Functional Sympatholysis Research Articles

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 ( ), possibly associated with disease-related changes in sympathetic (α-adrenergic) signaling. Thus, in seven patients with HFpEF (70±6years, 3 female/4 male) and seven controls (CON) (66±3years, 3 female/4 male), we examined changes (%Δ) in leg blood flow (LBF, Doppler ultrasound) and leg 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 10W). 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 10W (16%±8% vs. 8%±5%; P<0.05). PHEN increased leg in HFpEF (10%±3%, 11%±6%, 15%±7% at 0, 5 and 10W; 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 10W; P<0.05) and was positively related to LBF, leg oxygen delivery, leg , and the PHEN-induced increase in LBF (P<0.05). Together, these data indicate that excessive α-adrenergic vasoconstriction restrains blood flow and limits 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.

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.

Sex Difference in Reliance on CYP450 Contribution to Functional Sympatholysis in Healthy, Young Adults

Sex differences in functional sympatholysis have provided conflicting results, in both animal and human models. Further, investigations into underlying functional sympatholysis mechanisms between the sexes are largely unexplored. PURPOSE: To investigate if functional sympatholysis differs between biological sexes and to explore whether CYP450 inhibition alters functional sympatholysis between sexes. METHODS: Healthy males (n=12, 21±2 years) and females (n=12, 23±3 years, tested in early follicular phase) participated in three study visits (2 experimental). 120min prior to experimental testing, a placebo (PLA, 250 mg microcrystalline cellulose) or CYP450 2C9 inhibitor (150 mg fluconazole, FLZ) were ingested in a randomized, counter-balanced design. Beat-to-beat forearm muscle blood flow (FBF, doppler ultrasound) and mean arterial pressure (MAP, finger photoplethysmography), were measured continuously to calculate vascular conductance (FVC= FBF/MAPx100mmHg). Measurements were taken during 5 minutes of rest and lower body negative pressure (LBNP; -20mmHg) in resting conditions for 2 minutes. Following 10 minutes of rest, steady state rhythmic handgrip exercise (EX; 20% maximum voluntary contraction; 2 sec contraction, 1 sec relaxation duty cycles) was applied for 7 minutes with LBNP added for the last 2 minutes (Ex+LBNP). The magnitude of functional sympatholysis (%) was calculated as (functional sympatholysis (%) = Δ%FVC Ex+LBNP - Δ%FVC LBNP). Impairment to functional sympatholysis in response to CYP450 inhibition was calculated as the difference in functional sympatholysis (%) between the PLA and the FLZ condition. Results: Data are means ± SD. The magnitude of functional sympatholysis was not different between sexes in either PLA (Males: 33.91±20.91 % vs. Females: 38.43±21.71 %, P: 0.61 d: 0.21) nor FLZ (Males: 30.83±29.95 % vs. Females: 18.40±26.56 %, P: 0.23 r: 0.026). The impairment to functional sympatholysis in response to CYP450 inhibition was not different between sexes (Males: -3.07±20.02% vs. Women: -20.02±39.80%, P: 0.23, d: 0.5) with a moderate to large effect size. However, a two one-sided test (TOST) was used to test whether sexes had equivalent in responses to CYP450 inhibition and revealed non-significance ( t(18.86) = 0.574, p = 0.714) and thus non-equivalence. CONCLUSION: Our findings support previous investigation confirming no sex differences in functional sympatholysis in humans. However, our results indicate that sexes did not show equivalent responses to CYP450 inhibition despite non-significance. These findings advocate that females may be more reliant of CYP450 pathways for functional sympatholysis then men and future exploration is warranted. 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.

Skeletal muscle contractions attenuate the norepinephrine-related reduction in interstitial oxygen pressures in the rat spinotrapezius

Increased sympathetic nerve activity with exercise plays a key role in the control of arterial blood pressure and redistribution of blood flow. Within the microcirculation, elevated sympathetic outflow promotes the release of norepinephrine and smooth muscle contraction. Exercise-induced attenuation of sympathetic vasoconstriction (i.e., functional sympatholysis) is thus critical for active muscle oxygen delivery-utilization matching. However, the extent to which muscle contractions impact noradrenergic regulation of interstitial oxygen pressures (PO2is) is unknown. This is important because, as dictated by Fick’s law, PO2is provides the sole driving force for oxygen diffusion into the myocyte thereby supporting oxidative phosphorylation. PURPOSE: To determine the effects of norepinephrine administration on resting and contracting rat skeletal muscle PO2is. We tested the hypothesis that topical application (superfusion) of norepinephrine would result in blunted decreases in skeletal muscle PO2is during contractions compared to rest. METHODS: PO2is was determined via phosphorescence quenching in the exposed spinotrapezius muscle of anesthetized male Sprague-Dawley rats (n=7, 2-3 months old) at rest and during electrically-induced submaximal contractions (0.5 Hz, 3-6 V) for 6 minutes in response to norepinephrine superfusion (5x10−4 M). Functional sympatholysis was determined as the difference between the norepinephrine-induced changes in PO2is at rest and during the contraction steady-state (i.e., 2 min following the onset of muscle contractions) using (i) the absolute PO2is fall with norepinephrine (ΔPO2is), and (ii) the area under the PO2is curve plotted as a function of time (AUC). The tail (caudal) artery was cannulated for measurements of mean arterial pressure (MAP) and heart rate (HR). RESULTS: Consistent with our hypothesis, skeletal muscle contractions attenuated norepinephrine-evoked reductions in spinotrapezius interstitial oxygenation as evaluated by ΔPO2is (rest: -8.7±1.4, contractions: -4.3±1.1 mmHg, p&lt;0.05) and AUC (rest: 2246±368, contractions: 1066±273 mmHg·s, p&lt;0.05). Although MAP increased significantly with norepinephrine both at rest (+5±1%, p&lt;0.05) and during contractions (+8±1%, p&lt;0.05), this mild hypertensive response to norepinephrine was not different between conditions (p&gt;0.05). Norepinephrine superfusion did not affect HR at rest or during contractions (p&gt;0.05). CONCLUSIONS: That muscle contractions blunted the decrease in spinotrapezius PO2is with norepinephrine compared to the resting condition is consistent with functional sympatholysis. The latter serves to match active muscle O2 delivery-utilization in the face of elevated sympathetic nerve activity during exercise. The current results indicate that a novel combination of phosphorescence quenching (interstitial oxygenation, PO2is) and superfusion techniques may be used to evaluate functional sympatholysis in the intact rat spinotrapezius muscle. Showalter Research Trust, Purdue University. 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.

Functional sympatholysis of Neuropeptide Y-mediated vasoconstriction in humans

Metabolic inhibition of sympathetic vasoconstriction (functional sympatholysis) is essential for adequate perfusion of skeletal muscle during exercise. Functional sympatholysis occurs primarily through post-junctional attenuation of norepinephrine and α-adrenergic-mediated vasoconstriction, though other non-adrenergic neurotransmitters are co-released with norepinephrine. Neuropeptide Y (NPY) is a neurotransmitter that elicits robust vasoconstriction and is co-released with norepinephrine during exercise in an intensity dependent manner. Evidence from animal models indicate that NPY is sensitive to metabolic inhibition, however metabolic inhibition of NPY-mediated vasoconstriction has not been tested in humans. We tested the hypothesis that NPY-mediated vasoconstriction would be attenuated during handgrip exercise in humans. In 10 healthy adults (7M:3F, age: 30±7 years, BMI: 24±5) we measured forearm blood flow (Doppler ultrasound), blood pressure (brachial artery catheter), and calculated changes in forearm vascular conductance (FVC) to local intra-arterial infusions of phenylephrine (PE; α1-agonist) or NPY (Y1R-agonist) during 1) intra-arterial infusion of sodium nitroprusside (SNP; nitric oxide donor) to serve as a non-metabolic vasodilatory control or 2) dynamic rhythmic handgrip exercise (EX; 15% maximal voluntary contraction). The vasoconstrictor responses to PE and NPY were quantified as a percent ΔFVC during each condition (SNP vs. EX) and were compared by two-way RM-ANOVA. The magnitude of sympatholysis (ΔFVC during exercise expressed as a percent of control) for PE and NPY was compared by a paired t-test. As expected, the vasoconstrictor response to PE was attenuated during handgrip exercise compared to SNP (ΔFVC: SNP: -50±24 vs. EX: -17±9 %; p=0.002). Similarly, NPY-mediated vasoconstriction was blunted during handgrip exercise compared to SNP (ΔFVC: SNP: -32±22 vs. EX: -11±7 %; p=0.029). There was no difference in the magnitude of sympatholysis between PE and NPY (PE: 68±18, NPY: 52±34 %; p=0.28). These data indicate that NPY-mediated vasoconstriction is sensitive to metabolic inhibition in humans, and the magnitude of sympatholysis is not different from α1-adrenergic vasoconstriction. Funding: NIH R00HL153777. 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.

Impaired functional sympatholysis during exercise in HFpEF: toward understanding exercise intolerance.

Impaired functional sympatholysis during exercise in HFpEF: toward understanding exercise intolerance.

A single high-fat Western meal modulates vascular responsiveness to sympathetic activation at rest and during exercise in humans: a randomized controlled trial.

A single high-fat Western meal transiently reduces endothelium-dependent vasodilation at rest but the interaction with sympathetic vasoconstrictor activity during exercise remains unknown. Herein, we tested the hypothesis that a single high-fat Western meal would impair the ability of contracting skeletal muscle to offset vascular responsiveness to sympathetic activation during exercise, termed functional sympatholysis. In 18 (10F/8M) healthy young adults, forearm blood flow (Doppler ultrasound) and beat-to-beat arterial pressure (photoplethysmography) were measured during lower-body negative pressure (LBNP; -20 mmHg) applied at rest and simultaneously during low- (15% maximum contraction) and moderate-intensity (30% maximum contraction) rhythmic handgrip. The magnitude of sympatholysis was calculated as the difference of LBNP-induced changes in forearm vascular conductance (FVC) between handgrip and rest. Experiments were performed preprandial and 1h, 2h, and 3h after a high- or low-fat meal. In the preprandial state, LBNP decreased resting FVC (∆-54±10%), and these responses were attenuated during low- (∆-17±7%) and moderate-intensity handgrip (∆-8±6%). Following a high-fat meal, LBNP induced attenuated decreases in resting FVC (3h postprandial: ∆-47±10%, P=0.002 vs. preprandial), and blunted attenuation of FVC during low- (3h postprandial: ∆-23±8%, P=0.001 vs. preprandial) and moderate-intensity handgrip (3h postprandial: ∆-16±6%, P<0.001 vs. preprandial). The high-fat meal attenuated the magnitude of sympatholysis during low- (preprandial: 38±7% vs. 3h postprandial: 23±8%, P<0.001) and moderate-intensity handgrip (preprandial: 46±11% vs. 3h postprandial: 31±10%, P<0.001). The low-fat meal had no impact on these responses. In conclusion, a single high-fat Western meal modulates sympathetic vasoconstriction at rest and during low- and moderate-intensity handgrip exercise in young healthy adults.

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.

A single high-fat meal impairs functional sympatholysis in young healthy adults

Introduction: A high-fat diet increases the risk of cardiovascular disease and mortality, but the underlying mechanisms remain incompletely understood. During exercise, sympathetically mediated vasoconstriction is dampened in active skeletal muscle due to the counteracting vasodilatory effects of metabolic by-products from muscle contraction. This phenomenon, termed functional sympatholysis, plays a critical role in the redistribution of cardiac output to contracting skeletal muscles to meet the metabolic demands of exercise. Whether a high-fat meal modulates the magnitude of sympatholysis is currently unknown. Herein, we tested the hypothesis that a high-fat meal would acutely impair functional sympatholysis in young healthy adults. Methods: In a randomized, controlled, and cross-over design, 14 healthy adults (8 women, 26±4 years), consumed either a high- (1030kcals, 91g fat) or a low-fat (1006kcals, 1g fat) meal on two separate days. Forearm blood flow (FBF; Doppler ultrasound) and beat-to-beat mean arterial pressure (MAP; finger photoplethysmography) were measured during sympathetic activation induced by -20 mmHg lower-body negative pressure (LBNP) applied at rest and simultaneously during rhythmic handgrip exercise at 15% and 30% maximum voluntary contraction (MVC). Measurements were taken pre-meal and at 1-, 2-, and 3-hours post-prandial. Forearm vascular conductance (FVC) was calculated as FBF/MAP and the magnitude of sympatholysis as the difference of LBNP-induced changes in FVC between handgrip and rest. Results: Prior to the high-fat meal, LBNP decreased resting FVC (Δ-57±10%), and this response was attenuated during exercise at 15% (Δ-18±8%) and 30% MVC (Δ-9±7%). After the high-fat meal, LBNP induced smaller decreases in resting FVC (1h: Δ-43±12%, 2h: Δ-45±12%, 3h: Δ-49±8%, all P≤0.01 vs. pre), but during exercise greater decreases in FVC were observed at both 15% (1h: Δ-22±10%, 2h: Δ-23±8%, 3h: Δ-25±8%, all P≤0.04 vs. pre) and 30% MVC (1h: Δ-15±8%, 2h: Δ-15±6%, 3h: Δ-16±6%, all P≤0.04 vs. pre). As a result, the high-fat meal decreased the magnitude of sympatholysis during rhythmic handgrip exercise at 15% (pre-prandial: 39±8%, 1h: 20±10%, 2h: 21±8%, 3h: 24±7%, all P≤0.001) and 30% MVC (pre-prandial: 48±11%, 1h: 28±10%, 2h: 30±11%, 3h: 33±8%, all P≤0.001). The low-fat meal did not change the magnitude of sympatholysis at either 15% (pre-prandial: 37±6%, 1h: 39±6%, 2h: 38±7%, 3h: 37±8%, all P≥0.07) or 30% MVC (pre-prandial: 45±10%, 1h: 47±8%, 2h: 47±9%, 3h: 46±7%, all P≥0.39). Conclusion: These findings demonstrate the capacity of a high-fat meal to acutely impair the ability of contracting skeletal muscle to offset the vascular responsiveness to α1-adrenoreceptor activation during low- and moderate-intensity rhythmic handgrip exercise. Natural Science and Engineering Research Council (NSERC) of Canada Discovery Grant (P.J.M) This is the full abstract presented at the American Physiology Summit 2023 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.

The Effect of Aerobic Exercise Training on Functional Sympatholysis in Patients with Chronic Kidney Disease

Intro: Patients with chronic kidney disease (CKD) experience exercise intolerance that is associated with increased risk of cardiovascular disease and poor quality of life. Sympathetic vasoconstriction is markedly blunted during exercise to optimize blood flow to the metabolically active muscles, termed functional sympatholysis. Impaired functional sympatholysis is observed in models with cardiovascular disease and is suggested to contribute to exercise intolerance by limiting adequate muscle blood flow and tissue oxygen delivery to the exercising muscles. Chronic endurance exercise training is known to alter sympathetic regulation of muscle blood flow. Therefore, we investigated the hypotheses that 1) functional sympatholysis is impaired in CKD patients and 2) an aerobic exercise training alters functional sympatholysis status in CKD patents. Method: 79 sedentary individuals with CKD stages III-IV(62±9 yr, 94% hypertensive) and 21 age-matched controls (59±7 yr, 85% hypertensive) were enrolled. Lower body negative pressure (LBNP) at -20mmHg was used to induce sympathetic activation. Rhythmic handgrip exercise at 30% of maximal voluntary contraction was performed for 3 minutes followed by 4 additional minutes with concomitant LBNP. Forearm vasoconstrictor responses were continuously monitored using near-infrared spectroscopy to detect changes in muscle tissue saturation index (TSI, calculated as % of total labile signal). A subset of CKD patients (n=49) underwent 12 weeks of aerobic cycling exercise (spinning at 85% of maximal heart rate reserve, n=29) or stretching (active control intervention, n=20)that performed 45 min/session 3d/wk and matched for duration. Results: At rest, TSI was significantly decreased in response to LBNP, with no difference between CKD and controls (dTSI: -8.0±5.9, -5.6±9.1 % respectively, p&lt;0.05 from rest for all). During handgrip exercise, the LBNP-induced decrease in TSI was abolished with no difference between CKD and controls (dTSI: -0.9±13.2, 0.8±12.1 %; p&gt;0.05 from rest for all). No significant changes were observed in TSI responses to LBNP during handgrip exercise after 12 weeks of cycling (dTSI: -3.6±13.8 to -3.7±12.2 %; p=0.966) or stretching (dTSI: -2.0±11.7 to -0.9±10.6 %; p=0.748) in CKD patients. Conclusion: Sympathetic vasoconstriction was blunted in the exercising forearm muscles of CKD patients, indicating intact functional sympatholysis during a small muscle upper limb exercise. Aerobic exercise training did not alter functional sympatholysis in CKD patients. Determining the exercise training prescription that has beneficial effects on autonomic regulation and exercise tolerance in CKD warrants further investigation. NIH R01HL135183; NIH R61AT10457; NIH NCATS KL2TR002381, NIH T32 DK00756; NIH F32HL147547; VA Merit I01CX001065. This is the full abstract presented at the American Physiology Summit 2023 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.

Alpha 1-adrenergic-mediated vasoconstriction and functional sympatholysis in patients with heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) is characterized by peripheral vascular dysregulation at both rest and during exercise. Prior studies have demonstrated that blood flow to skeletal muscle is closely regulated by sympathetic (α-adrenergic) vasoconstriction and the interaction of vasoactive substances released locally during exercise that counteract α-adrenergic vasoconstriction. This phenomenon, known as “functional sympatholysis” helps facilitate exercise hyperemia and has been shown to decline via aging and disease-related changes in α1-adrenergic function but this has not been studied in HFpEF. Thus, the overall objective of this study was to characterize peripheral α1-adrenergic vasoconstriction in HFpEF. We hypothesized that both resting α1-mediated vasoconstriction and the magnitude of functional sympatholysis would be diminished in HFpEF compared to healthy, age-matched control (CON) subjects.Six patients with HFpEF (3 male, 70±3 yrs) and six CON (3 male, 67±1 yrs) completed this study. Leg blood flow (LBF, ultrasound Doppler) was evaluated at rest and during dynamic single leg knee-extension exercise (0, 5 and 10 Watts) before and after local, intra-arterial administration of Phenylephrine (PE, α1-adrenergic agonist). PE-mediated changes in LBF (%Δ) at rest and during exercise were compared between groups. The magnitude of functional sympatholysis was calculated as the difference between PE-induced %Δ LBF from rest to exercise.At rest, the PE-induced %Δ LBF was smaller in HFpEF (HFpEF: -17±2% vs. CON: -31±8%, P=0.01). While PE-induced %Δ LBF were similar between groups during exercise, when these responses were compared to %Δ LBF at rest, the calculated magnitude of sympatholysis was significantly reduced in HFpEF (0W: -4±3%, 5W: -5±4%, 10W: -8±3%) compared to CON (0W: -17±7%, 5W: -20±8%, 10W: -25±6%). These data demonstrate a marked diminution in both α1-adrenergic responsiveness at rest and the ability to attenuate α1-adrenergic mediated vasoconstriction during exercise (functional sympatholysis) in patients with HFpEF. Diminished α1-adrenergic signaling and impaired functional sympatholysis may represent an important aspect of HFpEF pathophysiology that likely contribute to vascular dysregulation in this patient group. Funded in part by grants from the National Institutes of Health (T32HL139451) and the Veterans Health Administration (I01RX001311, IK2RX003670, IK2CX002114). This is the full abstract presented at the American Physiology Summit 2023 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.

Contribution of cytochrome P450 2C9 pathways to functional sympatholysis in healthy young adults

In humans exercise hyperemia is adequately maintained despite elevated sympathetic outflow through an endothelium dependent, yet nitric oxide and prostaglandin independent mechanism(s) known as functional sympatholysis. Evidence suggests that functional sympatholysis has an endothelium-dependent hyperpolarization-like component, however, investigations into hyperpolarization pathways, such as, cytochrome P450 2C9 are scant. PURPOSE: To determine if cytochrome P450 inhibition attenuates functional sympatholysis in healthy young adults. METHODS: 21 participants (23 ± 3 years, 11 Females, tested in early follicular phase) participated in three study visits (1 familiarization, 2 experimental visits, random, single blind, counter-balanced) separated by ≥72 hours. Participants completed 5min baseline, 2min resting lower body negative pressure (LBNP, -20mmHg, to elevate sympathetic outflow), 7min of dynamic handgrip exercise at 20% effort with minutes 5-7 having LBNP stimulus superimposed (Ex+LBNP) in the supine position, exercising limb at heart-level. 10 min of rest separated each event. 120min prior to testing a placebo (PLA, 250 mg microcrystalline cellulose) or cytochrome P450 2C9 inhibitor (150 mg fluconazole, FLZ) were ingested. Forearm blood flow (FBF, Echo and Doppler ultrasound) and mean arterial pressure (MAP, finger photoplethysmography) were measured continuously and use to calculate forearm vascular conductance (FVC= FBF/MAPx100mmHg). RESULTS: All data are mean percent change in FVC (%Δ) ± SD. FVC decreased (p&lt;0.001) from baseline during LBNP with no effect of FLZ (PLA: -31.8 ± 19.4 vs. FLZ: -36.1 ± 25.1 p:0.53, d: 0.17). Interestingly, FVC increased from baseline to exercise (p&lt;0.001) with no effect of FLZ (PLA: 819.6 ± 503.9 vs. FLZ: 709.1 ± 459.2 p: 0.36, d: 0.20). However, FVC was unaltered during Ex+LBNP in PLA from steady-state exercise (4.5 ± 13.9, p:0.32) but was significantly reduced with FLZ (–11.5 ± 13.3, p&gt;0.001) with a large effect size between conditions (p&gt;0.001, d: 1.1). All responses were similar between the sexes (p:0.5, d:0.3). CONCLUSION: Our results indicate that in young healthy adults, inhibition of cytochrome P450 pathways attenuated vascular opposition to superimposed sympathetic stress during dynamic handgrip exercise. Thus, cytochrome P450 pathways are mechanistically involved in functional sympatholysis in young healthy humans. This is the full abstract presented at the American Physiology Summit 2023 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.

Ischemic Preconditioning Acutely Improves Functional Sympatholysis during Handgrip Exercise in Healthy Males but not Females.

Ischemic preconditioning (IPC), a procedure that involves the cyclic induction of limb ischemia and reperfusion via tourniquet inflation, has been reported to improve exercise capacity and performance, but the underlying mechanisms remain unclear. During exercise, sympathetically mediated vasoconstriction is dampened in active skeletal muscle. This phenomenon, termed functional sympatholysis, plays a critical role in maintaining oxygen delivery to working skeletal muscle and may contribute to determining exercise capacity. Herein, we investigate the effects of IPC on functional sympatholysis in humans. In 20 (10M/10F) healthy young adults, forearm blood flow (Doppler ultrasound) and beat-to-beat arterial pressure (finger photoplethysmography) were measured during lower body negative pressure (LBNP; -20 mm Hg) applied at rest and simultaneously during rhythmic handgrip exercise (30% maximum contraction) before and after local IPC (4 × 5-min 220 mm Hg) or sham (4 × 5-min 20 mm Hg). Forearm vascular conductance (FVC) was calculated as forearm blood flow/mean arterial pressure and the magnitude of sympatholysis as the difference of LBNP-induced changes in FVC between handgrip and rest. At baseline, LBNP decreased FVC (females [F] = ∆-41% ± 19%; males [M] = ∆-44% ± 10%), and these responses were attenuated during handgrip (F = ∆-8% ± 9%; M = ∆-8% ± 7%). After IPC, LBNP induced similar decreases in resting FVC (F = ∆-37% ± 19%; M = ∆-44% ± 13%). However, during handgrip, this response was further attenuated in males (∆-3% ± 9%, P = 0.02 vs pre) but not females (∆-5% ± 10%, P = 0.13 vs pre), which aligned with an IPC-mediated increase in sympatholysis (M-pre = 36% ± 10% vs post = 40% ± 9%, P = 0.01; F-pre = 32% ± 15% vs post = 32% ± 14%, P = 0.82). Sham IPC had no effect on any variables. These findings highlight a sex-specific effect of IPC on functional sympatholysis and provide evidence of a potential mechanism underlying the beneficial effects of IPC on human exercise performance.

Exaggerated pressor responses, but unaltered blood flow regulation and functional sympatholysis during lower limb exercise in young, non-Hispanic black males.

Young non-Hispanic black (BL) males have displayed lower blood flow (BF) and vascular conductance (VC), but intact functional sympatholysis, during upper limb exercise when compared to non-Hispanic white (WH) males. This study sought to explore if similar differences were also present in the lower limbs. Thirteen young BL males and thirteen WH males completed one visit comprised of rhythmic lower limb (plantar flexion) exercise as well as upper limb (handgrip) exercise for a limb-specific comparison. Limb BF, mean arterial pressure (MAP), and VC were evaluated at three submaximal workloads (8, 16, and 24kg). To determine potential limb differences in functional sympatholysis, the impact of sympathetic nervous system activation (via cold-pressor test (CPT)) was evaluated at rest and during steady state exercise (30% of maximal voluntary contraction) on a subsequent visit. MAP responses to lower and upper limb exercise were elevated in young BL males (vs WH males), resulting in significantly lower VC responses in the upper limb, but not the lower limb. Further, BL males, when compared to WH males, revealed no differences in functional sympatholysis, evident by similar responses in both the exercising leg and arm VC during CPT. The findings of the current study indicate that although elevated MAP responses were observed during both lower and upper limb exercise in young BL males, vascular conductance was only hindered in the upper limbs. This may potentially highlight enhanced compensatory mechanisms in the lower limb (vs upper limb) to maintain perfusion in young BL males.

Assessing functional sympatholysis during rhythmic handgrip exercise using Doppler ultrasound and near-infrared spectroscopy: sex differences and test-retest reliability.

The effects of sympathetic activity on vasoconstriction are dampened in active skeletal muscle during exercise, a phenomenon termed functional sympatholysis. Limited work has examined the influence of sex on the magnitude of sympatholysis or the test-retest reliability of measurements. In 16 women and 15 men, forearm blood flow (FBF; Doppler ultrasound), muscle oxygenation (near-infrared spectroscopy, NIRS), and beat-to-beat mean arterial pressure (MAP; photoplethysmography) were measured during lower-body negative pressure (LBNP; -20 mmHg) at rest and simultaneously during rhythmic handgrip exercise (30% maximum contraction). Measures were taken twice within the same visit (separated by 15 min) and repeated on a second visit. Forearm vascular conductance (FVC) was calculated as FBF/MAP. The magnitude of sympatholysis was calculated as the difference of LBNP-induced changes between handgrip and rest. LBNP decreased FBF (Δ-45 ± 15%), FVC (Δ-45 ± 16%), and muscle oxygenation (Δ-14 ± 11%); however, these responses were attenuated when LBNP was applied during rhythmic handgrip exercise (Δ-7 ± 9%, Δ-9 ± 10%, and Δ-6 ± 9%, respectively). The magnitude of sympatholysis was not different between men and women (FBF: 40 ± 16% vs. 35 ± 9%, P = 0.37; FVC: 38 ± 16% vs. 35 ± 11%, P = 0.53; muscle oxygenation: 5 ± 9% vs. 11 ± 10%, P = 0.11). Furthermore, sympatholysis measurements demonstrated good to excellent intraday (intraclass-correlation coefficients; ICC ≥ 0.85) and interday (ICC ≥ 0.72) test-retest reliability (all P ≤ 0.01) in both sexes. The coefficients of variation were larger with NIRS (68-91%) than with Doppler ultrasound (16%-22%) assessments of functional sympatholysis. Collectively, these findings demonstrate that assessments of functional sympatholysis are not impacted by biological sex and that Doppler ultrasound-derived measures of sympatholysis have better within-subject reliability than NIRS-derived measures in young healthy adults.

Does sympathetic vasoconstriction contribute to metabolism: Perfusion matching in exercising skeletal muscle?

The process of matching skeletal muscle blood flow to metabolism is complex and multi-factorial. In response to exercise, increases in cardiac output, perfusion pressure and local vasodilation facilitate an intensity-dependent increase in muscle blood flow. Concomitantly, sympathetic nerve activity directed to both exercising and non-active muscles increases as a function of exercise intensity. Several studies have reported the presence of tonic sympathetic vasoconstriction in the vasculature of exercising muscle at the onset of exercise that persists through prolonged exercise bouts, though it is blunted in an exercise-intensity dependent manner (functional sympatholysis). The collective evidence has resulted in the current dogma that vasoactive molecules released from skeletal muscle, the vascular endothelium, and possibly red blood cells produce local vasodilation, while sympathetic vasoconstriction restrains vasodilation to direct blood flow to the most metabolically active muscles/fibers. Vascular smooth muscle is assumed to integrate a host of vasoactive signals resulting in a precise matching of muscle blood flow to metabolism. Unfortunately, a critical review of the available literature reveals that published studies have largely focused on bulk blood flow and existing experimental approaches with limited ability to reveal the matching of perfusion with metabolism, particularly between and within muscles. This paper will review our current understanding of the regulation of sympathetic vasoconstriction in contracting skeletal muscle and highlight areas where further investigation is necessary.

Reflex Sympathetic Vasoconstriction And Functional Sympatholysis In Patients With Heart Failure With Preserved Ejection Fraction.

INTRODUCTION: Vascular impairments resulting in blood flow limitations to exercising muscle likely contribute to marked exercise intolerance in patients with heart failure with preserved ejection fraction (HFpEF). Blood flow to exercising muscle is modulated by the interaction of sympathetic nervous system (SNS)-mediated vasoconstriction and factors related to muscle contraction that counteract this vasoconstriction, so-called “functional sympatholysis”. While there is emerging evidence for SNS overactivity during exercise in patients with HFpEF, functional sympatholysis has not been assessed in this patient group. PURPOSE: To determine the effect of SNS-mediated vasoconstriction on exercising muscle blood flow and the efficacy of functional sympatholysis in HFpEF vs. age-matched control (CON) subjects. METHODS: 4 CON (2 male, 70 ± 2 yrs) and 5 Class II/III patients with HFpEF (2 male, 70 ± 4 yrs) participated in this study. Forearm blood flow (FBF, ultrasound Doppler), mean arterial blood pressure (MAP, Finometer), and forearm vascular conductance (FVC) were evaluated at rest and during rhythmic handgrip exercise (30% maximum voluntary contraction, 3-min bout, 1 Hz) before and after SNS activation evoked by lower body negative pressure (LBNP -20 mmHg). Hemodynamic changes (%Δ) in response to LBNP at rest and during exercise were compared between groups, and the magnitude of functional sympatholysis was calculated as the difference between LBNP-induced %Δ FVC from rest to exercise. RESULTS: At rest, %Δ FBF in response to LBNP was similar between CON vs. HFpEF (-24 ± 4% vs. 17 ± 5%), while %Δ FVC was significantly greater in CON vs. HFpEF (-23 ± 4% vs. -10 ± 3%, P < 0.05). During exercise, LBNP reduced FBF (-1 ± 2% vs. -6 ± 3%) and FVC (-1 ± 3% vs. -6 ± 3%) to a similar degree between groups. However, the magnitude of sympatholysis was significantly greater in CON vs. HFpEF (-23 ± 4% vs. -3 ± 4%, P < 0.05). LBNP-induced changes in MAP were not different between CON vs. HFpEF at rest (-1 ± 2% vs. -3 ± 1%) or during exercise (0 ± 2% vs. 1 ± 2%). CONCLUSION: These data suggest impaired functional sympatholysis in HFpEF, indicating that SNS-mediated vasoconstriction may contribute to blood flow limitations underpinning exercise intolerance in HFpEF

Intra and inter‐day reproducibility of the functional sympatholysis protocol during rhythmic handgrip exercise in healthy humans

IntroductionThe effects of increased sympathetic activity on vasoconstriction are blunted in active skeletal muscle during exercise. This mechanism, termed functional sympatholysis (FSY), plays a vital role in the maintenance of blood perfusion and oxygen delivery to meet the metabolic demands of active skeletal muscle. In humans, FSY can be determined experimentally by applying low‐levels of lower‐body negative pressure (LBNP) to unload the cardiopulmonary baroreceptors and induce a reflex sympathoexcitation at rest and simultaneously during rhythmic handgrip exercise. In young healthy individuals, LBNP causes a pronounced vasoconstrictor response at rest, but not during exercise in the working forearm. Although this paradigm has been used extensively to assess the FSY in humans, to date, no studies have examined the reproducibility of this protocol. Therefore, the aim of the present study is to assess the within‐visit and between‐visit reproducibility of the FSY protocol in healthy adults.MethodsTwenty‐three (12 females) young, healthy adults (26 ± 4 years; 73 ± 13 kg [mean ± SD]) participated in the study. Forearm blood flow (Doppler ultrasound) and beat‐to‐beat BP (finger photoplethysmography) were measured continuously at rest and during 6‐min of rhythmic handgrip exercise at 30% of maximum voluntary contraction. LBNP at ‐20 mmHg was applied for 2 min at rest and from the 3rd to the 5th min of handgrip. Participants performed two visits separated by ≥48h where two FSY bouts were completed. Forearm vascular conductance (FVC) was calculated as FBF/Mean blood pressure. The FSY index (FSYi) was calculated using the following formula: FSYi = (∆%handgrip ‐ ∆%rest)/100. Intraclass correlation coefficients (ICC; two‐way mixed effects, average measures, and absolute agreement) were used as a primary reproducibility measure (ICC values &gt;0.75 are considered very good).ResultsAt rest, LBNP induced a decrease in forearm blood flow (Visit 1 trial 1: ∆‐43±16%; Visit 1 trial 2: ∆‐38±10%; Visit 2: ∆‐39±14%) while these responses were attenuated during LBNP+handgrip (Visit 1 trial 1: ∆‐7±7%; Visit 1 trial 2: ∆‐5±8%; Visit 2: ∆‐4±7%). Each of these responses was similar across the trials (all P≥0.17). The FSYi calculated based on the forearm blood flow responses demonstrated excellent intra (ICC=0.93 [95% CI: 0.82‐0.97]; P&lt;0.001) and inter‐day (ICC=0.94 [95% CI: 0.85‐0.97]; P&lt;0.001) reproducibility. Similarly, FVC responses demonstrated very good intra (ICC=0.81 [95% CI: 0.55‐0.92]; P&lt;0.001) and inter‐day (ICC=0.86 [95% CI: 0.65‐0.95]; P&lt;0.001) reproducibility.ConclusionThis is the first study to demonstrate that Doppler ultrasound‐derived assessments of FSY using LBNP and forearm handgrip exercise possess high intra‐ and inter‐day reproducibility in young, healthy adults.

ATP Attenuates α1‐Adrenergic and Peptidergic Vasocontraction in Isolated Porcine Cerebral Arteries

IntroductionCompeting influences regulate blood flow control in the brain. In the skeletal muscle vasculature, purinergic ATP signaling attenuates α1‐adrenergic and peptidergic‐induced vascular smooth muscle contraction (i.e., phenomenon referred to as functional sympatholysis); however, whether this occurs in the cerebrovasculature remains unknown. The purpose of this experiment was to examine the effect of ATP on vascular responses to α1‐adrenergic‐ and peptidergic‐receptor activation in cerebral arteries. We hypothesized that ATP would attenuate the α1‐adrenergic‐ and peptidergic‐mediated vasocontraction in isolated pial arteries.MethodsFemale pigs (n=5) were euthanized and their brains harvested. Thereafter, 1A branches of the middle cerebral artery were dissected for wire‐myography. Dose‐response curves for the α1‐adrenergic agonist phenylephrine (PE;1e‐10‐1e‐4M) and the peptidergic agonist neuropeptide Y (NPY;1e‐12‐1e‐6M) were performed in the absence or presence of ATP (1e‐6M). Paired one‐tailed t‐tests were used to compare the overall magnitude of contraction (area under the curve; AUC) and physiological maximal responses between conditions.ResultsData are mean±SD. The AUC of PE‐mediated cerebral vasocontraction was attenuated by ATP (untreated=67±25 vs. ATP pre‐treatment=33±23 AU; p=0.03). However, the mean reduction in the maximal response to PE in arteries pre‐treated with ATP was not significant (untreated=43±15 vs. ATP pre‐treatment=27±11%; p=0.10). The AUC (untreated=62±31 vs. ATP pre‐treatment=37±23 AU) as well as maximal NPY‐mediated cerebral vasocontraction (untreated=65±20 vs. ATP pre‐treatment=49±21%) was attenuated by ATP (all p≤0.05).ConclusionThese data indicate ATP attenuates PE‐ and NPY‐mediated vasocontraction in isolated pial arteries. Thus, similar to the skeletal muscle vasculature, purinergic signaling attenuates vasoreactivity to α1‐adrenergic and peptidergic receptor activation in the cerebrovasculature. Functional sympatholysis may assist in coupling cerebral blood flow to brain metabolism under basal conditions as well as in the setting of heightened sympatho‐excitation.

Passive leg movement-induced vasodilation and exercise-induced sympathetic vasoconstriction.

The role of nitric oxide (NO) as a modulator of functional sympatholysis has been debated in the literature, but the preponderance of evidence suggests that the magnitude of NO-mediated dilation is restrained by sympathetic vasoconstriction. Therefore, we hypothesized that passive leg movement (PLM)-induced vasodilation, which is predominantly NO-mediated, would be attenuated by an exercise-induced increase in muscle sympathetic nerve activity (MSNA). To test this hypothesis, MSNA, leg blood flow (LBF), and mean arterial blood pressure (MAP) were measured and leg vascular conductance (LVC) calculated in 9 healthy subjects (30±3yr), during PLM with and without sympathoexcitation evoked by arm-cranking exercise (ACE), at 25, 50, and 75% of maximal capacity. During this incremental intensity ACE, MSNA increased significantly (26±2, 34±3, and 41±5 bursts/100 HB, respectively). LVC during PLM fell markedly (~1.2ml/min/mmHg) with each increase in ACE intensity, and there was a strong relationship (r=0.92; p<0.05) between ∆MSNA and ∆Peak LVC induced by the three intensities of ACE. Thus, as anticipated, this study reveals that the, NO-mediated, PLM-induced vasodilation, is significantly and proportionally attenuated by exercise-induced MSNA. This finding highlights the dominant role of MSNA in regulating skeletal muscle vascular conductance.