Muscle cyclo-oxygenase-2 pathway contributes to the exaggerated muscle mechanoreflex in rats with congestive heart failure

Protein kinase C epsilon contributes to chronic mechanoreflex sensitization in rats with heart failure

We investigated second-messenger signalling components linked to the stimulation of Gq protein-coupled receptors (e.g. thromboxane A2 and bradykinin B2 receptors) on the sensory endings of thin fibre muscle afferents in the chronic mechanoreflex sensitization in rats with myocardial infarction-induced heart failure with reduced ejection fraction (HF-rEF). We hypothesized that injection of either the inositol 1,4,5-trisphosphate (IP3) receptor antagonist xestospongin C (5 µg) or the PKCε translocation inhibitor PKCe141 (45 µg) into the arterial supply of the hindlimb would reduce the increase in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) evoked during 30 s of 1 Hz dynamic hindlimb muscle stretch in decerebrate, unanaesthetized HF-rEF rats but not sham-operated controls (SHAM). Ejection fraction was significantly reduced in HF-rEF (45 (19)%) compared to SHAM (80 (9)%; P < 0.001) rats. In HF-rEF rats (n = 3M/2F), IP3 receptor blockade had no effect on the peak ΔRSNA (pre: 99 (74)%; post: 133 (79)%; P = 0.974) or peak ΔMAP response to stretch (peak ΔMAP: pre: 32 (14) mmHg; post: 36 (21) mmHg; P = 0.719). Conversely, in another group of HF-rEF rats (n = 4M/3F), the PKCε translocation inhibitor reduced the peak ΔRSNA (pre: 110 (77)%; post: 62 (58)%; P = 0.029) and peak ΔMAP response to stretch (pre: 30 (20) mmHg; post: 17 (16) mmHg; P = 0.048). In SHAM counterparts, neither drug affected the mechanoreflex responses. Our findings highlight PKCε, but not IP3 receptors, as a significant second-messenger in the chronic mechanoreflex sensitization in HF-rEF which may play a crucial role in the exaggerated sympathetic response to exercise in this patient population. KEY POINTS: Skeletal muscle contraction results in an exaggerated reflex increase in sympathetic nerve activity in heart failure patients with reduced ejection fraction (HF-rEF) compared to healthy individuals, contributing to increased cardiovascular risk and impaired tolerance for mild exercise. The exaggerated reflex sympathetic responses in HF-rEF may be attributed to a chronic sensitization of mechanically sensitive thin fibre muscle afferents mediated, at least in part, by stimulation of Gq protein-coupled thromboxane A2 and bradykinin B2 receptors on muscle afferent sensory endings. The specific Gq protein-linked signalling mechanisms that produce the chronic mechanoreflex sensitization in HF-rEF have not been investigated but may involve inositol 1,4,5-trisphosphate (IP3) receptors and/or protein kinase C epsilon (PKCε). Here we demonstrate that PKCε, but not IP3 receptors, within the sensory endings of thin fibre muscle afferents plays a role in the sensitization of mechanically sensitive thin fibre muscle afferents in rats with HF-rEF.

Cyclooxygenase products contribute to the exaggerated exercise pressor reflex evoked by static muscle contraction in male UCD-type 2 diabetes mellitus rats

Cyclooxygenase (COX) products of arachidonic acid metabolism, specifically prostaglandins, play a role in evoking and transmitting the exercise pressor reflex in health and disease. Individuals with type 2 diabetes mellitus (T2DM) have an exaggerated exercise pressor reflex; however, the mechanisms for this exaggerated reflex are not fully understood. We aimed to determine the role played by COX products in the exaggerated exercise pressor reflex in T2DM rats. The exercise pressor reflex was evoked by static muscle contraction in unanesthetized, decerebrate, male, adult University of California Davis (UCD)-T2DM (n = 8) and healthy Sprague-Dawley (n = 8) rats. Changes (Δ) in peak mean arterial pressure (MAP) and heart rate (HR) during muscle contraction were compared before and after intra-arterial injection of indomethacin (1 mg/kg) into the contracting hindlimb. Data are presented as means ± SD. Inhibition of COX activity attenuated the exaggerated peak MAP (Before: Δ32 ± 13 mmHg and After: Δ18 ± 8 mmHg; P = 0.004) and blood pressor index (BPi) (Before: Δ683 ± 324 mmHg·s and After: Δ361 ± 222 mmHg·s; P = 0.006), but not HR (Before: Δ23 ± 8 beats/min and After Δ19 ± 10 beats/min; P = 0.452) responses to muscle contraction in T2DM rats. In healthy rats, COX activity inhibition did not affect MAP, HR, or BPi responses to muscle contraction. Inhibition of COX activity significantly reduced local production of prostaglandin E2 in T2DM and healthy rats. We conclude that peripheral inhibition of COX activity attenuates the pressor response to muscle contraction in T2DM rats, suggesting that COX products partially contribute to the exaggerated exercise pressor reflex in those with T2DM.NEW & NOTEWORTHY We compared the pressor and cardioaccelerator responses to static muscle contraction before and after inhibition of cyclooxygenase (COX) activity within the contracting hindlimb in decerebrate, unanesthetized type 2 diabetic mellitus (T2DM) and healthy rats. The pressor responses to muscle contraction were attenuated after peripheral inhibition of COX activity in T2DM but not in healthy rats. We concluded that COX products partially contribute to the exaggerated pressor reflex in those with T2DM.

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.

Enhanced Respiratory Frequency Response to Lower Limb Mechanoreceptors Activation in Patients with Chronic Obstructive Pulmonary Disease

PURPOSE

To investigate the mechanoreflex control of respiration and circulation in patients with chronic obstructive pulmonary disease (COPD).

METHODS

Twenty-eight patients with moderate-to-severe COPD (mean ± SD: 67.0 ± 7.9 yr, 10 women) and 14 age- and sex-matched controls (67.9 ± 2.6 yr, 7 women) participated in the study. Their dominant knee was passively moved to stimulate mechanoreceptors, whereas vastus lateralis surface electrical activity checked active contractions. A differential pressure flowmeter, an electrocardiogram, and a servo-controlled finger photoplethysmograph acquired cardiorespiratory data. To gain insight into the mechanoreflex arc, we further analyzed reduced/oxidized glutathione ratio and mechanoreceptor-related gene expression in a vastus lateralis biopsy of additional nine patients (63.9 ± 8.1 yr, 33% women) and eight controls (62.9 ± 9.1 yr, 38% women).

RESULTS

Patients with COPD had a greater peak respiratory frequency response (COPD: Δ = 3.2 ± 2.3 vs Controls: 1.8 ± 1.2 cycles per minute, P = 0.036) and a smaller peak tidal volume response to passive knee movement than controls. Ventilation, heart rate, stroke volume, and cardiac output peak responses, and total peripheral resistance nadir response, were unaltered by COPD. In addition, patients had a diminished glutathione ratio (COPD: 13.3 ± 3.8 vs controls: 20.0 ± 5.5 a.u., P = 0.015) and an augmented brain-derived neurotrophic factor expression (COPD: 2.0 ± 0.7 vs controls: 1.1 ± 0.4 a.u., P = 0.002) than controls. Prostaglandin E receptor 4, cyclooxygenase 2, and Piezo1 expression were similar between groups.

CONCLUSIONS

Respiratory frequency response to mechanoreceptors activation is increased in patients with COPD. This abnormality is possibly linked to glutathione redox imbalance and augmented brain-derived neurotrophic factor expression within locomotor muscles, which could increase mechanically sensitive afferents' stimulation and sensitivity.

Novel mechanosensory role for acid sensing ion channel subtype 1a in evoking the exercise pressor reflex in rats with heart failure

Mechanical and metabolic signals associated with skeletal muscle contraction stimulate the sensory endings of thin fibre muscle afferents, which, in turn, generates reflex increases in sympathetic nerve activity (SNA) and blood pressure (the exercise pressor reflex; EPR). EPR activation in patients and animals with heart failure with reduced ejection fraction (HF-rEF) results in exaggerated increases in SNA and promotes exercise intolerance. In the healthy decerebrate rat, a subtype of acid sensing ion channel (ASIC) on the sensory endings of thin fibre muscle afferents, namely ASIC1a, has been shown to contribute to the metabolically sensitive portion of the EPR (i.e. metaboreflex), but not the mechanically sensitive portion of the EPR (i.e. the mechanoreflex). However, the role played by ASIC1a in evoking the EPR in HF-rEF is unknown. We hypothesized that, in decerebrate, unanaesthetized HF-rEF rats, injection of the ASIC1a antagonist psalmotoxin-1 (PcTx-1; 100 ng) into the hindlimb arterial supply would reduce the reflex increase in renal SNA (RSNA) evoked via 30 s of electrically induced static hindlimb muscle contraction, but not static hindlimb muscle stretch (model of mechanoreflex activation isolated from contraction-induced metabolite-production). We found that PcTx-1 reduced the reflex increase in RSNA evoked in response to muscle contraction (n = 8; mean (SD) ∫ΔRSNA pre: 1343 (588) a.u.; post: 816 (573) a.u.; P = 0.026) and muscle stretch (n = 6; ∫ΔRSNA pre: 688 (583) a.u.; post: 304 (370) a.u.; P = 0.025). Our data suggest that, in HF-rEF rats, ASIC1a contributes to activation of the exercise pressor reflex and that contribution includes a novel role for ASIC1a in mechanosensation that is not present in healthy rats. KEY POINTS: Skeletal muscle contraction results in exaggerated reflex increases in sympathetic nerve activity in heart failure patients compared to healthy counterparts, which likely contributes to increased cardiovascular risk and impaired tolerance for even mild exercise (i.e. activities of daily living) for patients suffering with this condition. Activation of acid sensing ion channel subtype 1a (ASIC1a) on the sensory endings of thin fibre muscle afferents during skeletal muscle contraction contributes to reflex increases in sympathetic nerve activity and blood pressure, at least in healthy subjects. In this study, we demonstrate that ASIC1a on the sensory endings of thin fibre muscle afferents plays a role in both the mechanical and metabolic components of the exercise pressor reflex in male rats with heart failure. The present data identify a novel role for ASIC1a in evoking the exercise pressor reflex in heart failure and may have important clinical implications for heart failure patients.

Thromboxane A2 receptors contribute to the exaggerated exercise pressor reflex in male rats with heart failure

Mechanical and metabolic signals associated with skeletal muscle contraction stimulate the sensory endings of thin fiber muscle afferents and produce reflex increases in sympathetic nerve activity and blood pressure during exercise (i.e., the exercise pressor reflex; EPR). The EPR is exaggerated in patients and animals with heart failure with reduced ejection fraction (HF-rEF) and its activation contributes to reduced exercise capacity within this patient population. Accumulating evidence suggests that the exaggerated EPR in HF-rEF is partially attributable to a sensitization of mechanically activated channels produced by thromboxane A2 receptors (TxA2 -Rs) on those sensory endings; however, this has not been investigated. Accordingly, the purpose of this investigation was to determine the role played by TxA2 -Rs on the sensory endings of thin fiber muscle afferents in the exaggerated EPR in rats with HF-rEF induced by coronary artery ligation. In decerebrate, unanesthetized rats, we found that injection of the TxA2 -R antagonist daltroban (80 μg) into the arterial supply of the hindlimb reduced the pressor response to 30 s of electrically induced 1 Hz dynamic hindlimb muscle contraction in HF-rEF (n = 8, peak ∆MAP pre: 22 ± 3; post: 14 ± 2 mmHg; p = 0.01) but not sham (n = 10, peak ∆MAP pre: 13 ± 3; post: 11 ± 2 mmHg; p = 0.68) rats. In a separate group of HF-rEF rats (n = 4), we found that the systemic (intravenous) injection of daltroban had no effect on the EPR (peak ΔMAP pre: 26 ± 7; post: 25 ± 7 mmHg; p = 0.50). Our data suggest that TxA2 -Rs on thin fiber muscle afferents contribute to the exaggerated EPR evoked in response to dynamic muscle contraction in HF-rEF.

Exaggerated sympathetic and cardiovascular responses to dynamic mechanoreflex activation in rats with heart failure: Role of endoperoxide 4 and thromboxane A2 receptors

The primary purpose of this investigation was to determine the role played by endoperoxide 4 receptors (EP4-R) and thromboxane A₂ receptors (TxA₂-R) during isolated dynamic muscle mechanoreflex activation in rats with heart failure with reduced ejection fraction (HF-rEF) and sham-operated healthy controls. We found that injection of the EP4-R antagonist L-161,982 (1 μg) into the arterial supply of the hindlimb had no effect on the peak pressor response to dynamic hindlimb muscle stretch in HF-rEF (n = 6, peak ∆MAP pre: 27 ± 7; post: 27 ± 4 mm Hg; P = 0.99) or sham (n = 6, peak ∆MAP pre: 15 ± 3; post: 13 ± 3 mm Hg; P = 0.67) rats. In contrast, injection of the TxA₂-R antagonist daltroban (80 μg) into the arterial supply of the hindlimb reduced the pressor response to dynamic hindlimb muscle stretch in HF-rEF (n = 11, peak ∆MAP pre: 28 ± 4; post: 16 ± 2 mm Hg; P = 0.02) but not sham (n = 8, peak ∆MAP pre: 17 ± 3; post: 16 ± 3; P = 0.84) rats. Our data suggest that TxA₂-Rs on thin fibre muscle afferents contribute to the exaggerated mechanoreflex in HF-rEF.

No effect of endoperoxide 4 or thromboxane A2 receptor blockade on static mechanoreflex activation in rats with heart failure

NEW FINDINGS

What is the central question of this study? Do endoperoxide 4 and thromboxane A₂ receptors, which are receptors for cyclooxygenase products of arachidonic metabolism, on thin fibre muscle afferents play a role in the chronic mechanoreflex sensitization present in rats with heart failure with reduced ejection fraction (HF-rEF)? What is the main finding and its importance? The data do not support a role for endoperoxide 4 receptors or thromboxane A₂ receptors in the chronic mechanoreflex sensitization in HF-rEF rats.

ABSTRACT

We investigated the role of cyclooxygenase metabolite-associated endoperoxide 4 receptors (EP4-R) and thromboxane A₂ receptors (TxA₂ -R) on thin fibre muscle afferents in the chronic mechanoreflex sensitization in rats with myocardial infarction-induced heart failure with reduced ejection fraction (HF-rEF). We hypothesized that injection of either the EP4-R antagonist L-161,982 (1 µg) or the TxA₂ -R antagonist daltroban (80 µg) into the arterial supply of the hindlimb would reduce the increase in blood pressure and renal sympathetic nerve activity (RSNA) evoked in response to 30 s of static hindlimb skeletal muscle stretch (a model of isolated mechanoreflex activation) in decerebrate, unanaesthetized HF-rEF rats but not sham-operated control rats (SHAM). Ejection fraction was significantly reduced in HF-rEF (45 ± 11%) compared to SHAM (83 ± 6%; P < 0.01) rats. In SHAM and HF-rEF rats, we found that the EP4-R antagonist had no effect on the peak increase in mean arterial pressure (peak ΔMAP SHAM n = 6, pre: 15 ± 7, post: 15 ± 9, P = 0.99; HF-rEF n = 9, pre: 30 ± 11, post: 32 ± 15 mmHg, P = 0.84) or peak increase in RSNA (peak ΔRSNA SHAM pre: 33 ± 14, post: 47 ± 31%, P = 0.94; HF-rEF, pre: 109 ± 47, post: 139 ± 150%, P = 0.76) response to stretch. Similarly, in SHAM and HF-rEF rats, we found that the TxA₂ -R antagonist had no effect on the peak ΔMAP (SHAM n = 7, pre: 13 ± 7, post: 19 ± 14, P = 0.15; HF-rEF n = 14, pre: 24 ± 13, post: 21 ± 13 mmHg, P = 0.47) or peak ΔRSNA (SHAM pre: 52 ± 43, post: 57 ± 67%, P = 0.94; HF-rEF, pre: 108 ± 93, post: 88 ± 72%, P = 0.30) response to stretch. The data do not support a role for EP4-Rs or TxA₂ -Rs in the chronic mechanoreflex sensitization in HF-rEF.

Thromboxane A2 receptors mediate chronic mechanoreflex sensitization in a rat model of simulated peripheral artery disease

The exercise pressor reflex is a feedback autonomic and cardiovascular control mechanism evoked by mechanical and metabolic signals within contracting skeletal muscles. The mechanically sensitive component of the reflex (the mechanoreflex) is exaggerated in patients with peripheral artery disease (PAD) and in a rat model of simulated PAD in which a femoral artery is chronically ligated. Products of cyclooxygenase enzyme activity have been shown to chronically sensitize the mechanoreflex in PAD, but the identity of the muscle afferent receptors that mediate the sensitization is unclear. We hypothesized that injection of the endoperoxide 4 receptor (EP4-R) antagonist L161982 or the thromboxane A₂ receptor (TxA₂-R) antagonist daltroban into the arterial supply of the hindlimb would reduce the pressor response to repetitive, dynamic hindlimb skeletal muscle stretch (a model of isolated mechanoreflex activation) in rats with a femoral artery that was ligated ~72 h before the experiment but not in rats with freely perfused femoral arteries. We found that EP4-R blockade had no effect on the pressor response (peak Δmean arterial pressure) to stretch in freely perfused (n = 6, pre: 14 ± 2, post: 15 ± 2 mmHg, P = 0.97) or ligated (n = 8, pre: 29 ± 4, post: 29 ± 6 mmHg, P = 0.98) rats. In contrast, TxA₂-R blockade had no effect on the pressor response to stretch in freely perfused rats (n = 6, pre: 16 ± 3, post: 17 ± 4 mmHg, P = 0.99) but significantly reduced the response in ligated rats (n = 11, pre: 29 ± 4, post: 17 ± 5 mmHg, P < 0.01). We conclude that TxA₂-Rs contribute to chronic mechanoreflex sensitization in the chronic femoral artery-ligated rat model of simulated PAD.NEW & NOTEWORTHY We demonstrate that thromboxane A₂ receptors, but not endoperoxide 4 receptors, on the sensory endings of thin fiber muscle afferents contribute to the chronic sensitization of the muscle mechanoreflex in rats with a ligated femoral artery (a model of simulated peripheral artery disease). The data may have important implications for our understanding of blood pressure control during exercise in patients with peripheral artery disease.

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.

Metabo- and mechanoreceptor expression in human heart failure: Relationships with the locomotor muscle afferent influence on exercise responses

NEW FINDINGS

What is the central question of this study? How do locomotor muscle metabo- and mechanoreceptor expression compare in heart failure patients and controls? Do relationships exist between the protein expression and cardiopulmonary responses during exercise with locomotor muscle neural afferent feedback inhibition? What is the main finding and its importance? Heart failure patients exhibited greater protein expression of transient receptor potential vanilloid type 1 and cyclooxygenase-2 than controls. These findings are important as they identify receptors that may underlie the augmented locomotor muscle neural afferent feedback in heart failure.

ABSTRACT

Heart failure patients with reduced ejection fraction (HFrEF) exhibit abnormal locomotor group III/IV afferent feedback during exercise; however, the underlying mechanisms are unclear. Therefore, the purpose of this study was to determine (1) metabo- and mechanoreceptor expression in HFrEF and controls and (2) relationships between receptor expression and changes in cardiopulmonary responses with afferent inhibition. Ten controls and six HFrEF performed 5 min of cycling exercise at 65% peak workload with lumbar intrathecal fentanyl (FENT) or placebo (PLA). Arterial blood pressure and catecholamines were measured via radial artery catheter. A vastus lateralis muscle biopsy was performed to quantify cyclooxygenase-2 (COX-2), purinergic 2X₃ (P2X₃ ), transient receptor potential vanilloid type 1 (TRP_V 1), acid-sensing ion channel 3 (ASIC₃ ), Piezo 1 and Piezo 2 protein expression. TRP_V 1 and COX-2 protein expression was greater in HFrEF than controls (both P < 0.04), while P2X₃ , ASIC₃ , and Piezo 1 and 2 were not different between groups (all P > 0.16). In all participants, COX-2 protein expression was related to the percentage change in ventilation (r = -0.66) and mean arterial pressure (MAP) (r = -0.82) (both P < 0.01) with FENT (relative to PLA) during exercise. In controls, TRP_V 1 protein expression was related to the percentage change in systolic blood pressure (r = -0.77, P = 0.02) and MAP (r = -0.72, P = 0.03) with FENT (relative to PLA) during exercise. TRP_V 1 and COX-2 protein levels are elevated in HFrEF compared to controls. These findings suggest that the elevated TRP_V 1 and COX-2 expression may contribute to the exaggerated locomotor muscle afferent feedback during cycling exercise in HFrEF.

Investigation of the mechanisms of cyclooxygenase-mediated mechanoreflex sensitization in a rat model of simulated peripheral artery disease

Mechanical and metabolic stimuli within contracting skeletal muscles reflexly increase sympathetic nervous system activity and blood pressure. That reflex, termed the exercise pressor reflex, is exaggerated in patients with peripheral artery disease (PAD) and in a rat PAD model with a chronically ligated femoral artery. The cyclooxygenase (COX) pathway contributes to the exaggerated pressor response during rhythmic skeletal muscle contractions in patients with PAD, but the specific mechanism(s) of the COX-mediated exaggeration are not known. In decerebrate, unanesthetized rats with a chronically ligated femoral artery ("ligated" rats), we hypothesized that hindlimb arterial injection of the COX inhibitor indomethacin would reduce the pressor response during 1-Hz dynamic hindlimb skeletal muscle stretch; a model of the activation of the mechanical component of the exercise pressor reflex (i.e., the mechanoreflex). In ligated rats (n = 7), indomethacin reduced the pressor response during stretch (control: 30 ± 4; indomethacin: 12 ± 3 mmHg; P < 0.01), whereas there was no effect in rats with "freely perfused" femoral arteries (n = 6, control: 18 ± 5; indomethacin: 17 ± 5 mmHg; P = 0.87). In ligated rats (n = 4), systemic indomethacin injection had no effect on the pressor response during stretch. Femoral artery ligation had no effect on skeletal muscle COX protein expression or activity or concentration of the COX metabolite prostaglandin E₂. Conversely, femoral artery ligation increased expression of the COX metabolite receptors endoperoxide 4 and thromboxane A₂-R in dorsal root ganglia tissue. We conclude that, in ligated rats, the COX pathway sensitizes the peripheral endings of mechanoreflex afferents, which occurs principally as a result of increased expression of COX metabolite receptors.NEW & NOTEWORTHY We demonstrate that the mechanoreflex is sensitized by the cyclooxygenase (COX) pathway within hindlimb skeletal muscles in the rat chronic femoral artery ligation model of simulated peripheral artery disease (PAD). The mechanism of sensitization appears attributable to increased receptors for COX metabolites on sensory neurons and not increased concentration of COX metabolites. Our data may carry important clinical implications for patients with PAD who demonstrate exaggerated increases in blood pressure during exercise compared with healthy counterparts.

Cyclooxygenase inhibition does not impact the pressor response during static or dynamic mechanoreflex activation in healthy decerebrate rats

Passive limb movement and limb muscle stretch in humans and animals are common experimental strategies used to investigate activation of the muscle mechanoreflex independent of contraction-induced metabolite production. Cyclooxygenase (COX) metabolites, however, are produced by skeletal muscle stretch in vitro and have been found to impact various models of mechanoreflex activation. Whether COX metabolites influence the decerebrate rat triceps surae muscle stretch mechanoreflex model remains unknown. We examined the effect of rat triceps surae muscle stretch on the interstitial concentration of the COX metabolite prostaglandin E₂ (PGE₂). Interstitial PGE₂ concentration was increased above baseline values by 4 min of both static (38% increase, P = 0.01) and dynamic (56% increase, P < 0.01) triceps surae muscle stretch (n = 10). The 4-min protocol was required to collect enough microdialysis fluid for PGE₂ detection. The finding that skeletal muscle stretch in vivo was capable of producing COX metabolites prompted the hypothesis that intra-arterial administration of the COX inhibitor indomethacin (1 mg/kg) would reduce the pressor and cardioaccelerator responses evoked during 30 s (the duration most commonly used in the rat mechanoreflex model) of static and dynamic rat triceps surae muscle stretch. We found that indomethacin had no effect (P > 0.05, n = 9) on the pressor or cardioaccelerator response during 30 s of either static or dynamic stretch. We conclude that, despite the possibility of increased COX metabolite concentration, COX metabolites do not activate or sensitize thin-fiber muscle afferents stimulated during 30 s of static or dynamic hindlimb skeletal muscle stretch in healthy rats.

Chronic femoral artery ligation exaggerates the pressor and sympathetic nerve responses during dynamic skeletal muscle stretch in decerebrate rats

Mechanical and metabolic signals arising during skeletal muscle contraction reflexly increase sympathetic nerve activity and blood pressure (i.e., the exercise pressor reflex). In a rat model of simulated peripheral artery disease in which a femoral artery is chronically (~72 h) ligated, the mechanically sensitive component of the exercise pressor reflex during 1-Hz dynamic contraction is exaggerated compared with that found in normal rats. Whether this is due to an enhanced acute sensitization of mechanoreceptors by metabolites produced during contraction or involves a chronic sensitization of mechanoreceptors is unknown. To investigate this issue, in decerebrate, unanesthetized rats, we tested the hypothesis that the increases in mean arterial blood pressure and renal sympathetic nerve activity during 1-Hz dynamic stretch are larger when evoked from a previously "ligated" hindlimb compared with those evoked from the contralateral "freely perfused" hindlimb. Dynamic stretch provided a mechanical stimulus in the absence of contraction-induced metabolite production that closely replicated the pattern of the mechanical stimulus present during dynamic contraction. We found that the increases in mean arterial blood pressure (freely perfused: 14 ± 1 and ligated: 23 ± 3 mmHg, P = 0.02) and renal sympathetic nerve activity were significantly greater during dynamic stretch of the ligated hindlimb compared with the increases during dynamic stretch of the freely perfused hindlimb. These findings suggest that the exaggerated mechanically sensitive component of the exercise pressor reflex found during dynamic muscle contraction in this rat model of simulated peripheral artery disease involves a chronic sensitizing effect of ligation on muscle mechanoreceptors and cannot be attributed solely to acute contraction-induced metabolite sensitization. NEW & NOTEWORTHY We found that the pressor and sympathetic nerve responses during dynamic stretch were exaggerated in rats with a ligated femoral artery (a model of peripheral artery disease). Our findings provide mechanistic insights into the exaggerated exercise pressor reflex in this model and may have important implications for peripheral artery disease patients.

Muscle mechanoreflex activation via passive calf stretch causes renal vasoconstriction in healthy humans

Reflex renal vasoconstriction occurs during exercise, and renal vasoconstriction in response to upper-limb muscle mechanoreflex activation has been documented. However, the renal vasoconstrictor response to muscle mechanoreflex activation originating from lower limbs, with and without local metabolite accumulation, has not been assessed. Eleven healthy young subjects (26 ± 1 yr; 5 men) underwent two trials involving 3-min passive calf muscle stretch (mechanoreflex) during 7.5-min lower-limb circulatory occlusion (CO). In one trial, 1.5-min 70% maximal voluntary contraction isometric calf exercise preceded CO to accumulate metabolites during CO and stretch (mechanoreflex and metaboreflex; 70% trial). A control trial involved no exercise before CO (mechanoreflex alone; 0% trial). Beat-to-beat renal blood flow velocity (RBFV; Doppler ultrasound), mean arterial blood pressure (MAP; photoplethysmographic finger cuff), and heart rate (electrocardiogram) were recorded. Renal vascular resistance (RVR), an index of renal vasoconstriction, was calculated as MAP/RBFV. All baseline cardiovascular variables were similar between trials. Stretch increased RVR and decreased RBFV in both trials (change from CO with stretch: RVR - 0% trial = Δ 10 ± 2%, 70% trial = Δ 7 ± 3%; RBFV - 0% trial = Δ -3.8 ± 1.1 cm/s, 70% trial = Δ -2.7 ± 1.5 cm/s; P < 0.05 for RVR and RBFV). These stretch-induced changes were of similar magnitudes in both trials, e.g., with and without local metabolite accumulation, as well as when thromboxane production was inhibited. These findings suggest that muscle mechanoreflex activation via passive calf stretch causes renal vasoconstriction, with and without muscle metaboreflex activation, in healthy humans.

ASIC3 contributes to the blunted muscle metaboreflex in heart failure

INTRODUCTION

During exercise, the sympathetic nervous system is activated and blood pressure and HR increase. In heart failure (HF), the muscle metaboreceptor contribution to sympathetic outflow is attenuated and the mechanoreceptor contribution is accentuated. Previous studies suggest that lactic acid stimulates acid-sensing channel subtype 3 (ASIC3), inducing a neurally mediated pressor response. Thus, we hypothesized that the pressor response to ASIC3 stimulation is smaller in HF rats because of attenuation in expression and function of ASIC3 in sensory nerves.

METHODS

Lactic acid was injected into the arterial blood supply of the hind limb to stimulate ASIC3 in muscle afferent nerves and evoke muscle metaboreceptor response in control rats and HF rats. In addition, western blot analysis was used to examine expression of ASIC3 in dorsal root ganglion (DRG) and patch clamp to examine current response with ASIC3 activation.

RESULTS

Lactic acid (4 μmol·kg) increased mean arterial pressure by 28 ± 5 mm Hg in controls (n = 6) but only by 16 ± 3 mm Hg (P < 0.05 vs control) in HF (n = 8). In addition, HF decreased the protein levels of ASIC3 in DRG (optical density, 1.03 ± 0.02 in control, vs 0.79 ± 0.03 in HF; P < 0.05; n = 6 in each group). The peak current amplitude of dorsal DRG neuron in response to ASIC3 stimulation is smaller in HF rats than that in control rats.

CONCLUSIONS

Compared with those in controls, cardiovascular responses to lactic acid administered into the hind limb muscles are blunted in HF rats owing to attenuated ASIC3. This suggests that ASIC3 plays a role in engagement in the attenuated metaboreceptor component of the exercise pressor reflex in HF.

Molecular basis for the improvement in muscle metaboreflex and mechanoreflex control in exercise-trained humans with chronic heart failure

Previous studies have demonstrated that muscle mechanoreflex and metaboreflex controls are altered in heart failure (HF), which seems to be due to changes in cyclooxygenase (COX) pathway and changes in receptors on afferent neurons, including transient receptor potential vanilloid type-1 (TRPV1) and cannabinoid receptor type-1 (CB1). The purpose of the present study was to test the hypotheses: 1) exercise training (ET) alters the muscle metaboreflex and mechanoreflex control of muscle sympathetic nerve activity (MSNA) in HF patients. 2) The alteration in metaboreflex control is accompanied by increased expression of TRPV1 and CB1 receptors in skeletal muscle. 3) The alteration in mechanoreflex control is accompanied by COX-2 pathway in skeletal muscle. Thirty-four consecutive HF patients with ejection fractions <40% were randomized to untrained (n = 17; 54 ± 2 yr) or exercise-trained (n = 17; 56 ± 2 yr) groups. MSNA was recorded by microneurography. Mechanoreceptors were activated by passive exercise and metaboreceptors by postexercise circulatory arrest (PECA). COX-2 pathway, TRPV1, and CB1 receptors were measured in muscle biopsies. Following ET, resting MSNA was decreased compared with untrained group. During PECA (metaboreflex), MSNA responses were increased, which was accompanied by the expression of TRPV1 and CB1 receptors. During passive exercise (mechanoreflex), MSNA responses were decreased, which was accompanied by decreased expression of COX-2, prostaglandin-E2 receptor-4, and thromboxane-A2 receptor and by decreased in muscle inflammation, as indicated by increased miRNA-146 levels and the stable NF-κB/IκB-α ratio. In conclusion, ET alters muscle metaboreflex and mechanoreflex control of MSNA in HF patients. This alteration with ET is accompanied by alteration in TRPV1 and CB1 expression and COX-2 pathway and inflammation in skeletal muscle.

Influence of the metaboreflex on arterial blood pressure in heart failure patients

BACKGROUND

Feedback from active locomotor muscles contributes to the exercise pressor response in healthy humans, and is thought to be more prominent in heart failure (HF). The purpose of this study was to examine the influence of metaboreflex stimulation on arterial pressure in HF.

METHODS

Eleven HF patients (51 ± 5 years, New York Heart Association Class I/II, left ventricular ejection fraction 32 ± 3%) and 11 controls (42 ± 3 years) were recruited. Participants completed two exercise sessions on separate days: (1) symptom limited graded exercise test; and (2) constant work rate cycling (60% peak oxygen consumption,V˙O2) for 4 minutes with 2 minutes passive recovery. Recovery was randomized to normal or locomotor muscle regional circulatory occlusion (RCO). Mean arterial pressure (MAP), systolic pressure (SBP), diastolic pressure, heart rate (HR) and V˙O2 were measured at rest, end-exercise and recovery. O2 pulse (V˙O2/HR) and the rate pressure product (RPP = HR × SBP) were calculated.

RESULTS

In response to RCO, mean arterial pressure and SBP increased in HF compared with CTLs (6.8 ± 5.8% vs -3.0 ± 7.8%, P < .01 and 3.4 ± 6.4% vs -12.7 ± 10.4%, P < .01, respectively), with no difference in diastolic pressure (P = .61). HF patients had a smaller reduction in HR and RPP, but also displayed a larger decrease in O2 pulse consequent to locomotor metaboreflex stimulation (P < .05, for all).

CONCLUSION

RCO resulted in a markedly increased pressor response in HF relative to controls, due primarily to an increase of SBP and attenuated cardiac recovery as noted by the persistent elevation in HR.

Autonomic dysfunction in muscular dystrophy: a theoretical framework for muscle reflex involvement

Muscular dystrophies are a heterogeneous group of genetically inherited disorders whose most prominent clinical feature is progressive degeneration of skeletal muscle. In several forms of the disease, the function of cardiac muscle is likewise affected. The primary defect in this group of diseases is caused by mutations in myocyte proteins important to cellular structure and/or performance. That being stated, a growing body of evidence suggests that the development of autonomic dysfunction may secondarily contribute to the generation of skeletal and cardio-myopathy in muscular dystrophy. Indeed, abnormalities in the regulation of both sympathetic and parasympathetic nerve activity have been reported in a number of muscular dystrophy variants. However, the mechanisms mediating this autonomic dysfunction remain relatively unknown. An autonomic reflex originating in skeletal muscle, the exercise pressor reflex, is known to contribute significantly to the control of sympathetic and parasympathetic activity when stimulated. Given the skeletal myopathy that develops with muscular dystrophy, it is logical to suggest that the function of this reflex might also be abnormal with the pathogenesis of disease. As such, it may contribute to or exacerbate the autonomic dysfunction that manifests. This possibility along with a basic description of exercise pressor reflex function in health and disease are reviewed. A better understanding of the mechanisms that possibly underlie autonomic dysfunction in muscular dystrophy may not only facilitate further research but could also lead to the identification of new therapeutic targets for the treatment of muscular dystrophy.

Renal vasoconstriction is augmented during exercise in patients with peripheral arterial disease

Peripheral arterial disease (PAD) patients have augmented blood pressure increases during exercise, heightening their cardiovascular risk. However, it is unknown whether patients have exaggerated renal vasoconstriction during exercise and if oxidative stress contributes to this response. Eleven PAD patients and 10 controls (CON) performed 4-min mild, rhythmic, plantar flexion exercise of increasing intensity (0.5-2 kg) with each leg (most and least affected in PAD). Eight patients also exercised with their most affected leg during ascorbic acid (AA) infusion. Renal blood flow velocity (RBFV; Doppler ultrasound), mean arterial blood pressure (MAP; Finometer), and heart rate (HR; electrocardiogram [ECG]) were measured. Renal vascular resistance (RVR), an index of renal vasoconstriction, was calculated as MAP/RBFV. Baseline RVR and MAP were similar while HR was higher in PAD than CON (2.08 ± 0.23 vs. 1.87 ± 0.20 au, 94 ± 3 vs. 93 ± 3 mmHg, and 72 ± 3 vs. 59 ± 3 bpm [P < 0.05] for PAD and CON, respectively). PAD had greater RVR increases during exercise than CON, specifically during the first minute (PAD most: 26 ± 5% and PAD least: 17 ± 5% vs. CON: 3 ± 3%; P < 0.05). AA did not alter baseline RVR, MAP, or HR. AA attenuated the augmented RVR increase in PAD during the first minute of exercise (PAD most: 33 ± 4% vs. PAD most with AA: 21 ± 4%; P < 0.05). In conclusion, these findings suggest that PAD patients have augmented renal vasoconstriction during exercise, with oxidative stress contributing to this response.