Augmentation of endothelium-dependent vasodilatory signalling improves functional sympatholysis in contracting muscle of older adults

Elevated sympathetic-mediated vasoconstriction at rest but intact functional sympatholysis during exercise in heart failure with reduced ejection fraction

Patients with heart failure with reduced ejection fraction (HFrEF) have exaggerated sympathoexcitation and impaired peripheral vascular conductance. Evidence demonstrating consequent impaired functional sympatholysis is limited in HFrEF. This study aimed to determine the magnitude of reduced limb vascular conductance during sympathoexcitation and whether functional sympatholysis would abolish such reductions in HFrEF. Twenty patients with HFrEF and 22 age-matched controls performed the cold pressor test (CPT) [left foot 2-min in -0.5 (1)°C water] alone and with right handgrip exercise (EX + CPT). Right forearm vascular conductance (FVC), forearm blood flow (FBF), and mean arterial pressure (MAP) were measured. Patients with HFrEF had greater decreases in %ΔFVC and %ΔFBF during CPT (both P < 0.0001) but not EX + CPT (P = 0.449, P = 0.199) compared with controls, respectively. %ΔFVC and %ΔFBF decreased from CPT to EX + CPT in patients with HFrEF (both P < 0.0001) and controls (P = 0.018, P = 0.015), respectively. MAP increased during CPT and EX + CPT in both groups (all P < 0.0001). MAP was greater in controls than in patients with HFrEF during EX + CPT (P = 0.025) but not CPT (P = 0.209). In conclusion, acute sympathoexcitation caused exaggerated peripheral vasoconstriction and reduced peripheral blood flow in patients with HFrEF. Handgrip exercise abolished sympathoexcitatory-mediated peripheral vasoconstriction and normalized peripheral blood flow in patients with HFrEF. These novel data reveal intact functional sympatholysis in the upper limb and suggest that exercise-mediated, local control of blood flow is preserved when cardiac limitations that are cardinal to HFrEF are evaded with dynamic handgrip exercise.NEW & NOTEWORTHY Patients with HFrEF demonstrate impaired peripheral blood flow regulation, evidenced by heightened peripheral vasoconstriction that reduces limb blood flow in response to physiological sympathoexcitation (cold pressor test). Despite evidence of exaggerated sympathetic vasoconstriction, patients with HFrEF demonstrate a normal hyperemic response to moderate-intensity handgrip exercise. Most importantly, acute, simultaneous handgrip exercise restores normal limb vasomotor control and vascular conductance during acute sympathoexcitation (cold pressor test), suggesting intact functional sympatholysis in patients with HFrEF.

α-Adrenergic regulation of skeletal muscle blood flow during exercise in patients with heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) has been characterized by lower blood flow to exercising limbs and lower peak oxygen utilization (V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ ), possibly associated with disease-related changes in sympathetic (α-adrenergic) signaling. Thus, in seven patients with HFpEF (70 ± 6 years, 3 female/4 male) and seven controls (CON) (66 ± 3 years, 3 female/4 male), we examined changes (%Δ) in leg blood flow (LBF, Doppler ultrasound) and legV ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ to intra-arterial infusion of phentolamine (PHEN, α-adrenergic antagonist) or phenylephrine (PE, α1-adrenergic agonist) at rest and during single-leg knee-extension exercise (0, 5 and 10 W). At rest, the PHEN-induced increase in LBF was not different between groups, but PE-induced reductions in LBF were lower in HFpEF (-16% ± 4% vs. -26% ± 5%, HFpEF vs. CON; P < 0.05). During exercise, the PHEN-induced increase in LBF was greater in HFpEF at 10 W (16% ± 8% vs. 8% ± 5%; P < 0.05). PHEN increased legV ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ in HFpEF (10% ± 3%, 11% ± 6%, 15% ± 7% at 0, 5 and 10 W; P < 0.05) but not in controls (-1% ± 9%, -4% ± 2%, -1% ± 5%; P = 0.24). The 'magnitude of sympatholysis' (PE-induced %Δ LBF at rest - PE-induced %Δ LBF during exercise) was lower in patients with HFpEF (-6% ± 4%, -6% ± 6%, -7% ± 5% vs. -13% ± 6%, -17% ± 5%, -20% ± 5% at 0, 5 and 10 W; P < 0.05) and was positively related to LBF, leg oxygen delivery, legV ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ , and the PHEN-induced increase in LBF (P < 0.05). Together, these data indicate that excessive α-adrenergic vasoconstriction restrains blood flow and limitsV ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ of the exercising leg in patients with HFpEF, and is related to impaired functional sympatholysis in this patient group. KEY POINTS: Sympathetic (α-adrenergic)-mediated vasoconstriction is exaggerated during exercise in patients with heart failure with preserved ejection fraction (HFpEF), which may contribute to limitations of blood flow, oxygen delivery and oxygen utilization in the exercising muscle. The ability to adequately attenuate α1-adrenergic vasoconstriction (i.e. functional sympatholysis) within the vasculature of the exercising muscle is impaired in patients with HFpEF. These observations extend our current understanding of HFpEF pathophysiology by implicating excessive α-adrenergic restraint and impaired functional sympatholysis as important contributors to disease-related impairments in exercising muscle blood flow and oxygen utilization in these patients.

Age-related alterations in the cardiovascular responses to acute exercise in males and females: role of the exercise pressor reflex

Autonomic adjustments of the cardiovascular system are critical for initiating and sustaining exercise by facilitating the redistribution of blood flow and oxygen delivery to meet the metabolic demands of the active skeletal muscle. Afferent feedback from active skeletal muscles evokes reflex increases in sympathetic nerve activity and blood pressure (BP) (i.e., exercise pressor reflex) and contributes importantly to these primary neurovascular adjustments to exercise. When altered, this reflex contributes significantly to the exaggerated sympathetic and BP response to exercise observed in many cardiovascular-related diseases, highlighting the importance of examining the reflex and its underlying mechanism(s). A leading risk factor for the pathogenesis of cardiovascular disease in both males and females is aging. Although regular exercise is an effective strategy for mitigating the health burden of aging, older adults face a greater risk of experiencing an exaggerated cardiovascular response to exercise. However, the role of aging in mediating the exercise pressor reflex remains highly controversial, as conflicting findings have been reported. This review aims to provide a brief overview of the current understanding of the influence of aging on cardiovascular responses to exercise, focusing on the role of the exercise pressor reflex and proposing future directions for research. We reason that this review will serve as a resource for health professionals and researchers to stimulate a renewed interest in this critical area.

Carbohydrate ingestion attenuates the reduction in complex cognitive function and cerebral blood flow during prolonged passive heat stress in humans

PURPOSE

To determine whether carbohydrate ingestion would reduce cognitive dysfunction in humans following long duration passive heat stress (PHS) versus consuming electrolytes alone.

METHODS

Fifteen young (27 ± 4 y) healthy adults were exposed to 120 min of PHS through the use of a liquid perfused suit (50 °C) on two randomized visits. Subjects consumed fluids supplemented with electrolytes (E) or electrolytes + carbohydrates (E + C). Pre- and post-heat stress, body mass (BM) and plasma osmolality (pOsm) were measured. Heart rate (HR), blood pressure (BP), Physiological Strain Index (PSI), core temperature (Tc), plasma glucose, respiration rate (RR), end-tidal CO2 (PetCO2) and internal carotid artery (ICA) blood flow were recorded at baseline and every 15 min of heat stress. Cognitive function was assessed via the Automated Neuropsychological Assessment Metric at baseline and at 30- and 120 min during heat stress.

RESULTS

There were no significant differences between fluid conditions for BM, pOsm, PSI, Tc, RR or PetCO2. Plasma glucose was ∼75% greater in the E + C condition compared to the E condition after 90 min of PHS (P < 0.05). Cognitive function (120 min) was impaired following PHS only in E condition (P < 0.05) and performance on complex cognitive tasks were better by ∼22-340% in the E + C vs. E (P < 0.05). Compared to the E condition, HR and BP were lower and ICA blood flow, vascular conductance, and glucose delivery was ∼90% greater in the E + C after 90 min of PHS (P < 0.05).

CONCLUSIONS

These data are the first to demonstrate that carbohydrate ingestion may have a protective effect on cognitive function during long duration PHS. Furthermore, this protection was associated with preserved ICA blood flow and glucose delivery to the brain.

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.

Global REACH 2018: the adaptive phenotype to life with chronic mountain sickness and polycythaemia

KEY POINTS

Humans suffering from polycythaemia undergo multiple circulatory adaptations including changes in blood rheology and structural and functional vascular adaptations to maintain normal blood pressure and vascular shear stresses, despite high blood viscosity. During exercise, several circulatory adaptations are observed, especially involving adrenergic and non-adrenergic mechanisms within non-active and active skeletal muscle to maintain exercise capacity, which is not observed in animal models. Despite profound circulatory stress, i.e. polycythaemia, several adaptations can occur to maintain exercise capacity, therefore making early identification of the disease difficult without overt symptomology. Pharmacological treatment of the background heightened sympathetic activity may impair the adaptive sympathetic response needed to match local oxygen delivery to active skeletal muscle oxygen demand and therefore inadvertently impair exercise capacity.

ABSTRACT

Excessive haematocrit and blood viscosity can increase blood pressure, cardiac work and reduce aerobic capacity. However, past clinical investigations have demonstrated that certain human high-altitude populations suffering from excessive erythrocytosis, Andeans with chronic mountain sickness, appear to have phenotypically adapted to life with polycythaemia, as their exercise capacity is comparable to healthy Andeans and even with sea-level inhabitants residing at high altitude. By studying this unique population, which has adapted through natural selection, this study aimed to describe how humans can adapt to life with polycythaemia. Experimental studies included Andeans with (n = 19) and without (n = 17) chronic mountain sickness, documenting exercise capacity and characterizing the transport of oxygen through blood rheology, including haemoglobin mass, blood and plasma volume and blood viscosity, cardiac output, blood pressure and changes in total and local vascular resistances through pharmacological dissection of α-adrenergic signalling pathways within non-active and active skeletal muscle. At rest, Andeans with chronic mountain sickness had a substantial plasma volume contraction, which alongside a higher red blood cell volume, caused an increase in blood viscosity yet similar total blood volume. Moreover, both morphological and functional alterations in the periphery normalized vascular shear stress and blood pressure despite high sympathetic nerve activity. During exercise, blood pressure, cardiac work and global oxygen delivery increased similar to healthy Andeans but were sustained by modifications in both non-active and active skeletal muscle vascular function. These findings highlight widespread physiological adaptations that can occur in response to polycythaemia, which allow the maintenance of exercise capacity.

ATP and acetylcholine interact to modulate vascular tone and α1-adrenergic vasoconstriction in humans

The vascular endothelium senses and integrates numerous inputs to regulate vascular tone. Recent evidence reveals complex signal processing within the endothelium, yet little is known about how endothelium-dependent stimuli interact to regulate blood flow. We tested the hypothesis that combined stimulation of the endothelium with adenosine triphosphate (ATP) and acetylcholine (ACh) elicits greater vasodilation and attenuates α₁-adrenergic vasoconstriction compared with combination of ATP or ACh with the endothelium-independent dilator sodium nitroprusside (SNP). We assessed forearm vascular conductance (FVC) in young adults (6 women, 7 men) during local intra-arterial infusion of ATP, ACh, or SNP alone and in the following combinations: ATP + ACh, SNP + ACh, and ATP + SNP, wherein the second dilator was coinfused after attaining steady state with the first dilator. By design, each dilator evoked a similar response when infused separately (ΔFVC, ATP: 48 ± 4; ACh: 57 ± 6; SNP: 53 ± 6 mL·min^-1·100 mmHg^-1; P ≥ 0.62). Combined infusion of the endothelium-dependent dilators evoked greater vasodilation than combination of either dilator with SNP (ΔFVC from first dilator, ATP + ACh: 45 ± 9 vs. SNP + ACh: 18 ± 7 and ATP + SNP: 26 ± 4 mL·min^-1·100 mmHg^-1, P < 0.05). Phenylephrine was subsequently infused to evaluate α₁-adrenergic vasoconstriction. Phenylephrine elicited less vasoconstriction during infusion of ATP or ACh versus SNP (ΔFVC, -25 ± 3 and -29 ± 4 vs. -48 ± 3%; P < 0.05). The vasoconstrictor response to phenylephrine was further diminished during combined infusion of ATP + ACh (-13 ± 3%; P < 0.05 vs. ATP or ACh alone) and was less than that observed when either dilator was combined with SNP (SNP + ACh: -26 ± 3%; ATP + SNP: -31 ± 4%; both P < 0.05 vs. ATP + ACh). We conclude that endothelium-dependent agonists interact to elicit vasodilation and limit α₁-adrenergic vasoconstriction in humans.NEW & NOTEWORTHY The results of this study highlight the vascular endothelium as a critical site for integration of vasomotor signals in humans. To our knowledge, this is the first study to demonstrate that combined stimulation of the endothelium with ATP and ACh results in enhanced vasodilation compared with combination of either ATP or ACh with an endothelium-independent dilator. Furthermore, we show that ATP and ACh interact to modulate α₁-adrenergic vasoconstriction in human skeletal muscle in vivo.

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.