Peripheral δ-opioid receptors attenuate the exercise pressor reflex

Sympathetic Nerve Activity and Blood Pressure Response to Exercise in Peripheral Artery Disease: From Molecular Mechanisms, Human Studies, to Intervention Strategy Development

Sympathetic nerve activity (SNA) regulates the contraction of vascular smooth muscle and leads to a change in arterial blood pressure (BP). It was observed that SNA, vascular contractility, and BP are heightened in patients with peripheral artery disease (PAD) during exercise. The exercise pressor reflex (EPR), a neural mechanism responsible for BP response to activation of muscle afferent nerve, is a determinant of the exaggerated exercise-induced BP rise in PAD. Based on recent results obtained from a series of studies in PAD patients and a rat model of PAD, this review will shed light on SNA-driven BP response and the underlying mechanisms by which receptors and molecular mediators in muscle afferent nerves mediate the abnormalities in autonomic activities of PAD. Intervention strategies, particularly non-pharmacological strategies, improving the deleterious exercise-induced SNA and BP in PAD, and enhancing tolerance and performance during exercise will also be discussed.

The exercise pressor reflex: An update

The exercise pressor reflex is a feedback mechanism engaged upon stimulation of mechano- and metabosensitive skeletal muscle afferents. Activation of these afferents elicits a reflex increase in heart rate, blood pressure, and ventilation in an intensity-dependent manner. Consequently, the exercise pressor reflex has been postulated to be one of the principal mediators of the cardiorespiratory responses to exercise. In this updated review, we will discuss classical and recent advancements in our understating of the exercise pressor reflex function in both human and animal models. Particular attention will be paid to the afferent mechanisms and pathways involved during its activation, its effects on different target organs, its potential role in the abnormal cardiovascular response to exercise in diseased states, and the impact of age and biological sex on these responses. Finally, we will highlight some unanswered questions in the literature that may inspire future investigations in the field.

Ischemic preconditioning of the muscle reduces the metaboreflex response of the knee extensors

Purpose

This study investigated the effect of ischemic preconditioning (IP) on metaboreflex activation following dynamic leg extension exercise in a group of healthy participants.

Method

Seventeen healthy participants were recruited. IP and SHAM treatments (3 × 5 min cuff occlusion at 220 mmHg or 20 mmHg, respectively) were administered in a randomized order to the upper part of exercising leg’s thigh only. Muscle pain intensity (MP) and pain pressure threshold (PPT) were monitored while administrating IP and SHAM treatments. After 3 min of leg extension exercise at 70% of the maximal workload, a post-exercise muscle ischemia (PEMI) was performed to monitor the discharge group III/IV muscle afferents via metaboreflex activation. Hemodynamics were continuously recorded. MP was monitored during exercise and PEMI.

Results

IP significantly reduced mean arterial pressure compared to SHAM during metaboreflex activation (mean ± SD, 109.52 ± 7.25 vs. 102.36 ± 7.89 mmHg) which was probably the consequence of a reduced end diastolic volume (mean ± SD, 113.09 ± 14.25 vs. 102.42 ± 9.38 ml). MP was significantly higher during the IP compared to SHAM treatment, while no significant differences in PPT were found. MP did not change during exercise, but it was significantly lower during the PEMI following IP (5.10 ± 1.29 vs. 4.00 ± 1.54).

Conclusion

Our study demonstrated that IP reduces hemodynamic response during metaboreflex activation, while no effect on MP and PPT were found. The reduction in hemodynamic response was likely the consequence of a blunted venous return.

Cardiovascular Control During Exercise: The Connectivity of Skeletal Muscle Afferents to the Brain

The exercise pressor reflex (EPR) is engaged upon the activation of group III/IV skeletal muscle afferents and is one of the principal mediators of cardiovascular responses to exercise. This review explores the hypothesis that afferent signals from EPR communicate via GABAergic contacts within the brain stem to evoke parasympathetic withdrawal and sympathoexcitation to increase cardiac output, peripheral resistance, and blood pressure during exercise.

ASIC1a does not play a role in evoking the metabolic component of the exercise pressor reflex in a rat model of peripheral artery disease

The role of the ASIC1a in evoking the exercise pressor reflex in rats with simulated peripheral artery disease is unknown. This prompted us to determine whether ASIC1a plays a role in evoking the exaggerated exercise pressor reflex in decerebrated rats with simulated peripheral artery disease. To simulate peripheral artery disease, we ligated the left femoral artery 72 h before the experiment. The right femoral artery was freely perfused and used as a control. To test our hypothesis, we measured the effect of injecting two ASIC1a blockers into the arterial supply of the triceps surae muscles with and without the femoral artery ligated on the reflex pressor responses to 1) static contraction of the triceps surae muscles, 2) calcaneal tendon stretch, and 3) intra-arterial injection of diprotonated phosphate (pH 6.0). We found that the ASIC1a blockers psalmotoxin-1 (200 ng/kg) and mambalgin-1 (6.5 μg/kg) decreased the pressor responses to static contraction as well as the peak pressor responses to injection of diprotonated phosphate when these responses were evoked from the freely perfused hindlimb. In contrast, ASIC1a blockers only decreased the peak pressor responses evoked by injection of diprotonated phosphate in the hindlimb circulation with simulated peripheral artery disease. This inhibitory effect was less than the one measured from the healthy hindlimb. Independently of the hindlimb of interest, ASIC1a blockers had no effect on the pressor responses to tendon stretch. Our results do not support the hypothesis that ASIC1a play a role in evoking the exercise pressor reflex arising from a hindlimb with simulated peripheral artery disease.NEW & NOTEWORTHY The role of ASIC1a in evoking the metabolic component of the exercise pressor reflex in peripheral artery disease is unknown. Using a within-rat experimental design, we found that the contribution of ASIC1a decreased in a rat model of peripheral artery disease. These results have key implications to help finding better treatments and improve morbidity, quality of life, and mortality in patients with peripheral artery disease.

Effects of remote limb ischemic conditioning on muscle strength in healthy young adults: A randomized controlled trial

Remote limb ischemic conditioning (RLIC) is a clinically feasible method in which brief, sub-lethal bouts of ischemia protects remote organs or tissues from subsequent ischemic injury. A single session of RLIC can improve exercise performance and increase muscle activation. The purpose of this study, therefore, was to assess the effects of a brief, two-week protocol of repeated RLIC combined with strength training on strength gain and neural adaptation in healthy young adults. Participants age 18–40 years were randomized to receive either RLIC plus strength training (n = 15) or sham conditioning plus strength training (n = 15). Participants received RLIC or sham conditioning over 8 visits using a blood pressure cuff on the dominant arm with 5 cycles of 5 minutes each alternating inflation and deflation. Visits 3–8 paired conditioning with wrist extensors strength training on the non-dominant (non-conditioned) arm using standard guidelines. Changes in one repetition maximum (1 RM) and electromyography (EMG) amplitude were compared between groups. Both groups were trained at a similar workload. While both groups gained strength over time (P = 0.001), the RLIC group had greater strength gains (9.38 ± 1.01 lbs) than the sham group (6.3 ± 1.08 lbs, P = 0.035). There was not a significant group x time interaction in EMG amplitude (P = 0.231). The RLIC group had larger percent changes in 1 RM (43.8% vs. 26.1%, P = 0.003) and EMG amplitudes (31.0% vs. 8.6%, P = 0.023) compared to sham conditioning. RLIC holds promise for enhancing muscle strength in healthy young and older adults, as well as clinical populations that could benefit from strength training.

ASIC1a plays a key role in evoking the metabolic component of the exercise pressor reflex in rats

The role of the acid-sensing ion channel 1a (ASIC1a) in evoking the exercise pressor reflex is unknown, despite the fact that ASIC1a is opened by decreases in pH in the physiological range. This fact prompted us to test the hypothesis that ASIC1a plays an important role in evoking the exercise pressor reflex in decerebrated rats with freely perfused hindlimb muscles. To test this hypothesis, we measured the effect of injecting two ASIC1a blockers into the arterial supply of the triceps surae muscles on the reflex pressor responses to four maneuvers, namely 1) static contraction of the triceps surae muscles (i.e., the exercise pressor reflex), 2) calcaneal tendon stretch, 3) intra-arterial injection of lactic acid, and 4) intra-arterial injection of diprotonated phosphate. We found that the 2 ASIC1a blockers, psalmotoxin-1 (200 ng/kg) and mambalgin-1 (6.5 μg/kg), decreased the pressor responses to static contraction as well as the peak pressor responses to injection of lactic acid and diprotonated phosphate. In contrast, neither ASIC1a blocker had any effect on the pressor responses to tendon stretch. Importantly, we found that ASIC1a blockade significantly decreased the pressor response to static contraction after a latency of at least 8 s. Our results support the hypothesis that ASIC1a plays a key role in evoking the metabolic component of the exercise pressor reflex.NEW & NOTEWORTHY The role played by acid-sensing ion channel 1a (ASIC1a) in evoking the exercise pressor reflex remains unknown. In decerebrated rats with freely perfused femoral arteries, blocking ASIC1a with psalmotoxin-1 or mambalgin-1 significantly attenuated the pressor response to static contraction, lactic acid, and diprotonated phosphate injection but had no effect on the pressor response to stretch. We conclude that ASIC1a plays a key role in evoking the exercise pressor reflex by responding to contraction-induced metabolites, such as protons.

Stimulation of spinal δ-opioid receptors attenuates the exercise pressor reflex in decerebrate rats

A reflex arising from contracting hindlimb muscle is responsible in part for the increases in arterial pressure and heart rate evoked by exercise. The afferent arm of this reflex comprises group III and IV afferents. δ-Opioid receptors are expressed predominately on the spinal endings of group III afferents, whereas μ-opioid receptors are expressed predominately on the spinal endings of group IV afferents. Using stimuli that activated group III afferents, namely static contraction, calcaneal tendon stretch, and lactic acid injection into the superficial epigastric artery, we tested the hypothesis that, in rats with either patent or ligated femoral arteries, activation of pre- and postsynaptic δ-opioid receptors in the dorsal horn attenuated pressor reflex responses to these stimuli. In rats with patent arteries or ligated femoral arteries, [d-Pen^2,5]enkephalin (DPDPE), a δ-opioid agonist injected intrathecally (10 μg in 10 μl), significantly attenuated the pressor responses to contraction, stretch, and lactic acid (all P < 0.05). Naltrindole, a δ-opioid receptor antagonist, prevented the attenuation. In contrast, DPDPE did not attenuate the pressor response to capsaicin injection into the superficial epigastric artery in either group of rats (both P > 0.05). Intrathecal injection of saline (10 μl), the vehicle for DPDPE, had no effect on the pressor responses in either group of rats. We conclude that activation of spinal δ-opioid receptors attenuates reflexes evoked by group III afferents in both freely perfused and ligated 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.

Evaluation of Analgesia, Tolerance, and the Mechanism of Action of Morphine-6-O-Sulfate Across Multiple Pain Modalities in Sprague-Dawley Rats

BACKGROUND

Morphine-6-O-sulfate (M6S) is a mixed μ/δ-opioid receptor (OR) agonist and potential alternative to morphine for treatment of chronic multimodal pain.

METHODS

To provide more support for this hypothesis, the antinociceptive effects of M6S and morphine were compared in tests that access a range of pain modalities, including hot plate threshold (HPT), pinprick sensitivity threshold (PST) and paw pressure threshold tests.

RESULTS

Acutely, M6S was 2- to 3-fold more potent than morphine in HPT and PST tests, specifically, derived from best-fit analysis of dose-response relationships of morphine/M6S half-effective dose (ED50) ratios (lower, upper 95% confidence interval [CI]) were 2.8 (2.0-5.8) in HPT and 2.2 (2.1, 2.4) in PST tests. No differences in analgesic drug potencies were detected in the PPT test (morphine/M6S ED50 ratio 1.2 (95% CI, 0.8-1.4). After 7 to 9 days of chronic treatment, tolerance developed to the antinociceptive effects of morphine, but not to M6S, in all 3 pain tests. Morphine-tolerant rats were not crosstolerant to M6S. The antinociceptive effects of M6S were not sensitive to κ-OR antagonists. However, the δ-OR antagonist, naltrindole, blocked M6S-induced antinociception by 55% ± 4% (95% CI, 39-75) in the HPT test, 94% ± 4% (95% CI, 84-105) in the PST test, and 5% ± 17% (95% CI, -47 to 59) or 51% ± 14% (95% CI, 14-84; 6 rats per each group) in the paw pressure threshold test when examined acutely or after 7 days of chronic treatment, respectively.

CONCLUSIONS

Activity via δ-ORs thus appears to be an important determinant of M6S action. M6S also exhibited favorable antinociceptive and tolerance profiles compared with morphine in 3 different antinociceptive assays, indicating that M6S may serve as a useful alternative for rotation in morphine-tolerant subjects.

The efficacy of transcutaneous electrical nerve stimulation on the improvement of walking distance in patients with peripheral arterial disease with intermittent claudication: study protocol for a randomised controlled trial: the TENS-PAD study

Background

In patients with peripheral arterial disease (PAD), walking improvements are often limited by early pain onset due to vascular claudication. It would thus appear interesting to develop noninvasive therapeutic strategies, such as transcutaneous electrical nerve stimulation (TENS), to improve the participation of PAD patients in rehabilitation programmes, and thus improve their quality of life. Our team recently tested the efficacy of a single 45-min session of 10-Hz TENS prior to walking. TENS significantly delayed pain onset and increased the pain-free walking distance in patients with class-II PAD. We now seek to assess the efficacy of a chronic intervention that includes the daily use of TENS for 3 weeks (5 days a week) on walking distance in Leriche-Fontaine stage-II PAD patients.

Methods/design

This is a prospective, double-blind, multicentre, randomised, placebo-controlled trial. One hundred subjects with unilateral PAD (Leriche-Fontaine stage II) will be randomised into two groups (1:1). For the experimental group (TENS group): the treatment will consist of stimulation of the affected leg (at a biphasic frequency of 10 Hz, with a pulse width of 200 μs, maximal intensity below the motor threshold) for 45 min per day, in the morning before the exercise rehabilitation programme, for 3 weeks, 5 days per week. For the control group (SHAM group): the placebo stimulation will be delivered according to the same modalities as for the TENS group but with a voltage level automatically falling to zero after 10 s of stimulation. First outcome: walking distance without pain. Secondary outcomes: transcutaneous oxygen pressure (TcPO₂) measured during a Strandness exercise test, peak oxygen uptake (VO₂ peak), endothelial function (EndoPAT®), Ankle-brachial Pressure Index, Body Mass Index, lipid profile (LDL-C, HDL-C, triglycerides), fasting glycaemia, HbA1c level, and the WELCH questionnaire.

Discussion

TENS-PAD is the first randomised controlled trial that uses transcutaneous electrical therapy as an adjuvant technique to improve vascular function in the treatment of PAD. If the results are confirmed, this technique could be incorporated into the routine care in cardiovascular rehabilitation centers and used in the long term by patients to improve their walking capacity.

Trial registration

ClinicalTrials.gov, ID: NCT02678403. Registered on 9 February 2016. Sponsor: Toulouse University Hospital.

Electronic supplementary material

The online version of this article (doi:10.1186/s13063-017-1997-1) contains supplementary material, which is available to authorized users.

Purinergic P2X Receptors and Heightened Exercise Pressor Reflex in Peripheral Artery Disease

Arterial blood pressure (BP) and vasoconstriction regulated by sympathetic nerve activity (SNA) are heightened during exercise in patients with peripheral artery disease (PAD). The exercise pressor reflex is considered as a neural mechanism responsible for the exaggerated autonomic responses to exercise in PAD. A series of studies have employed a rat model of PAD to examine signal pathways at receptor and cellular levels by which the exercise pressor reflex is amplified. This review will summarize results obtained from recent human and animal studies with respect to contribution of muscle afferents to augmented SNA and BP responses in PAD. The role played by adenosine triphosphate (ATP) and ATP sensitive purinergic P2X receptors will be emphasized.

Effects of ischemic preconditioning on short-duration cycling performance

It has been demonstrated that ischemic preconditioning (IPC) improves endurance performance. However, the potential benefits during anaerobic events and the mechanism(s) underlying these benefits remain unclear. Fifteen recreational cyclists were assessed to evaluate the effects of IPC of the upper thighs on anaerobic performance, skeletal muscle activation, and metabolic responses during a 60-s sprint performance. After an incremental test and a familiarization visit, subjects were randomly submitted in visits 3 and 4 to a performance protocol preceded by intermittent bilateral cuff inflation (4 × (5 min of blood flow restriction + 5 min reperfusion)) at either 220 mm Hg (IPC) or 20 mm Hg (control). To increase data reliability, each intervention was replicated, which was also in a random manner. In addition to the mean power output, the pulmonary oxygen uptake, blood lactate kinetics, and quadriceps electromyograms (EMGs) were analyzed during performance and throughout 45 min of passive recovery. After IPC, performance was improved by 2.1% compared with control (95% confidence intervals of 0.8% to 3.3%, P = 0.001), followed by increases in (i) the accumulated oxygen deficit, (ii) the amplitude of blood lactate kinetics, (iii) the total amount of oxygen consumed during recovery, and (iv) the overall EMG amplitude (P < 0.05). In addition, the ratio between EMG and power output was higher during the final third of performance after IPC (P < 0.05). These results suggest an increased skeletal muscle activation and a higher anaerobic contribution as the ultimate responses of IPC on short-term exercise performance.

Intrathecal fentanyl abolishes the exaggerated blood pressure response to cycling in hypertensive men

KEY POINTS

The increase in blood pressure observed during physical activities is exaggerated in patients with hypertension, exposing them to a higher cardiovascular risk. Neural signals from the skeletal muscles appear to be overactive, resulting in this abnormal response in hypertensive patients. In the present study, we tested whether the attenuation of these neural signals in hypertensive patients could normalize their abnormal increase in blood pressure during physical activity. Attenuation of the neural signals from the leg muscles with intrathecal fentanyl injection reduced the blood pressure of hypertensive men during cycling exercise to a level comparable to that of normotensive men. Skeletal muscle afferent overactivity causes the abnormal cardiovascular response to exercise and was reverted in this experimental model, appearing as potential target for treatment. Hypertensive patients present an exaggerated increase in blood pressure and an elevated cardiovascular risk during exercise. Although controversial, human studies suggest that group III and IV skeletal muscle afferents might contribute to this abnormal response. In the present study, we investigated whether attenuation of the group III and IV muscle afferent signal of hypertensive men eliminates the exaggerated increase in blood pressure occurring during exercise. Eight hypertensive men performed two sessions of 5 min of cycling exercise at 40 W. Between sessions, the subjects were provided with a lumbar intrathecal injection of fentanyl, a μ-opioid receptor agonist, aiming to attenuate the central projection of opioid-sensitive group III and IV muscle afferent nerves. The cardiovascular response to exercise of these subjects was compared with that of six normotensive men. During cycling, the hypertensive group demonstrated an exaggerated increase in blood pressure compared to the normotensive group (mean ± SEM: +17 ± 3 vs. +8 ± 1 mmHg, respectively; P < 0.05), whereas the increase in heart rate, stroke volume, cardiac output and vascular conductance was similar (P > 0.05). Fentanyl inhibited the blood pressure response to exercise in the hypertensive group (+11 ± 2 mmHg) to a level comparable to that of the normotensive group (P > 0.05). Moreover, fentanyl increased the responses of vascular conductance and stroke volume to exercise (P < 0.05), whereas the heart rate response was attenuated (P < 0.05) and the cardiac output response was maintained (P > 0.05). The results of the present study show that attenuation of the exercise pressor reflex normalizes the blood pressure response to cycling exercise in hypertensive individuals.

Oscillatory blood pressure response to the onset of cycling exercise in men: role of group III/IV muscle afferents

NEW FINDINGS

What is the central question of this study? Neural feedback from group III/IV muscle afferents has a key role in regulation of cardiovascular responses to exercise. Blood pressure oscillates in the first seconds of dynamic exercise, but the contribution of muscle afferent feedback to this pattern is unclear. What is the main finding and its importance? We demonstrate that attenuation of group III/IV muscle afferent feedback by spinal fentanyl impairs the pressor response after 10 s of moderate leg cycling exercise, but this afferent feedback does not appear to be necessary for induction of the oscillatory pattern of blood pressure at the onset of exercise. We investigated whether attenuation of the central projections of group III/IV skeletal muscle afferents via lumbar intrathecal administration of the μ-opioid receptor agonist fentanyl affects the oscillatory blood pressure (BP) response to the onset of dynamic exercise. Eight healthy, recreationally active men (28 ± 3 years old) performed 40 s of cycling at 80 W (60 r.p.m.) before (control) and after fentanyl administration, while heart rate, stroke volume, cardiac output, systolic, mean and diastolic BP and total vascular conductance were continuously monitored. Sytolic and mean BP responses to cycling included an initial increase (from 0 to 3 s), followed by a transient decrease below resting levels (from 3 to 10 s) and then a sustained increase (>10 s). In the presence of fentanyl, systolic and mean BP responses closely matched those in control conditions in the first 10 s, but were blunted thereafter (P < 0.05). In contrast, fentanyl did not modify the heart rate, stroke volume, cardiac output, diastolic BP or total vascular conductance responses to 40 s of cycling (P > 0.05). These findings suggest that during moderate leg cycling exercise the muscle afferents contribute to the BP response after ∼10 s, but that they do not appear to be implicated in the oscillation of BP at the onset of exercise.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

The exercise pressor reflex and peripheral artery disease

The exercise pressor reflex contributes to increases in cardiovascular and ventilatory function during exercise. These reflexive increases are caused by both mechanical stimulation and metabolic stimulation of group III and IV afferents with endings in contracting skeletal muscle. Patients with peripheral artery disease (PAD) have an augmented exercise pressor reflex. Recently, an animal model of PAD was established which allows further investigation of possible mechanisms involved in this augmented reflex. Earlier studies have identified ASIC3 channels, bradykinin receptors, P2X receptors, endoperoxide receptors, and thromboxane receptors as playing a role in evoking the exercise pressor reflex in healthy rats. This review focuses on recent studies using a rat model of PAD in order to determine possible mechanisms contributing to the exaggerated exercise pressor reflex seen in patients with this disease.

Effects of peripheral and spinal κ-opioid receptor stimulation on the exercise pressor reflex in decerebrate rats

The exercise pressor reflex is greater in rats with ligated femoral arteries than it is in rats with freely perfused femoral arteries. The exaggerated reflex in rats with ligated arteries is attenuated by stimulation of μ-opioid and δ-opioid receptors on the peripheral endings of thin-fiber muscle afferents. The effect of stimulation of κ-opioid receptors on the exercise pressor reflex is unknown. We tested the hypothesis that stimulation of κ-opioid receptors attenuates the exercise pressor reflex in rats with ligated, but not freely perfused, femoral arteries. The pressor responses to static contraction were compared before and after femoral arterial or intrathecal injection of the κ-opioid receptor agonist U62066 (1, 10, and 100 μg). Femoral arterial injection of U62066 did not attenuate the pressor responses to contraction in either group of rats. Likewise, intrathecal injection of U62066 did not attenuate the pressor response to contraction in rats with freely perfused femoral arteries. In contrast, intrathecal injection of 10 and 100 μg of U62066 attenuated the pressor response to contraction in rats with ligated femoral arteries, an effect that was blocked by prior intrathecal injection of the κ-opioid receptor antagonist nor-binaltorphimine. In rats with ligated femoral arteries, the pressor response to stimulation of peripheral chemoreceptors by sodium cyanide was not changed by intrathecal U62066 injections, indicating that these injections had no direct effect on the sympathetic outflow. We conclude that stimulation of spinal, but not peripheral, κ-opioid receptors attenuates the exaggerated exercise pressor reflex in rats with ligated femoral arteries.

Nerve Growth Factor, Muscle Afferent Receptors and Autonomic Responsiveness with Femoral Artery Occlusion

The exercise pressor reflex is a neural control mechanism responsible for the cardiovascular responses to exercise. As exercise is initiated, thin fiber muscle afferent nerves are activated by mechanical and metabolic stimuli arising in the contracting muscles. This leads to reflex increases in arterial blood pressure and heart rate primarily through activation of sympathetic nerve activity (SNA). Studies of humans and animals have indicated that the exercise pressor reflex is exaggerated in a number of cardiovascular diseases. For the last several years, a series of studies have employed a rodent model to examine the mechanisms at receptor and cellular levels by which responses of SNA and blood pressure to static exercise are heightened in peripheral artery disease (PAD), one of the most common cardiovascular disorders. Specifically, femoral artery occlusion is used to study intermittent claudication that is observed in human PAD. Our studies have demonstrated that the receptors on thin fiber muscle afferents including transient receptor potential vanilloid type 1 (TRPV1), purinergic P2X3 and acid sensing ion channel subtype 3 (ASIC3) are engaged in augmented autonomic responses this disease. This review will present some of recent results in regard with several receptors in muscle sensory neurons in contribution to augmented autonomic responses in PAD. We will emphasize the role played by nerve growth factor (NGF) in regulating those sensory receptors in the processing of amplified exercise pressor reflex. Also, we will discuss the role played by hypoxia-inducible facor-1α regarding the enhanced autonomic reflex with femoral artery occlusion. The purpose of this review is to focus on a theme namely that PAD accentuates reflexively autonomic responses to exercise and further address regulatory mechanisms leading to abnormal autonomic responsiveness.