Functional sympatholysis during exercise in patients with type 2 diabetes with intact response to acetylcholine

The cardiovascular changes underlying a low cardiac output with exercise in patients with type 2 diabetes mellitus

The significant morbidity and premature mortality of type 2 diabetes mellitus (T2DM) is largely associated with its cardiovascular consequences. Focus has long been on the arterial atheromatosis of DM giving rise to early stroke and myocardial infarctions, whereas less attention has been given to its non-ischemic cardiovascular consequences. Irrespective of ischemic changes, T2DM is associated with heart failure (HF) most commonly with preserved ejection fraction (HFpEF). Largely due to increasing population ages, hypertension, obesity and T2DM, HFpEF is becoming the most prevalent form of heart failure. Unfortunately, randomized controlled trials of HFpEF have largely been futile, and it now seems logical to address the important different phenotypes of HFpEF to understand their underlying pathophysiology. In the early phases, HFpEF is associated with a significantly impaired ability to increase cardiac output with exercise. The lowered cardiac output with exercise results from both cardiac and peripheral causes. T2DM is associated with left ventricular (LV) diastolic dysfunction based on LV hypertrophy with myocardial disperse fibrosis and significantly impaired ability for myocardial blood flow increments with exercise. T2DM is also associated with impaired ability for skeletal muscle vasodilation during exercise, and as is the case in the myocardium, such changes may be related to vascular rarefaction. The present review discusses the underlying phenotypical changes of the heart and peripheral vascular system and their importance for an adequate increase in cardiac output. Since many of the described cardiovascular changes with T2DM must be considered difficult to change if fully developed, it is suggested that patients with T2DM are early evaluated with respect to their cardiovascular compromise.

Neural control of the circulation during exercise in heart failure with reduced and preserved ejection fraction

Patients with heart failure with reduced (HFrEF) and preserved ejection fraction (HFpEF) exhibit severe exercise intolerance that may be due, in part, to inappropriate cardiovascular and hemodynamic adjustments to exercise. Several neural mechanisms and locally released vasoactive substances work in concert through complex interactions to ensure proper adjustments to meet the metabolic demands of the contracting skeletal muscle. Specifically, accumulating evidence suggests that disease-related alterations in neural mechanisms (e.g., central command, exercise pressor reflex, arterial baroreflex, and cardiopulmonary baroreflex) contribute to heightened sympathetic activation and impaired ability to attenuate sympathetic vasoconstrictor responsiveness that may contribute to reduced skeletal muscle blood flow and severe exercise intolerance in patients with HFrEF. In contrast, little is known regarding these important aspects of physiology in patients with HFpEF, though emerging data reveal heightened sympathetic activation and attenuated skeletal muscle blood flow during exercise in this patient population that may be attributable to dysregulated neural control of the circulation. The overall goal of this review is to provide a brief overview of the current understanding of disease-related alterations in the integrative neural cardiovascular responses to exercise in both HFrEF and HFpEF phenotypes, with a focus on sympathetic nervous system regulation during exercise.

Evidence of impaired functional sympatholysis in patients with heart failure with preserved ejection fraction

Exercising muscle blood flow is reduced in patients with heart failure with a preserved ejection fraction (HFpEF), which may be related to disease-related changes in the ability to overcome sympathetic nervous system (SNS)-mediated vasoconstriction during exercise, (i.e., "functional sympatholysis"). Thus, in 12 patients with HFpEF (69 ± 7 yr) and 11 healthy controls (Con, 69 ± 4 yr), we examined forearm blood flow (FBF), mean arterial pressure (MAP), and forearm vascular conductance (FVC) during rhythmic handgrip exercise (HG) at 30% of maximum voluntary contraction with or without lower-body negative pressure (LBNP, -20 mmHg) to increase SNS activity and elicit peripheral vasoconstriction. SNS-mediated vasoconstrictor responses were determined as LBNP-induced changes (%Δ) in FVC, and the "magnitude of sympatholysis" was calculated as the difference between responses at rest and during exercise. At rest, the LBNP-induced change in FVC was significantly lesser in HFpEF compared with Con (HFpEF: -9.5 ± 5.5 vs. Con: -21.0 ± 8.0%; P < 0.01). During exercise, LBNP-induced %ΔFVC was significantly attenuated in Con compared with rest (HG: -5.8 ± 6.0%; P < 0.05) but not in HFpEF (HG: -9.9 ± 2.5%; P = 0.88). Thus, the magnitude of sympatholysis was lesser in HFpEF compared with Con (HFpEF: 0.4 ± 4.7 vs. Con: -15.2 ± 11.8%; P < 0.01). These data demonstrate a diminished ability to attenuate SNS-mediated vasoconstriction in HFpEF and provide new evidence suggesting impaired functional sympatholysis in this patient group.NEW & NOTEWORTHY Data from the current study suggest that functional sympatholysis, or the ability to adequately attenuate sympathetic nervous system (SNS)-mediated vasoconstriction during exercise, is impaired in patients with heart failure with preserved ejection fraction (HFpEF). These observations extend the current understanding of HFpEF pathophysiology by implicating inadequate functional sympatholysis as an important contributor to reduced exercising muscle blood flow in this patient group.

Neurovascular Dysregulation During Exercise in Type 2 Diabetes

Emerging evidence suggests that type 2 diabetes (T2D) may impair the ability to properly adjust the circulation during exercise with augmented blood pressure (BP) and an attenuated contracting skeletal muscle blood flow (BF) response being reported. This review provides a brief overview of the current understanding of these altered exercise responses in T2D and the potential underlying mechanisms, with an emphasis on the sympathetic nervous system and its regulation during exercise. The research presented support augmented sympathetic activation, heightened BP, reduced skeletal muscle BF, and impairment in the ability to attenuate sympathetically mediated vasoconstriction (i.e., functional sympatholysis) as potential drivers of neurovascular dysregulation during exercise in T2D. Furthermore, emerging evidence supporting a contribution of the exercise pressor reflex and central command is discussed along with proposed future directions for studies in this important area of research.

Greater α1-adrenergic-mediated vasoconstriction in contracting skeletal muscle of patients with type 2 diabetes

Findings presented in this article are the first to show patients with type 2 diabetes mellitus have blunted hyperemic and vasodilatory responses to dynamic handgrip exercise. Moreover, we illustrate greater α1-adrenergic-mediated vasoconstriction may contribute to our initial observations. Collectively, these data suggest patients with type 2 diabetes may have impaired functional sympatholysis, which can contribute to their reduced exercise capacity.

Recent advances in exercise pressor reflex function in health and disease

Autonomic alterations at the onset of exercise are critical to redistribute cardiac output towards the contracting muscles while preventing a fall in arterial pressure due to excessive vasodilation within the contracting muscles. Neural mechanisms responsible for these adjustments include central command, the exercise pressor reflex, and arterial and cardiopulmonary baroreflexes. The exercise pressor reflex evokes reflex increases in sympathetic activity to the heart and systemic vessels and decreases in parasympathetic activity to the heart, which increases blood pressure (BP), heart rate, and total peripheral resistance through vasoconstriction of systemic vessels. In this review, we discuss recent advancements in our understanding of exercise pressor reflex function in health and disease. Specifically, we discuss emerging evidence suggesting that sympathetic vasoconstrictor drive to the contracting and non-contracting skeletal muscle is differentially controlled by central command and the metaboreflex in healthy conditions. Further, we discuss evidence from animal and human studies showing that cardiovascular diseases, including hypertension, diabetes, and heart failure, lead to an altered exercise pressor reflex function. We also provide an update on the mechanisms thought to underlie this altered exercise pressor reflex function in each of these diseases. Although these mechanisms are complex, multifactorial, and dependent on the etiology of the disease, there is a clear consensus that several mechanisms are involved. Ultimately, approaches targeting these mechanisms are clinically significant as they provide alternative therapeutic strategies to prevent adverse cardiovascular events while also reducing symptoms of exercise intolerance.

An Incremental Sit-to-Stand Exercise for Evaluating Physical Capacity in Older Patients with Type 2 Diabetes

Exercise is recommended for older patients with type 2 diabetes mellitus (T2DM), and increased physical activity contributes to better management of their condition. The conventional exercise test with treadmill or cycle ergometer (CE) for assessing physical capacity, such as peak oxygen uptake (VO₂) and anaerobic threshold (AT), is not always usable for older patients with T2DM. The incremental sit-to-stand (ISTS) exercise is an incremental exercise test using external signals to control the sit-to-stand rate in a given time frame and can be performed in a small space using only a chair. This study aimed to examine the validity of the physical capacity assessment during the ISTS exercise, based on the relationships between the ISTS performance, peak VO₂, AT on ISTS exercise and CE test, in older patients with T2DM. Twenty-two patients with T2DM (10 men, 12 women; mean age, 68.0 years; range, 61-77 years) performed ISTS exercise (according to an existing protocol) and CE test in a randomized manner. Peak VO₂, AT, and completion time were determined for the ISTS exercise and CE test. Peak VO₂ during ISTS exercise was significantly associated with that during the CE test (r = 0.89, p < 0.01). The completion time on the ISTS exercise was significantly associated with peak VO₂ (r = 0.80, p < 0.01) and AT on the ISTS exercise (r = 0.78, p < 0.01). The ISTS exercise is a useful tool to determine the physical capacity and estimate peak VO₂ and AT in older people with T2DM.

Exaggerated exercise pressor reflex in type 2 diabetes: Potential role of oxidative stress

Type 2 diabetes mellitus (T2DM) leads to exaggerated cardiovascular responses to exercise, in part due to an exaggerated exercise pressor reflex. Accumulating data suggest excessive oxidative stress contributes to an exaggerated exercise pressor reflex in cardiovascular-related diseases. Excessive oxidative stress is also a primary underlying mechanism for the development and progression of T2DM. However, whether oxidative stress plays a role in mediating the exaggerated exercise pressor reflex in T2DM is not known. Therefore, this review explores the potential role of oxidative stress leading to increased activation of the afferent arm of the exercise pressor reflex. Several lines of evidence support direct and indirect effects of oxidative stress on the exercise pressor reflex. For example, intramuscular ROS may directly and indirectly (by attenuating contracting muscle blood flow) increase group III and IV afferent activity. Oxidative stress is a primary underlying mechanism for the development of neuropathic pain, which in turn is associated with increased group III and IV afferent activity. These are the same type of afferents that evoke muscle pain and the exercise pressor reflex. Furthermore, oxidative stress-induced release of inflammatory mediators may modulate afferent activity. Collectively, these alterations may result in a positive feedback loop that further amplifies the exercise pressor reflex. An exaggerated reflex increases the risk of adverse cardiovascular events. Thus, identifying the contribution of oxidative stress could provide a potential therapeutic target to reduce this risk in T2DM.

Preserved ability to blunt sympathetically-mediated vasoconstriction in exercising skeletal muscle of young obese humans

Sympathetic vasoconstriction is attenuated in exercising muscles to assist in matching of blood flow with metabolic demand. This "functional sympatholysis" may be impaired in young obese individuals due to greater sympathetic activation and/or reduced local vasodilatory capacity of both small and large arteries, but this remains poorly understood. We tested the hypothesis that functional sympatholysis is impaired in obese individuals compared with normal-weight counterparts. In 36 obese and normal-weight young healthy adults (n = 18/group), we measured forearm blood flow and calculated forearm vascular conductance (FVC) responses to reflex increases in sympathetic nerve activity induced by lower body negative pressure (LBNP) at rest and during rhythmic handgrip exercise at 15% and 30% of the maximal voluntary contraction (MVC). FVC was normalized to lean forearm mass. In normal-weight individuals, LBNP evoked a decrease in FVC (-16.1 ± 5.7%) in the resting forearm, and the reduction in FVC (15%MVC: -8.1 ± 3.3%; 30%MVC: -1.0 ± 4.0%) was blunted during exercise in an intensity-dependent manner (P < 0.05). Similarly, in obese individuals, LBNP evoked a comparable decrease in FVC (-10.9 ± 5.7%) in the resting forearm, with the reduction in FVC (15%MVC: -9.7 ± 3.3%; 30%MVC: -0.3 ± 4.0%) also blunted during exercise in an intensity-dependent manner (P < 0.05). The magnitude of sympatholysis was similar between groups (P > 0.05) and was intensity-dependent (P < 0.05). Our findings suggest that functional sympatholysis is not impaired in young obese individuals without overt cardiovascular diseases.

Recovery of rate-pressure product and cardiac mortality in coronary artery disease patients with type 2 diabetes

AIMS

To investigate prognostic significance of post-exercise recovery of rate-pressure product (RPP) in patients with stable coronary artery disease (CAD) and type 2 diabetes (T2D).

METHODS

Patients with angiographically documented CAD and T2D (n = 697) underwent symptom-limited bicycle exercise test. Exercise capacity (EC), heart rate, blood pressure and RPP responses to peak exercise and recovery (2' and 5' after cessation of exercise) were analyzed. Cardiac death was the primary and sudden cardiac death (SCD) secondary endpoint.

RESULTS

During a median follow-up of 76 months, 49 cardiac deaths (7.0%) and 28 SCDs (4.0%) were observed. The recovery of RPP at 5' was the strongest univariate predictor of cardiac death (hazard ratio [HR]: 2.55 per SD decrease, 95%CI: 1.82-3.58, p < 0.001) and SCD (HR: 2.34, 95%CI: 1.51-3.62, p < 0.001). In multivariate analysis, it remained significantly associated to cardiac death and SCD without (HR: 1.66, 95%CI: 1.14-2.41, p < 0.01 and HR: 1.75, 95%CI: 1.08-2.85, p < 0.05, respectively) and with additional adjustment for EC and peak RPP (HR: 1.45, 95%CI: 1.09-1.92, p < 0.05 and HR: 1.52, 95%CI: 1.01-2.27, p < 0.05, respectively).

CONCLUSIONS

The recovery of RPP after exercise is a potent predictor of cardiac death in patients with CAD and T2D. It provides significant prognostic information beyond EC and peak RPP.

Reduced skeletal-muscle perfusion and impaired ATP release during hypoxia and exercise in individuals with type 2 diabetes

AIMS/HYPOTHESIS

Plasma ATP is a potent vasodilator and is thought to play a role in the local regulation of blood flow. Type 2 diabetes is associated with reduced tissue perfusion. We aimed to examine whether individuals with type 2 diabetes have reduced plasma ATP concentrations compared with healthy control participants (case-control design).

METHODS

We measured femoral arterial and venous plasma ATP levels with the intravascular microdialysis technique during normoxia, hypoxia and one-legged knee-extensor exercise (10 W and 30 W) in nine participants with type 2 diabetes and eight control participants. In addition, we infused acetylcholine (ACh), sodium nitroprusside (SNP) and ATP into the femoral artery to assess vascular function and ATP signalling.

RESULTS

Individuals with type 2 diabetes had a lower leg blood flow (LBF; 2.9 ± 0.1 l/min) compared with the control participants (3.2 ± 0.1 l/min) during exercise (p < 0.05), in parallel with lower venous plasma ATP concentration (205 ± 35 vs 431 ± 72 nmol/l; p < 0.05). During systemic hypoxia, LBF increased from 0.35 ± 0.04 to 0.54 ± 0.06 l/min in control individuals, whereas it did not increase (0.25 ± 0.04 vs 0.31 ± 0.03 l/min) in the those with type 2 diabetes and was lower than in the control individuals (p < 0.05). Hypoxia increased venous plasma ATP levels in both groups (p < 0.05), but the increase was higher in control individuals (90 ± 26 nmol/l) compared to those with type 2 diabetes (18 ± 5 nmol/l). LBF and vascular conductance were lower during ATP (0.15 and 0.4 μmol min^-1 [kg leg mass]^-1) and ACh (100 μg min^-1 [kg leg mass]^-1) infusion in individuals with type 2 diabetes compared with the control participants (p < 0.05), whereas there was no difference during SNP infusion.

CONCLUSIONS/INTERPRETATION

These findings demonstrate that individuals with type 2 diabetes have lower plasma ATP concentrations during exercise and hypoxia compared with control individuals, and this occurs in parallel with lower blood flow. Moreover, individuals with type 2 diabetes have a reduced vasodilatory response to infused ATP. These impairments in the ATP system are both likely to contribute to the reduced tissue perfusion associated with type 2 diabetes.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02001766.

Functional sympatholysis is impaired in end-stage renal disease

Patients with end-stage renal disease (ESRD) have decreased exercise capacity and exercise intolerance that contribute to cardiovascular risk. One potential mechanism underlying exercise intolerance in ESRD is impaired ability to oppose sympathetically mediated vasoconstriction within exercising skeletal muscle (i.e., functional sympatholysis, FS). We hypothesized that ESRD patients have impaired FS compared with healthy (CON) and hypertensive (HTN) controls and that impaired FS is related to circulating levels of the uremic toxin asymmetric dimethyl arginine (ADMA), an endogenous nitric oxide synthase inhibitor. Near-infrared spectroscopy-derived oxygen tissue saturation index (TSI) of the forearm muscle was measured continuously in 33 participants (9 CON, 14 HTN, 10 ESRD) at rest and during low-dose (-20 mmHg) lower body negative pressure (LBNP), moderate rhythmic handgrip exercise, and LBNP with concomitant handgrip exercise (LBNP+handgrip). Resting muscle TSI was lower in ESRD than in CON and HTN groups (CON = 67.8 ± 1.9%, HTN = 67.2 ± 1.1%, ESRD = 62.7 ± 1.5%, P = 0.03). Whereas CON and HTN groups had an attenuation in sympathetically mediated reduction in TSI during LBNP + handgrip compared with LBNP alone (P ≤ 0.05), this response was not present in ESRD (P = 0.71), suggesting impaired FS. There was no difference in plasma [ADMA] between groups (CON = 0.47 ± 0.05 µmol/l, HTN = 0.42 ± 0.06 µmol/l, ESRD = 0.63 ± 0.14 µmol/l, P = 0.106) and no correlation between plasma [ADMA] and resting muscle TSI (P = 0.84) or FS (P = 0.75). Collectively, these findings suggest that ESRD patients have lower muscle perfusion at rest and impaired FS but that these derangements are not related to circulating [ADMA].

Exercise intolerance in Type 2 diabetes: is there a cardiovascular contribution?

Physical activity is critically important for Type 2 diabetes management, yet adherence levels are poor. This might be partly due to disproportionate exercise intolerance. Submaximal exercise tolerance is highly sensitive to muscle oxygenation; impairments in exercising muscle oxygen delivery may contribute to exercise intolerance in Type 2 diabetes since there is considerable evidence for the existence of both cardiac and peripheral vascular dysfunction. While uncompromised cardiac output during submaximal exercise is consistently observed in Type 2 diabetes, it remains to be determined whether an elevated cardiac sympathetic afferent reflex could sympathetically restrain exercising muscle blood flow. Furthermore, while deficits in endothelial function are common in Type 2 diabetes and are often cited as impairing exercising muscle oxygen delivery, no direct evidence in exercise exists, and there are several other vasoregulatory mechanisms whose dysfunction could contribute. Finally, while there are findings of impaired oxygen delivery, conflicting evidence also exists. A definitive conclusion that Type 2 diabetes compromises exercising muscle oxygen delivery remains premature. We review these potentially dysfunctional mechanisms in terms of how they could impair oxygen delivery in exercise, evaluate the current literature on whether an oxygen delivery deficit is actually manifest, and correspondingly identify key directions for future research.

Muscle α-adrenergic responsiveness during exercise and ATP-induced vasodilation in chronic obstructive pulmonary disease patients

Sympathetic vasoconstriction is blunted in exercising muscle (functional sympatholysis) but becomes attenuated with age. We tested the hypothesis that functional sympatholysis is further impaired in chronic obstructive pulmonary disease (COPD) patients. We determined leg blood flow and calculated leg vascular conductance (LVC) during 1) femoral-arterial Tyramine infusion (evokes endogenous norepinephrine release, 1 µmol·min^-1·kg leg mass^-1), 2) one-legged knee extensor exercise with and without Tyramine infusion [10 W and 20% of maximal workload (WL_max)], 3) ATP (0.05 µmol·min^-1·kg leg mass^-1) and Tyramine infusion, and 4) incremental ATP infusions (0.05, 0.3, and 3.0 µmol·min^-1·kg leg mass^-1). We included 10 patients with moderate to severe COPD and 8 age-matched healthy control subjects. Overall, leg blood flow and LVC were lower in COPD patients during exercise ( P < 0.05). Tyramine reduced LVC in both groups at 10-W exercise (COPD: -3 ± 1 ml·min^-1·mmHg^-1 and controls: -3 ± 1 ml·min^-1·mmHg^-1, P < 0.05) and 20% WL_max (COPD: -4 ± 1 ml·min^-1·mmHg^-1 and controls: -3 ± 1 ml·min^-1·mmHg^-1, P < 0.05) with no difference between groups. Incremental ATP infusions induced dose-dependent vasodilation with no difference between groups, and, in addition, the vasoconstrictor response to Tyramine infused together with ATP was not different between groups (COPD: -0.03 ± 0.01 l·min^-1·kg leg mass^-1 vs.

CONTROLS

-0.04 ± 0.01 l·min^-1·kg leg mass^-1, P > 0.05). Compared with age-matched healthy control subjects, the vasodilatory response to ATP is intact in COPD patients and their ability to blunt sympathetic vasoconstriction (functional sympatholysis) as evaluated by intra-arterial Tyramine during exercise or ATP infusion is maintained. NEW & NOTEWORTHY The ability to blunt sympathetic vasoconstriction in exercising muscle and ATP-induced dilation in chronic obstructive pulmonary disease patients remains unexplored. Chronic obstructive pulmonary disease patients demonstrated similar sympathetic vasoconstriction in response to intra-arterial Tyramine during exercise and ATP-induced vasodilation compared with age-matched healthy control subjects.

Lack of independent effect of type 2 diabetes beyond characteristic comorbidities and medications on small muscle mass exercising muscle blood flow and exercise tolerance

Persons with type 2 diabetes (T2D) are believed to have reduced exercise tolerance; this may be partly due to impaired exercising muscle blood flow (MBF). Whether there is an impact of T2D on exercising MBF within the typical constellation of comorbidities (hypertension, dyslipidemia, obesity) and their associated medications has not been investigated. We tested the hypothesis that small muscle mass exercise tolerance is reduced in persons with T2D versus Controls (matched for age, body mass index, fitness, comorbidities, non-T2D medications) and that this is related to blunted MBF. Eight persons with T2D and eight controls completed a forearm critical force (fCF_impulse) test as a measure of exercise tolerance (10-min intermittent maximal effort forearm contractions; the average contraction impulse in the last 30 sec quantified fCF_impulse). Forearm blood flow (FBF; ultrasound) and mean arterial pressure (MAP; finger photoplethysmography) were measured; forearm vascular conductance (FVK) was calculated. Data are means ± SD, T2D versus Control. fCF_impulse was not different between groups (136.9 ± 47.3  N·sec vs. 163.1 ± 49.7 N·sec, P = 0.371) nor was the ΔFBF from rest to during exercise at fCF_impulse (502.9 ± 144.6 vs. 709.1 ± 289.2 mL/min, P = 0.092), or its determinants ΔFVK and ΔMAP (both P > 0.05), although there was considerable interindividual variability. ΔFBF was strongly related to fCF_impulse (r = 0.727, P = 0.002), providing support for the relationship between oxygen delivery and exercise tolerance. We conclude that small muscle mass exercising MBF and exercise tolerance are not impaired in representative persons with T2D versus appropriately matched controls. This suggests that peripheral vascular control impairment does not contribute to reduced exercise tolerance in this population.

Intravascular ATP and the regulation of blood flow and oxygen delivery in humans

Regulation of vascular tone is a complex response that integrates multiple signals that allow for blood flow and oxygen supply to match oxygen demand appropriately. Here, we discuss the potential role of intravascular adenosine triphosphate (ATP) as a primary factor in these responses and put forth the hypothesis that deficient ATP release contributes to impairments in vascular control exhibited in aged and diseased populations.

Independent effect of type 2 diabetes beyond characteristic comorbidities and medications on immediate but not continued knee extensor exercise hyperemia

We tested the hypothesis that type 2 diabetes (T2D), when present in the characteristic constellation of comorbidities (obesity, hypertension, dyslipidemia) and medications, slows the dynamic adjustment of exercising muscle perfusion and blunts the steady state relative to that of controls matched for age, body mass index, fitness, comorbidities, and non-T2D medications. Thirteen persons with T2D and 11 who served as controls performed rhythmic single-leg isometric quadriceps exercise (rest-to-6 kg and 6-to-12 kg transitions, 5 min at each intensity). Measurements included leg blood flow (LBF, femoral artery ultrasound), mean arterial pressure (MAP, finger photoplethysmography), and leg vascular conductance (LVK, calculated). Dynamics were quantified using mean response time (MRT). Measures of amplitude were also used to compare response adjustment: the change from baseline to 1) the peak initial response (greatest 1-s average in the first 10 s; ΔLBFPIR, ΔLVKPIR) and 2) the on-transient (average from curve fit at 15, 45, and 75 s; ΔLBFON, ΔLVKON). ΔLBFPIR was significantly blunted in T2D vs. control individuals (P = 0.037); this was due to a tendency for reduced ΔLVKPIR (P = 0.063). In contrast, the overall response speed was not different between groups (MRT P = 0.856, ΔLBFON P = 0.150) nor was the change from baseline to steady state (P = 0.204). ΔLBFPIR, ΔLBFON, and LBF MRT did not differ between rest-to-6 kg and 6-to-12 kg workload transitions (all P > 0.05). Despite a transient amplitude impairment at the onset of exercise, there is no robust or consistent effect of T2D on top of the comorbidities and medications typical of this population on the overall dynamic adjustment of LBF, or the steady-state levels achieved during low- or moderate-intensity exercise.

Effect of acute inspiratory muscle exercise on blood flow of resting and exercising limbs and glucose levels in type 2 diabetes

To evaluate the effects of inspiratory loading on blood flow of resting and exercising limbs in patients with diabetic autonomic neuropathy. Ten diabetic patients without cardiovascular autonomic neuropathy (DM), 10 patients with cardiovascular autonomic neuropathy (DM-CAN) and 10 healthy controls (C) were randomly assigned to inspiratory muscle load of 60% or 2% of maximal inspiratory pressure (PImax) for approximately 5 min, while resting calf blood flow (CBF) and exercising forearm blood flow (FBF) were measured. Reactive hyperemia was also evaluated. From the 20 diabetic patients initially allocated, 6 wore a continuous glucose monitoring system to evaluate the glucose levels during these two sessions (2%, placebo or 60%, inspiratory muscle metaboreflex). Mean age was 58 ± 8 years, and mean HbA1c, 7.8% (62 mmol/mol) (DM and DM-CAN). A PImax of 60% caused reduction of CBF in DM-CAN and DM (P<0.001), but not in C, whereas calf vascular resistance (CVR) increased in DM-CAN and DM (P<0.001), but not in C. The increase in FBF during forearm exercise was blunted during 60% of PImax in DM-CAN and DM, and augmented in C (P<0.001). Glucose levels decreased by 40 ± 18.8% (P<0.001) at 60%, but not at 2%, of PImax. A negative correlation was observed between reactive hyperemia and changes in CVR (Beta coefficient = -0.44, P = 0.034). Inspiratory muscle loading caused an exacerbation of the inspiratory muscle metaboreflex in patients with diabetes, regardless of the presence of neuropathy, but influenced by endothelial dysfunction. High-intensity exercise that recruits the diaphragm can abruptly reduce glucose levels.

Impaired dynamic cerebral autoregulation at rest and during isometric exercise in type 2 diabetes patients

Type 2 diabetes mellitus patients (T2D) have elevated risk of stroke, suggesting that cerebrovascular function is impaired. Herein, we examined dynamic cerebral autoregulation (CA) at rest and during exercise in T2D patients and determined whether underlying systemic oxidative stress is associated with impairments in CA. Middle cerebral artery blood velocity and arterial blood pressure (BP) were measured at rest and during 2-min bouts of low- and high-intensity isometric handgrip performed at 20% and 40% maximum voluntary contraction, respectively, in seven normotensive and eight hypertensive T2D patients and eight healthy controls. Dynamic CA was estimated using the rate of regulation (RoR). Total reactive oxygen species (ROS) and superoxide levels were measured at rest. There were no differences in RoR at rest or during exercise between normotensive and hypertensive T2D patients. However, when compared with controls, T2D patients exhibited lower RoR at rest and during low-intensity handgrip indicating impaired dynamic CA. Moreover, the RoR was further reduced by 29 ± 4% during high-intensity handgrip in T2D patients (0.307 ± 0.012/s rest vs. 0.220 ± 0.014/s high intensity; P < 0.01), although well maintained in controls. T2D patients demonstrated greater baseline total ROS and superoxide compared with controls, both of which were negatively related to RoR during handgrip (e.g., total ROS: r = -0.71, P < 0.05; 40% maximum voluntary contraction). Collectively, these data demonstrate impaired dynamic CA at rest and during isometric handgrip in T2D patients, which may be, in part, related to greater underlying systemic oxidative stress. Additionally, dynamic CA is blunted further with high intensity isometric contractions potentially placing T2D patients at greater risk for cerebral events during such activities.

Interactive effect of acute sympathetic activation and exercise intensity on the dynamic response characteristics of vascular conductance in the human calf muscle

PURPOSE

The effect of acute activation of the sympathetic nervous system on the dynamic response of muscle hyperaemia during exercise at different intensities is not clear.

METHODS

To explore this, six men performed 16, 5-min bouts of intermittent calf contractions at two intensities (25 and 50 % MVC) and two levels of sympathetic activation (CPT cold pressor test, CON control). Mean arterial pressure (MAP) and leg vascular conductance (LVC leg blood flow/MAP) were measured during rest and contractions (3 s intervals), and dynamic response characteristics of LVC were estimated using curve-fitting and empirical modeling.

RESULTS

MAP was ~20 % greater (P ≤ 0.05) during CPT than CON before and during initial contractions at both intensities. At 25 % MVC, CPT reduced the exercise-induced change in LVC (0.109 vs 0.125 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), an effect attributed to the reduction in the amplitude of the fast growth phase (0.091 vs 0.128 1 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05). At 50 % MVC, CPT also blunted the fast growth phase (0.147 vs 0.189 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), but the total change in LVC during exercise was unaffected because of a significant reduction in the amplitude of the rapid decay phase and tendency (P = 0.1) for a lower amplitude of the slow decay phase.

CONCLUSION

Increased sympathetic constraint of vasodilation persists during initial contractions but is overcome at the high intensity by a mechanism apparently related to hyperaemic decay.

Hemodynamic responses during graded and constant-load plantar flexion exercise in middle-aged men and women with type 2 diabetes

We tested the hypotheses that type 2 diabetes (T2D) impairs the 1) leg hemodynamic responses to an incremental intermittent plantar-flexion exercise and 2) dynamic responses of leg vascular conductance (LVC) during low-intensity (30% maximal voluntary contraction, MVC) and high-intensity (70% MVC) constant-load plantar-flexion exercise in the supine posture. Forty-four middle-aged individuals with T2D (14 women), and 35 healthy nondiabetic (ND) individuals (18 women) were tested. Leg blood flow (LBF) was measured between each contraction using venous occlusion plethysmography. During the incremental test peak force (Fpeak) relative to MVC was significantly reduced ( P < 0.05) in men and women with T2D compared with their respective nondiabetic counterparts. Peak LBF and the slope of LBF relative to percentage Fpeak were also reduced ( P < 0.05) in women with T2D compared with healthy women (peak blood flow, 460.6 ± 126.8 vs. 628.3 ± 347.7 ml/min; slope, 3.78 ± 1.74 vs. 5.85 ± 3.14 ml·min−1·%Fpeak−1) and in men with T2D compared with nondiabetic men (peak blood flow, 621.7 ± 241.3 vs. 721.2 ± 359.7 ml/min; slope, 5.75 ± 2.66 vs. 6.33 ± 3.63 ml·min−1·%Fpeak−1). During constant-load contractions at 30% MVC T2D did not affect the dynamic responses of LVC (LBF/MAP). However, at 70% MVC [completed by a subgroup of participants (20 with T2D, 6 women; 13 ND, 6 women)] the time constant of the second growth phase of LVC was longer and the amplitude of the first growth phase was lower ( P < 0.05 for both) in men and women with T2D. The results suggest that the T2D-induced impairments in performance of the leg muscles are related to reductions in blood flow in both men and women.

Inefficient functional sympatholysis is an overlooked cause of malperfusion in contracting skeletal muscle

Contracting skeletal muscle can overcome sympathetic vasoconstrictor activity (functional sympatholysis), which allows for a blood supply that matches the metabolic demand. This ability is thought to be mediated by locally released substances that modulate the effect of noradrenaline (NA) on the α-receptor. Tyramine induces local NA release and can be used in humans to investigate the underlying mechanisms and physiological importance of functional sympatholysis in the muscles of healthy and diseased individuals as well as the impact of the active muscles' training status. In sedentary elderly men, functional sympatholysis and muscle blood flow are impaired compared to young men, but regular physical activity can prevent these age related impairments. In young subjects, two weeks of leg immobilization causes a reduced ability for functional sympatholysis, whereas the trained leg maintained this function. Patients with essential hypertension have impaired functional sympatholysis in the forearm, and reduced exercise hyperaemia in the leg, but this can be normalized by aerobic exercise training. The effect of physical activity on the local mechanisms that modulate sympathetic vasoconstriction is clear, but it remains uncertain which locally released substance(s) block the effect of NA and how this is accomplished. NO and ATP have been proposed as important inhibitors of NA mediated vasoconstriction and presently an inhibitory effect of ATP on NA signalling via P2 receptors appears most likely.

A Mechanism-Based Approach to Prevention of and Therapy for Fibromyalgia

Fibromyalgia syndrome (FMS) is characterized by pain referred to deep tissues. Diagnosis and treatment of FMS are complicated by a variable coexistence with regional pain, fatigue, sleep disruption, difficulty with mentation, and depression. The widespread, deep pain of FMS can be a consequence of chronic psychological stress with autonomic dysregulation. Stress acts centrally to facilitate pain and acts peripherally, via sympathetic vasoconstriction, to establish painful muscular ischemia. FMS pain, with or without a coexistent regional pain condition, is stressful, setting up a vicious circle of reciprocal interaction. Also, stress interacts reciprocally with systems of control over depression, mentation, and sleep, establishing FMS as a multiple-system disorder. Thus, stress and the ischemic pain it generates are fundamental to the multiple disorders of FMS, and a therapeutic procedure that attenuates stress and peripheral vasoconstriction should be highly beneficial for FMS. Physical exercise has been shown to counteract peripheral vasoconstriction and to attenuate stress, depression, and fatigue and improve mentation and sleep quality. Thus, exercise can interrupt the reciprocal interactions between psychological stress and each of the multiple-system disorders of FMS. The large literature supporting these conclusions indicates that exercise should be considered strongly as a first-line approach to FMS therapy.

Vasodilator interactions in skeletal muscle blood flow regulation

During exercise, oxygen delivery to skeletal muscle is elevated to meet the increased oxygen demand. The increase in blood flow to skeletal muscle is achieved by vasodilators formed locally in the muscle tissue, either on the intraluminal or on the extraluminal side of the blood vessels. A number of vasodilators have been shown to bring about this increase in blood flow and, importantly, interactions between these compounds seem to be essential for the precise regulation of blood flow. Two compounds stand out as central in these vasodilator interactions: nitric oxide (NO) and prostacyclin. These two vasodilators are both stimulated by several compounds, e.g. adenosine, ATP, acetylcholine and bradykinin, and are affected by mechanically induced signals, such as shear stress. NO and prostacyclin have also been shown to interact in a redundant manner where one system can take over when formation of the other is compromised. Although numerous studies have examined the role of single and multiple pharmacological inhibition of different vasodilator systems, and important vasodilators and interactions have been identified, a large part of the exercise hyperaemic response remains unexplained. It is plausible that this remaining hyperaemia may be explained by cAMP- and cGMP-independent smooth muscle relaxation, such as effects of endothelial derived hyperpolarization factors (EDHFs) or through metabolic modulation of sympathetic effects. The nature and role of EDHF as well as potential novel mechanisms in muscle blood flow regulation remain to be further explored to fully elucidate the regulation of exercise hyperaemia.

Contribution of intravascular versus interstitial purines and nitric oxide in the regulation of exercise hyperaemia in humans

The regulation of blood flow to skeletal muscle involves a complex interaction between several locally formed vasodilators that are produced both in the skeletal muscle interstitium and intravascularly. The gas nitric oxide (NO) and the purines ATP and adenosine, are potent vasodilators that are formed by multiple cell types and released into the skeletal muscle interstitium and in plasma in response to muscle contraction. Cellular sources of ATP and NO in plasma are erythrocytes and endothelial cells, whereas interstitial sources are skeletal muscle cells and endothelial cells. Adenosine originates primarily from extracellular degradation of ATP. During exercise the concentrations of ATP and adenosine increase markedly in the interstitium with smaller increases occurring in plasma, and thus the interstitial concentration during exercise is severalfold higher than in plasma. The concentration of NO metabolites (NOx) in interstitium and plasma does not change during exercise and is similar in the two compartments. Adenosine and NO have been shown to contribute to exercise hyperaemia whereas the role of ATP remains unclear due to lack of specific purinergic receptor blockers. The relative role of intravascular versus interstitial vasodilators is not known but evidence suggests that both compartments are important. In cardiovascular disease, a reduced capacity to form adenosine in the muscle interstitium may be a contributing factor in increased peripheral vascular resistance.

Role of nitric oxide and prostanoids in the regulation of leg blood flow and blood pressure in humans with essential hypertension: effect of high-intensity aerobic training

We examined the role of nitric oxide (NO) and prostanoids in the regulation of leg blood flow and systemic blood pressure before and after 8 weeks of aerobic high-intensity training in individuals with essential hypertension (n = 10) and matched healthy control subjects (n = 11). Hypertensive subjects were found to have a lower (P < 0.05) blood flow to the exercising leg than normotensive subjects (30 W: 2.92 ± 0.16 vs. 3.39 ± 0.37 l min(−1)). Despite the lower exercise hyperaemia, pharmacological inhibition of the NO and prostanoid systems reduced leg blood flow to a similar extent during exercise in the two groups and vascular relaxation to the NO-dependent vasodilator acetylcholine was also similar between groups. High-intensity aerobic training lowered (P < 0.05) resting systolic (∼9 mmHg) and diastolic (∼12 mmHg) blood pressure in subjects with essential hypertension, but this effect of training was abolished when the NO and prostanoid systems were inhibited. Skeletal muscle vascular endothelial NO synthase uncoupling, expression and phosphorylation status were similar in the two groups before and after training. These data demonstrate that a reduction in exercise hyperaemia in hypertensive subjects is not associated with a reduced capacity of the NO and prostanoid systems to induce vasodilatation or with altered acetylcholine-induced response. However, our data suggest that the observed reduction in blood pressure is related to a training-induced change in the tonic effect of NO and/or prostanoids on vascular tone.