Insulin differentially regulates systemic and skeletal muscle vascular resistance

Are strategies to increase muscle mass and strength as effective in people with type 2 diabetes?

People with type 2 diabetes (T2D) have a 2-3-time higher risk of developing sarcopenia, a musculoskeletal disease marked by a progressive loss of skeletal muscle mass and strength, compared to people without T2D. This narrative review examines the effectiveness of lifestyle interventions in enhancing muscle mass and strength in people with T2D, emphasizing their growing importance with advancements in obesity treatments. PubMed and Google Scholar were utilized to identify the most relevant published studies based on the authors' knowledge. The maintenance of skeletal muscle strength and mass in people with T2D is becoming more prominent due to the advent of weight loss therapies such as low-energy diets, bariatric surgery and pharmacotherapies. Although the weight loss is to be commended, a large proportion (20-50%) of the weight loss comes from lean mass, indicative of a loss in muscle mass. There are currently no pharmacotherapies to increase, or mitigate the loss of, lean mass, with lifestyle strategies prominent in this arena. Resistance exercise is the most effective method to increase muscle mass and strength in people with T2D, but there is some evidence of an anabolic resistance. Aerobic exercise and increased dietary protein intake may result in small increases in muscle mass and strength, with no evidence of an anabolic resistance to these stimuli. Exercise and protein supplementation can increase, or aid in the retention of, muscle strength and mass in individuals with T2D, but further research is needed to explore their benefits in patients undergoing concomitant pharmaceutical and surgical treatments.

Elevated blood flow in people with type 1 and type 2 diabetes

AIMS

The study aimed to evaluate blood flow (BF) and microvascular function in the forearm of people with type 1 and type 2 diabetes at rest and after ischemia. Microvascular function plays a crucial role in regulating BF in peripheral tissues based on metabolic demand.

METHODS

People with diabetes and sex-matched healthy controls were recruited. Brachial artery diameter and blood velocity were continuously measured at rest and after ischemia by an automatic tracking system. BF and vascular conductance were then calculated.

RESULTS

Forty-nine people with diabetes and 49 controls were enrolled. BF at rest and after ischemia was significantly higher in people with diabetes than controls: Type 1, 243 ± 116 and 631 ± 233 ml/min; controls, 180 ± 106 and 486 ± 227 ml/min; Type 2, 332 ± 149 and 875 ± 293 ml/min; controls 222 ± 106 and 514 ± 224 ml/min. Vascular conductance was significantly higher in Type 2 than in controls at rest and after ischemia.

CONCLUSIONS

People with diabetes exhibited significantly increased BF, with Type 2 also showing heightened vascular conductance. Activating metabolic pathways triggered by hyperglycemia may lead to distinct vascular redistribution, potentially impairing blood flow over time. These findings of the study underscore the importance of understanding overall vascular dynamics in diabetes and its implications for vascular health.

The impact of type 2 diabetes duration on serum asymmetric dimethylarginine and C-reactive protein concentration in Bosnian patients

Objective. The present study assessed the impact of type 2 diabetes mellitus (T2DM) duration on the serum asymmetric dimethylarginine (ADMA) and C-reactive protein (CRP) concentration in Bosnian patients.

Methods. Participants for this cross-sectional study were randomly selected from the Family Medicine Clinic (Sarajevo, Bosnia and Herzegovina). Serum ADMA concentration was determined by ELISA. Serum high-sensitivity (hs-CRP) was determined by particle-enhanced immunonephelometry. ANOVA test followed by Scheffe post-hoc test or Kruskal-Wallis test followed by Man-Whitney test were used for statistical analysis.

Results. The study included 38 patients in up to 10 years diabetes duration (≤10 years T2DM) group, 22 patients in greater than 10 years diabetes duration (>10 years T2DM) group, and 60 controls. Serum ADMA concentration in the >10 years T2DM group (1.81±0.15 μmol/L) was significantly higher compared to serum ADMA concentration in the ≤10 years T2DM group (1.38±0.41 μmol/L; p<0.001) and in controls (0.62±0.15 μmol/L; p<0.001). A significant difference in serum ADMA concentration was found between the <10 years T2DM group and the controls (p<0.001). The serum CRP concentration in the >10 years T2DM group [5.95 (4.20–9.12) mg/L] was significantly higher compared to serum CRP concentration in the <10 years T2DM group [2.35 (1.40–4.30) mg/L; p<0.001] and controls [0.85 (0.50–1.30) mg/L; p<0.001]. Significant difference in serum CRP concentration was observed between the <10 years T2DM group and controls (p<0.001).

Conclusions. The present study showed an increase in the serum ADMA and CRP concentrations with the advancement of T2DM. These results suggest that ADMA and CRP may serve as indicators of endothelial dysfunction and chronic low-grade inflammation progression in patients with T2DM. Larger prospective studies are required to confirm the observed findings.

Sucrose dampens caffeine-induced blood pressure elevations - A randomized crossover pilot study in healthy, non-obese men

Purpose

Sales for sugar-sweetened and caffeinated beverages are still rising globally and their consumption has been linked to the development of cardiovascular diseases. However, direct evidence from human interventional studies in response to such beverages is still scarce.

Methods

Seven young, non-obese men participated in a randomized crossover study where four test drinks [60 g sucrose + 50 mg caffeine, 60 g sucrose + caffeine-placebo, 50 mg caffeine, and caffeine-placebo] were investigated. Each drink was brought to a total volume of 500 mL with water. Continuous and beat-to-beat hemodynamic monitoring was conducted for 30 min baseline and continued for 90 min after the ingestion of each drink. Measurements included blood pressure, heart rate, stroke volume, cardiac output, total peripheral resistance, index of contractility, and double product.

Results

Two-factor ANOVA analysis revealed significant treatment-by-time effects for diastolic blood pressure, heart rate, stroke volume, cardiac output, total peripheral resistance, index of contractility, and double product (all p < 0.01). Diastolic blood pressure and total peripheral resistance increased significantly to caffeine-only (all p < 0.05), while sucrose + caffeine-placebo and sucrose + caffeine both decreased resistance responses (all p < 0.05). Cardiac output increased significantly to sucrose + caffeine-placebo and sucrose + caffeine (all p < 0.05), and on trend for heart rate, stroke volume, and index of contractility (all p between 0.05 and 0.09).

Conclusion

In young, non-obese men, a caffeinated and sucrose-sweetened beverage at concentrations similar to classical commercial Cola products exhibited distinct hemodynamic actions where the presence of sucrose dampened caffeine-induced blood pressure elevations, but at the expense of a tendency to increase cardiac work.

Oral and intravenous glucose administration elicit opposing microvascular blood flow responses in skeletal muscle of healthy people: role of incretins

Abstract

Insulin infusion increases skeletal muscle microvascular blood flow (MBF) in healthy people but is impaired during insulin resistance. However, we have shown that eliciting insulin secretion via oral glucose loading in healthy people impairs muscle MBF, whilst others have demonstrated intravenous glucose infusion stimulates MBF. We aimed to show that the route of glucose administration (oral versus intravenous) influences muscle MBF, and explore potential gut‐derived hormones that may explain these divergent responses. Ten healthy individuals underwent a 120 min oral glucose tolerance test (OGTT; 75 g glucose) and on a subsequent occasion an intravenous glucose tolerance test (IVGTT, bypassing the gut) matched for similar blood glucose excursions. Femoral artery and thigh muscle microvascular (contrast‐enhanced ultrasound) haemodynamics were measured at baseline and during the OGTT/IVGTT. Plasma insulin, C‐peptide, glucagon, non‐esterified fatty acids and a range of gut‐derived hormones and incretins (gastric inhibitory polypeptide (GIP) and glucagon‐like peptide‐1(GLP‐1)) were measured at baseline and throughout the OGTT/IVGTT. The IVGTT increased whereas the OGTT impaired MBF (1.3‐fold versus 0.5‐fold from baseline, respectively, P = 0.0006). The impairment in MBF during the OGTT occurred despite producing 2.8‐fold higher plasma insulin concentrations (P = 0.0001). The change in MBF from baseline (ΔMBF) negatively correlated with ΔGIP concentrations (r = −0.665, P < 0.0001). The natural log ratio of incretins GLP‐1:GIP was positively associated with ΔMBF (r = 0.658, P < 0.0001), suggesting they have opposing actions on the microvasculature. Postprandial hyperglycaemia per se does not acutely determine opposing microvascular responses between OGTT and IVGTT. Incretins may play a role in modulating skeletal muscle MBF in humans.

Key points

Insulin or mixed nutrient meals stimulate skeletal muscle microvascular blood flow (MBF) to aid in the delivery of nutrients; however, this vascular effect is lost during insulin resistance.

Food/drinks containing large glucose loads impair MBF in healthy people; however, this impairment is not observed when glucose is infused intravenously (bypassing the gut).

We investigated skeletal muscle MBF responses to a 75 g oral glucose tolerance test and intravenous glucose infusion and aimed to identify potential gut hormones responsible for glucose‐mediated changes in MBF.

Despite similar blood glucose concentrations, orally ingested glucose impaired, whereas intravenously infused glucose augmented, skeletal muscle MBF. The incretin gastric inhibitory polypeptide was negatively associated with MBF, suggestive of an incretin‐mediated MBF response to oral glucose ingestion.

This work provides new insight into why diets high in glucose may be detrimental to vascular health and provides new avenues for novel treatment strategies targeting microvascular dysfunction.

Cocoa Flavanols Adjuvant to an Oral Nutritional Supplement Acutely Enhances Nutritive Flow in Skeletal Muscle without Altering Leg Glucose Uptake Kinetics in Older Adults

Ageing is associated with postprandial muscle vascular and metabolic dysfunction, suggesting vascular modifying interventions may be of benefit. Reflecting this, we investigated the impact of acute cocoa flavanol (450-500 mg) intake (versus placebo control) on vascular (via ultrasound) and glucose/insulin metabolic responses (via arterialised/venous blood samples and ELISA) to an oral nutritional supplement (ONS) in twelve healthy older adults (50% male, 72 ± 4 years), in a crossover design study. The cocoa condition displayed significant increases in m. vastus lateralis microvascular blood volume (MBV) in response to feeding at 180 and 240-min after ONS consumption (baseline: 1.00 vs. 180 min: 1.09 ± 0.03, p = 0.05; 240 min: 1.13 ± 0.04, p = 0.002), with MBV at these timepoints significantly higher than in the control condition (p < 0.05). In addition, there was a trend (p = 0.058) for MBV in m. tibialis anterior to increase in response to ONS in the cocoa condition only. Leg blood flow and vascular conductance increased, and vascular resistance decreased in response to ONS (p < 0.05), but these responses were not different between conditions (p > 0.05). Similarly, glucose uptake and insulin increased in response to ONS (p < 0.05) comparably between conditions (p > 0.05). Thus, acute cocoa flavanol supplementation can potentiate oral feeding-induced increases in MBV in older adults, but this improvement does not relay to muscle glucose uptake.

Cardiovascular responses to a glucose drink in young male individuals with overweight/obesity and mild alterations in glucose metabolism, but without impaired glucose tolerance

PURPOSE

Little is known about whether mild aberrations in glucose metabolism, which are seen in overweight/obese subjects (OW/OB) without impaired glucose tolerance, affect regulator control elements for blood pressure homeostasis.

METHODS

Hence, we measured in age-matched male subjects with normal weight (n = 16; BMI = 22.4 kg m^-2) and OW/OB (n = 11; BMI = 28.6 kg m^-2) continuous beat-to-beat blood pressure, heart rate, stroke volume, myocardial contractility and baroreflex sensitivity during a 30 min baseline and for 120 min after the ingestion of 75 g glucose dissolved in 300 mL tap water (OGTT). Blood samples for the assessment of plasma glucose and insulin were collected at baseline and every 30 min after the drink and homeostatic model assessment of insulin resistance (HOMA-IR) was calculated.

RESULTS

At baseline, glucose (5.3 ± 0.4 SD vs 5.0 ± 0.4 mmol L^-1; p = 0.01), insulin (7.4 ± 0.4 vs 3.7 ± 2.7 mU L^-1; p = 0.02) and HOMA-IR (1.8 ± 1.3 vs 0.8 ± 0.6; p = 0.01) were significantly higher in subjects with OW/OB, but none classified as having impaired glucose tolerance (plasma glucose levels < 7.8 mmol L^-1 at 120 min post-OGTT) or hypertension (all < 130/80 mmHg at baseline). In response to the glucose drink, and in comparison to subjects with normal weight, we observed in subjects with OW/OB a trend towards increased plasma insulin levels (+7445 ± 4858 vs. +4968 ± 1924 mU h L^-1; p = 0.08), which was not seen for blood glucose (p = 0.59). Moreover, subjects with OW/OB showed impaired peripheral vasodilation, diminished heart rate and myocardial contractility responses but increased peripheral pulse pressure (all p < 0.05).

CONCLUSIONS

Young male subjects with OW/OB, but without glucose intolerance or hypertension, showed attenuated peripheral vasodilation and diminished cardiac responses to a glucose drink.

Exercise capacity and cardiac function in adolescents born post-term

There is some evidence that children born post-term (≥42 weeks of gestation) have metabolic abnormalities that may be associated with an increased risk of adverse health outcomes in adulthood. However, there are no data as to whether adolescents born post-term display alterations in aerobic capacity or cardiovascular function. We studied 48 adolescents (56% males) in Auckland (New Zealand) with a mean age of 14.3 years (SD = 1.7): 25 born post-term and 23 born at term (37-41 weeks of gestation). Assessments included metabolic markers in blood, whole body DXA scans, 24-hour ambulatory blood pressure monitoring, maximal exercise capacity, as well as cardiac MRI scan at rest and during submaximal exercise. Exercise capacity was lower in the post-term than in control participants (44.5 vs 47.8 ml/kgffm/min; p = 0.04). There were no differences in left ventricular volumes at rest and during exercise between groups. The 24-hour ambulatory blood pressure monitoring also showed no differences between the two groups. Being born post-term was associated with reduced exercise capacity, but with no observed differences in central cardiac function. We speculate that the reduction in exercise capacity may be due to changes in the peripheral vascular system.

Perspective: Cardiovascular Responses to Sugar-Sweetened Beverages in Humans: A Narrative Review with Potential Hemodynamic Mechanisms

Cardiovascular diseases are still the primary cause of mortality worldwide, with high blood pressure and type 2 diabetes as major promoters. Over the past 3 decades, almost in parallel with the rise in cardiovascular disease incidence, the consumption of sugar-sweetened beverages (SSBs) has increased. In this context, SSBs are potential contributors to weight gain and increase the risk for elevations in blood pressure, type 2 diabetes, coronary heart disease, and stroke. Nevertheless, the mechanisms underlying the cardiovascular and metabolic responses to SSBs, in particular on blood pressure, are poorly understood. We discuss and propose potential mechanisms underlying differential effects of sugars on postprandial blood pressure regulation; provide evidence for additional molecular contributors, i.e., fibroblast growth factor 21, towards sugar-induced cardiovascular responses; and discuss potential cardiovascular neutral sugars. Furthermore, we explore whether pre-existing glucose intolerance in humans exacerbates the cardiovascular responses to SSBs, thus potentially aggravating the cardiovascular risk in already-susceptible individuals.

Identifying protein biomarkers in predicting disease severity of dengue virus infection using immune-related protein microarray

Dengue virus is one of the most widespread flaviviruses that re-emerged throughout recent decades. The progression from mild dengue to severe dengue (SD) with the complications such as vascular leakage and hemorrhage increases the fatality rate of dengue. The pathophysiology of SD is not entirely clear. To investigate potential biomarkers that are suggestive of pathogenesis of SD, a small panel of serum samples selected from 1 healthy individual, 2 dengue patients without warning signs (DWS-), 2 dengue patients with warning signs (DWS+), and 5 patients with SD were subjected to a pilot analysis using Sengenics Immunome protein array. The overall fold changes of protein expressions and clustering heat map revealed that PFKFB4, TPM1, PDCL3, and PTPN20A were elevated among patients with SD. Differential expression analysis identified that 29 proteins were differentially elevated greater than 2-fold in SD groups than DWS- and DWS+. From the 29 candidate proteins, pathways enrichment analysis also identified insulin signaling and cytoskeleton pathways were involved in SD, suggesting that the insulin pathway may play a pivotal role in the pathogenesis of SD.

Endothelial insulin receptors differentially control insulin signaling kinetics in peripheral tissues and brain of mice

Insulin receptors (IRs) on endothelial cells may have a role in the regulation of transport of circulating insulin to its target tissues; however, how this impacts on insulin action in vivo is unclear. Using mice with endothelial-specific inactivation of the IR gene (EndoIRKO), we find that in response to systemic insulin stimulation, loss of endothelial IRs caused delayed onset of insulin signaling in skeletal muscle, brown fat, hypothalamus, hippocampus, and prefrontal cortex but not in liver or olfactory bulb. At the level of the brain, the delay of insulin signaling was associated with decreased levels of hypothalamic proopiomelanocortin, leading to increased food intake and obesity accompanied with hyperinsulinemia and hyperleptinemia. The loss of endothelial IRs also resulted in a delay in the acute hypoglycemic effect of systemic insulin administration and impaired glucose tolerance. In high-fat diet-treated mice, knockout of the endothelial IRs accelerated development of systemic insulin resistance but not food intake and obesity. Thus, IRs on endothelial cells have an important role in transendothelial insulin delivery in vivo which differentially regulates the kinetics of insulin signaling and insulin action in peripheral target tissues and different brain regions. Loss of this function predisposes animals to systemic insulin resistance, overeating, and obesity.

Substantial Inter-Subject Variability in Blood Pressure Responses to Glucose in a Healthy, Non-obese Population

Aim: A large inter-subject variability in the blood pressure (BP) response to glucose drinks has been reported. However, the underlying factors remain elusive and we hypothesized that accompanying changes in glucose metabolism affect these BP responses.

Methods: Cardiovascular and glycemic changes in response to a standard 75 g oral-glucose-tolerance-test were investigated in 30 healthy, non-obese males. Continuous cardiovascular monitoring, including beat-to-beat BP, electrocardiographically deduced heart rate and impedance cardiography, was performed during a 30 min baseline and continued up to 120 min after glucose ingestion. Blood samples were taken at baseline, 30, 60, 90, and 120 min for the assessment of glucose, insulin and c-peptide. Additionally, we evaluated body composition by using validated bioelectrical impedance techniques.

Results: Individual overall changes (i.e., averages over 120 min) for systolic BP ranged from −4.9 to +4.7 mmHg, where increases and decreases were equally distributed (50%). Peak changes (i.e., peak averages over 10 min intervals) for systolic BP ranged from −1.3 to +9.5 mmHg, where 93% of subjects increased systolic BP above baseline values (similar for diastolic BP) whilst 63% of subjects increased peak systolic BP by more than 4 mmHg. Changes in peak systolic BP were negatively associated with the calculated Matsuda-index of insulin sensitivity (r = −0.39, p = 0.04) but with no other evaluated parameter including body composition. Moreover, besides a trend toward an association between overall changes in systolic BP and total fat mass percentage (r = +0.32, p = 0.09), no association was found between other body composition parameters and overall BP changes.

Conclusion: Substantial inter-subject variability in BP changes was observed in a healthy, non-obese subpopulation in response to an oral glucose load. In 63% of subjects, peak systolic BP increased by more than a clinically relevant 4 mmHg. Peak systolic BP changes, but not overall BP changes, correlated with insulin sensitivity, with little influence of body composition.

Postprandial Effects of Breakfast Glycemic Index on Vascular Function among Young Healthy Adults: A Crossover Clinical Trial

This study aimed to evaluate the postprandial effects of high and low glycemic index (GI) breakfasts on vascular function. It was a crossover trial that included 40 young healthy adults (50% women), aged 20-40 years, who were recruited at primary care settings. They consumed three experimental breakfasts in randomized order, each one separated by a 1-week washout period: (1) control conditions (only water); (2) low GI (LGI) breakfast (29.4 GI and 1489 KJ energy); and (3) high GI (HGI) breakfast (64.0 GI and 1318 KJ energy). Blood samples were collected at 60 and 120 min after each breakfast to determine glucose and insulin levels. Vascular parameters were measured at 15 min intervals. Augmentation index (AIx) was studied as a primary outcome. Secondary outcomes comprised glucose, insulin, heart rate (HR) and pulse pressures (PPs). We found a trend toward increased AIx, HR and PPs for the HGI versus the LGI breakfast. A significant interaction between the type of breakfast consumed and all measured parameters was identified (p < 0.05) except for central PP. Stratifying data by sex, this interaction remained significant for AIx and augmentation pressure only in males (p < 0.05). In conclusion, breakfast GI could affect postprandial vascular responses in young healthy adults.

The Higher the Insulin Resistance the Lower the Cardiac Output in Men with Type 1 Diabetes During the Maximal Exercise Test

BACKGROUND

The aim of this study was to assess the hemodynamic parameters analyzed in bioimpedance cardiography during maximal exercise in patients with type 1 diabetes differing in insulin resistance.

METHODS

The study group consisted of 40 men with type 1 diabetes. Tissue sensitivity to insulin was assessed on the basis of the glucose disposal rate (GDR) analyzed during hyperinsulinemic-euglycemic clamp. Patients were divided into groups with GDR <4.5 mg/kg/min (G1 group-lower insulin sensitivity) and GDR ≥4.5 mg/kg/min (G2 group-higher insulin sensitivity). During the exercise test, the heart rate, systolic volume, cardiac output, cardiac index were measured by the impedance meter (PhysioFlow).

RESULTS

Compared with the G2 group, the G1 group had a lower cardiac output (CO): during exercise 8.6 (IQR 7.7-10.0) versus 12.8 (IQR 10.8-13.7) L/min; P < 0.0001, at the maximal effort 13.1 (IQR 12.2-16.7) versus 18.6 (IQR 16.9-20.2) L/min; P = 0.001, and during observation after exercise 8.4 (IQR 6.3-9.6) versus 11.9 (IQR 10.1-13.1) L/min; P < 0.0001. We noticed a positive correlation of GDR and cardiac output: during the exercise test (r = 0.63, P = 0.0002), at the maximal effort (Rs 0.56, P = 0.001), and during observation after the exercise test (r = 0.72, P < 0.0001). In multivariate logistic regression, cardiac output during exercise and during observation was associated with high GDR, regardless of the age and duration of diabetes [OR: 1.98 (95% CI 1.10-3.56), P = 0.02 and OR: 1.91 (95% CI 1.05-3.48), P = 0.03; respectively].

CONCLUSION

In nonobese subjects with type 1 diabetes, with good metabolic control, insulin resistance is associated with cardiac hemodynamic parameters assessed during and after exercise. The higher the insulin resistance the lower the cardiac output during maximal exercise in men with type 1 diabetes.

Blood pressure in Warmblood horses before and during a euglycemic-hyperinsulinemic clamp

Background

Insulin resistance (IR) in humans is related to hypertension and impaired vasodilation. Insulin administration has been shown to lower blood pressure both in insulin resistant as well as in insulin sensitive individuals. The aim of the study was to investigate the association between insulin sensitivity and alterations in blood pressure in healthy horses before and after a euglycemic-hyperinsulinemic clamp (EHC). A 3-h EHC was performed in 13 healthy horses (11 mares, 2 geldings). Blood samples for measurement of plasma glucose and insulin were collected before the start of the EHC, every 10 min during the EHC and immediately after the EHC. Mean, systolic- and diastolic blood pressure was measured before and during the final 10 min of the EHC using an indirect high-definition oscillometric monitor (HDO, horse model) applied to the middle of the coccygeal artery. Five consecutive measurements were made in each horse and on each occasion. Insulin and glucose data from the EHC were used to calculate the mean rate of glucose disposal per unit of insulin during steady state (M/I ratio). Insulin resistance was defined as a M/I ratio <5 mg/kg/min/mUL (Lindåse et al. in Am J Vet Res 77:300–309, 2016).

Results

Insulin administration decreased systolic, diastolic and mean arterial pressure in all horses. The M/I ratio for all horses was negatively correlated with the decrease in systolic blood pressure (r² = 0.55, P = 0.004) and mean arterial pressure (r² = 0.31, P = 0.048) but not diastolic blood pressure (r² = 0.12, P = 0.26). Eight horses were defined as insulin resistant (IR) and five horses had normal insulin sensitivity. The five horses with normal insulin sensitivity showed a greater decrease in systolic blood pressure (−17.0 ± 7.4 vs. −3.4 ± 4.6 mmHg, P = 0.001) and MAP (19.2 ± 14.7 vs. 6.9 ± 8.7 mmHg, P = 0.04) than IR horses. There was no difference in the decrease in diastolic blood pressure between groups (16 ± 12.8 vs. 8.9 ± 12.1 mmHg, P = 0.17).

Conclusions

This study indicates that there is a relationship between insulin sensitivity and systolic and MAP in horses. However, studies on a larger number of horses are needed to confirm this association.

Energy Drinks and Their Impact on the Cardiovascular System: Potential Mechanisms

Globally, the popularity of energy drinks is steadily increasing. Scientific interest in their effects on cardiovascular and cerebrovascular systems in humans is also expanding and with it comes a growing number of case reports of adverse events associated with energy drinks. The vast majority of studies carried out in the general population report effects on blood pressure and heart rate. However, inconsistencies in the current literature render it difficult to draw firm conclusions with regard to the effects of energy drinks on cardiovascular and cerebrovascular variables. These inconsistencies are due, in part, to differences in methodologies, volume of drink ingested, and duration of postconsumption measurements, as well as subject variables during the test. Recent well-controlled, randomized crossover studies that used continuous beat-to-beat measurements provide evidence that cardiovascular responses to the ingestion of energy drinks are best explained by the actions of caffeine and sugar, with little influence from other ingredients. However, a role for other active constituents, such as taurine and glucuronolactone, cannot be ruled out. This article reviews the potentially adverse hemodynamic effects of energy drinks, particularly on blood pressure and heart rate, and discusses the mechanisms by which their active ingredients may interact to adversely affect the cardiovascular system. Research areas and gaps in the literature are discussed with particular reference to the use of energy drinks among high-risk individuals.

Analysis of cardiorespiratory phase coupling and cardiovascular autonomic responses during food ingestion

The present study analyzed whether the phase coherency (λ) of respiratory sinus arrhythmia (RSA) is altered by food ingestion in healthy young subjects. After 5min of resting control, 13 healthy volunteers were asked to eat a solid meal with access to water at their own pace, followed by 5min of the postprandial state. The R-R interval (RRI), beat-to-beat blood pressure (BP), and respiratory activity were recorded using electrocardiography, a Finapres device, and inductance plethysmography, respectively. The stroke volume was calculated by the pulse-contour method from continuous BP measurement, and the cardiac output (CO) was obtained by multiplying the stroke volume by the heart rate. From the oscillatory signals of RSA and respiration, λ was computed; additionally, frequency domain indexes of the heart rate variability (HRV) were calculated using a short-time Fourier transform. A steady-state 3-min resting period (R), food ingestion period (FOOD), and the first 2-min and the last 3-min of the post prandial period were analyzed separately. We also compared the responses to gum chewing (GUM) and water intake (WATER) using the same protocol on separate days. A shortening of RRI and increases in BP and CO were observed in FOOD compared to R, suggesting a shift of sympathovagal balance toward sympathetic activation. Similar responses but smaller magnitudes were observed in the GUM condition, whereas only transient shortening of RRI was observed in the WATER condition. The HRV indexes did not show any significant changes in response to GUM and WATER but sympathovagal balance was shifted in favor of sympathetic dominance in FOOD. λ decreased during all of the conditions. There was a significant negative correlation between λ and the indirect measure of sympathovagal balance. These results suggest that ingestion of food induces enhanced cardiac sympathetic activity and that a phase coherence of RSA could provide a sensitive measure for evaluating the cardiac autonomic profile.

Acute effects of hyperinsulinemia and hyperglycemia on vascular inflammatory biomarkers and endothelial function in overweight and obese humans

We investigated the separate and combined effects of hyperglycemia and hyperinsulinemia on markers of endothelial function, proinflammatory and proatherothrombotic responses in overweight/obese nondiabetic humans. Twenty-two individuals (13 F/9 M, BMI 30.1 ± 4.1 kg/m(2)) were studied during four randomized, single-blind protocols. The pancreatic clamp technique was combined with 4-h glucose clamps consisting of either 1) euinsulinemia-euglycemia, 2) euinsulinemia-hyperglycemia, 3) hyperinsulinemia-hyperglycemia, or 4) hyperinsulinemia-euglycemia. Insulin levels were higher (998 ± 66 vs. 194 ± 22 pmol/l) during hyperinsulinemia compared with euinsulinemia. Glucose levels were 11.1 mmol/l during hyperinsulinemia compared with 5.1 ± 0.1 mmol/l during euglycemia. VCAM, ICAM, P-selectin, E-selectin, IL-6, adiponectin, and PAI-1 responses were all increased (P < 0.01-0.0001), and endothelial function was decreased (P < 0.0005) during euinsulinemia-hyperglycemia compared with other protocols. Hyperinsulinemia in the presence of hyperglycemia prevented the increase in proinflammatory and proatherothrombotic markers while also normalizing vascular endothelial function. We conclude that 4 h of moderate hyperglycemia can result in increases of proinflammatory markers (ICAM, VCAM, IL-6, E-selectin), platelet activation (P-selectin), reduced fibrinolytic balance (increased PAI-1), and disordered endothelial function in a group of obese and overweight individuals. Hyperinsulinemia prevents the actions of moderate hyperglycemia to reduce endothelial function and increase proinflammatory and proatherothrombotic markers.

The vascular endothelium and human diseases

Alterations of endothelial cells and the vasculature play a central role in the pathogenesis of a broad spectrum of the most dreadful of human diseases, as endothelial cells have the key function of participating in the maintenance of patent and functional capillaries. The endothelium is directly involved in peripheral vascular disease, stroke, heart disease, diabetes, insulin resistance, chronic kidney failure, tumor growth, metastasis, venous thrombosis, and severe viral infectious diseases. Dysfunction of the vascular endothelium is thus a hallmark of human diseases. In this review the main endothelial abnormalities found in various human diseases such as cancer, diabetes mellitus, atherosclerosis, and viral infections are addressed.

Local NOS inhibition impairs vascular and metabolic actions of insulin in rat hindleg muscle in vivo

Insulin stimulates microvascular recruitment in skeletal muscle, and this vascular action augments muscle glucose disposal by ∼40%. The aim of the current study was to determine the contribution of local nitric oxide synthase (NOS) to the vascular actions of insulin in muscle. Hooded Wistar rats were infused with the NOS inhibitor N(ω)-nitro-L-arginine methylester (L-NAME, 10 μM) retrogradely via the epigastric artery in one leg during a systemic hyperinsulinemic-euglycemic clamp (3 mU·min(-1)·kg(-1) × 60 min) or saline infusion. Femoral artery blood flow, microvascular blood flow (assessed from 1-methylxanthine metabolism), and muscle glucose uptake (2-deoxyglucose uptake) were measured in both legs. Local L-NAME infusion did not have any systemic actions on blood pressure or heart rate. Local L-NAME blocked insulin-stimulated changes in femoral artery blood flow (84%, P < 0.05) and microvascular recruitment (98%, P < 0.05), and partially blocked insulin-mediated glucose uptake in muscle (reduced by 34%, P < 0.05). L-NAME alone did not have any metabolic effects in the hindleg. We conclude that insulin-mediated microvascular recruitment is dependent on local activation of NOS in muscle and that this action is important for insulin's metabolic actions.

Effect of Opuntia humifusa supplementation and acute exercise on insulin sensitivity and associations with PPAR-γ and PGC-1α protein expression in skeletal muscle of rats

This study examined whether Opuntia humifusa (O. humifusa), which is a member of the Cactaceae family, supplementation and acute swimming exercise affect insulin sensitivity and associations with PPAR-γ and PGC-1α protein expression in rats. Thirty-two rats were randomly divided into four groups (HS: high fat diet sedentary group, n = 8; HE: high fat diet acute exercise group, n = 8; OS: 5% O. humifusa supplemented high fat diet sedentary group, n = 8; OE: 5% O. humifusa supplemented high fat diet acute exercise group, n = 8). Rats in the HE and OE swam for 120 min. before being sacrificed. Our results indicated that serum glucose level, fasting insulin level and homeostasis model assessment of insulin resistance (HOMA-IR) in OS were significantly lower compared to those of the HS (p < 0.01, p < 0.05, p < 0.05). In addition, PPAR-γ protein expression in the OS and OE was significantly higher than that of the HS and HE, respectively (p < 0.05, p < 0.01). PGC-1α and GLUT-4 protein expressions in the OS were significantly higher compared to those of the HS (p < 0.05, p < 0.05). From these results, O. humifusa supplementation might play an important role for improving insulin sensitivity through elevation of PPAR-γ, PGC-1α, and GLUT-4 protein expression in rat skeletal muscle.

The effect of endogenously released glucose, insulin, glucagon-like peptide 1, ghrelin on cardiac output, heart rate, stroke volume, and blood pressure

Background

Ingestion of a meal increases the blood flow to the gastrointestinal organs and affects the heart rate (HR), blood pressure and cardiac output (CO), although the mechanisms are not known. The aim of this study was to evaluate the effect of endogenously released glucose, insulin, glucagon-like peptide 1 (GLP-1), ghrelin on CO, HR, stroke volume (SV), and blood pressure.

Methods

Eleven healthy men and twelve healthy women ((mean ± SEM) aged: 26 ± 0.2 y; body mass index: 21.8 ± 0.1 kg/m²)) were included in this study. The CO, HR, SV, systolic and diastolic blood pressure, antral area, gastric emptying rate, and glucose, insulin, GLP-1 and ghrelin levels were measured.

Results

The CO and SV at 30 min were significantly higher, and the diastolic blood pressure was significantly lower, than the fasting in both men and women (P < 0.05). In men, significant correlations were found between GLP-1 level at 30 min and SV at 30 min (P = 0.015, r = 0.946), and between ghrelin levels and HR (P = 0.013, r = 0.951) at 110 min. Significant correlations were also found between the change in glucose level at 30 min and the change in systolic blood pressure (P = 0.021, r = -0.681), and the change in SV (P = 0.008, r = -0.748) relative to the fasting in men. The insulin 0-30 min AUC was significantly correlated to the CO 0-30 min AUC (P = 0.002, r = 0.814) in men. Significant correlations were also found between the 0-120 min ghrelin and HR AUCs (P = 0.007, r = 0.966) in men. No statistically significant correlations were seen in women.

Conclusions

Physiological changes in the levels of glucose, insulin, GLP-1 and ghrelin may influence the activity of the heart and the blood pressure. There may also be gender-related differences in the haemodynamic responses to postprandial changes in hormone levels. The results of this study show that subjects should not eat immediately prior to, or during, the evaluation of cardiovascular interventions as postprandial affects may affect the results, leading to erroneous interpretation of the cardiovascular effects of the primary intervention.

Trial registration number

NCT01027507

The Correlations between Extremity Circumferences with Total and Regional Amounts of Skeletal Muscle and Muscle Strength in Obese Women with Type 2 Diabetes

BACKGROUND

Insulin resistance is related to central obesity and the amount of skeletal muscle. A simple and practical anthropometric marker for muscle mass is not known, although waist circumference (WC) is used as an indicator of abdominal obesity. The aims of this study were to investigate whether arm (AC) and thigh circumferences (TC) can be used as an indicator of muscle mass and if they are related to muscle strength.

METHODS

A total of 110 obese (body mass index [BMI]≥25 kg/m(2)) women with type 2 diabetes were enrolled, and WC, AC, and TC were measured. Abdominal visceral fat (AVF), subcutaneous fat (ASF), and total fat (ATF) were assessed by computed tomography, regional muscle (MM), and fat mass by dual-energy X-ray absorptiometry, muscle strength by one repetition maximum (1RM) of both extremities (chest and leg press) and insulin resistance by K(ITT).

RESULTS

The mean age was 56.2±7.3 years, duration of diabetes was 4.2±4.4 years, and BMI was 27.2±2.8 kg/m(2). WC was correlated with ATF, AVF, and ASF (r=0.728, P<0.001; r=0.515, P<0.001; r=0.608, P<0.001, respectively). Arm MM was correlated with AC (r=0.500, P<0.001), and leg MM with TC (r=0.291, P=0.002). Upper 1RM was related to AC/WC ratio (r=0.359, P<0.001), and lower 1RM was to TC/WC ratio (r=0.286, P=0.003). Insulin resistance had significant relations with AVF, WC, and total MM (r=-0.262, P=0.008; r=-0.217, P=0.029; r=0.160, P=0.031, respectively).

CONCLUSION

The muscle mass was related to extremity circumferences, and muscle strength was to extremity/waist circumference ratio in obese women with type 2 diabetes.

Relationship between postprandial changes in cardiac left ventricular function, glucose and insulin concentrations, gastric emptying, and satiety in healthy subjects

Background

The digestion of food is known to alter the hemodynamics of the body significantly. The purpose of this study was to study the postprandial changes in stroke volume (SV), cardiac output (CO) and left ventricular (LV) longitudinal systolic and diastolic functions measured with tissue Doppler imaging, in relation to gastric emptying rate (GER), satiety, and glucose and insulin concentrations in healthy subjects.

Methods

Twenty-three healthy subjects were included in this study. The fasting and postprandial changes at 30 min and 110 min in CO, heart rate (HR) and blood pressure were measured. Moreover, tissue Doppler imaging systolic (S'), early (E') and late (A') mitral annular diastolic velocities were measured in the septal (s) and lateral (l) walls. Glucose and insulin concentrations, and satiety were measured before and 15, 30, 45, 60, 90, and 120 min after the start of the meal. The GER was calculated as the percentage change in the antral cross-sectional area 15-90 min after ingestion of the meal.

Results

This study show that both CO, systolic longitudinal ventricular velocity of the septum (S's) and lateral wall (S'l), the early diastolic longitudinal ventricular velocity of the lateral wall (E'l), the late diastolic longitudinal ventricular velocity of the septum (A's) and lateral wall (A'l) increase significantly, and were concomitant with increased satiety, antral area, glucose and insulin levels. The CO, HR and SV at 30 min were significantly higher, and the diastolic blood pressure was significantly lower, than the fasting. The satiety was correlated to HR and diastolic blood pressure. The insulin level was correlated to HR.

Conclusions

This study shows that postprandial CO, HR, SV and LV longitudinal systolic and diastolic functions increase concomitantly with increased satiety, antral area, and glucose and insulin levels. Therefore, patients should not eat prior to, or during, cardiac evaluation as the effects of a meal may affect the results and their interpretation.

Trial Registration

ClinicalTrials.gov: NCT01027507

Daily physical activity enhances reactivity to insulin in skeletal muscle arterioles of hyperphagic Otsuka Long-Evans Tokushima Fatty rats

Insulin-mediated glucose disposal is dependent on the vasodilator effects of insulin. In type 2 diabetes, insulin-stimulated vasodilation is impaired as a result of an imbalance in NO and ET-1 production. We tested the hypothesis that chronic voluntary wheel running (RUN) prevents impairments in insulin-stimulated vasodilation associated with obesity and type 2 diabetes independent of the effects of RUN on adiposity by randomizing Otsuka Long Evans Tokushima Fatty (OLETF) rats, a model of hyperphagia-induced obesity and type 2 diabetes, to 1) RUN, 2) caloric restriction (CR; diet adjusted to match body weights of RUN group), or 3) sedentary control (SED) groups (n = 8/group) at 4 wk. At 40 wk, NO- and ET-1-mediated vasoreactivity to insulin (1-1,000 μIU/ml) was assessed in the presence of a nonselective ET-1 receptor blocker (tezosentan) or a NO synthase (NOS) inhibitor [N(G)-nitro-L-arginine methyl ester (L-NAME)], respectively, in second-order arterioles isolated from the white portion of the gastrocnemius muscle. Body weight, fasting plasma glucose, and hemoglobin A1c were lower in RUN and CR than SED (P < 0.05); however, the glucose area under the curve (AUC) following the intraperitoneal glucose tolerance test was lower only in the RUN group (P < 0.05). Vasodilator responses to all doses of insulin were greater in RUN than SED or CR in the presence of a tezosentan (P < 0.05), but group differences in vasoreactivity to insulin with coadministration of L-NAME were not observed. We conclude daily wheel running prevents obesity and type 2 diabetes-associated declines in insulin-stimulated vasodilation in skeletal muscle arterioles through mechanisms that appear to be NO mediated and independent of attenuating excess adiposity in hyperphagic rats.

The effects of resistance training on muscle and body fat mass and muscle strength in type 2 diabetic women

BACKGROUND

Our goal was to investigate the effects of low intensity resistance training on body fat, muscle mass and strength, cardiovascular fitness, and insulin sensitivity in type 2 diabetes.

METHODS

Twenty-eight overweight women with type 2 diabetes were randomly assigned to a resistance training group (RG, n = 13) or a control group (CG, n = 15). RG performed resistance training using elastic bands, of which strength was equal to 40 to 50% of one repetition maximum (1RM), for three days per week. Each exercise consisted of three sets for 60 minutes. We assessed abdominal fat using computed tomography, muscle mass using dual-energy X-ray absorptiometry, and muscle strength using Keiser's chest and leg press. Insulin sensitivity was measured using the insulin tolerance test, and aerobic capacity was expressed as oxygen uptake at the anaerobic threshold (AT-VO(2)) before and after the 12-week exercise program.

RESULTS

The age of participants was 56.4 +/- 7.1 years, duration of diabetes was 5.9 +/- 5.5 years, and BMI was 27.4 +/- 2.5 kg/m(2), without significant differences between two groups. During intervention, a greater increase in muscle mass and greater decreases in both total fat mass and abdominal fat were observed in RG compared to those of CG (P = 0.015, P = 0.011, P = 0.010, respectively). Increase in 1RM of upper and lower extremities was observed in the RG (P = 0.004, P = 0.040, respectively), without changes in AT-VO(2) and insulin resistance in either group.

CONCLUSION

In conclusion, the low intensity resistance training was effective in increasing muscle mass and strength and reducing total fat mass without change of insulin sensitivity in type 2 diabetic patients.

Lipid peroxidation in relation to ageing and the role of endogenous aldehydes in diabetes and other age-related diseases

Lipid intermediates which are generated by ROS have drawn more attention after it was found that lipid peroxidation and lipid-radical cycles are two alternative processes. In biological membranes alpha-tocopherol and cytochrome b5, as known, act synergistically to overcome free radical injury and to form lipid-radical cycles. These cycles activate membrane proteins, protect membrane lipids from oxidation and prevent from formation of endogenous aldehydes. Experimental and clinical evidence accumulated for 5-6 years suggests that endogenous aldehydes, such as malonic dialdehyde (MDA) and methylglyoxal (MG), are the major initiators of the metabolic disorders. The age-related diseases emerge when cells cannot control formation of aldehydes and/or cannot abolish the negative effect of methylglyoxal on their metabolism. If the efficiency of the glyoxalase system is insufficient toxic aldehydes cause cumulative damage over a lifetime. In this paper, we provide evidence to consider ageing as a process in which lipid-radical cycles gradually substitute for lipid peroxidation. There are always two opposing tendencies or actions which counteract each other - actions of melatonin, lipid-radical cycles and the glyoxalase system (anti-ageing effect) and negative actions of the toxic aldehydes (pro-ageing effect). Life span is determined by the balance of two opposing processes.

Fructose ingestion acutely elevates blood pressure in healthy young humans

Overconsumption of fructose, particularly in the form of soft drinks, is increasingly recognized as a public health concern. The acute cardiovascular responses to ingesting fructose have not, however, been well-studied in humans. In this randomized crossover study, we compared cardiovascular autonomic regulation after ingesting water and drinks containing either glucose or fructose in 15 healthy volunteers (aged 21–33 yr). The total volume of each drink was 500 ml, and the sugar content 60 g. For 30 min before and 2 h after each drink, we recorded beat-to-beat heart rate, arterial blood pressure, and cardiac output. Energy expenditure was determined on a minute-by-minute basis. Ingesting the fructose drink significantly increased blood pressure, heart rate, and cardiac output but not total peripheral resistance. Glucose ingestion resulted in a significantly greater increase in cardiac output than fructose but no change in blood pressure and a concomitant decrease in total peripheral resistance. Ingesting glucose and fructose, but not water, significantly increased blood pressure variability and decreased cardiovagal baroreflex sensitivity. Energy expenditure increased by a similar amount after glucose and fructose ingestion, but fructose elicited a significantly greater increase in respiratory quotient. These results show that ingestion of glucose and fructose drinks is characterized by specific hemodynamic responses. In particular, fructose ingestion elicits an increase in blood pressure that is probably mediated by an increase in cardiac output without compensatory peripheral vasodilatation.

Skeletal muscle capillary responses to insulin are abnormal in late-stage diabetes and are restored by angiotensin-converting enzyme inhibition

Acute physiological hyperinsulinemia increases skeletal muscle capillary blood volume (CBV), presumably to augment glucose and insulin delivery. We hypothesized that insulin-mediated changes in CBV are impaired in type 2 diabetes mellitus (DM) and are improved by angiotensin-converting enzyme inhibition (ACE-I). Zucker obese diabetic rats (ZDF, n = 18) and control rats (n = 9) were studied at 20 wk of age. One-half of the ZDF rats were treated with quinapril (ZDF-Q) for 15 wk prior to study. CBV and capillary flow in hindlimb skeletal muscle were measured by contrast-enhanced ultrasound (CEU) at baseline and at 30 and 120 min after initiation of a euglycemic hyperinsulinemic clamp (3 mU.min(-1).kg(-1)). At baseline, ZDF and ZDF-Q rats were hyperglycemic and hyperinsulinemic vs. controls. Glucose utilization in ZDF rats was 60-70% lower (P < 0.05) than in controls after 30 and 120 min of hyperinsulinemia. In ZDF-Q rats, glucose utilization was impaired at 30 min but similar to controls at 120 min. Basal CBV was lower in ZDF and ZDF-Q rats compared with controls (13 +/- 4, 7 +/- 3, and 9 +/- 2 U, respectively). With hyperinsulinemia, CBV increased by about twofold in control animals at 30 and 120 min, did not change in ZDF animals, and increased in ZDF-Q animals only at 120 min to a level similar to controls. Anatomic capillary density on immunohistology was not different between groups. We conclude that insulin-mediated capillary recruitment in skeletal muscle, which participates in glucose utilization, is impaired in animals with DM and can be partially reversed by chronic ACE-I therapy.

Aldehydes and disturbance of carbohydrate metabolism: some consequences and possible approaches to its normalization

There are many well-documented errors of metabolism involving genetic defects that affect carbohydrate utilization. The array of disorders includes the defective utilization of glucose, as well as enzymatic deficiencies in glycolysis and gluconeogenesis, and the pentose phosphate pathway. Besides, there is considerable literature about metabolic syndrome and diabetes. However, the main problem of their origin remains obscure. Also, it is presently beyond doubt that there are various causes of insulin resistance. The development of insulin resistance may be associated not only with insulin production disorders or presence of insulin antagonists but also with modification of the number of receptors and sensitivity of peripheral tissues. The insulin resistance originates from insulin signal transmission defects at its initial stages. It is presently uncertain which mechanisms of adaptation regulation are activated or should be activated under hyperglycemia conditions. This is the main problem of the selection of strategy of hyperglycemia treatment but it is important that aldehydes - the secondary products of lipid peroxidation and protein glycation (malondialdehyde and methylglyoxal) - make a contribution to abnormal metabolism. As far as the role of methylglyoxal in inhibition of antioxidant enzymes is concerned, the involvement of the ketoaldehyde in such processes as oxidative stress, cell proliferation control, and carbohydrate metabolism disorders does not cast any doubt.

Mixed meal and light exercise each recruit muscle capillaries in healthy humans

Intense exercise and insulin each increases total limb blood flow and recruits muscle capillaries, presumably to facilitate nutrient exchange. Whether mixed meals or light exercise likewise recruits capillaries is unknown. We fed 18 (9 M, 9 F) healthy volunteers a 480-kcal liquid mixed meal. Plasma glucose, insulin, brachial artery flow, and forearm muscle microvascular blood volume were measured before and after the meal. Brachial artery flow and microvascular volume were also examined with light (25% max), moderate (50%), and heavy (80%) forearm contraction every 20 s in 5 (4 M, 1 F) healthy adults. After the meal, glucose and insulin rose modestly (to approximately 7 mM and approximately 270 pM) and peaked by 30 min, whereas brachial artery blood flow (P < 0.05) and the microvascular volume (P < 0.01) each increased significantly by 60 min, and microvascular flow velocity did not change. For exercise, both 50 and 80%, but not 25% maximal handgrip, increased average forearm and brachial artery blood flow (P < 0.01). Flow increased immediately after each contraction and declined toward basal over 15 s. Exercise at 25% max increased microvascular volume threefold (P < 0.01) without affecting microvascular flow velocity or total forearm blood flow. Forearm exercise at 80% maximal grip increased both microvascular volume and microvascular flow velocity (P < 0.05 each). We conclude that light exercise and simple meals each markedly increases muscle microvascular volume, thereby expanding the endothelial surface for nutrient exchange, and that capillary recruitment is an important physiological response to facilitate nutrient/hormone delivery in healthy humans.

Effects of insulin on left ventricular function during dynamic exercise in overweight and obese subjects

AIMS

We designed this study in order to determine the effect of insulin on cardiac function in overweight and obese subjects during exercise.

METHODS AND RESULTS

The cardiac function of 62 normal glucose tolerant subjects, aged 30-40 and divided into normal weight (group 1, n=22, BMI 20-24.9 kg/m(2)), overweight (group 2, n=20, BMI 25-29.9 kg/m(2)), and obesity (group 3, n=20, BMI 30-35 kg/m(2)) was evaluated at rest and during dynamic exercise through angiocardioscintigraphy, when on hyperinsulinaemic euglycaemic clamp (test A) and when on normal saline infusion (test B). Left ventricular function at rest was statistically greater (P<0.05) in both tests in overweight and obese subjects compared with normal weight controls, with no statistical difference (P=0.057) within groups between insulin and normal saline infusion. During exercise, cardiac function improved in all the subjects in both tests. The increase was lower in overweight and obese patients, even if statistically significant only in obese vs. control subjects in both tests (P<0.05). Insulin sensitivity showed a significant correlation (P< or =0.001) with left ventricular ejection fraction (LVEF) at rest and with change in LVEF during clamp.

CONCLUSION

Our findings suggest a metabolic pathogenesis for the impaired LV function in obesity.

Differing physiological effects of epinephrine in type 1 diabetes and nondiabetic humans

Acute increases of the key counterregulatory hormone epinephrine can be modified by a number of physiological and pathological conditions in type 1 diabetic patients (T1DM). However, it is undecided whether the physiological effects of epinephrine are also reduced in T1DM. Therefore, the aim of this study was to determine whether target organ (liver, muscle, adipose tissue, pancreas, cardiovascular) responses to epinephrine differ between healthy subjects and T1DM patients. Thirty-four age- and weight-matched T1DM (n = 17) and healthy subjects (n = 17) underwent two randomized, single-blind, 2-h hyperinsulinemic euglycemic clamp studies with (Epi) and without epinephrine infusion. Muscle biopsy was performed at the end of each study. Epinephrine levels during Epi were similar in all groups (4,039 +/- 384 pmol/l). Glucose (5.3 +/- 0.06 mmol/l) and insulin levels (462 +/- 18 pmol/l) were also similar in all groups during the glucose clamps. Glucagon responses to Epi were absent in T1DM and significantly reduced compared with healthy subjects. Endogenous glucose production during the final 30 min was significantly greater during Epi in healthy subjects compared with T1DM (8.4 +/- 1.3 vs. 4.4 +/- 0.6 micromol.kg(-1).min(-1), P = 0.041). Glucose uptake showed almost a twofold greater decrease with Epi in healthy subjects vs. T1DM (Delta31 +/- 2 vs. Delta17 +/- 2 nmol.kg(-1).min(-1), respectively, P = 0.026). Glycerol, beta-hydroxybutyrate, and nonesterified fatty acid (NEFA) all increased significantly more in T1DM compared with healthy subjects. Increases in systolic blood pressure were greater in healthy subjects, but reductions of diastolic blood pressure were greater in T1DM patients with Epi. Reduction of glycogen synthase was significantly greater during epinephrine infusion in T1DM vs. healthy subjects. In summary, despite equivalent epinephrine, insulin, and glucose levels, changes in glucose flux, glucagon, and cardiovascular responses were greater in healthy subjects compared with T1DM. However, T1DM patients had greater lipolytic responses (glycerol and NEFA) during Epi. Thus we conclude that there is a spectrum of significant in vivo physiological differences of epinephrine action at the liver, muscle, adipose tissue, pancreas, and cardiovascular system between T1DM and healthy subjects.

Heart failure: the frequent, forgotten, and often fatal complication of diabetes

There is a high frequency of heart failure (HF) accompanied by an increased mortality risk for patients with diabetes. The poor prognosis of these patients has been explained by an underlying diabetic cardiomyopathy exacerbated by hypertension and ischemic heart disease. In these patients, activation of the sympathetic nervous system results in increased myocardial utilization of fatty acids and induction of fetal gene programs, decreasing myocardial function. Activation of the renin-angiotensin system results in myocardial remodeling. It is imperative for physicians to intercede early to stop the progression of HF, yet at least half of patients with left ventricular dysfunction remain undiagnosed and untreated until advanced disease causes disability. This delay is largely because of the asymptomatic nature of early HF, which necessitates more aggressive assessment of HF risk factors and early clinical signs. Utilization of beta-blockade, ACE inhibitors, or possibly angiotensin receptor blockers is essential in preventing remodeling with its associated decline in ventricular function. beta-Blockers not only prevent, but may also reverse, cardiac remodeling. Glycemic control may also play an important role in the therapy of diabetic HF. The adverse metabolic side effects that have been associated with beta-adrenergic inhibitors in the diabetic patient may be circumvented by use of a third-generation beta-blocker. Prophylactic utilization of ACE inhibitors and beta-blockers to avoid, rather than await, the need to treat HF should be considered in high-risk diabetic patients.

Inhibiting NOS blocks microvascular recruitment and blunts muscle glucose uptake in response to insulin

We examined the effects of inhibiting nitric oxide synthase with Nomega-nitro-l-arginine-methyl ester (l-NAME) on total hindlimb blood flow, muscle microvascular recruitment, and hindlimb glucose uptake during euglycemic hyperinsulinemia in vivo in the rat. We used two independent methods to measure microvascular perfusion. In one group of animals, microvascular recruitment was measured using the metabolism of exogenously infused 1-methylxanthine (1-MX), and in a second group contrast-enhanced ultrasound (CEU) was used. Limb glucose uptake was measured by arterial-venous concentration differences after 2 h of insulin infusion. Saline alone did not alter femoral artery flow, glucose uptake, or 1-MX metabolism. Insulin (10 mU.min-1.kg-1) significantly increased hindlimb total blood flow (0.69 +/- 0.02 to 1.22 +/- 0.11 ml/min, P < 0.05), glucose uptake (0.27 +/- 0.05 to 0.95 +/- 0.08 micromol/min, P < 0.05), 1-MX uptake (5.0 +/- 0.5 to 8.5 +/- 1.0 nmol/min, P < 0.05), and skeletal muscle microvascular volume measured by CEU (10.0 +/- 1.6 to 15.0 +/- 1.2 video intensity units, P < 0.05). Addition of l-NAME to insulin completely blocked the effect of insulin on both total limb flow and microvascular recruitment (measured using either 1-MX or CEU) and blunted glucose uptake by 40% (P < 0.05). We conclude that insulin specifically recruits flow to the microvasculture in skeletal muscle via a nitric oxide-dependent pathway and that this may be important to insulin's overall action to regulate glucose disposal.

Blood flow and muscle metabolism: a focus on insulin action

The vascular system controls the delivery of nutrients and hormones to muscle, and a number of hormones may act to regulate muscle metabolism and contractile performance by modulating blood flow to and within muscle. This review examines evidence that insulin has major hemodynamic effects to influence muscle metabolism. Whole body, isolated hindlimb perfusion studies and experiments with cell cultures suggest that the hemodynamic effects of insulin emanate from the vasculature itself and involve nitric oxide-dependent vasodilation at large and small vessels with the purpose of increasing access for insulin and nutrients to the interstitium and muscle cells. Recently developed techniques for detecting changes in microvascular flow, specifically capillary recruitment in muscle, indicate this to be a key site for early insulin action at physiological levels in rats and humans. In the absence of increases in bulk flow to muscle, insulin may act to switch flow from nonnutritive to the nutritive route. In addition, there is accumulating evidence to suggest that insulin resistance of muscle in vivo in terms of impaired glucose uptake could be partly due to impaired insulin-mediated capillary recruitment. Exercise training improves insulin-mediated capillary recruitment and glucose uptake by muscle.

Direct evidence for insulin-induced capillary recruitment in skin of healthy subjects during physiological hyperinsulinemia

It has been proposed that insulin-mediated changes in muscle perfusion modulate insulin-mediated glucose uptake. However, the putative effects of insulin on the microcirculation that permit such modulation have not been studied in humans. We examined the effects of systemic hyperinsulinemia on skin microvascular function in eight healthy nondiabetic subjects. In addition, the effects of locally administered insulin on skin blood flow were assessed in 10 healthy subjects. During a hyperinsulinemic clamp, we measured leg blood flow with venous occlusion plethysmography, skin capillary density with capillaroscopy, endothelium-(in)dependent vasodilatation of skin microcirculation with iontophoresis of acetylcholine and sodium nitroprusside combined with laser Doppler fluxmetry, and skin vasomotion by Fourier analysis of microcirculatory blood flow. To exclude nonspecific changes in the hemodynamic variables, a time-volume control study was performed. Insulin iontophoresis was used to study the local effects of insulin on skin blood flow. Compared to the control study, systemic hyperinsulinemia caused an increase in leg blood flow (-0.54 +/- 0.93 vs. 1.97 +/- 1.1 ml. min(-1). dl(-1); P < 0.01), an increase in the number of perfused capillaries in the resting state (-3.7 +/- 3.0 vs. 3.4 +/- 1.4 per mm(2); P < 0.001) and during postocclusive reactive hyperemia (-0.8 +/- 2.2 vs. 5.1 +/- 3.7 per mm(2); P < 0.001), an augmentation of the vasodilatation caused by acetylcholine (722 +/- 206 vs. 989 +/- 495%; P < 0.05) and sodium nitroprusside (618 +/- 159 vs. 788 +/- 276%; P < 0.05), and a change in vasomotion by increasing the relative contribution of the 0.01- to 0.02-Hz and 0.4- to 1.6-Hz spectral components (P < 0.05). Compared to the control substance, locally administered insulin caused a rapid increase ( approximately 13.5 min) in skin microcirculatory blood flow (34.4 +/- 42.5 vs. 82.8 +/- 85.7%; P < 0.05). In conclusion, systemic hyperinsulinemia in skin 1) induces recruitment of capillaries, 2) augments nitric oxide-mediated vasodilatation, and 3) influences vasomotion. In addition, locally administered insulin 4) induces a rapid increase in total skin blood flow, independent of systemic effects.

Dose-dependent vasodilating effects of insulin on adenosine-stimulated myocardial blood flow

In the peripheral vasculature, insulin induces time- and dose-dependent vasodilation. We have recently demonstrated that insulin potentiates adenosine-stimulated myocardial blood flow. However, it is unknown whether insulin's effects on the coronary vasculature are dose dependent. In this study, we quantitated myocardial blood flow and adenosine-stimulated coronary flow (140 microg.kg(-1).min(-1) for 5 min) in 10 healthy men (age, 32 +/- 6 years; BMI, 24.1 +/- 1.8 kg/m(2)) using positron emission tomography and (15)O-labeled water. Hyperemic myocardial blood flow was measured in the basal state, during euglycemic physiological hyperinsulinemia (serum insulin approximately 65 mU/l) and during supraphysiological hyperinsulinemia (serum insulin approximately 460 mU/l). Basal myocardial blood flow was 0.84 +/- 0.17 ml.g(-1).min(-1). Physiological hyperinsulinemia increased the adenosine-stimulated flow by 20% (from 3.92 +/- 1.17 to 4.72 +/- 0.96 ml.g(- 1).min(-1); P < 0.05). Supraphysiological hyperinsulinemia further enhanced the adenosine-stimulated flow by 19% (to 5.61 +/- 1.03 ml.g(-1).min(-1); P < 0.05). These effects were not explained by changes in systemic hemodynamics, since coronary resistance decreased during each insulin infusion (P < 0.05). In addition, hyperemic myocardial blood flow responses during insulin stimulation were positively correlated with whole-body glucose uptake. The results demonstrate that insulin is able to enhance hyperemic myocardial blood flow in a dose-dependent manner in healthy subjects. These effects might contribute to the known beneficial dose-dependent effects of insulin on myocardial ischemia.

Systemic and local adrenergic regulation of muscle glucose utilization during hypoglycemia in healthy subjects

Adrenergic responses are crucial for hypoglycemic recovery. Epinephrine increases glucose production, lipolysis, and peripheral insulin resistance as well as blood flow and glucose delivery. Sympathetic activation causes vasoconstriction and reduces glucose delivery. To determine the effects of alpha- and beta-adrenergic activity on muscle glucose uptake during hypoglycemia, we studied forearm blood flow (FBF) (plethysmography), arteriovenous glucose difference (AV-diff), and forearm glucose uptake (FGU) during insulin infusion with 60 min of euglycemia followed by 60 min of hypoglycemia. Twelve healthy subjects (27 plus minus 5 years of age) were randomized to intravenous propranolol (IV PROP, 80 microg/min), intravenous phentolamine (IV PHEN, 500 microg/min), intra-arterial propranolol (IA PROP, 25 microg/min), intra-arterial phentolamine (IA PHEN, 12 microg/min per 100 ml forearm tissue), and saline (SAL). FBF increased during hypoglycemia with SAL (P < 0.001) but not with IA or IV PROP. FGU (P = 0.015) and AV-diff (P = 0.099) fell during hypoglycemia with IA PROP but not with IV PROP. FBF increased during hypoglycemia with IA and IV PHEN (P < 0.005). AV-diff fell during hypoglycemia with IA and IV PHEN (P < 0.01), but FGU was unchanged. Blood pressure fell (P < 0.001), and adrenergic and neuroglycopenic symptoms increased with IV PHEN (P < 0.01). Thus, systemic but not local propranolol prevents a decrease in forearm glucose extraction during hypoglycemia, suggesting that epinephrine increases peripheral muscular insulin resistance through systemic effects. alpha-Adrenergic activation inhibits vasodilation and helps maintain brain glucose delivery.

Cardiac responses to insulin-induced hypoglycemia in nondiabetic and intensively treated type 1 diabetic patients

Insulin-induced hypoglycemia occurs commonly in intensively treated patients with type 1 diabetes, but the cardiovascular consequences of hypoglycemia in these patients are not known. We studied left ventricular systolic [left ventricular ejection fraction (LVEF)] and diastolic [peak filling rate (PFR)] function by equilibrium radionuclide angiography during insulin infusion (12 pmol. kg(-1). min(-1)) under either hypoglycemic (approximately 2.8 mmol/l) or euglycemic (approximately 5 mmol/l) conditions in intensively treated patients with type 1 diabetes and healthy nondiabetic subjects (n = 9 for each). During hypoglycemic hyperinsulinemia, there were significant increases in LVEF (DeltaLVEF = 11 +/- 2%) and PFR [DeltaPFR = 0.88 +/- 0.18 end diastolic volume (EDV)/s] in diabetic subjects as well as in the nondiabetic group (DeltaLVEF = 13 +/- 2%; DeltaPFR = 0.79 +/- 0.17 EDV/s). The increases in LVEF and PFR were comparable overall but occurred earlier in the nondiabetic group. A blunted increase in plasma catecholamine, cortisol, and glucagon concentrations occurred in response to hypoglycemia in the diabetic subjects. During euglycemic hyperinsulinemia, LVEF also increased in both the diabetic (DeltaLVEF = 7 +/- 1%) and nondiabetic (DeltaLVEF = 4 +/- 2%) groups, but PFR increased only in the diabetic group. In the comparison of the responses to hypoglycemic and euglycemic hyperinsulinemia, only the nondiabetic group had greater augmentation of LVEF, PFR, and cardiac output in the hypoglycemic study (P < 0.05 for each). Thus intensively treated type 1 diabetic patients demonstrate delayed augmentation of ventricular function during moderate insulin-induced hypoglycemia. Although diabetic subjects have a more pronounced cardiac response to hyperinsulinemia per se than nondiabetic subjects, their response to hypoglycemia is blunted.

Pharmacological modulation of endothelial function by insulin in the rat aorta

Nitric oxide (NO)-mediated vasodilation induced by hyperinsulinaemia might involve an indirect action which promotes agonist-stimulated endothelial function. Our aim was to attempt to demonstrate such modulation of endothelium-dependent vasodilation by insulin in the rat isolated aorta. We found that vasodilation in response to acetylcholine, but not to adenosine diphosphate (ADP), histamine or the calcium ionophore A23187, was modestly enhanced after 20-min pretreatment with human insulin (100 nM) whereas endothelium-independent responses to the NO donor sodium nitroprusside were not significantly affected. Human insulin thus has the acute pharmacological action of selectively enhancing muscarinic receptor-mediated endothelial function in rat aortic vascular smooth muscle in-vitro.

Mechanisms of insulin resistance in experimental hyperinsulinemic dogs

This study was undertaken to characterize the insulin resistance and the mechanism thereof caused by chronic hyperinsulinemia produced in dogs by surgically diverting the veins of the pancreas from the portal vein to the vena cava. Pancreatic venous diversion (PVD, n = 8) caused a sustained increase in arterial insulin and decrease in portal insulin concentration compared with the control group (n = 6). Hyperinsulinemic euglycemic clamps were conducted 4 wk after surgery. The increase in the glucose disposal rate (GDR) was significantly less in the PVD group (39.0+/-5.0 vs. 27.9+/-3.2 micromol/kg/min, P < 0.01) compared with the control group, but the suppression of hepatic glucose production by insulin was similar for both groups. Muscle insulin receptor tyrosine kinase activity (IR-TKA) increased from 6.2+/-0.4 to 20.3+/-2.7 in the control group, but from 5.8+/-0.5 to only 12.7+/-1.7 fmol P/fmol IR in the PVD group (P < 0.01). With respect to the periphery, the time to half-maximum response (t1/2a) for arterial insulin was the same for both groups, whereas the t1/2a for lymph insulin (30+/-3 vs. 40+/-4 min, P < 0.05) and GDR (29+/-3 vs. 66+/-10 min, P < 0.01) were greater for the PVD group. Chronic hyperinsulinemia led to marked peripheral insulin resistance characterized by decreased insulin-stimulated GDR, and impaired activation of GDR kinetics due, in part, to reduced IR-TKA. Transendothelial insulin transport was impeded and was responsible for one third of the kinetic defect in insulin-resistant animals, while slower intracellular mechanisms of GDR were responsible for the remaining two thirds.

Mechanisms mediating insulin-induced hypotension in rats. A role for nitric oxide and autonomic mediators

The mechanisms associated with insulin-induced cardiovascular inhibitory responses were evaluated in untreated normal rats and in normal rats pretreated with an antagonist of nitric oxide (NO) production (L-NAME), with cholinergic, alpha- and beta-adrenergic antagonists, or after ganglionic blockade. Male Wistar rats were anesthetized with a mixture of urethane and alpha-chloralose and placed on a electric heating pad. The femoral artery and vein were cannulated for measurements of mean arterial pressure (MAP), heart rate, plasma glucose, blood sampling, and intravenous injections. Intravenous injection of insulin (5.0 U/kg) in untreated rats resulted in a significant and sustained decrease in arterial blood pressure (average 24%) and in a slight decrease in heart rate. These cardiovascular responses were blocked by L-NAME and by the cholinergic antagonist atropine, suggesting an involvement of NO and the cholinergic receptors, or an effect of insulin on the central nervous system parasympathetic center. The ganglionic blocker hexamethonium attenuated the insulin-induced response. On the other hand, the hypotensive effect of insulin persisted after sympathetic blockade with the alpha-1 antagonist prazosin and the beta-1 antagonist atenolol. We conclude that the insulin-induced decrease in blood pressure is due to both increased cholinergic outflow and to NO production and that an enhanced sympathetic activity possibly mediated by a reactive release of norepinephrine or epinephrine modulates this response.

Insulin-stimulated production of nitric oxide is inhibited by wortmannin. Direct measurement in vascular endothelial cells

Hypertension is associated with insulin-resistant states such as diabetes and obesity. Nitric oxide (NO) contributes to regulation of blood pressure. To gain insight into potential mechanisms linking hypertension with insulin resistance we directly measured and characterized NO production from human umbilical vein endothelial cells (HUVEC) in response to insulin using an amperometric NO-selective electrode. Insulin stimulation of HUVEC resulted in rapid, dose-dependent production of NO with a maximal response of approximately 100 nM NO (200,000 cells in 2 ml media; ED50 approximately 500 nM insulin). Although HUVEC have many more IGF-1 receptors than insulin receptors (approximately 400,000, and approximately 40,000 per cell respectively), a maximally stimulating dose of IGF-1 generated a smaller response than insulin (40 nM NO; ED50 approximately 100 nM IGF-1). Stimulation of HUVEC with PDGF did not result in measurable NO production. The effects of insulin and IGF-1 were completely blocked by inhibitors of either tyrosine kinase (genestein) or nitric oxide synthase (L-NAME). Wortmannin (an inhibitor of phosphatidylinositol 3-kinase [PI 3-kinase]) inhibited insulin-stimulated production of NO by approximately 50%. Since PI 3-kinase activity is required for insulin-stimulated glucose transport, our data suggest that NO is a novel effector of insulin signaling pathways that are also involved with glucose metabolism.

Obesity/insulin resistance is associated with endothelial dysfunction. Implications for the syndrome of insulin resistance

To test the hypothesis that obesity/insulin resistance impairs both endothelium-dependent vasodilation and insulin-mediated augmentation of endothelium-dependent vasodilation, we studied leg blood flow (LBF) responses to graded intrafemoral artery infusions of methacholine chloride (MCh) or sodium nitroprusside (SNP) during saline infusion and euglycemic hyperinsulinemia in lean insulin-sensitive controls (C), in obese insulin-resistant subjects (OB), and in subjects with non-insulin-dependent diabetes mellitus (NIDDM). MCh induced increments in LBF were approximately 40% and 55% lower in OB and NIDDM, respectively, as compared with C (P < 0.05). Euglycemic hyperinsulinemia augmented the LBF response to MCh by - 50% in C (P < 0.05 vs saline) but not in OB and NIDDM. SNP caused comparable increments in LBF in all groups. Regression analysis revealed a significant inverse correlation between the maximal LBF change in response to MCh and body fat content. Thus, obesity/insulin resistance is associated with (a) blunted endothelium-dependent, but normal endothelium-independent vasodilation and (b) failure of euglycemic hyperinsulinemia to augment endothelium-dependent vasodilation. Therefore, obese/insulin-resistant subjects are characterized by endothelial dysfunction and endothelial resistance to insulin's effect on enhancement of endothelium-dependent vasodilation. This endothelial dysfunction could contribute to the increased risk of atherosclerosis in obese insulin-resistant subjects.

Obesity/insulin resistance is associated with endothelial dysfunction. Implications for the syndrome of insulin resistance

To test the hypothesis that obesity/insulin resistance impairs both endothelium-dependent vasodilation and insulin-mediated augmentation of endothelium-dependent vasodilation, we studied leg blood flow (LBF) responses to graded intrafemoral artery infusions of methacholine chloride (MCh) or sodium nitroprusside (SNP) during saline infusion and euglycemic hyperinsulinemia in lean insulin-sensitive controls (C), in obese insulin-resistant subjects (OB), and in subjects with non-insulin-dependent diabetes mellitus (NIDDM). MCh induced increments in LBF were approximately 40% and 55% lower in OB and NIDDM, respectively, as compared with C (P < 0.05). Euglycemic hyperinsulinemia augmented the LBF response to MCh by - 50% in C (P < 0.05 vs saline) but not in OB and NIDDM. SNP caused comparable increments in LBF in all groups. Regression analysis revealed a significant inverse correlation between the maximal LBF change in response to MCh and body fat content. Thus, obesity/insulin resistance is associated with (a) blunted endothelium-dependent, but normal endothelium-independent vasodilation and (b) failure of euglycemic hyperinsulinemia to augment endothelium-dependent vasodilation. Therefore, obese/insulin-resistant subjects are characterized by endothelial dysfunction and endothelial resistance to insulin's effect on enhancement of endothelium-dependent vasodilation. This endothelial dysfunction could contribute to the increased risk of atherosclerosis in obese insulin-resistant subjects.

NG-monomethyl-L-arginine inhibits the blood flow but not the insulin-like response of forearm muscle to IGF- I: possible role of nitric oxide in muscle protein synthesis

In human skeletal muscle, insulin-like growth factor-I (IGF-I) exerts both growth hormone-like (increase in protein synthesis) and insulin-like (decrease in protein degradation and increase in glucose uptake) actions and augments forearm blood flow two- to threefold. This study was designed to address whether (a) the increase in blood flow due to IGF-I could be blocked by an inhibitor of nitric oxide synthase; and (b) the metabolic actions of IGF-I were altered by use of a nitric oxide synthase inhibitor. Forearm blood flow, glucose, lactate, oxygen, nitrite, and phenylalanine balances and phenylalanine kinetics were studied in a total of 17 healthy, adult volunteers after an overnight fast in two different protocols. In protocol 1, after basal samples IGF-I was infused alone for 4 h with samples repeated during the last 30 min. After the 4-h sample period, NG-monomethyl-L-arginine (L-NMMA) was infused into the brachial artery for 2 h to bring flow back to baseline and repeat samples were taken (6 h). In response to IGF-I alone, forearm blood flow rose from 3.8 +/- 1.0 (bas) to 7.9 +/- l.9 (4 h) ml/min/100 ml (P < 0.01) and was reduced back to baseline by L-NMMA at 6 h (P < 0.01). In protocol 1, IGF-I alone increased forearm nitrite release at 4 h (P < 0.03), which was reduced back to baseline by L-NMMA at 6 h (P < 0.05). Despite the reduction in flow with L-NMMA, IGF+L-NMMA yielded increases in glucose uptake (P < 0.005), lactate release (P < 0.04), oxygen uptake (P < 0.01), and a positive shift in phenylalanine balance (P < 0.01) due to both an increase in muscle protein synthesis (P < 0.02) and a decrease in protein degradation (P < 0.03). In protocol 2, L-NMMA was coinfused with IGF-I for 6 h, with the dose titrated to keep blood flow +/- 25% of baseline. Coinfusion of L-NMMA restrained blood flow to baseline and also yielded the same, significant metabolic effects, except that no significant increase in muscle protein synthesis was detected. These observations suggest: (a) that IGF-I increases blood flow through a nitric oxide-dependent mechanism; (b) that total blood flow does not affect the insulin-like response of muscle to IGF-I; and (c) that nitric oxide may be required for the protein synthetic (growth hormone-like) response of muscle to IGF-I.

Vascular actions of insulin in health and disease

Insulin has well known metabolic effects. However, depending on the magnitude and duration of the insulin stimulus, this hormone can also produce vasodilation and vascular smooth muscle growth. The association of hyperinsulinemia with the metabolic disorders of obesity and non-insulin-dependent diabetes, as well as with the cardiovascular pathologies of hypertension and atherosclerosis, has led to suggestions that perhaps elevated insulin levels are causally related to these diseases. Alternatively, insulin resistance may develop following an increase in skeletal muscle vascular resistance, with or without hypertension, such that a reduction in skeletal muscle blood flow leads to an attenuated glucose delivery and uptake. These hypotheses are explored in this review by examining the effects of insulin on vascular smooth muscle tissue during both acute and prolonged exposure. An interaction among hyperinsulinemia, hyperglycemia, and hyperlipidemia associated with the insulin resistant state is described whereby insulin resistance can be both a cause and a result of elevated vascular resistance. The association between blood flow and insulin stimulated glucose uptake suggests that therapeutic intervention against the development of skeletal muscle vascular resistance should occur early in individuals generally predisposed to cardiovascular pathology in order to attenuate, or avoid, insulin resistance and its sequelae.