Effect of centrally acting angiotensin converting enzyme inhibitor on the exercise-induced increases in muscle sympathetic nerve activity

Abnormal neurovascular control during central and peripheral chemoreceptors stimulation in heart failure patients with preserved ejection fraction

PURPOSE

Central and peripheral chemoreceptors are hypersensitized in patients with heart failure with reduced ejection fraction. Whether this autonomic alteration occurs in patients with heart failure with preserved ejection fraction (HFpEF) remains little known. We test the hypothesis that the central and peripheral chemoreflex control of muscle sympathetic nerve activity (MSNA) is altered in HFpEF.

METHODS

Patients aged 55-80 years with symptoms of heart failure, body mass index ≤ 35 kg/m2, left ventricular ejection fraction > 50%, left atrial volume index > 34 mL/m2, left ventricular early diastolic filling velocity and early diastolic tissue velocity of mitral annulus ratio (E/e' index) ≥ 13, and BNP levels > 35 pg/mL were included in the study (HFpEF, n = 9). Patients without heart failure with preserved ejection fraction (non-HFpEF, n = 9), aged-paired, were also included in the study. Peripheral chemoreceptors stimulation (10% O2 and 90% N2, with CO2 titrated) and central chemoreceptors stimulation (7% CO2 and 93% O2) were conducted for 3 min. MSNA was evaluated by microneurography technique, and forearm blood flow (FBF) by venous occlusion plethysmography.

RESULTS

During hypoxia, MSNA responses were greater (p < 0.001) and FBF responses were lower in patients with HFpEF (p = 0.006). Likewise, MSNA responses during hypercapnia were higher (p < 0.001) and forearm vascular conductance (FVC) levels were lower (p = 0.030) in patients with HFpEF.

CONCLUSIONS

Peripheral and central chemoreflex controls of MSNA are hypersensitized in patients with HFpEF, which seems to contribute to the increase in MSNA in these patients. In addition, peripheral and central chemoreceptors stimulation in patients with HFpEF causes muscle vasoconstriction.

Control of blood pressure in the cold: differentiation of skin and skeletal muscle vascular resistance

NEW FINDINGS

What is the central question of this study? Why does blood pressure increases during cold air exposure? Specifically, what is the contribution of skin and skeletal muscle vascular resistance during whole body versus isolated face cooling? What is the main finding and its importance? Whole-body cooling caused an increase in blood pressure through an increase in skeletal muscle and cutaneous vascular resistance. However, isolated mild face cooling caused an increase in blood pressure predominately via an increase in cutaneous vasoconstriction.

ABSTRACT

The primary aim of this investigation was to determine the individual contribution of the cutaneous and skeletal muscle circulations to the cold-induced pressor response. To address this, we examined local vascular resistances in the cutaneous and skeletal muscle of the arm and leg. Thirty-four healthy individuals underwent three different protocols, whereby cold air to clamp skin temperature (27°C) was passed over (1) the whole-body, (2) the whole-body, but with the forearm pre-cooled to clamp cutaneous vascular resistance, and (3) the face. Cold exposure applied to the whole body or isolated to the face increased mean arterial pressure (all, P < 0.001) and total peripheral resistance (all, P < 0.047) compared to thermal neutral baseline. Whole-body cooling increased femoral (P < 0.005) and brachial artery resistance (P < 0.003) compared to thermoneutral baseline. Moreover, when the forearm was pre-cooled to remove the contribution of cutaneous resistance (P = 0.991), there was a further increase in lower arm vasoconstriction (P = 0.036) when whole-body cooling was superimposed. Face cooling also caused a reflex increase in lower arm cutaneous (P = 0.009) and brachial resistance (P = 0.050), yet there was no change in femoral resistance (P = 0.815) despite a reflex increase in leg cutaneous resistance (P = 0.010). Cold stress causes an increase in blood pressure through a change in total peripheral resistance that is largely due to cutaneous vasoconstriction with face cooling, but there is additional vasoconstriction in the skeletal muscle vasculature with whole-body cooling.

Global Reach 2018: sympathetic neural and hemodynamic responses to submaximal exercise in Andeans with and without chronic mountain sickness

Andeans with chronic mountain sickness (CMS) and polycythemia have similar maximal oxygen uptakes to healthy Andeans. Therefore, this study aimed to explore potential adaptations in convective oxygen transport, with a specific focus on sympathetically mediated vasoconstriction of nonactive skeletal muscle. In Andeans with (CMS+, n = 7) and without (CMS-, n = 9) CMS, we measured components of convective oxygen delivery, hemodynamic (arterial blood pressure via intra-arterial catheter), and autonomic responses [muscle sympathetic nerve activity (MSNA)] at rest and during steady-state submaximal cycling exercise [30% and 60% peak power output (PPO) for 5 min each]. Cycling caused similar increases in heart rate, cardiac output, and oxygen delivery at both workloads between both Andean groups. However, at 60% PPO, CMS+ had a blunted reduction in Δtotal peripheral resistance (CMS-, -10.7 ± 3.8 vs. CMS+, -4.9 ± 4.1 mmHg·L-1·min-1; P = 0.012; d = 1.5) that coincided with a greater Δforearm vasoconstriction (CMS-, -0.2 ± 0.6 vs. CMS+, 1.5 ± 1.3 mmHg·mL-1·min-1; P = 0.008; d = 1.7) and a rise in Δdiastolic blood pressure (CMS-, 14.2 ± 7.2 vs. CMS+, 21.6 ± 4.2 mmHg; P = 0.023; d = 1.2) compared with CMS-. Interestingly, although MSNA burst frequency did not change at 30% or 60% of PPO in either group, at 60% Δburst incidence was attenuated in CMS+ (P = 0.028; d = 1.4). These findings indicate that in Andeans with polycythemia, light intensity exercise elicited similar cardiovascular and autonomic responses compared with CMS-. Furthermore, convective oxygen delivery is maintained during moderate-intensity exercise despite higher peripheral resistance. In addition, the elevated peripheral resistance during exercise was not mediated by greater sympathetic neural outflow, thus other neural and/or nonneural factors are perhaps involved.NEW & NOTEWORTHY During submaximal exercise, convective oxygen transport is maintained in Andeans suffering from polycythemia. Light intensity exercise elicited similar cardiovascular and autonomic responses compared with healthy Andeans. However, during moderate-intensity exercise, we observed a blunted reduction in total peripheral resistance, which cannot be ascribed to an exaggerated increase in muscle sympathetic nerve activity, indicating possible contributions from other neural and/or nonneural mechanisms.

Low-dose fentanyl does not alter muscle sympathetic nerve activity, blood pressure, or tolerance during progressive central hypovolemia

Hemorrhage is a leading cause of battlefield and civilian trauma deaths. Several pain medications, including fentanyl, are recommended for use in the prehospital (i.e., field setting) for a hemorrhaging solider. However, it is unknown whether fentanyl impairs arterial blood pressure (BP) regulation, which would compromise hemorrhagic tolerance. Thus, the purpose of this study was to test the hypothesis that an analgesic dose of fentanyl impairs hemorrhagic tolerance in conscious humans. Twenty-eight volunteers (13 females) participated in this double-blinded, randomized, placebo-controlled trial. We conducted a presyncopal limited progressive lower body negative pressure test (LBNP; a validated model to simulate hemorrhage) following intravenous administration of fentanyl (75 µg) or placebo (saline). We quantified tolerance as a cumulative stress index (mmHg·min), which was compared between trials using a paired, two-tailed t test. We also compared muscle sympathetic nerve activity (MSNA; microneurography) and beat-to-beat BP (photoplethysmography) during the LBNP test using a mixed effects model [time (LBNP stage) × trial]. LBNP tolerance was not different between trials (fentanyl: 647 ± 386 vs. placebo: 676 ± 295 mmHg·min, P = 0.61, Cohen's d = 0.08). Increases in MSNA burst frequency (time: P < 0.01, trial: P = 0.29, interaction: P = 0.94) and reductions in mean BP (time: P < 0.01, trial: P = 0.50, interaction: P = 0.16) during LBNP were not different between trials. These data, the first to be obtained in conscious humans, demonstrate that administration of an analgesic dose of fentanyl does not alter MSNA or BP during profound central hypovolemia, nor does it impair tolerance to this simulated hemorrhagic insult.

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.

Perindopril Reduces Arterial Pressure and Does Not Inhibit Exercise-Induced Angiogenesis in Spontaneously Hypertensive Rats

ABSTRACT

Sympathetic activity, arteriolar structure, and angiogenesis are important mechanisms modulating hypertension and this study aimed to analyze the effects of perindopril treatment, associated or not with exercise training, on the mechanisms that control blood pressure (BP) in hypertensive rats. Spontaneously hypertensive rats (SHR) were allocated into 4 groups: 1/sedentary (S); 2/perindopril (P, 3.0 mg/kg/d); 3/trained (T); and 4/trained + perindopril (TP). Wistar rats were used as normotensive sedentary control group. SHR were assigned to undergo a treadmill training (T) or were kept sedentary. Heart rate, BP, sympathetic activity to the vessels (LF-SBP), and skeletal muscle and myocardial morphometric analyses were performed. BP was significantly lower after all 3 strategies, compared with S and was accompanied by lower LF-SBP (-76%, -53%, and -44%, for P, T, and TP, respectively). Arteriolar vessel wall cross-sectional area was lower after treatments (-56%, -52%, and -56%, for P, T, and TP, respectively), and only TP presented higher arteriolar lumen area. Capillary rarefaction was present in soleus muscle and myocardium in S group and both trained groups presented higher vessel density, although perindopril attenuated this increase in soleus muscle. Although myocyte diameter was not different between groups, myocardial collagen deposition area, higher in S group, was lower after 3 strategies. In conclusion, we may suggest that perindopril could be an option for the hypertensive people who practice exercise and need a specific pharmacological treatment to reach a better BP control, mainly because training-induced angiogenesis is an important response to facilitate blood flow perfusion and oxygen uptake and perindopril did not attenuate this response.

Low-dose ketamine affects blood pressure, but not muscle sympathetic nerve activity, during progressive central hypovolemia without altering tolerance

KEY POINTS

Haemorrhage is the leading cause of battlefield and civilian trauma deaths. Given that a haemorrhagic injury on the battlefield is almost always associated with pain, it is paramount that the administered pain medication does not disrupt the physiological mechanisms that are beneficial in defending against the haemorrhagic insult. Current guidelines from the US Army's Committee on Tactical Combat Casualty Care (CoTCCC) for the selection of pain medications administered to a haemorrhaging soldier are based upon limited scientific evidence, with the clear majority of supporting studies being conducted on anaesthetized animals. Specifically, the influence of low-dose ketamine, one of three analgesics employed in the pre-hospital setting by the US Army, on haemorrhagic tolerance in humans is unknown. For the first time in conscious males and females, the findings of the present study demonstrate that the administration of an analgesic dose of ketamine does not compromise tolerance to a simulated haemorrhagic insult. Increases in muscle sympathetic nerve activity during progressive lower-body negative pressure were not different between trials. Despite the lack of differences for muscle sympathetic nerve activity responses, mean blood pressure and heart rate were higher during moderate hypovolemia after ketamine vs. placebo administration.

ABSTRACT

Haemorrhage is the leading cause of battlefield and civilian trauma deaths. For a haemorrhaging soldier, there are several pain medications (e.g. ketamine) recommended for use in the prehospital, field setting. However, the data to support these recommendations are primarily limited to studies in animals. Therefore, it is unknown whether ketamine adversely affects physiological mechanisms responsible for maintenance of arterial blood pressure (BP) during haemorrhage in humans. In humans, ketamine has been demonstrated to raise resting BP, although it has not been studied with the concomitant central hypovolemia that occurs during haemorrhage. Thus, the present study aimed to test the hypothesis that ketamine does not impair haemorrhagic tolerance in humans. Thirty volunteers (15 females) participated in this double-blinded, randomized, placebo-controlled trial. A pre-syncopal limited progressive lower-body negative pressure (LBNP; a validated model for simulating haemorrhage) test was conducted following the administration of ketamine (20 mg) or placebo (saline). Tolerance was quantified as a cumulative stress index and compared between trials using a paired, two-tailed t test. We compared muscle sympathetic nerve activity (MSNA; microneurography), beat-to-beat BP (photoplethysmography) and heart rate (electrocardiogram) responses during the LBNP test using a mixed effects model (time [LBNP stage] × drug). Tolerance to the LBNP test was not different between trials (Ketamine: 635 ± 391 vs. Placebo: 652 ± 360 mmHg‧min, p = 0.77). Increases in MSNA burst frequency (time: P < 0.01, trial: p = 0.27, interaction: p = 0.39) during LBNP stages were no different between trials. Despite the lack of differences for MSNA responses, mean BP (time: P < 0.01, trial: P < 0.01, interaction: p = 0.01) and heart rate (time: P < 0.01, trial: P < 0.01, interaction: P < 0.01) were higher during moderate hypovolemia after ketamine vs. placebo administration (P < 0.05 for all, post hoc), but not at the end of LBNP. These data, which are the first to be obtained in conscious humans, demonstrate that the administration of low-dose ketamine does not impair tolerance to simulated haemorrhage or mechanisms responsible for maintenance of BP.

Mechanisms of sympathetic restraint in human skeletal muscle during exercise: role of α-adrenergic and nonadrenergic mechanisms

Sympathetic vasoconstriction is mediated by α-adrenergic receptors under resting conditions. During exercise, increased sympathetic nerve activity (SNA) is directed to inactive and active skeletal muscle; however, it is unclear what mechanism(s) are responsible for vasoconstriction during large muscle mass exercise in humans. The aim of this study was to determine the contribution of α-adrenergic receptors to sympathetic restraint of inactive skeletal muscle and active skeletal muscle during cycle exercise in healthy humans. In ten male participants (18-35 yr), mean arterial pressure (intra-arterial catheter) and forearm vascular resistance (FVR) and conductance (FVC) were assessed during cycle exercise (60% total peak workload) alone and during combined cycle exercise + handgrip exercise (HGE) before and after intra-arterial blockade of α- and β-adrenoreceptors via phentolamine and propranolol, respectively. Cycle exercise caused vasoconstriction in the inactive forearm that was attenuated ~80% with adrenoreceptor blockade (%ΔFVR, +81.7 ± 84.6 vs. +9.7 ± 30.7%; P = 0.05). When HGE was performed during cycle exercise, the vasodilatory response to HGE was restrained by ~40% (ΔFVC HGE, +139.3 ± 67.0 vs. cycle exercise: +81.9 ± 66.3 ml·min^-1·100 mmHg^-1; P = 0.03); however, the restraint of active skeletal muscle blood flow was not due to α-adrenergic signaling. These findings highlight that α-adrenergic receptors are the primary, but not the exclusive mechanism by which sympathetic vasoconstriction occurs in inactive and active skeletal muscle during exercise. Metabolic activity or higher sympathetic firing frequencies may alter the contribution of α-adrenergic receptors to sympathetic vasoconstriction. Finally, nonadrenergic vasoconstrictor mechanisms may be important for understanding the regulation of blood flow during exercise.NEW & NOTEWORTHY Sympathetic restraint of vascular conductance to inactive skeletal muscle is critical to maintain blood pressure during moderate- to high-intensity whole body exercise. This investigation shows that cycle exercise-induced restraint of inactive skeletal muscle vascular conductance occurs primarily because of activation of α-adrenergic receptors. Furthermore, exercise-induced vasoconstriction restrains the subsequent vasodilatory response to hand-grip exercise; however, the restraint of active skeletal muscle vasodilation was in part due to nonadrenergic mechanisms. We conclude that α-adrenergic receptors are the primary but not exclusive mechanism by which sympathetic vasoconstriction restrains blood flow in humans during whole body exercise and that metabolic activity modulates the contribution of α-adrenergic receptors.

Arterial Baroreflex Resetting During Exercise in Humans: Underlying Signaling Mechanisms

The arterial baroreflex (ABR) resets during exercise in an intensity-dependent manner to operate around a higher blood pressure with maintained sensitivity. This review provides a historical perspective of ABR resetting and the involvement of other neural reflexes in mediating exercise resetting. Furthermore, we discuss potential underlying signaling mechanisms that may contribute to exercise ABR resetting in physiological and pathophysiological conditions.

Muscle pump-induced inhibition of sympathetic vasomotor outflow during low-intensity leg cycling is attenuated by muscle metaboreflex activation

Muscle sympathetic nerve activity (MSNA) decreases during leg cycling at low intensity because of muscle pump-induced increases in venous return and loading of the cardiopulmonary baroreceptors. However, MSNA increases during leg cycling when exercise is above moderate intensity or for a long duration, suggesting that the sympathoinhibitory effect of the cardiopulmonary baroreflex can be overridden by a powerful sympathoexcitatory drive, such as the skeletal muscle metaboreflex. Therefore, we tested the hypothesis that high-intensity muscle metaboreflex activation attenuates muscle pump-induced inhibition of MSNA during leg cycling. MSNA (left radial nerve) was recorded during graded isolation of the muscle metaboreflex in the forearm with postexercise ischemia (PEI) after low (PEI-L)- and high (PEI-H)-intensity isometric handgrip exercise (20% and 40% maximum voluntary contraction, respectively). Leg cycling (15–20 W) was performed alone and during each PEI trial (PEI-L+Cycling, PEI-H+Cycling). Cycling alone induced a significant decrease in MSNA burst frequency (BF) and total activity (TA). MSNA BF and TA also decreased when cycling was performed during PEI-L. However, the magnitude of decrease in MSNA during PEI-L+Cycling [∆BF: –19 ± 2% ( P < 0.001), ∆TA: –25 ± 4% ( P < 0.001); mean ± SE] was less than that during cycling alone [∆BF: –39 ± 5% ( P = 0.003), ∆TA: –45 ± 5% ( P = 0.002)]. More importantly, MSNA did not decrease during cycling with PEI-H [∆BF: –1 ± 2% ( P = 0.845), ∆TA: +2 ± 3% ( P = 0.959)]. These results suggest that muscle pump-induced inhibition of sympathetic vasomotor outflow during low-intensity leg cycling is attenuated by muscle metaboreflex activation in an intensity-dependent manner.

NEW & NOTEWORTHY There are no available data concerning the interaction between the sympathoinhibitory effect of muscle pump-induced cardiopulmonary baroreflex loading during leg cycling and the sympathoexcitatory influence of the muscle metaboreflex. In this study, muscle metaboreflex activation attenuated the inhibition of muscle sympathetic nerve activity (MSNA) during leg cycling. This may explain, in part, the response of MSNA to graded-intensity dynamic exercise in which low-intensity leg cycling inhibits MSNA whereas high-intensity exercise elicits graded sympathoexcitation.

Muscle sympathetic nerve activity during exercise

Appropriate cardiovascular adjustment is necessary to meet the metabolic demands of working skeletal muscle during exercise. The sympathetic nervous system plays a crucial role in the regulation of arterial blood pressure and blood flow during exercise, and several important neural mechanisms are responsible for changes in sympathetic vasomotor outflow. Changes in sympathetic vasomotor outflow (i.e., muscle sympathetic nerve activity: MSNA) in inactive muscles during exercise differ depending on the exercise mode (static or dynamic), intensity, duration, and various environmental conditions (e.g., hot and cold environments or hypoxic). In 1991, Seals and Victor [6] reviewed MSNA responses to static and dynamic exercise with small muscle mass. This review provides an updated comprehensive overview on the MSNA response to exercise including large-muscle, dynamic leg exercise, e.g., two-legged cycling, and its regulatory mechanisms in healthy humans.