P2X receptor-mediated muscle pressor reflex in myocardial infarction

Sex differences in the purinergic 2 receptor-mediated blood pressure response to treadmill exercise in rats with simulated peripheral artery disease

We investigated the role played by ATP-sensitive purinergic 2 (P2) receptors in evoking the pressor response to treadmill exercise in male and female rats with and without femoral arteries that were ligated for ∼72 h to induce simulated peripheral artery disease (PAD). We hypothesized that PPADS (P2 receptor antagonist, 10 mg iv) would reduce the pressor response to 4 min of treadmill exercise (15 m·min-1, 1° incline) and steady-state exercise plasma norepinephrine (NE) values in male and female rats, and that the magnitude of effect of PPADS would be greater in rats with simulated PAD ("ligated") than in sham-operated rats. In males, PPADS significantly reduced the difference between steady-state exercise and baseline mean arterial pressure (ΔMAP) response to treadmill exercise in sham (n = 8; pre-PPADS: 12 ± 2, post-PPADS: 1 ± 5 mmHg; P = 0.037) and ligated (n = 4; pre-PPADS: 20 ± 2, post-PPADS: 11 ± 3 mmHg; P = 0.028) rats with a similar magnitude of effect observed between groups (P = 0.720). In females, PPADS had no effect on the ΔMAP response to treadmill exercise in sham (n = 6; pre-PPADS: 9 ± 2, post-PPADS: 7 ± 2 mmHg; P = 0.448) or ligated (n = 6; pre-PPADS: 15 ± 2, post-PPADS: 16 ± 3 mmHg; P = 0.684) rats. When NE values were grouped by sex independent of ligation/sham status, PPADS significantly reduced plasma NE in male (P = 0.016) and female (P = 0.027) rats. The data indicate that P2 receptors contribute to the sympathetic response to exercise in both male and female rats but that the sympathoexcitatory role for P2 receptors translates into an obligatory role in the blood pressure response to exercise in male but not in female rats.NEW & NOTEWORTHY Here, we demonstrate that purinergic 2 (P2) receptors contribute significantly to the blood pressure response to treadmill exercise in male rats both with and without simulated PAD induced by femoral artery ligation. We found no role for P2 receptors in the blood pressure response to treadmill exercise in female rats, thus revealing clear sex differences in P2 receptor-mediated blood pressure control during exercise.

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.

Bradykinin Contributes to Sympathetic and Pressor Responses Evoked by Activation of Skeletal Muscle Afferents P2X in Heart Failure

BACKGROUND/AIMS

Published data suggest that purinergic P2X receptors of muscle afferent nerves contribute to the enhanced sympathetic nervous activity (SNA) and blood pressure (BP) responses during static exercise in heart failure (HF). In this study, we examined engagement of bradykinin (BK) in regulating responses of SNA and BP evoked by P2X stimulation in rats with HF. We further examined cellular mechanisms responsible for BK. We hypothesized that BK potentiates P2X currents of muscle dorsal root ganglion (DRG) neurons, and this effect is greater in HF due to upregulation of BK kinin B2 and P2X3 receptor. As a result, BK amplifies muscle afferents P2X-mediated SNA and BP responses.

METHODS

Renal SNA and BP responses were recorded in control rats and rats with HF. Western Blot analysis and patch-clamp methods were employed to examine the receptor expression and function of DRG neurons involved in the effects of BK.

RESULTS

BK injected into the arterial blood supply of the hindlimb muscles heightened the reflex SNA and BP responses induced by P2X activation with α,β-methylene ATP to a greater degree in HF rats. In addition, HF upregulated the protein expression of kinin B2 and P2X3 in DRG and the prior application of BK increased the magnitude of α,β-methylene ATP-induced currents in muscle DRG neurons from HF rats.

CONCLUSION

BK plays a facilitating role in modulating muscle afferent P2X-engaged reflex sympathetic and pressor responses. In HF, P2X responsivness is augmented due to increases in expression of kinin B2 and P2X3 receptors and P2X current activity.

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.

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.

Blood flow restriction training and the exercise pressor reflex: a call for concern

Blood flow restriction (BFR) training (also known as Kaatsu training) is an increasingly common practice employed during resistance exercise by athletes attempting to enhance skeletal muscle mass and strength. During BFR training, blood flow to the exercising muscle is mechanically restricted by placing flexible pressurizing cuffs around the active limb proximal to the working muscle. This maneuver results in the accumulation of metabolites (e.g., protons and lactic acid) in the muscle interstitium that increase muscle force and promote muscle growth. Therefore, the premise of BFR training is to simulate and receive the benefits of high-intensity resistance exercise while merely performing low-intensity resistance exercise. This technique has also been purported to provide health benefits to the elderly, individuals recovering from joint injuries, and patients undergoing cardiac rehabilitation. Since the seminal work of Alam and Smirk in the 1930s, it has been well established that reductions in blood flow to exercising muscle engage the exercise pressor reflex (EPR), a reflex that significantly contributes to the autonomic cardiovascular response to exercise. However, the EPR and its likely contribution to the BFR-mediated cardiovascular response to exercise is glaringly missing from the scientific literature. Inasmuch as the EPR has been shown to generate exaggerated increases in sympathetic nerve activity in disease states such as hypertension (HTN), heart failure (HF), and peripheral artery disease (PAD), concerns are raised that BFR training can be used safely for the rehabilitation of patients with cardiovascular disease, as has been suggested. Abnormal BFR-induced and EPR-mediated cardiovascular complications generated during exercise could precipitate adverse cardiovascular or cerebrovascular events (e.g., cardiac arrhythmia, myocardial infarction, stroke and sudden cardiac death). Moreover, although altered EPR function in HTN, HF, and PAD underlies our concern for the widespread implementation of BFR, use of this training mechanism may also have negative consequences in the absence of disease. That is, even normal, healthy individuals performing resistance training exercise with BFR are potentially at increased risk for deleterious cardiovascular events. This review provides a brief yet detailed overview of the mechanisms underlying the autonomic cardiovascular response to exercise with BFR. A more complete understanding of the consequences of BFR training is needed before this technique is passively explored by the layman athlete or prescribed by a health care professional.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Nerve growth factor decreases in sympathetic and sensory nerves of rats with chronic heart failure

Nerve growth factor (NGF) plays a critical role in the maintenance and survival of both sympathetic and sensory nerves. Also, NGF can regulate receptor expression and neuronal activity in the sympathetic and sensory neurons. Abnormalities in NGF regulation are observed in patients and animals with heart failure (HF). Nevertheless, the effects of chronic HF on the levels of NGF within the sympathetic and sensory nerves are not known. Thus, the ELISA method was used to assess the levels of NGF in the stellate ganglion (SG) and dorsal root ganglion (DRG) neurons of control rats and rats with chronic HF induced by myocardial infarction. Our data show for the first time that the levels of NGF were significantly decreased (P < 0.05) in the SG and DRG neurons 6-20 weeks after ligation of the coronary artery. In addition, a close relation was observed between the NGF levels and the left ventricular function. In conclusion, chronic HF impairs the expression of NGF in the sympathetic and sensory nerves. Given that sensory afferent nerves are engaged in the sympathetic nervous responses to somatic stimulation (i.e. muscle activity during exercise) via a reflex mechanism, our data indicate that NGF is likely responsible for the development of muscle reflex-mediated abnormal sympathetic responsiveness observed in chronic HF.

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.

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.

Exaggerated exercise pressor reflex in adults with moderately elevated systolic blood pressure: role of purinergic receptors

The neurocirculatory responses to exercise are exaggerated in hypertension, increasing cardiovascular risk, yet the mechanisms remain incompletely understood. The aim of this study was to examine the in vitro effectiveness of pyridoxal-5-phosphate as a purinergic (P2) receptor antagonist in isolated murine dorsal root ganglia (DRG) neurons and the in vivo contribution of P2 receptors to the neurocirculatory responses to exercise in older adults with moderately elevated systolic blood pressure (BP). In vitro, pyridoxal-5-phosphate attenuated the ATP-induced increases in [Ca2+]i (73 ± 15 vs. 11 ± 3 nM; P < 0.05). In vivo, muscle sympathetic nerve activity (MSNA; peroneal microneurography) and arterial BP (Finometer) were assessed during exercise pressor reflex activation (static handgrip followed by postexercise ischemia; PEI) during a control trial (normal saline) and localized P2 receptor blockade (pyridoxal-5-phosphate). Compared with normotensive adults (63 ± 2 yr, 117 ± 2/70 ± 2 mmHg), adults with moderately elevated systolic BP (65 ± 1 yr, 138 ± 5/79 ± 3 mmHg) demonstrated greater increases in MSNA and BP during handgrip and PEI. Compared with the control trial, local antagonism of P2 receptors during PEI partially attenuated MSNA (39 ± 4 vs. 34 ± 5 bursts/min; P < 0.05) in adults with moderately elevated systolic BP. In conclusion, these data demonstrate pyridoxal-5-phosphate is an effective P2 receptor antagonist in isolated DRG neurons, which are of particular relevance to the exercise pressor reflex. Furthermore, these findings indicate that exercise pressor reflex function is exaggerated in older adults with moderately elevated systolic BP and further suggest a modest role of purinergic receptors in evoking the abnormally large reflex-mediated increases in sympathetic activity during exercise in this clinical population.

Bradykinin B2 receptor contributes to the exaggerated muscle mechanoreflex in rats with femoral artery occlusion

Static muscle contraction activates the exercise pressor reflex, which in turn increases sympathetic nerve activity (SNA) and blood pressure (BP). Bradykinin (BK) is considered as a muscle metabolite responsible for modulation of the sympathetic and cardiovascular responses to muscle contraction. Prior studies have suggested that kinin B2 receptor mediates the effects of BK on the reflex SNA and BP responses during stimulation of skeletal muscle afferents. In patients with peripheral artery disease and a rat model with femoral artery ligation, amplified SNA and BP responses to static exercise were observed. This dysfunction of the exercise pressor reflex has previously been shown to be mediated, in part, by muscle mechanoreflex overactivity. Thus, in this report, we determined whether kinin B2 receptor contributes to the augmented mechanoreflex activity in rats with 24 h of femoral artery occlusion. First, Western blot analysis was used to examine protein expression of B2 receptors in dorsal root ganglion tissues of control limbs and ligated limbs. Our data show that B2 receptor displays significant overexpression in ligated limbs as compared with control limbs (optical density: 0.94 ± 0.02 in control and 1.87 ± 0.08 after ligation, P < 0.05 vs. control; n = 6 in each group). Second, mechanoreflex was evoked by muscle stretch and the reflex renal SNA (RSNA) and mean arterial pressure (MAP) responses to muscle stretch were examined after HOE-140, a B2 receptors blocker, was injected into the arterial blood supply of the hindlimb muscles. The results demonstrate that the stretch-evoked reflex responses were attenuated by administration of HOE-140 in control rats and ligated rats; however, the attenuating effects of HOE-140 were significantly greater in ligated rats, i.e., after 5 μg/kg of HOE-140 RSNA and MAP responses evoked by 0.5 kg of muscle tension were attenuated by 43% and 25% in control vs. 54% and 34% in ligation (P < 0.05 vs. control group; n = 11 in each group). In contrast, there was no significant difference in B1 receptor expression in both experimental groups, and arterial injection of R-715, a B1 receptors blocker, had no significant effects on RSNA and MAP responses evoked by muscle stretch. Accordingly, results obtained from this study support our hypothesis that heightened kinin B2 receptor expression in the sensory nerves contributes to the exaggerated muscle mechanoreflex in rats with femoral artery occlusion.

Augmented P2X response and immunolabeling in dorsal root ganglion neurons innervating skeletal muscle following femoral artery occlusion

The responsiveness of sensory neurons to muscle metabolites is altered under the conditions of insufficient limb blood supply in some diseases, such as peripheral artery disease. The purpose of this study was to examine ATP-induced current with activation of purinergic P2X subtypes P2X₃ and P2X₂/₃ in dorsal root ganglion (DRG) neurons of control limbs and limbs with 24 h of femoral artery occlusion using whole cell patch-clamp methods. Also, dual-labeling immunohistochemistry was employed to determine existence of P2X₃ expression in DRG neurons of thin-fiber afferents. DRG neurons from 4- to 6-wk-old rats were labeled by injecting the fluorescence tracer DiI into the hindlimb muscles 4-5 days before the recording experiments. Transient (P2X₃), mixed (P2X₃ and P2X₂/₃), and sustained (P2X₂/₃) current responses to α,β-methylene ATP (a P2X receptor agonist) are observed in small and medium DRG neurons, and size distribution of DRG neurons is similar in control and occluded limbs. However, the peak current amplitude of DRG neuron induced by stimulation of P2X₃ and/or P2X₂/₃ is larger in occluded limbs than that in control limbs. Moreover, the percentage of DRG neurons with P2X₃ transient currents is greater after arterial occlusion compared with control. In addition, a rapid desensitization was observed in DRG neurons with transient currents, but not with sustained currents in control and occluded groups. Furthermore, results from immunofluorescence experiments show that femoral artery occlusion primarily augments P2X₃ expression within DRG neurons projecting C-fiber afferents. Overall, these findings suggest that 1) greater ATP-induced currents with activation of P2X₃ and P2X₂/₃ are developed when hindlimb arterial blood supply is deficient under ischemic conditions and 2) increased P2X₃ expression is largely observed in C-fibers of DRG neurons after hindlimb vascular insufficiency.

Exaggerated Pressor Response in Relation to Attenuated Muscle Temperature Response during Contraction in Ischemic Heart Failure

It is known that muscle temperature (T_m) increases with exercise. The purpose of this study was to examine if contraction-induced increase in T_m was altered in rats with heart failure (HF) induced by chronic myocardial infraction (MI) as compared with healthy control animals. A temperature probe was inserted in the triceps surae muscle to continuously measure T_m throughout experiments. Static muscle contraction was induced by electrical stimulation of the sciatic nerve for 1 min. As baseline T_m was 34°C, contraction increased temperature by 1.6 ± 0.18°C in nine health control rats and by 1.0 ± 0.15°C in 10 MI rats (P < 0.05 vs. control). Note that there were no differences in developed muscle tension and muscle weight between the two groups. In addition, muscle contraction increased mean arterial pressure by 23 ± 3 mmHg in control rats and by 31 ± 3 mmHg in MI rats (P < 0.05 vs. control). A regression analysis further shows that there is an inverse liner relationship between the pressor response and static contraction-induced increase in T_m. Our data suggest that T_m increase evoked by contraction is impaired in MI rats. The abnormal alteration in T_m likely modifies the reflex cardiovascular responses in MI via mechanisms of temperature-sensitive receptors on muscle afferent nerves.

Muscle afferent receptors engaged in augmented sympathetic responsiveness in peripheral artery disease

The exercise pressor reflex (EPR) 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 (BP) and heart rate primarily through activation of sympathetic nerve activity (SNA). Studies of humans and animals have indicated that the EPR is exaggerated in a number of cardiovascular diseases. For the last several years, studies have specifically employed a rodent model to examine the mechanisms at receptor and cellular levels by which responses of SNA and BP to static exercise are heightened in peripheral artery disease (PAD), one of the most common cardiovascular disorders. A rat model of this disease has well been established. Specifically, femoral artery occlusion is used to study intermittent claudication that is observed in human PAD. The receptors on thin fiber muscle afferents that are engaged in this disease include transient receptor potential vanilloid type 1 (TRPV1), purinergic P2X, and acid sensing ion channel (ASIC). The role played by nerve growth factor in regulating those sensory receptors in the processing of amplified EPR was also investigated. The purpose of this review is to focus on a theme namely that PAD accentuates autonomic reflex responses to exercise and further address regulatory mechanisms leading to abnormal sympathetic responsiveness. This review will present some of recent results in regard with several receptors in muscle sensory neurons in contribution to augmented autonomic reflex responses in PAD. Review of the findings from recent studies would lead to a better understanding in integrated processing of sympathetic nervous system in PAD.

Role for NGF in augmented sympathetic nerve response to activation of mechanically and metabolically sensitive muscle afferents in rats with femoral artery occlusion

Arterial blood pressure and heart rate responses to static contraction of the hindlimb muscles are greater in rats whose femoral arteries were previously ligated than in control rats. Also, the prior findings demonstrate that nerve growth factor (NGF) is increased in sensory neurons-dorsal root ganglion (DRG) neurons of occluded rats. However, the role for endogenous NGF in engagement of the augmented sympathetic and pressor responses to stimulation of mechanically and/or metabolically sensitive muscle afferent nerves during static contraction after femoral artery ligation has not been specifically determined. In the present study, both afferent nerves and either of them were activated by muscle contraction, passive tendon stretch, and arterial injection of lactic acid into the hindlimb muscles. Data showed that femoral occlusion-augmented blood pressure response to contraction was significantly attenuated by a prior administration of the NGF antibody (NGF-Ab) into the hindlimb muscles. The effects of NGF neutralization were not seen when the sympathetic nerve and pressor responses were evoked by stimulation of mechanically sensitive muscle afferent nerves with tendon stretch in occluded rats. In addition, chemically sensitive muscle afferent nerves were stimulated by lactic acid injected into arterial blood supply of the hindlimb muscles after the prior NGF-Ab, demonstrating that the reflex muscle responses to lactic acid were significantly attenuated. The results of this study further showed that NGF-Ab attenuated an increase in acid-sensing ion channel subtype 3 (ASIC3) of DRG in occluded rats. Moreover, immunohistochemistry was employed to examine the number of C-fiber and A-fiber DRG neurons. The data showed that distribution of DRG neurons with different thin fiber phenotypes was not notably altered when NGF was infused into the hindlimb muscles. However, NGF increased expression of ASIC3 in DRG neurons with C-fiber but not A-fiber. Overall, these data suggest that 1) NGF is amplified in sensory nerves of occluded rats and contributes to augmented reflex sympathetic and blood pressure responses evoked by stimulation of chemically, but not mechanically, sensitive muscle afferent nerves and 2) NGF likely plays a role in modulating the muscle metaboreflex via enhancement of ASIC3 expression in C-fiber of DRG neurons.

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

Cyclo-oxygenase (COX) enzymes are responsible for the formation from arachidonic acid of prostaglandins, among other metabolites. Prior studies have suggested that inhibition of the COX pathway attenuates the responses of sympathetic nerve activity and blood pressure during static muscle contraction. Static muscle contraction activates the exercise pressor reflex, which in turn increases sympathetic nerve activity and blood pressure. Also, COX products contribute to exaggeration of the exercise pressor reflex in heart failure (HF). This dysfunction of the exercise pressor reflex has previously been shown to be mediated primarily by muscle mechanoreflex overactivity. It is well known that COX-1 and COX-2 are two isoforms of the enzyme that lead to formation of these important biological mediators involved in the muscle reflex. Thus, in the present study, we determined whether the COX-1 and/or COX-2 pathway contribute(s) to the augmented mechanoreflex activity in HF. First, Western blot analysis was employed to examine protein expression of COX-1 and COX-2 in skeletal muscle tissue of control rats and rats with HF induced by myocardial infarction. Our data show that there is no significant difference in COX-1 expression in both experimental groups. However, COX-2 displays significant overexpression in rats with HF compared with control rats (optical density 1.06 ± 0.05 in control and 1.6 ± 0.05 in HF, P < 0.05 versus control). Second, the mechanoreflex was evoked by passive tendon stretch, and the reflex sympathetic and pressor responses to muscle stretch were examined after COX-1 and COX-2 inhibitors (FR-122047 and SC-236) were individually injected into the arterial blood supply of the hindlimb muscles. The results demonstrate that the stretch-evoked reflex responses in rats with HF were significantly attenuated by administration of SC-236, but not by FR-122047, i.e. renal sympathetic nerve activity and mean arterial pressure responses evoked by 0.5 kg of muscle tension were 52.3 ± 8.9% and 19 ± 1.4 mmHg, respectively, in control conditions and 26.4 ± 5.6% and 5.7 ± 1.6 mmHg (P < 0.05 versus control group) after 0.25 mg kg(-1) of SC-236. Muscle stretch-evoked renal sympathetic nerve activity and mean arterial pressure responses were 51.8 ± 8.2% and 18.7 ± 1.2 mmHg, respectively, in control conditions and 48.3 ± 5.3% and 17.5 ± 1.9 mmHg (P > 0.05 versus control group) after 1.0 mg kg(-1) of FR-122047. Accordingly, the results obtained from this study support our hypothesis that heightened COX-2 expression within the hindlimb muscles contributes to the exaggerated muscle mechanoreflex in congestive HF.

Acid-sensing ion channel subtype 3 function and immunolabelling increases in skeletal muscle sensory neurons following femoral artery occlusion

Sympathetic nerve activity and arterial blood pressure responses to static hindlimb muscle contractions are greater in rats with femoral arteries that were previously ligated (24-72 h earlier) than in control rats. Studies further demonstrate that acid-sensing ion channel subtype 3 (ASIC(3)) in thin-fibre muscle afferents contributes to the amplified reflex muscle responses observed in occluded rats, probably due to enhanced ASIC(3) expression in muscle sensory neurons. The purpose of this study was to characterize acid-induced current with activation of ASIC(3) in dorsal root ganglion (DRG) neurons of control rats and rats with 24 h of femoral occlusion using whole-cell patch clamp methods. Also, immunohistochemistry was employed to examine existence of ASIC(3) expression in DRG neurons of thin-fibre afferents. DRG neurons from 4- to 6-week-old rats were labelled by injecting the fluorescence tracer DiI into the hindlimb muscles 4-5 days prior to the recording experiments. The results of this study show that ∼90% of current responses evoked by pH 6.7 in DRG neurons innervating the hindlimb muscles are ASIC(3)-like. The peak current amplitude to pH 6.7 is significantly attenuated with application of rAPETx2, a specific ASIC(3) antagonist. In addition, ASIC(3)-like current responses to pH 6.7 are observed in small, medium and large DRG neurons, and size distribution of DRG neurons is similar in control and occluded animals. However, the peak current amplitude of DRG neuron response induced by ASIC(3) stimulation is larger in occluded rats than that in control rats. Moreover, the percentage of DRG neurons with ASIC(3)-like currents is greater after arterial occlusion compared with control. Furthermore, results from double immunofluorescence experiments show that femoral artery occlusion mainly augments ASIC(3) expression within DRG neurons projecting C-fibre afferents. Taken together, these data suggest that (1) the majority of current responses to pH 6.7 are ASIC(3)-like in DRG neurons with nerve endings in the hindlimb muscles, (2) a greater acid-induced current responding to pH 6.7 develops when hindlimb arterial blood supply is deficient under ischaemic conditions, and (3) increased ASIC(3) expression is largely observed in thin C-fibres of DRG neurons after hindlimb ischaemia.

Cardiovascular regulation by skeletal muscle reflexes in health and disease

Heart rate and blood pressure are elevated at the onset and throughout the duration of dynamic or static exercise. These neurally mediated cardiovascular adjustments to physical activity are regulated, in part, by a peripheral reflex originating in contracting skeletal muscle termed the exercise pressor reflex. Mechanically sensitive and metabolically sensitive receptors activating the exercise pressor reflex are located on the unencapsulated nerve terminals of group III and group IV afferent sensory neurons, respectively. Mechanoreceptors are stimulated by the physical distortion of their receptive fields during muscle contraction and can be sensitized by the production of metabolites generated by working skeletal myocytes. The chemical by-products of muscle contraction also stimulate metaboreceptors. Once activated, group III and IV sensory impulses are transmitted to cardiovascular control centers within the brain stem where they are integrated and processed. Activation of the reflex results in an increase in efferent sympathetic nerve activity and a withdrawal of parasympathetic nerve activity. These actions result in the precise alterations in cardiovascular hemodynamics requisite to meet the metabolic demands of working skeletal muscle. Coordinated activity by this reflex is altered after the development of cardiovascular disease, generating exaggerated increases in sympathetic nerve activity, blood pressure, heart rate, and vascular resistance. The basic components and operational characteristics of the reflex, the techniques used in human and animals to study the reflex, and the emerging evidence describing the dysfunction of the reflex with the advent of cardiovascular disease are highlighted in this review.

Contribution of nerve growth factor to upregulation of P2X₃ expression in DRG neurons of rats with femoral artery occlusion

Femoral artery occlusion augments the sympathetic nerve and pressor responses to muscle contraction and muscle metabolites injected into the arterial blood supply of the hindlimb muscles in rats. The underlying mechanism by which these reflex responses are enhanced after muscle vascular insufficiency is unclear. Purinergic P2X(3) receptor has been reported to contribute to the metabolic component of the exercise pressor reflex. Thus the purpose of this study was to examine if chronic femoral occlusion would alter the expression of P2X(3) in dorsal root ganglion (DRG) neurons of rats. Also, P2X(3)-mediated sympathetic responsiveness was examined after femoral occlusion. In addition, the role played by nerve growth factor (NGF) in regulating the expression and response of P2X(3) was examined. Western blot analysis showed that 24 h of femoral ligation increased the levels of P2X(3) (optical density: 0.93 ± 0.07 in control and 1.37 ± 0.10 after occlusion; P < 0.05 vs. control). The fluorescence immunohistochemistry further demonstrated that the occlusion elevated P2X(3) expression in DRG neurons (percentage of P2X(3)-positive cells: 33 ± 3% in control and 51 ± 3% in occlusion; P < 0.05 vs. control). Furthermore, the results showed that responses of renal sympathetic nerve activity and blood pressure to stimulation of P2X were greater in occluded rats than responses in control rats by injection of α,β-methylene ATP into the arterial blood supply of the hindlimb muscle. Finally, infusion of NGF in the hindlimb muscles of healthy rats increased P2X(3) (optical density: 0.98 ± 0.12 in control and 1.37 ± 0.16 with NGF; P < 0.05 vs. control). The pressor response to injection of α,β-methylene ATP was increased in the rats with NGF infusion. Likewise, blocking NGF attenuated exaggeration of the reflex response induced by α,β-methylene ATP in occluded rats. The findings of this study suggest that the levels of P2X(3) in primary afferent neurons are upregulated as the blood supply to the hindlimb is deficient under ischemic conditions, leading to augmentation of the muscle reflex. NGF is closely related to increases in P2X(3) receptor expression and response.

Alteration in skeletal muscle afferents in rats with chronic heart failure

An exaggerated exercise pressor reflex (EPR) contributes to exercise intolerance and excessive sympatho-excitation in the chronic heart failure (CHF) state. However, the components of this reflex that are responsible for the exaggerated EPR in CHF remain unknown. To determine whether muscle afferent function is altered in CHF, we recorded the discharge of group III and IV afferents in response to static contraction, passive stretch and hindlimb intra-arterial injection of capsaicin in sham and CHF rats. We also investigated the roles of purinergic 2X receptor (P2X) and the transient receptor potential vanilloid 1 (VR1) in mediating the altered sensitivity of muscle afferents. Compared with sham rats, CHF rats exhibited greater responses of group III afferents to contraction and stretch whereas the responses of group IV afferents to contraction and capsaicin were blunted. Hindlimb intra-arterial infusion of pyridoxal phosphate-6-azophenyl-2,4-disulfonic acid (PPADS), a P2X antagonist, attenuated the responses of group III afferents to contraction and stretch in CHF rats to a greater extent than in sham rats. Western blot data showed that P2X3 receptors were significantly upregulated in doral root ganglion (DRG) of CHF rats whereas VR1 receptors were significantly downregulated. Immunohistochemical evidence showed that immunostaining of the P2X3 receptors was more intense in both IB4-positive (C-fibre) and NF200-positive (A-fibre) neurons in DRG of CHF rats whereas the immunostaining of the VR1 receptors was decreased in IB4-positive neurons. These data suggest that group III afferents are sensitized whereas group IV afferents are desensitized in CHF, which is related to the dysfunction of P2X and VR1 receptors.

Femoral artery occlusion increases expression of ASIC3 in dorsal root ganglion neurons

Acid-sensing ion channels (ASICs) in sensory nerves are responsive to increases in the levels of protons in the extracellular medium. Prior studies suggest that the muscle metabolite, lactic acid, plays a role in reflex sympathetic and cardiovascular responses via stimulation of thin muscle afferent nerves. Also, femoral artery occlusion augments the reflex sympathetic nerve response in rats. ASIC3 is a main subtype to appear in sensory nerves in mediating the response induced by increases in protons in the interstitial space of contracting muscles. Thus, in this article, we hypothesized that femoral occlusion increases the expression of ASIC3 in primary afferent neurons innervating muscles, and this contributes to the exaggerated reflex sympathetic responses. Femoral occlusion/vascular insufficiency of the hindlimb muscles was induced by the femoral artery ligation in rats. First, Western blot analysis shows that 24-72 h of femoral artery ligation significantly increased the expression of ASIC3 protein in dorsal root ganglion (optical density, 1.0 ± 0.07 in control vs. 1.65 ± 0.1 after 24 h of occlusion, P < 0.05; n = 6 in each group). There were no significant differences for increases in ASIC3 24 and 72 h postocclusion. Second, experiments using fluorescent immunohistochemistry and retrograde-labeling technique show that a greater percentage of ASIC3 staining neurons are localized in muscle-innervating dorsal root ganglion neurons after the arterial occlusion (78 ± 3% in 24 h post occlusion vs. 59 ± 5% in control, P < 0.05; n = 6 in each group). Third, the reflex responses in renal sympathetic nerve and arterial blood pressure induced by the stimulation of ASIC were examined after an injection of lactic acid into the arterial blood supply of hindlimb muscles of control rats and ligated rats. The results demonstrate that the sympathetic and pressor responses to lactic acid were significantly augmented after femoral occlusion compared with those in the control group. The data of this study suggest that enhanced ASIC3 expression in muscle afferent nerves contributes to the exaggerated reflex sympathetic and pressor responses to lactic acid as seen in arterial occlusion.

Mechanical stretch induces nerve sprouting in rat sympathetic neurocytes

Sympathetic nerve sprouting (SNS) has been shown to occur after myocardial infarction (MI) and heart failure (HF) and is known to be responsible for the development of lethal arrhythmias. During MI or HF intracardiac cells are exposed to increased mechanical stretch. Molecular mechanisms which trigger sympathetic neural growth are largely unknown. Therefore, this study aimed to investigate the impact of mechanical stretch on rat neonatal sympathetic neurocytes of the superior cervical ganglion (SCG). Mechanical stretch resulted in an increased growth of sympathetic neurocytes. Furthermore, we could demonstrate that SCG neurocytes express nerve growth factor (NGF), ciliary neurotrophic factor (CNTF), neurotrophin-3 (NT-3) and glial derived neurotrophic factor (GDNF) on mRNA and protein level. An increased NGF and CNTF expression, a down-regulated GDNF expression and an unchanged NT-3 expression were identified in the neurocyte cell culture supernatant of neurocytes exposed to mechanical stretch. However, neither brain derived neurotrophic factor (BDNF) mRNA and protein was expressed in SCG neurocytes, nor BDNF could be detected in the cell culture supernatant of SCG neurons. By anti-neurotrophin neutralizing experiments NGF and CNTF were identified as important stretch-induced growth-inducing factors. Losartan, an angiotensin-II type 1 receptor inhibitor, abolished the stretch-induced increase of NGF and CNTF expression and thereby prevented the stretch-induced neural growth. This study provides new molecular mechanisms by which the inhibitory effect of angiotensin-II type 1 receptor blockers on the neural/arrhythmogenic remodeling can be explained. However, further in-vivo studies are required to address this important issue.

Interstitial adenosine triphosphate modulates muscle afferent nerve-mediated pressor reflex

Previous work has shown that muscle contraction elevates interstitial adenosine triphosphate concentration ([ATP]i), which is likely due to the release of ATP from active skeletal muscle. ATP activation of purinergic receptors P2X on thin muscle afferent fibers further enhances cardiovascular responses to contraction. Thus, the purposes of this study were: (1) to examine the mechanisms by which ATP is released from muscle in response to mechanical stimulation; and (2) to study the effects of interstitial ATP concentrations on modulating pressor response to muscle contraction. Static contraction of the triceps surae muscle was evoked by electrical stimulation (at 5 HZ and 2.5 times motor threshold) of the tibial nerve in 9 anesthetized cats. Muscle interstitial ATP samples were collected from microdialysis probes inserted into the muscles. Dialysate ATP concentrations were determined using the luciferin-luciferase assay. In a control experiment, contraction was induced after 0.5 ml of saline was injected into the arterial blood supply of the hindlimb muscles. This increased [ATP]i by 220% (P < 0.05 vs. baseline). After gadolinium (1 mM), a blocker of mechanically sensitive channels, was injected into the muscles, contraction increased [ATP]i by 112% (P < 0.05 vs. control). In contrast, glibenclamide (an inhibitor of the ATP-binding cassette protein), monensin, and brefeldin A, which interfere with vesicular formation (or trafficking) and inhibit exocytosis, did not significantly affect ATP release by muscle contraction. In addition, a regression analysis showed that [ATP]i was linearly related to the pressor response to muscle contraction. The data suggest that ATP release from skeletal muscle is mediated via involvement of mechanosensitive channels. These findings further support a physiological role for release of ATP in modulating cardiovascular responses during static muscle contraction.

Sympathetic nerve responses to muscle contraction and stretch in ischemic heart failure

Congestive heart failure (CHF) induces abnormal regulation of peripheral blood flow during exercise. Previous studies have suggested that a reflex from contracting muscle is disordered in this disease. However, there has been very little investigation of the muscle reflex regulating sympathetic outflows in CHF. Myocardial infarction (MI) was induced by the coronary artery ligation in rats. Echocardiography was performed to determine fractional shortening (FS), an index of the left ventricular function. We examined renal and lumbar sympathetic nerve activities (RSNA and LSNA, respectively) during 1-min repetitive (1- to 4-s stimulation to relaxation) contraction or stretch of the triceps surae muscles. During these interventions, the RSNA and LSNA responded synchronously as tension was developed. The RSNA and LSNA responses to contraction were significantly greater in MI rats (n = 13) with FS <30% than in control animals (n = 13) with FS >40% (RSNA: +49 +/- 7 vs. +19 +/- 4 a.u., P < 0.01; LSNA: +28 +/- 7 vs. +8 +/- 2 a.u., P < 0.01) at the same tension development. Stretch also increased the RSNA and LSNA to a larger degree in MI (n = 13) than in control animals (n = 13) (RSNA: +36 +/- 6 vs. +19 +/- 3 a.u., P < 0.05; LSNA: +24 +/- 3 vs. +9 +/- 2 a.u., P < 0.01). The data demonstrate that CHF exaggerates sympathetic nerve responses to muscle contraction as well as stretch. We suggest that muscle afferent-mediated sympathetic outflows contribute to the abnormal regulation of peripheral blood flow seen during exercise in CHF.

Effect of muscle interstitial pH on P2X and TRPV1 receptor-mediated pressor response

Activation of purinergic P2X receptors and transient receptor potential vanilloid type 1 (TRPV1) on muscle afferent nerve evokes the pressor response. Because P2X and TRPV1 receptors are sensitive to changes in pH, the aim of this study was to examine the effects of muscle acidification on those receptor-mediated cardiovascular responses. In decerebrate rats, the pH in the hindlimb muscle was adjusted by infusing acidic Ringer solutions into the femoral artery. Dialysate was then collected using microdialysis probes inserted into the muscles, and pH was measured. The interstitial pH was 7.53+/-0.01, 7.22+/-0.02, 6.94+/-0.04, and 6.59+/-0.03 in response to arterial infusion of the Ringer solution at pH 7.4, 6.5, 5.5, and 4.5, respectively. Femoral arterial injection of alpha,beta-methylene-ATP (P2X receptor agonist) in the concentration of 0.25 mM (volume, 0.15-0.25 ml; injection duration, 1 min) at the infused pH of 7.4, 6.5, and 5.5 increased mean arterial pressure (MAP) by 29+/-2, 24+/-3, and 21+/-3 mmHg, respectively (P<0.05, pH 5.5 vs. pH 7.4). When pH levels in the infused solution were 7.4, 6.5, 5.5, and 4.5, capsaicin (1 microg/kg), a TRPV1 agonist, was injected into the artery. This elevated MAP by 29+/-4, 33+/-2, 35+/-3, and 40+/-3 mmHg, respectively (P<0.05, pH 4.5 vs. pH 7.4). Furthermore, blocking acid-sensing ion channel (ASIC) blunted pH effects on TRPV1 response. Our data indicate that 1) muscle acidosis attenuates P2X-mediated pressor response but enhances TRPV1 response; 2) exaggerated TRPV1 response may require lower pH in muscle, and the effect is likely to be mediated via ASIC mechanisms. This study provides evidence that muscle pH may be important in modulating P2X and TRPV1 responsiveness in exercising muscle.