusion using contrast-enhanced ultrasound (CEU), 2) arterial stiffness using carotid-femoral pulse-wave velocity (cfPWV) and radial artery pulse wave analysis (PWA), and 3) metabolic insulin sensitivity by the glucose infusion rate (GIR) during a 2-h, 1 mU/min/kg euglycemic-insulin clamp. Subjects with T1DM were metabolically insulin resistant (GIR = 5.2 ± 0.7 vs. 6.6 ± 0.6 mg/min/kg, P < 0.001). Insulin increased muscle microvascular blood volume and flow in control (P < 0.001, for each) but not in subjects with T1DM. Metabolic insulin sensitivity correlated with increases of muscle microvascular perfused volume (P < 0.05). Baseline measures of vascular stiffness did not differ between groups. However, during hyperinsulinemia, cfPWV was greater (P < 0.02) in the T1DM group and the backward pulse wave pressure declined with insulin only in controls (P < 0.03), both indices indicating that insulin-induced vascular relaxation in controls only. Subjects with T1DM have muscle microvascular insulin resistance that may precede clinical microvascular disease.NEW & NOTEWORTHY Using contrast ultrasound and measures of vascular stiffness, we compared vascular and metabolic responses to insulin in patients with type 1 diabetes with age-matched controls. The patients with type 1 diabetes demonstrated both vascular and metabolic insulin resistance with more than half of the patients with diabetes having a paradoxical vasoconstrictive vascular response to insulin.

Nitric oxide-dependent micro- and macrovascular dysfunction occurs early in adolescents with type 1 diabetes

Arterial stiffness and endothelial dysfunction are both reported in children with type 1 diabetes (DM1) and may predict future cardiovascular events. In health, nitric oxide (NO) relaxes arteries and increases microvascular perfusion. The relationships between NO-dependent macro- and microvascular functional responses and arterial stiffness have not been studied in adolescents with DM1. Here, we assessed macro- and microvascular function in DM1 adolescents and age-matched controls at baseline and during an oral glucose challenge (OGTT). DM1 adolescents (n = 16) and controls (n = 14) were studied before and during an OGTT. At baseline, we measured: 1) large artery stiffness using both aortic augmentation index (AI) and carotid-femoral pulse wave velocity (cfPWV); 2) brachial flow-mediated dilation (FMD) and forearm endothelial function using postischemic flow velocity (PIFV); and 3) forearm muscle microvascular blood volume (MBV) using contrast-enhanced ultrasound. Following OGTT, AI, cfPWV, and MBV were reassessed at 60 min and MBV again at 120 min. Within individual and between-group, comparisons were made by paired and unpaired t tests or repeated measures ANOVA. Baseline FMD was lower (P = 0.02) in DM1. PWV at 0 and 60 min did not differ between groups. Baseline AI did not differ between groups but declined with OGTT only in controls (P = 0.02) and was lower than DM1 at 60 min (P < 0.03). Baseline MBV was comparable in DM1 and control groups, but declined in DM1 at 120 min (P = 0.01) and was lower than the control group (P < 0.03). There was an inverse correlation between plasma glucose and MBV at 120 min (r = -0.523, P < 0.01). No differences were noted between groups for V̇O2max (mL/min/kg), body fat (%), or body mass index (BMI). NO-dependent macro- and microvascular function, including FMD and AI, and microvascular perfusion, respectively, are impaired early in the course of DM1, precede increases of arterial stiffness, and may provide an early indicator of vascular risk.NEW & NOTEWORTHY This is the first study to show that type 1 diabetes impairs multiple nitric oxide-dependent vascular functions.

Predictors of arterial stiffness in adolescents and adults with type 1 diabetes: a cross-sectional study

INTRODUCTION

Individuals with type 1 diabetes have increased arterial stiffness compared with age-matched healthy controls. Our aim was to determine which hemodynamic and demographic factors predict arterial stiffness in this population.

RESEARCH DESIGN AND METHODS

Carotid-femoral pulse wave velocity (cfPWV) was examined in 41 young adults and adolescents with type 1 diabetes without microvascular complications. Two ordinary least squares regression analyses were performed to determine multivariate relationships between cfPWV (loge) and (1) age, duration of diabetes, sex, and hemoglobin A1c and (2) augmentation index (AIx), mean arterial pressure, flow-mediated dilation (FMD), and heart rate. We also examined differences in macrovascular outcome measures between sexes.

RESULTS

Age, sex, and FMD provided unique predictive information about cfPWV in these participants with type 1 diabetes. Despite having similar cardiovascular risk factors, men had higher cfPWV compared with women but no differences were observed in other macrovascular outcomes (including FMD and AIx).

CONCLUSIONS

Only age, sex, and FMD were uniquely associated with arterial stiffness in adolescents and adults with uncomplicated type 1 diabetes. Women had less arterial stiffness and similar nitric oxide-dependent endothelial function compared with men. Larger, prospective investigation is warranted to determine the temporal order of and sex differences in arterial dysfunction in type 1 diabetes.

Insulin-mediated muscle microvascular perfusion and its phenotypic predictors in humans

Insulin increases muscle microvascular perfusion and enhances tissue insulin and nutrient delivery. Our aim was to determine phenotypic traits that foretell human muscle microvascular insulin responses. Hyperinsulinemic euglycemic clamps were performed in 97 adult humans who were lean and healthy, had class 1 obesity without comorbidities, or controlled type 1 diabetes without complications. Insulin-mediated whole-body glucose disposal rates (M-value) and insulin-induced changes in muscle microvascular blood volume (ΔMBV) were determined. Univariate and multivariate analyses were conducted to examine bivariate and multivariate relationships between outcomes, ΔMBV and M-value, and predictor variables, body mass index (BMI), total body weight (WT), percent body fat (BF), lean body mass, blood pressure, maximum consumption of oxygen (VO2max), plasma LDL (LDL-C) and HDL cholesterol, triglycerides (TG), and fasting insulin (INS) levels. Among all factors, only M-value (r = 0.23, p = 0.02) and VO2max (r = 0.20, p = 0.047) correlated with ΔMBV. Conversely, INS (r = - 0.48, p ≤ 0.0001), BF (r = - 0.54, p ≤ 0.001), VO2max (r = 0.5, p ≤ 0.001), BMI (r = - 0.40, p < 0.001), WT (r = - 0.33, p = 0.001), LDL-C (r = - 0.26, p = 0.009), TG (r = - 0.25, p = 0.012) correlated with M-value. While both ΔMBV (p = 0.045) and TG (p = 0.03) provided significant predictive information about M-value in the multivariate regression model, only M-value was uniquely predictive of ΔMBV (p = 0.045). Thus, both M-value and VO2max correlated with ΔMBV but only M-value provided unique predictive information about ΔMBV. This suggests that metabolic and microvascular insulin responses are important predictors of one another, but most metabolic insulin resistance predictors do not predict microvascular insulin responses.

Insulin increases central aortic stiffness in response to hyperglycemia in healthy humans: A randomized four-arm study

INTRODUCTION

Increasing arterial stiffness is a feature of vascular aging that is accelerated by conditions that enhance cardiovascular risk, including diabetes mellitus. Multiple studies demonstrate divergence of carotid-femoral pulse wave velocity and augmentation index in persons with diabetes mellitus, though mechanisms responsible for this are unclear.

MATERIALS AND METHODS

We tested the effect of acutely and independently increasing plasma glucose, plasma insulin, or both on hemodynamic function and markers of arterial stiffness (including carotid-femoral pulse wave velocity, augmentation index, forward and backward wave reflection amplitude, and wave reflection magnitude) in a four-arm, randomized study of healthy young adults.

RESULTS

Carotid-femoral pulse wave velocity increased only during hyperglycemic-hyperinsulinemia (+0.36 m/s; p = 0.032), while other markers of arterial stiffness did not change (all p > 0.05). Heart rate (+3.62 bpm; p = 0.009), mean arterial pressure (+4.14 mmHg; p = 0.033), central diastolic blood pressure (+4.16 mmHg; p = 0.038), and peripheral diastolic blood pressure (+4.09 mmHg; p = 0.044) also significantly increased during hyperglycemic-hyperinsulinemia.

CONCLUSIONS

Hyperglycemic-hyperinsulinemia acutely increased cfPWV, heart rate, mean arterial pressure, and diastolic blood pressure in healthy humans, perhaps reflecting enhanced sympathetic tone. Whether repeated bouts of hyperglycemia with hyperinsulinemia contribute to chronically-enhanced arterial stiffness remains unknown.

Acute hyperglycaemia enhances both vascular endothelial function and cardiac and skeletal muscle microvascular function in healthy humans

KEY POINTS

Multiple clinical studies report that acute hyperglycaemia (induced by mixed meal or oral glucose) decreases arterial vascular function in healthy humans. Feeding, however, impacts autonomic output, blood pressure, and insulin and incretin secretion, which may themselves alter vascular function. No prior studies have examined the effect of acute hyperglycaemia on both macro- and microvascular function while controlling plasma insulin concentrations. Macrovascular and microvascular functional responses to euglycaemia and hyperglycaemia were compared. Octreotide was infused throughout both protocols to prevent endogenous insulin release. Acute hyperglycaemia (induced by intravenous glucose) enhanced brachial artery flow-mediated dilatation, increased skeletal muscle microvascular blood volume and flow, and expanded cardiac muscle microvascular blood volume. Compared to other published findings, the results suggest that vascular responses to acute hyperglycaemia differ based on the study population (i.e. normal weight vs. overweight/obese) and/or glucose delivery method (i.e. intravenous vs. oral glucose).

ABSTRACT

High glucose concentrations acutely provoke endothelial cell oxidative stress and are suggested to trigger diabetes-related macro- and microvascular injury in humans. Multiple clinical studies report that acute hyperglycaemia (induced by mixed meal or oral glucose) decreases arterial vascular function in healthy humans. Feeding, however, impacts autonomic output, blood pressure, and insulin and incretin secretion, which may each independently alter vascular function and obscure the effect of acute hyperglycaemia per se. Surprisingly, no studies have examined the acute effects of intravenous glucose-induced hyperglycaemia on both macro- and microvascular function while controlling plasma insulin concentrations. In this randomized study of healthy young adults, we compared macrovascular (i.e. brachial artery flow-mediated dilatation, carotid-femoral pulse wave velocity and post-ischaemic brachial artery flow velocity) and microvascular (heart and skeletal muscle perfusion by contrast-enhanced ultrasound) functional responses to euglycaemia and hyperglycaemia. Octreotide was infused throughout both protocols to prevent endogenous insulin release. Acute intravenous glucose-induced hyperglycaemia enhanced brachial artery flow-mediated dilatation (P = 0.004), increased skeletal muscle microvascular blood volume and flow (P = 0.001), and expanded cardiac muscle microvascular blood volume (P = 0.014). No measure of vascular function changed during octreotide-maintained euglycaemia. Our findings suggest that unlike meal-provoked acute hyperglycaemia, 4 h of intravenous glucose-induced hyperglycaemia enhances brachial artery flow-mediated dilatation, provokes cardiac and skeletal muscle microvascular function, and does not impair aortic stiffness. Previous findings of acute large artery vascular dysfunction during oral glucose or mixed meal ingestion may be due to differences in study populations and meal-induced humoral or neural factors beyond hyperglycaemia per se. (ClinicalTrials.gov number NCT03520569.).

Hyperglycemia does not Inhibit Insulin's Effects on Microvascular Perfusion in Healthy Humans: A Randomized Crossover Study

Diabetes mellitus accelerates vascular disease through multiple biochemical pathways driven by hyperglycemia, with insulin resistance and/or hyperinsulinemia also contributing. Persons with diabetes mellitus experience premature large vessel and microvascular disease when compared to normoglycemic controls. Currently there is a paucity of clinical data identifying how acutely the vasculature responds to hyperglycemia and whether other physiologic factors (e.g., vasoactive hormones) contribute. To our knowledge, no prior studies have examined the dynamic effects of acute hyperglycemia on insulin-mediated actions on both micro- and macrovascular function in the same subjects. In this randomized crossover trial, healthy young adults underwent two infusion protocols designed to compare the effects of insulin infusion during euglycemia and hyperglycemia on micro- and macrovascular function. Both euglycemic- and hyperglycemic-hyperinsulinemia increased skeletal (but not cardiac) muscle microvascular blood volume (each p<0.02) and blood flow significantly (each p<0.04), and these increases did not differ between protocols. Hyperglycemic-hyperinsulinemia trended towards increased carotid-femoral pulse wave velocity (indicating increased aortic stiffness; p= 0.065 after Bonferroni adjustment), while euglycemic-hyperinsulinemia did not. There were no changes in post-ischemic flow velocity or brachial artery flow-mediated dilation during either protocol. Plasma endothelin-1 levels significantly decreased during both protocols (each p<0.02). In this study, acute hyperglycemia for 4 hours did not inhibit insulin's ability to increase skeletal muscle microvascular perfusion but did provoke a slight increase in aortic stiffness. Hyperglycemia also did not adversely affect myocardial microvascular perfusion or endothelial function or prevent the decline of endothelin-1 during insulin infusion.