Muscle Microvascular Blood Flow Research Articles

Short-term high-calorie high-fat feeding induces hyperinsulinemia and blunts skeletal muscle microvascular blood flow in healthy humans.

Skeletal muscle microvascular blood flow (MBF) plays an important role in glucose disposal in muscle. Impairments in muscle MBF contribute to insulin resistance and pre-diabetes. Animal studies show that short-term (3 day) high-fat feeding blunts skeletal muscle MBF prior to impairing insulin-stimulated glucose disposal. It is not known if this occurs in humans. We investigated the temporal impact of a 7-day high-calorie high-fat (HCHF) diet intervention (+52% kJ; 41% fat) on fasting and postprandial cardiometabolic outcomes in 14 healthy adults (18-37 years). Metabolic health and vascular responses to a mixed meal challenge (MMC) were measured at pre- (day 0), mid- (day 4) and post-intervention (day 8). There were no significant differences in body weight, body fat %, fasting blood glucose, and fasting plasma insulin concentrations at pre-, mid- and post-intervention. Compared to pre-intervention there was a significant increase in insulin (but not glucose) total area under the curve, in response to the MMC at mid-intervention (p=0.041) and at post-intervention (p=0.028). Unlike at pre- and mid-intervention, at post-intervention muscle MBF decreased at 60 min (p=0.024) and 120 min (p=0.023) following the MMC. However, macrovascular blood flow was significantly increased from 0 to 60 mins (p<0.001) and 120 mins (p<0.001) following the MMC at pre-, mid- and post-intervention. Therefore, short-term HCHF feeding in healthy individuals leads to elevated postprandial insulin but not glucose levels and a blunting of meal-induced skeletal muscle MBF responses but not macrovascular blood flow responses.

BLUNTED SKELETAL MUSCLE MICROVASCULAR BLOOD FLOW IS A POTENTIAL UNDERLYING MECHANISM FOR EXERCISE INTOLERANCE IN ADULTS WITH ATRIAL FIBRILLATION

INTRODUCTION &amp; AIMS Approximately 45% of adults living with atrial fibrillation (AF) experience exercise intolerance (EI). However, the mechanisms of EI in AF are not well understood. We aimed to determine whether impaired skeletal muscle microvascular blood flow (MBF), not macrovascular, responses to peak exercise is a plausible explanation for EI in adults with AF. METHODS Adults with AF and healthy controls completed a Modified Bruce treadmill protocol to obtain peak oxygen uptake (V̇O2peak). Skeletal muscle microvascular blood volume, velocity, and flow in the vastus lateralis muscle was assessed using contrast enhanced ultrasound. Superficial femoral artery diameter, blood velocity and flow were assessed using 2D and Doppler ultrasound. Vascular measurements were collected at rest, immediately post-exercise, and 30 minutes post-exercise. RESULTS Nine adults with AF (age: 62±5 years, 66% females, BMI: 29.7±4.2 kg/m2, V̇O2peak: 24.3±6.1 mL/kg/min) and seven controls (age: 63±10 years, 57% females, BMI: 26.7±1.7 kg/m2, V̇O2peak: 31.0±7.5 mL/kg/min) participated. One participant was in AF during testing. A significant group x time interaction in skeletal muscle MBF (p=0.04) and near significant interaction in blood volume (capillary recruitment, p=0.08) was observed. Post-hoc analysis revealed adults with AF had a significantly blunted MBF at 30 minutes post-exercise (-1.3 fold versus control, p=0.01) and reduced microvascular blood volume (i.e. capillary recruitment) pre- and 30 minutes post-exercise (-6.6 fold [p=0.01] and -9.4 fold [p=0.01] versus controls). No differences were observed for changes in skeletal muscle microvascular blood velocity, or femoral artery diameter, blood velocity or flow. CONCLUSION Despite similar femoral artery blood flow responses, adults with AF have lower skeletal muscle MBF responses to peak exercise which may be driven by a reduced capillary recruitment in the skeletal muscle. Our findings provide new insight into vascular complications which may partially explain EI in those with AF.

EXPLORING THE ROLE OF SKELETAL MUSCLE MICROVASCULAR BLOOD FLOW IN EXERCISE CAPACITY AMONG INDIVIDUALS WITH CONTROLLED TYPE 2 DIABETES

INTRODUCTION &amp; AIMS The cardiac and macrovascular system is vital for human exercise capacity, yet the microvascular network is primarily responsible for delivering nutrients and oxygen to metabolically active tissues like skeletal muscle. Due to methodological limitations, few studies have investigated the role of muscle microvascular blood flow in determining exercise capacity or intolerance in individuals with chronic conditions such as type 2 diabetes. Our aim was to use modern contrast-enhanced ultrasound to directly measure microvascular blood flow in skeletal muscle at rest and after a maximal treadmill exercise test in individuals with controlled type 2 diabetes. METHODS Fifteen adults with controlled type 2 diabetes (age: 62.5 ± 8.0 years [M ± SD]; BMI: 31.6 ± 6.9 kg/m2; HbA1c: 6.7 ± 0.9%; 13 females and 2 male) attended the laboratory after a 2-hour fast and completed a Modified Bruce treadmill exercise test until exhaustion. Microvascular blood velocity, volume, and flow in the vastus lateralis muscle was assessed at rest, immediately post-exercise, and 30 minutes post-exercise via contrast-enhanced ultrasound during a constant-rate intravenous contrast agent infusion (DEFINITY®). Participants also completed a whole-body dual x-ray absorptiometry scan and a six-minute walk test (6MWT). RESULTS Microvascular blood velocity, volume, and flow increased immediately after exercise (∼230%, ∼79%, and ∼529%, respectively; all p&amp;lt;0.001), and remained elevated above baseline levels 30 minutes post-exercise (∼79%, ∼25%, and ∼139%, respectively; all p≤0.003). Resting and post-exercise skeletal muscle microvascular blood flow measures were not associated with VO2peak (ml/min/kg), time to exhaustion, or six-minute walk distance (all p&amp;gt;0.05). CONCLUSION Muscle microvascular blood flow remains elevated for up to 30 minutes following maximal aerobic exercise in individuals with controlled type 2 diabetes. The lack of association between muscle microvascular blood flow and exercise capacity is not clear but may indicate considerable blood flow redundancy or reserve within the vascular network.

Treatment of Hypotension with Phenylephrine Increases Cerebral Microvascular Perfusion in a Rat Model of Acute Hemodilution

Intraoperative blood loss is associated with reduced mean arterial blood pressure (MAP), which may endanger vital organ perfusion, including the brain. Phenylephrine (PE), an α1-receptor agonist, is clinically utilized to treat intraoperative hypotension through vasoconstriction of resistance arterioles. Clinical studies suggest that raising MAP with PE may decrease perfusion to vital organs such as the brain. Experimental studies suggest that lower α-receptor density in the cerebral resistance arteries may lead to preserved brain perfusion after PE administration. We hypothesize that treatment of hypotension with PE will result in preserved brain perfusion due to different microvascular blood flow and tissue oxygen tension (pO2) responses in the brain and skeletal muscle in an experimental rat model. With Animal Care and Use Committee approval in accordance with ARRIVE-2 guidelines, spontaneously breathing anesthetized rats (1.5% isoflurane) underwent hemodilution by exchanging 40-50% estimated blood volume 1:1 with saline over 10 minutes to achieve a 10-20 mmHg drop in MAP. MAP (tail artery) was measured continuously. Hypotension was treated with PE (10-20 μg/kg/min iv) to increase MAP to a target above baseline MAP. Shed blood was re-infused after stopping the PE infusion. Microvascular blood flow (Oxyflo; Oxford Optronix, Oxford, UK, n=7) or microvascular pO2 (OxyLED, Oxyphor G4, Oxygen Enterprises Ltd., Philadelphia, PA, n=6) were measured continuously in the cerebral cortex (burr hole) and biceps femoris muscle simultaneously in each rat. Results were analyzed via one or two-way ANOVA, with significance achieved at p&lt;0.05. Baseline and post-hemodilution MAP values were comparable in both groups. PE infusion increased MAP to a mean level near 100 mmHg in both groups (p&lt;0.05). Microvascular blood flow and tissue pO2 were greater in the brain as compared to the muscle at all time points (p&lt;0.01 for both). PE infusion resulted in an increase in cerebral microvascular blood flow from baseline (1465±689 vs. 687±453 blood perfusion units (BPU); p&lt;0.001) that was sustained after discontinuing PE and during blood reinfusion. No changes in muscle microvascular flow were observed after PE infusion (294±129 vs. 168±113 BPU). A sustained increase in cerebral microvascular pO2 was measured following PE infusion (50.2±8.6 vs. 40.6±4.6 mmHg; p=0.02), whereas muscle tissue pO2 did not change (22.2±5.6 vs. 25.0±2.5 mmHg). A differential blood flow and microvascular pO2 response was observed following PE infusion in brain and skeletal muscle. Both cerebral microvascular blood flow and tissue pO2 demonstrated sustained increases after PE infusion. By contrast, no changes were observed in muscle microvascular blood flow and pO2. These results suggest that treatment of hypotension with PE administration preferentially increased cerebral vs. muscle perfusion in an experimental model of acute blood loss and fluid resuscitation. Canadian Anesthesiologists’ Society and University of Toronto Merit Award Support. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

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.

Contribution of reduced skeletal muscle perfusion reserve to exercise intolerance in HFpEF

Abstract Methods Transthoracic echocardiography and skeletal muscle microvascular perfusion by contrast-enhanced ultrasound were performed at rest and immediately post exercise test in 77 HFpEF patients in NYHA class II and III, and in 20 age- and sex-matched individuals with a similar profile of comorbidities, but normal exercise tolerance. Exercise reserve of cardiac function parameters and skeletal muscle perfusion was calculated by subtracting resting value from exercise value. Results HFpEF patients demonstrated impairment of left ventricular (LV) and left atrial (LA) function reserve as reflected by lower exercise-induced increases in LV global longitudinal strain, LA reservoir strain, cardiac output and e’ mitral annular velocity, and higher increase in E/e’ ratio (Table). Similarly, skeletal muscle microvascular blood flow (MBF) reserve was lower in HFpEF patients than in the referents. There was no significant difference between the study group and the control group in resting MBF. MBF reserve was significantly associated with exercise capacity (r = 0.34; p = 0.004) and cardiac output reserve (r = 0.24; p = 0.04). In multivariable analysis including clinical and demographic variables, LV and LA components of cardiac functional reserve (LV global longitudinal strain, LA reservoir strain, cardiac output, e’ and E/e’) and MBF reserve, change from rest to exercise in MBF (beta 0.32, SE 0.10, p = 0.003) was in addition to patient age, sex and LV strain reserve among the independent correlates of exercise capacity. Conclusions Skeletal muscle perfusion reserve is impaired in HFpEF, and is associated with reduced exercise capacity independent of clinical, demographic and "central" cardiac factors. Accordingly, our results provide a rationale for extending the spectrum of pathophysiological milieu to be considered when planning patient management strategies in HFpEF over skeletal muscle-associated contributors to exercise intolerance.

Insufficient arterial driving pressure turns hypoxic microvascular remodeling into a catastrophe

Abstract The tissue microvasculature is known to remodel in response to ischemia. However, the benefit or harm of this process is not completely understood and may limit also the potential of taking control of ischemic diseases that remain the leading cause of death worldwide. In a series of clinical and preclinical studies, we have thus dissected the biological relevance and the pathomechanistic role of hypoxic microvascular remodeling in critical limb threatening ischemia (CLTI). High-resolution contrast enhanced ultrasound was used in 50 patients with severe CLTI scheduled for immediate revascularization to study ischemic muscle microvascular blood flow. Immunohistological analyses were performed for amputation muscle samples from a set of 40 CLTI patients to analyze microvascular changes with respect to the level of myofiber atrophy. Furthermore, the amputation muscle samples were used to find 3 paired samples from 20 patients representing microvascular transformation along worsening of ischemic muscle damage, and used for RNA sequencing to analyze mechanisms of microvascular transformation. Additionally, animal models for acute ischemia in C57Bl-mice (n=42) and hypercholesterolemic LDLR-/-ApoB100/100 mice (n=61) and chronic CLTI-like ischemia model in cholesterol-fed NZW rabbits (n=24) with AdVEGF-A or AdLacZ control gene transfer were employed to study: 1) the natural course of microvascular changes leading to efficient muscle regeneration; 2) the effect of reduced arterial driving pressure on muscle regeneration; and 3) the effect of reduced arterial driving pressure and microvascular transformation on muscle regeneration, respectively. Decreased capillary transit time (P&amp;lt;0.01) and a gradual arterialization of capillaries along worsening of ischemic tissue damage (P&amp;lt;0.001) were observed in CLTI muscle. CLTI muscle RNA sequencing further suggested that the capillary arterialization process is an orchestration of different growth factors, cell types and even physical forces in the hypoxic muscle. Furthermore, animal models of acute and chronic limb ischemia display post-ischemic capillary enlargement (P&amp;lt;0.05) as a primary microvascular modification that facilitates collateral maturation but under decreased arterial driving pressure can turn from a good intention to aggravating ischemic damage. These results depict a critical role for the microvasculature in determining ischemic muscle outcome after an initial arterial occlusion in CLTI. Hypoxic microvascular remodeling, according to this data, initially has a good purpose but can also cause harm given enough time and the wrong environment. Moreover, these results offer understanding to the differential natural recoveries of experimental animals and human patients from an ischemic exposure – information that is essential for designing better treatments for ischemic diseases.

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.

Contrast-enhanced ultrasound repeatability for the measurement of skeletal muscle microvascular blood flow.

What is the central question of this study? Contrast-enhanced ultrasound (CEUS) can be used to directly assess skeletal muscle perfusion but its day-to-day repeatability over time has not yet been validated: is CEUS a repeatable method for the measurement of skeletal muscle microvascular blood flow (MBF) at rest and in response to exercise, across independent assessment sessions? What is the main finding and its importance? A strong agreement between CEUS MBF measures across sessions suggests it is a repeatable method for assessing skeletal muscle perfusion over time. This validation provides confidence for incorporating these measures into longitudinal studies such as a chronic intervention or disease progression to gain further knowledge of skeletal muscle microvascular function. Contrast-enhanced ultrasound (CEUS) can be used to directly assess skeletal muscle perfusion. However, its repeatability over time has not yet been validated and therefore its use in longitudinal measures (i.e., exploring the impact of a chronic intervention or disease progression) is limited. This study aimed to determine the repeatability of CEUS for the measurement of skeletal muscle microvascular blood flow (MBF) at baseline and in response to exercise, across independent assessment sessions. Ten healthy volunteers (five female; 30±6years) had CEUS of the right vastus lateralis recorded in two separate sessions, 14days apart. Measurements were taken at baseline, during an isometric leg extension and during recovery. Acoustic intensity data from a region of interest were plotted as a replenishment curve to obtain blood volume (A) and flow velocity (β) values from a one-phase association non-linear regression of mean tissue echogenicity. Linear regression and Bland-Altman analyses of A and β values were performed, with significance assumed as P<0.05. Strong positive correlations were observed across sessions for all A and β values (both P<0.0001). Bland-Altman analysis showed a bias (SD) of -0.013±1.24 for A and -0.014±0.31 for β. A bias of 0.201±0.770 at baseline, 0.527±1.29 during contraction and -0.203±1.29 at recovery was observed for A, and -0.0328±0.0853 (baseline), -0.0446±0.206 (contraction) and 0.0382±0.233 (recovery) for β. A strong agreement between CEUS MBF measures across independent sessions suggests it to be a repeatable method for assessing skeletal muscle perfusion over time, and therefore facilitates wider use in longitudinal studies.

Effects of GLP-1 Infusion Upon Whole-body Glucose Uptake and Skeletal Muscle Perfusion During Fed-state in Older Men.

Ageing skeletal muscles become both insulin resistant and atrophic. The hormone glucagon-like peptide 1 (GLP-1) facilitates postprandial glucose uptake as well as augmenting muscle perfusion, independent of insulin action. We thus hypothesized exogenous GLP-1 infusions would enhance muscle perfusion and positively affect glucose metabolism during fed-state clamps in older people. Eight men (71 ± 1 years) were studied in a randomized crossover trial. Basal blood samples were taken before postprandial (fed-state) insulin and glucose clamps, accompanied by amino acid infusions, for 3 hours. Reflecting this, following insertions of peripheral and femoral vessels cannulae and baseline measurements, peripheral IV infusions of octreotide, insulin (Actrapid), 20% glucose, and mixed amino acids; Vamin 14-EF with or without a femoral arterial GLP-1 infusion were started. GLP-1, insulin, and C-peptide were measured by ELISA. Muscle microvascular blood flow was assessed via contrast enhanced ultrasound. Whole-body glucose handling was assayed by assessing glucose infusion rate parameters. Skeletal muscle microvascular blood flow significantly increased in response to GLP-1 vs feeding alone (5.0 ± 2.1 vs 1.9 ± 0.7 fold-change from basal, respectively; P = 0.008), while also increasing whole-body glucose uptake (area under the curve 16.9 ± 1.7 vs 11.4 ± 1.8 mg/kg-1/180 minutes-1, P = 0.02 ± GLP, respectively). The beneficial effects of GLP-1 on whole-body glycemic control are evident with insulin clamped at fed-state levels. GLP-1 further enhances the effects of insulin on whole-body glucose uptake in older men, underlining its role as a therapeutic target. The effects of GLP-1 in enhancing microvascular flow likely also affects other glucose-regulatory organs, reflected by greater whole-body glucose uptake.

Toward a Better Understanding of Muscle Microvascular Perfusion During Exercise in Patients With Peripheral Artery Disease: The Effect of Lower-Limb Revascularization

Purpose: Leg muscle microvascular blood flow (perfusion) is impaired in response to maximal exercise in patients with peripheral artery disease (PAD); however, during submaximal exercise, microvascular perfusion is maintained due to a greater increase in microvascular blood volume compared with that seen in healthy adults. It is unclear whether this submaximal exercise response reflects a microvascular impairment, or whether it is a compensatory response for the limited conduit artery flow in PAD. Therefore, to clarify the role of conduit artery blood flow, we compared whole-limb blood flow and skeletal muscle microvascular perfusion responses with exercise in patients with PAD (n=9; 60±7 years) prior to, and following, lower-limb endovascular revascularization. Materials and Methods: Microvascular perfusion (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after a 5 minute bout of submaximal intermittent isometric plantar-flexion exercise using contrast-enhanced ultrasound imaging. Exercise contraction-by-contraction whole-leg blood flow and vascular conductance were measured using strain-gauge plethysmography. Results: With revascularization there was a significant increase in whole-leg blood flow and conductance during exercise (p<0.05). Exercise-induced muscle microvascular perfusion response did not change with revascularization (pre-revascularization: 3.19±2.32; post-revascularization: 3.89±1.67 aU.s−1; p=0.38). However, the parameters that determine microvascular perfusion changed, with a reduction in the microvascular volume response to exercise (pre-revascularization: 6.76±3.56; post-revascularization: 2.42±0.69 aU; p<0.01) and an increase in microvascular flow velocity (pre-revascularization: 0.25±0.13; post-revascularization: 0.59±0.25 s−1; p=0.02). Conclusion: These findings suggest that patients with PAD compensate for the conduit artery blood flow impairment with an increase in microvascular blood volume to maintain muscle perfusion during submaximal exercise. Clinical Impact The findings from this study support the notion that the impairment in conduit artery blood flow in patients with PAD leads to compensatory changes in microvascular blood volume and flow velocity to maintain muscle microvascular perfusion during submaximal leg exercise. Moreover, this study demonstrates that these microvascular changes are reversed and become normalized with successful lower-limb endovascular revascularization.

Is vascular insulin resistance an early step in diet-induced whole-body insulin resistance?

There is increasing evidence that skeletal muscle microvascular (capillary) blood flow plays an important role in glucose metabolism by increasing the delivery of glucose and insulin to the myocytes. This process is impaired in insulin-resistant individuals. Studies suggest that in diet-induced insulin-resistant rodents, insulin-mediated skeletal muscle microvascular blood flow is impaired post-short-term high fat feeding, and this occurs before the development of myocyte or whole-body insulin resistance. These data suggest that impaired skeletal muscle microvascular blood flow is an early vascular step before the onset of insulin resistance. However, evidence of this is still lacking in humans. In this review, we summarise what is known about short-term high-calorie and/or high-fat feeding in humans. We also explore selected animal studies to identify potential mechanisms. We discuss future directions aimed at better understanding the ‘early’ vascular mechanisms that lead to insulin resistance as this will provide the opportunity for much earlier screening and timing of intervention to assist in preventing type 2 diabetes.

225-LB: Impaired Microvascular Responses May Explain Health Disparities and Impaired Metabolic Flexibility in Hispanic Residents of the Rio Grande Valley

Purpose: Impaired skeletal muscle microvascular blood flow (MBF) is one of the earliest pathophysiologies of type 2 diabetes (T2D) . People with a family history of T2D (FH+ are at greater risk for developing T2D than those with no family history (FH-) . Further, we have shown that normoglycemic Caucasian FH+ individuals have impaired skeletal muscle microvascular responses and metabolic flexibility (MF - ability to switch substrate oxidation upon stimulation) to a mixed meal challenge (MMC) when compared with matched FH- (p&amp;lt;0.01) , suggesting early vascular insulin resistance. Mexican Hispanic individuals have the highest prevalence of T2D in the US and make up ∼95% of Hispanic residents of the Rio Grande Valley (HRGV) where T2D diagnosis rates are ∼3x the national average. Therefore, we sought to determine if impaired MBF responses are present in healthy HRGV. Methods: 15 FH- and 13 FH+ HRGV were recruited to assess MF and MBF responses to a MMC. MF was measured via indirect calorimetry and MBF via contrast-enhanced ultrasound while fasting and again 60-min post MMC. Results: No changes in skeletal MBF were noted in FH- (1.21 ± 0.53 SEM and 0.86 ± 0.23 SEM fasting and postprandial MBF respectively) or FH+ (2.17 ± 0.98 SEM and 1.49 ± 0.66 SEM fasting and postprandial MBF respectively) groups. Abdominal adipose MBF responses increased in FH+ (-2.3 ± 0.71 SEM, p=0.004) , but not in FH- (-0.49 ± 0.39 SEM p=0.39) . FH+ display impaired postprandial metabolic flexibility (MF) responses during 60-min of testing post-MMC. Neither blood glucose responses (99.3 ± 1.3 SEM and 102.27 ± 2.1 SEM fasting and post-prandial respectively, p=0.2) nor body composition differed between FH groups. Conclusions: HRGV display impaired skeletal muscle MBF responses, indicating early vascular insulin resistance. Further, the impairment in MF in FH+ may be partially explained by increased adipose MBF responses, as adipose is less metabolically active than skeletal muscle. Disclosure R. D. Russell: None. Y. Tai: None. S. Marupudi: None.

Microvascular Blood Flow Response to Hyperinsulinemic‐Euglycemic Clamp in Zucker Diabetic Sprague Dawley Rat Model of Type 2 Diabetes

ObjectiveQuantify and characterize the skeletal muscle microvascular blood flow in Zucker Diabetic Sprague Dawley (ZDSD) rat model of T2D at rest and during a hyperinsulinemic‐euglycemic clamp.HypothesisChronic hyperglycemia in the ZDSD rat model of T2D causes blunted capillary hemodynamic response to hyperinsulinemic‐euglycemic clamp compared to the age and diet matched control Sprague Dawley (SD) rats.MethodsBiweekly blood glucose and conscious blood pressure measurements were collected from 7‐ to 25‐week‐old male SD (n=12) and ZDSD (n=18) rats. At 27 weeks of age, animals were fasted overnight prior to intravital video microscopy experiments. On the day of experiment animals were anaesthetized with an intraperitoneal injection of pentobarbital, tracheotomized, and carotid and external jugular cannulas were inserted to monitor physiological status and for administration of fluids. The extensor digitorum longus (EDL) muscle of the lower hind limb was isolated, blunt dissected and reflected over a glass coverslip on the stage of an inverted microscope. Recordings of the EDL were completed at baseline and during hyperinsulinemic‐euglycemic clamp. Hyperinsulinemic‐euglycemic clamp was achieved by continuous intravenous infusion of insulin (Humulin R 2U/hr/kg) and a 50% glucose mixture to titrate blood glucose to euglycemia (5‐7 mM). Mixed blood samples were collected from a tail clip to determine systemic blood glucose. Analysis of intravital videos were completed offline using custom MATLAB software. ZDSD rats that had fasting blood glucose of less than 20 mM at 19 weeks of age were excluded from the hyperglycemic group. Animals exhibiting aberrant microvascular blood flow at baseline or during euglycemic clamp were excluded from the data set. All animal protocols were approved by Memorial University of Newfoundland’s Animal Care and Use Committee.ResultsHyperglycemic ZDSD rats had a significantly higher blood glucose than SD controls from 17 to 25 weeks old and at the beginning of euglycemic clamp (26.13 ± 0.97 and 5.95 ± 0.37, p &lt; 0.0001, Fig 1A). SD rats had a significantly higher glucose infusion rate than ZDSD rats (22.58 ± 1.81 and 13.19 ± 2.76, p &lt; 0.0001, Fig 1B). SD rats had a significant increase in red blood cell (RBC) velocity (149.8 ± 56.71 and 210.0 ± 62.14 µm/s, p = 0.0051, Fig 2A), RBC supply rate (SR) (12.49 ± 8.23 and 19.91 ± 12.25 cells/s, p = 0.004, Fig 2B) and RBC oxygen saturation(sO2) (38.05 ± 11.65 and 45.70 ± 10.13, p = 0.014, Fig 2D) between baseline and euglycemia. However, ZDSD animals had no significant difference in RBC velocity (178.1 ± 63.91 and 194.7 ± 37.69 µm/s, p = 0.3683, Fig 2A), RBC supply rate (14.63 ± 7.55 and 20.03 ± 7.46 cells/s, p = 0.0704, Fig 2B), hematocrit (31.03 ± 3.23 and 33.02 ± 6.22 %, p = 0.4309, Fig 2C), and RBC sO2 (61.91 ± 11.40 and 64.00 ± 11.59, p = 0.5324 Fig 2D). The RBC sO2 was significantly higher in ZDSD than SD at baseline (61.91 ± 11.40 compared to 38.05 ± 11.65, p = 0.0166, Fig 2D).ConclusionSD rats had a robust increase in capillary hemodynamics during hyperinsulinemic‐euglycemic clamp. However, the skeletal muscle capillary hemodynamics of ZDSD rats did not significantly change between baseline and during euglycemia. These finding suggests a potential impairment in skeletal muscle microvascular blood flow response in this type 2 diabetic rodent mode. The elevated RBC sO2 in ZDSD rats may indicate a dysregulation of capillary blood flow in the ZDSD model.

Curcumin Enhances Fed-State Muscle Microvascular Perfusion but Not Leg Glucose Uptake in Older Adults.

Therapeutic interventions aimed at enhancing blood flow may combat the postprandial vascular and metabolic dysfunction that manifests with chronological ageing. We compared the effects of acute curcumin (1000 mg) coupled with an oral nutritional supplement (ONS, 7.5 g protein, 24 g carbohydrate and 6 g fat) versus a placebo and ONS (control) on cerebral and leg macrovascular blood flow, leg muscle microvascular blood flow, brachial artery endothelial function, and leg insulin and glucose responses in healthy older adults (n = 12, 50% male, 73 ± 1 year). Curcumin enhanced m. tibialis anterior microvascular blood volume (MBV) at 180 and 240 min following the ONS (baseline: 1.0 vs. 180 min: 1.08 ± 0.02, p = 0.01 vs. 240 min: 1.08 ± 0.03, p = 0.01), and MBV was significantly higher compared with the control at both time points (p < 0.05). MBV increased from baseline in the m. vastus lateralis at 240 min after the ONS in both groups (p < 0.05), and there were no significant differences between groups. Following the ONS, leg blood flow and leg vascular conductance increased, and leg vascular resistance decreased similarly in both conditions (p < 0.05). Brachial artery flow-mediated dilation and middle cerebral artery blood flow were unchanged in both conditions (p > 0.05). Similarly, the curcumin and control groups demonstrated comparable increases in glucose uptake and insulin in response to the ONS. Thus, acute curcumin supplementation enhanced ONS-induced increases in m. tibialis anterior MBV without potentiating m. vastus lateralis MBV, muscle glucose uptake, or systemic endothelial or macrovascular function in healthy older adults.

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

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.

Contrast Ultrasound Assessment of Skeletal Muscle Recruitable Perfusion after Permanent Left Ventricular Assist Device Implantation: Implications for Functional Recovery

In heart failure with reduced ejection fraction (HFrEF), abnormal regulation of skeletal muscle perfusion contributes to reduced exercise tolerance. The aim of this study was to test the hypothesis that improvement in functional status after permanent left ventricular assist device (LVAD) implantation in patients with HFrEF is related to improvement in muscle perfusion during work, which was measured using contrast-enhanced ultrasound (CEUS). CEUS perfusion imaging of calf muscle at rest and during low-intensity plantar flexion exercise (20W, 0.2Hz) was performed in patients with HFrEF (n=22) at baseline and 3months after placement of permanent LVADs. Parametric analysis of CEUS data was used to quantify muscle microvascular blood flow (MBF), blood volume index, and red blood cell flux rate. For subjects alive at 3months, comparisons were made between those with New York Heart Association functional class I or II (n=13) versus III or IV (n=7) status after LVAD. Subjects were followed for a median of 5.7years for mortality. Echocardiographic data before and after LVAD placement and LVAD parameters were similar in subjects classified with New York Heart Association functional class I-II versus functional class III-IV after LVAD. Skeletal muscle MBF at rest and during exercise before LVAD implantation was also similar between groups. After LVAD placement, resting MBF remained similar between groups, but during exercise those with New York Heart Association functional class I or II had greater exercise MBF (111±60 vs 52±38 intensity units/sec, P=.03), MBF reserve (median, 4.45 [3.95 to 6.80] vs 2.22 [0.98 to 3.80]; P=.02), and percentage change in exercise MBF (median, 73% [-28% to 83%] vs -45% [-80% to 26%]; P=.03). During exercise, increases in MBF were attributable to faster microvascular flux rate, with little change in blood volume index, indicating impaired exercise-mediated microvascular recruitment. The only clinical or echocardiographic feature that correlated with post-LVAD exercise MBF was a history of diabetes mellitus. There was a trend toward better survival in patients who demonstrated improvement in muscle exercise MBF after LVAD placement (P=.05). CEUS perfusion imaging can quantify peripheral vascular responses to advanced therapies for HFrEF. After LVAD implantation, improvement in functional class is seen in patients with improvements in skeletal muscle exercise perfusion and flux rate, implicating a change in vasoactive substances that control resistance arteriolar tone.

Green Tea Extract Concurrent with an Oral Nutritional Supplement Acutely Enhances Muscle Microvascular Blood Flow without Altering Leg Glucose Uptake in Healthy Older Adults.

Postprandial macro- and microvascular blood flow and metabolic dysfunction manifest with advancing age, so vascular transmuting interventions are desirable. In this randomised, single-blind, placebo-controlled, crossover trial, we investigated the impact of the acute administration of green tea extract (GTE; containing ~500 mg epigallocatechin-3-gallate) versus placebo (CON), alongside an oral nutritional supplement (ONS), on muscle macro- and microvascular, cerebral macrovascular (via ultrasound) and leg glucose/insulin metabolic responses (via arterialised/venous blood samples) in twelve healthy older adults (42% male, 74 ± 1 y). GTE increased m. vastus lateralis microvascular blood volume (MBV) at 180 and 240 min after ONS (baseline: 1.0 vs. 180 min: 1.11 ± 0.02 vs. 240 min: 1.08 ± 0.04, both p < 0.005), with MBV significantly higher than CON at 180 min (p < 0.05). Neither the ONS nor the GTE impacted m. tibialis anterior perfusion (p > 0.05). Leg blood flow and vascular conductance increased, and vascular resistance decreased similarly in both conditions (p < 0.05). Small non-significant increases in brachial artery flow-mediated dilation were observed in the GTE only and middle cerebral artery blood flow did not change in response to GTE or CON (p > 0.05). Glucose uptake increased with the GTE only (0 min: 0.03 ± 0.01 vs. 35 min: 0.11 ± 0.02 mmol/min/leg, p = 0.007); however, glucose area under the curve and insulin kinetics were similar between conditions (p > 0.05). Acute GTE supplementation enhances MBV beyond the effects of an oral mixed meal, but this improved perfusion does not translate to increased leg muscle glucose uptake in healthy older adults.

Six weeks of high-intensity interval training enhances contractile activity induced vascular reactivity and skeletal muscle perfusion in older adults

Impairments in muscle microvascular function are associated with the pathogenesis of sarcopenia and cardiovascular disease. High-intensity interval training (HIIT) is an intervention by which a myriad of beneficial skeletal muscle/cardiovascular adaptations have been reported across age, including capillarisation and improved endothelial function. Herein, we hypothesised that HIIT would enhance muscle microvascular blood flow and vascular reactivity to acute contractile activity in older adults, reflecting HIIT-induced vascular remodelling. In a randomised controlled-trial, twenty-five healthy older adults aged 65–85 years (mean BMI 27.0) were randomised to 6-week HIIT or a no-intervention control period of an equal duration. Measures of microvascular responses to a single bout of muscle contractions (i.e. knee extensions) were made in the m. vastus lateralis using contrast-enhanced ultrasound during a continuous intravenous infusion of Sonovue™ contrast agent, before and after the intervention period, with concomitant assessments of cardiorespiratory fitness and resting blood pressure. HIIT led to improvements in anaerobic threshold (13.2 ± 3.4 vs. 15.3 ± 3.8 ml/kg/min, P < 0.001), dynamic exercise capacity (145 ± 60 vs. 159 ± 59 W, P < 0.001) and resting (systolic) blood pressure (142 ± 15 vs. 133 ± 11 mmHg, P < 0.01). Notably, HIIT elicited significant increases in microvascular blood flow responses to acute contractile activity (1.8 ± 0.63 vs. 2.3 ± 0.8 (arbitrary contrast units (AU), P < 0.01)), with no change in any of these parameters observed in the control group. Six weeks HIIT improves skeletal muscle microvascular responsiveness to acute contractile activity in the form of active hyperaemia-induced by a single bout of resistance exercise. These findings likely reflect reports of enhanced large vessel distensibility, improved endothelial function, and muscle capillarisation following HIIT. Moreover, our findings illustrate that HIIT may be effective in mitigating deleterious alterations in muscle microvascular mediated aspects of sarcopenia.

Apocynin Improves Functional Microvascular Blood Flow in Sickle Cell Disease during Contractile Exercise

Background: Reactive oxygen species are thought to contribute to vascular dysfunction, including vaso-occlusion (VO), in sickle cell disease (SCD). It is unclear whether reduction of oxidative stress either prevents VO or improves microvascular blood flow (MBF) in key tissues. We hypothesized that apocynin, which is known to inhibit NADPH-oxidase, can reverse defects in skeletal muscle MBF at rest or during exercise. To test our hypothesis, quantitative perfusion imaging with contrast enhanced ultrasound (CEU) was performed in Townes SCD mice treated chronically with apocynin.Methods: Townes SCD mice (S/S), a model of severe disease produced by transfer of human α, γ, and βs globin genes, and control mice (A/A) were studied under normoxic conditions. On a separate day, mice were studied after exposure to hypoxia for 3 hrs at 8% O2 with reoxygentation for 1 hr. For each strain, half of the mice were treated with apocynin (50mg/kg/day p.o.) for ≥6 weeks prior to study. Quantitative CEU of the proximal hindlimb adductor skeletal muscle was performed at rest and during contractile exercise produced by electrosimulation at 2 Hz. CEU was performed during a continuous infusion of lipid-shelled decafluorobutane microbubbles and time-intensity data was analyzed after a destructive pulse sequence to assess microvascular blood volume (A-value), microvascular blood flux rate (β), and MBF (Axβ). Hemoglobin levels were drawn prior to CEU studies.Results: Blood hemoglobin was lower in untreated normoxic SCD than control mice (7.2±1.3 vs. 10.0±3.5 g/dL, p=0.04) and did not change significantly with either apocynin therapy or hypoxic conditions. Under normoxic conditions in untreated mice, SCD compared to control mice had higher resting skeletal muscle MBF (Ab: 5.2±3.4 vs 12.8±9.2 ml/min for A/A vs S/S, p=0.385) and microvascular flux rates (β: 0.10±0.04 vs 0.20±0.05 s-1, p=0.02), although statistical significance was reached only for the latter. Exercise-mediated flow reserve was greater in A/A than S/S mice (9.3 vs 3.3, p=0.02), although peak hyperemic flow was similar. Apocynin did not alter resting perfusion under normoxic conditions, but did increase exercise MBF in S/S mice (72.16±35.3 vs. 35.72±24.98 ml/min for treated vs untreated; p=0.04). Hypoxic stress did not produce any obvious behavioral signs of systemic VO. Compared to normoxic conditions, hypoxic stress produced an increase in both resting and exercise MBF in all cohorts. Again, apocynin therapy increased exercise MBF in S/S mice after hypoxia, although in contrast to normoxic conditions statistical significance was not reached.Conclusions: Under resting conditions, skeletal muscle perfusion in Townes SCD mice is increased compared to controls, which likely represents a compensatory response to anemia. This compensatory response is lost during exercise but is restored with chronic apocynin therapy. After hypoxic stress, there is an increase rather than decrease in muscle perfusion in Townes mice with only a minor effect of apocynin on exercise perfusion. These results suggest that anti-oxidant strategies in SCD could help normalize physiologic flow responses to exercise. DisclosuresNo relevant conflicts of interest to declare.