Exercise Pressor Reflex In Rats Research Articles

Blocking Interleukin‐6 Signal Pathway Attenuates Exaggerated Exercise Pressor Reflex in Rats with Femoral Artery Occlusion

A rat model of femoral artery occlusion was employed to study the exercise pressor reflex in peripheral artery disease. Prior studies have shown that pro‐inflammatory interleukin‐6 (IL‐6) is engaged in the amplified blood pressure response to static muscle contraction in occluded rats. IL‐6 complexes with soluble IL‐6 receptor (IL‐6R) to activate cells expressing the signal transducer glycoprotein (gp130). Thus, the current study was to determine the role played by IL‐6 activated signal complex (IL‐6‐gp130) in regulating the exercise pressor reflex in rats with femoral occlusion. Following 72 hours of femoral artery occlusion, the levels of protein expression of IL‐6R and gp130 were increased in dorsal root ganglion (DRG). In addition, we examined the pressor response to static muscle contraction after 62.5 ng/kg and 125 ng/kg of SC144 (a gp130 inhibitor) were injected into the arterial blood supply of the hindlimb muscle of control rats and occluded rats. We found that SC144 significantly attenuated the pressor response to contraction in occluded rats, but the effect of SC144 was not observed in control rats. In control rats, peak response of mean arterial pressure (MAP) was 15±2 mmHg after saline and 12±3 mmHg after 125 ng/kg of SC144 (n=6; P > 0.05 vs. saline). In occluded rats, peak MAP response was 25±5 mmHg after saline and 14±5 mmHg after 125 ng/kg of SC144 (n=6; P < 0.05 vs. saline). There was no difference in developed muscle tension between saline group and group injected with SC144 (528±28 g in saline control vs. 545±29 g with SC144 injection, P > 0.05). Overall, our data suggest that IL‐6 signal mechanisms in muscle sensory nerves contribute to the exaggerated sympathetic and pressor responsiveness observed during exercise in experimental rats with peripheral artery disease.Support or Funding InformationThis study was supported by NIH P01 HL134609This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Peripheral µ-opioid receptors attenuate the responses of group III and IV afferents to contraction in rats with simulated peripheral artery disease.

Patients with peripheral artery disease show an exaggerated pressor response to mild exercise, an effect attributable to the exercise pressor reflex, whose afferent arm comprises the thinly myelinated group III and unmyelinated group IV afferents. Previously, we found that DAMGO, a µ-opioid agonist injected into the femoral artery, attenuated the exaggerated exercise pressor reflex in rats with ligated femoral arteries, a preparation that simulates the blood flow patterns to muscle that is seen in patients with peripheral artery disease. Continuing this line of investigation, we recorded the responses of group III and IV afferents to static contraction before and after injecting DAMGO (1 µg) into the superficial epigastric artery in rats with patent femoral arteries and in rats with ligated femoral arteries. In rats with patent arteries, DAMGO did not change the responses to contraction of either group III ( n = 9; P = 0.83) or group IV ( n = 8; P = 0.34) afferents. In contrast, in rats with ligated femoral arteries, DAMGO injection (1 µg) significantly decreased the responses to contraction of both group III afferents ( n = 9, P < 0.01) and group IV afferents ( n = 9; P < 0.01). DAMGO did not significantly attenuate the responses of either group III or IV afferents to capsaicin in rats with either patent or ligated femoral arteries. These findings are in agreement with our previous studies that showed that peripheral DAMGO injection attenuated the exercise pressor reflex in rats with ligated femoral arteries but had only a modest effect on the exercise pressor reflex in rats with patent femoral arteries. NEW & NOTEWORTHY In an animal model of peripheral artery disease, we show that the µ-opioid agonist, DAMGO reduces the afferent response rate resulting from stimulated static contraction. These results suggest that peripherally active opioid agonists that do not cross the blood-brain barrier may be therapeutic for treatment of peripheral artery disease without the negative and addictive side effects associated with opioids in the central nervous system.

Local Injections of Superoxide Dismutase Attenuate the Exercise Pressor Reflex in Rats with Femoral Artery Occlusion.

The exercise pressor reflex is amplified in patients with peripheral artery disease (PAD) and in an experimental PAD model of rats induced by femoral artery occlusion. Heightened blood pressure worsens the restricted blood flow directed to the limbs in this disease. The purpose of this study was to determine the role played by muscle oxidative stress in regulating the augmented pressor response to static exercise in PAD. We hypothesized that limb ischemia impairs muscle superoxide dismutase (SOD) thereby leading to abnormal autonomic responsiveness observed in PAD animals, and a chronic compensation of SOD for anti-oxidation improves the exaggerated exercise pressor reflex. Our data show that femoral occlusion decreased the protein levels of SOD in ischemic muscle as compared with control muscle. Downregulation of SOD appeared to a greater degree in the oxidative (red) muscle than in the glycolytic (white) muscle under the condition of muscle ischemia. In addition, the exercise pressor response was assessed during electrically induced static contraction. The data demonstrates that the enhancement of the exercise pressor reflex was significantly attenuated after tempol (a mimetic of SOD, 30 mg over a period of 72 h) was administered into the occluded hindlimb. In the occluded rats, mean arterial pressure (MAP) response was 26 ± 3 mmHg with no tempol and 12 ± 2 mmHg with tempol application (P < 0.05 vs. group with no tempol; n = 6 in each group). There were no differences in muscle tension development (time-tension index: 12.1 ± 1.2 kgs with no tempol and 13.5 ± 1.1 kgs with tempol; P > 0.05 between groups). In conclusion, SOD is lessened in the ischemic muscles and supplement of SOD improves the amplified exercise pressor reflex, which is likely beneficial to the restricted blood flow to the limbs in PAD.

Effects of Type 1 Diabetes on the Exercise Pressor Reflex in Rats

Type 1 diabetes (T1DM) is known to cause both peripheral and autonomic neuropathy through mechanisms affecting thinly myelinated afferents. These same afferents may also play a role in the exercise pressor reflex suggesting that T1DM could alter this reflex as well. The purpose of this study was to establish the effects of type 1 diabetes on the pressor and cardioaccelerator responses evoked by statically contracting the hindlimb muscles at different time points of the disease and identify possible mechanisms responsible for these changes. We injected (i.p.) 50 mg/kg of Streptozotocin (STZ) or the vehicle (CTL) in either sex and waited 1wk (STZ: BW=267±12 g, glucose=438±24 mg/dL, HbA1C=6.5±0.3%; CTL: BW=328±16 g, glucose=167±10 mg/dL, HbA1C=4.4±0.1%), 3wks (STZ: BW=293±10 g, glucose=465±22 mg/dL, HbA1C=10±0.3%; CTL: BW=329±22 g, glucose=212±26 mg/dL, HbA1C=4.8±0.4%), or 6wks (STZ: BW=380±16 g, glucose=546±35 mg/dL, HbA1C=12±0.4%) before performing experiments. In unanaesthetized, decerebrate rats, we statically contracted the hindlimb muscles for 30s and measured changes in mean arterial pressure (MAP) and heart rate (HR). We then injected either α,β‐methylene ATP (20μg/kg; 200μl), a P2X receptor agonist, or lactic acid (24mM; 200μl), an ASIC3 channel agonist, into the arterial supply of the hindlimb and measured the evoked pressor and cardioaccelerator responses for each group. We found that the pressor (STZ: ΔMAP=25±5 mmHg, n=4; CTL: ΔMAP=13±2 mmHg, n=7; p=0.01) but not cardioaccelerator (STZ: ΔHR=23±9 bpm, n=4; CTL: ΔHR=13±3 bpm, n=7; p=0.10) response to static contraction was exaggerated 1wk after injecting STZ. Likewise, the pressor (STZ: ΔMAP=32±5 mmHg, n=8; CTL: ΔMAP=17±3 mmHg, n=6; p=0.02) but not cardioaccelerator (STZ: ΔHR=22±5 bpm, n=8; CTL: ΔHR=15±2 bpm, n=6; p=0.27) response to static contraction was exaggerated 3wks after injecting STZ. Conversely, both the pressor (6wks STZ: ΔMAP=5±2 mmHg, n=7) and cardioaccelerator (6wks STZ: ΔHR=1±2 bpm, n=7) responses to static contraction were significantly attenuated 6wks after injecting STZ compared to 1wk (MAP: p=0.02; HR: p=0.01) and 3wks after injecting STZ (MAP: p&lt;0.01; HR: p&lt;0.01). The developed tensions from contraction were similar within each comparison. Furthermore, we found that injecting lactic acid, but not α,β‐methylene ATP, into the arterial supply of the hindlimb evoked a greater pressor (STZ: ΔMAP=31±5 mmHg, n=9; CTL: ΔMAP=20±3 mmHg, n=13; p=0.04) and cardioaccelerator (STZ: ΔHR=13±3 bpm, n=9; CTL: ΔHR=3±2 bpm, n=13; p&lt;0.01) response 1wk after STZ injection compared to CTL. We conclude that the exercise pressor reflex is exaggerated in the early stages of T1DM but attenuated in the late stages of this disease. Additionally, ASIC3 channels but not P2X receptors may play a role in this exaggerated reflex 1wk after STZ injection.

Proteinase-Activated Receptor-2 Sensitivity of Amplified TRPA1 Activity in Skeletal Muscle Afferent Nerves and Exercise Pressor Reflex in Rats with Femoral Artery Occlusion

Background/Aims: Limb ischemia occurs in peripheral artery disease (PAD). Sympathetic nerve activity (SNA) that regulates blood flow directed to the ischemic limb is exaggerated during exercise in this disease, and transient receptor potential channel A1 (TRPA1) in thin-fiber muscle afferents contributes to the amplified sympathetic response. The purpose of the present study was to determine the role of proteinase-activated receptor-2 (PAR2) in regulating abnormal TRPA1 function and the TRPA1-mediated sympathetic component of the exercise pressor reflex. Methods: A rat model of femoral artery ligation was employed to study PAD. Dorsal root ganglion (DRG) tissues were obtained to examine the protein levels of PAR2 using western blot analysis. Current responses induced by activation of TRPA1 in skeletal muscle DRG neurons were characterized using whole-cell patch clamp methods. The blood pressure response to static exercise (i.e., muscle contraction) and stimulation of TRPA1 was also examined after a blockade of PAR2. Results: The expression of PAR2 was amplified in DRG neurons of the occluded limb, and PAR2 activation with SL-NH2 (a PAR2 agonist) increased the amplitude of TRPA1 currents to a greater degree in DRG neurons of the occluded limb. Moreover, FSLLRY-NH2 (a PAR antagonist) injected into the arterial blood supply of the hindlimb muscles significantly attenuated the pressor response to muscle contraction and TRPA1 stimulation in rats with occluded limbs. Conclusions: The PAR2 signal in muscle sensory nerves contributes to the amplified exercise pressor reflex via TRPA1 mechanisms in rats with femoral artery ligation. These findings provide a pathophysiological basis for autonomic responses during exercise activity in PAD, which may potentially aid in the development of therapeutic approaches for improvement of blood flow in this disease.

The mechano-gated channel inhibitor GsMTx4 reduces the exercise pressor reflex in decerebrate rats.

Mechanical and metabolic stimuli from contracting muscles evoke reflex increases in blood pressure, heart rate and sympathetic nerve activity. Little is known, however, about the nature of the mechano-gated channels on the thin fibre muscle afferents that contribute to evoke this reflex, termed the exercise pressor reflex. We determined the effect of GsMTx4, an inhibitor of mechano-gated Piezo channels, on the exercise pressor reflex evoked by intermittent contraction of the triceps surae muscles in decerebrated, unanaesthetized rats. GsMTx4 reduced the pressor, cardioaccelerator and renal sympathetic nerve responses to intermittent contraction but did not reduce the pressor responses to femoral arterial injection of compounds that stimulate the metabolically-sensitive thin fibre muscle afferents. Expression levels of Piezo2 channels were greater than Piezo1 channels in rat dorsal root ganglia. Our findings suggest that mechanically-sensitive Piezo proteins contribute to the generation of the mechanical component of the exercise pressor reflex in rats. Mechanical and metabolic stimuli within contracting skeletal muscles evoke reflex autonomic and cardiovascular adjustments. In cats and rats, gadolinium has been used to investigate the role played by the mechanical component of this reflex, termed the exercise pressor reflex. Gadolinium, however, has poor selectivity for mechano-gated channels and exerts multiple off-target effects. We tested the hypothesis that GsMTX4, a more selective mechano-gated channel inhibitor than gadolinium and a particularly potent inhibitor of mechano-gated Piezo channels, reduced the exercise pressor reflex in decerebrate rats. Injection of 10 μg of GsMTx4 into the arterial supply of the hindlimb reduced the peak pressor (control: 24 ± 5, GsMTx4: 12 ± 5 mmHg, P < 0.01), cardioaccelerator and renal sympathetic nerve responses to tendon stretch, a purely mechanical stimulus, but had no effect on the pressor responses to intra-arterial injection of α,β-methylene ATP or lactic acid. Moreover, injection of 10 μg of GsMTx4 into the arterial supply of the hindlimb reduced the peak pressor (control: 24 ± 2, GsMTx4: 14 ± 3 mmHg, P < 0.01), cardioaccelerator and renal sympathetic nerve responses to electrically-induced intermittent hindlimb muscle contractions. By contrast, injection of 10 μg of GsMTx4 into the jugular vein had no effect on the pressor, cardioaccelerator, or renal sympathetic nerve responses to contraction. Quantitative RT-PCR and western blot analyses indicated that both Piezo1 and Piezo2 channel isoforms were natively expressed in rat dorsal root ganglia tissue. We conclude that GsMTx4 reduced the exercise pressor reflex in decerebrate rats and that the reduction was attributable, at least in part, to its effect on mechano-gated Piezo channels.

Combined, but not individual, blockade of ASIC3, P2X, and EP4 receptors attenuates the exercise pressor reflex in rats with freely perfused hindlimb muscles.

In healthy humans, tests of the hypothesis that lactic acid, PGE2, or ATP plays a role in evoking the exercise pressor reflex proved controversial. The findings in humans resembled ours in decerebrate rats that individual blockade of the receptors to lactic acid, PGE2, and ATP had only small effects on the exercise pressor reflex provided that the muscles were freely perfused. This similarity between humans and rats prompted us to test the hypothesis that in rats with freely perfused muscles combined receptor blockade is required to attenuate the exercise pressor reflex. We first compared the reflex before and after injecting either PPADS (10 mg/kg), a P2X receptor antagonist, APETx2 (100 μg/kg), an activating acid-sensing ion channel 3 (ASIC) channel antagonist, or L161982 (2 μg/kg), an EP4 receptor antagonist, into the arterial supply of the hindlimb of decerebrated rats. We then examined the effects of combined blockade of P2X receptors, ASIC3 channels, and EP4 receptors on the exercise pressor reflex using the same doses, intra-arterial route, and time course of antagonist injections as those used for individual blockade. We found that neither PPADS (n = 5), APETx2 (n = 6), nor L161982 (n = 6) attenuated the reflex. In contrast, combined blockade of these receptors (n = 7) attenuated the peak (↓27%, P < 0.019) and integrated (↓48%, P < 0.004) pressor components of the reflex. Combined blockade injected intravenously had no effect on the reflex. We conclude that combined blockade of P2X receptors, ASIC3 channels, and EP4 receptors on the endings of thin fiber muscle afferents is required to attenuate the exercise pressor reflex in rats with freely perfused hindlimbs.

Aldosterone and Salt Loading Independently Exacerbate the Exercise Pressor Reflex in Rats.

The sympathetic and pressor responses to exercise are exaggerated in hypertension. Evidence suggests that an overactive exercise pressor reflex (EPR) contributes to this abnormal responsiveness. The mechanisms underlying this EPR overactivity are poorly understood. An increasing body of evidence suggests that aldosterone and excessive salt intake play a role in regulating resting sympathetic activity and blood pressure in hypertension. Therefore, each is a good candidate for the generation of EPR dysfunction in this disease. The purpose of this study was to examine whether excessive salt intake and chronic administration of aldosterone potentiate EPR function. Changes in mean arterial pressure and renal sympathetic nerve activity induced by EPR stimulation were examined in vehicle and aldosterone-treated (4 weeks via osmotic mini-pump) Sprague-Dawley rats given either water or saline (elevated salt load) to drink. When compared with vehicle/water-treated rats, stimulation of the EPR by muscle contraction evoked significantly greater increases in mean arterial pressure in vehicle/saline, aldosterone/water, and aldosterone/saline-treated animals (14±3, 29±3, 37±6, and 44±7 mm Hg/kg, respectively; P<0.01). A similar renal sympathetic nerve activity response profile was likewise produced (39±11%, 87±15%, 110±20%, and 151±25%/kg, respectively; P<0.01). The pressor and sympathetic responses to the individual activation of the mechanically and chemically sensitive components of the EPR were also augmented by both saline and aldosterone. These data provide the first direct evidence that both aldosterone and high salt intake elicit EPR overactivity. As such, each represents a potential mechanism by which sympathetic activity and blood pressure are augmented during exercise in hypertension.

Role played by interleukin-6 in evoking the exercise pressor reflex in decerebrate rats: effect of femoral artery ligation.

IL-6 signaling via the soluble IL-6 receptor (sIL-6r) has been shown to increase primary afferent responsiveness to noxious stimuli. This finding prompted us to test the hypothesis that IL-6 and sIL-6r would increase the exercise pressor reflex in decerebrate rats with freely perfused femoral arteries. We also tested the hypothesis that soluble glycoprotein (sgp)130, an inhibitor of IL-6/sIL-6r signaling, would decrease the exaggerated exercise pressor reflex that is found in decerebrate rats with ligated femoral arteries. In rats with freely perfused femoral arteries, coinjection of 50 ng of IL-6 and sIL-6r into the arterial supply of the hindlimb significantly increased the peak pressor response to static (control: 14 ± 3 mmHg and IL-6/sIL-6r: 17 ± 2 mmHg, P = 0.03) and intermittent isometric (control: 10 ± 2 mmHg and IL-6/sIL-6r: 15 ± 4 mmHg, P = 0.03) hindlimb muscle contraction. In rats with ligated femoral arteries, injection of 50 ng of sgp130 into the arterial supply of the hindlimb reduced the peak pressor response to static (control: 24 ± 2 mmHg and sgp130: 16 ± 3 mmHg, P = 0.01) and intermittent isometric (control: 16 ± 2 mmHg and sgp130: 13 ± 2 mmHg, P = 0.04) hindlimb muscle contraction, whereas there was no effect of sgp130 on the exercise pressor reflex in rats with freely perfused femoral arteries. We conclude that coinjection of exogenous IL-6 and sIL-6r increased the exercise pressor reflex in rats with freely perfused femoral arteries. More importantly, we also conclude that IL-6 and sIL-6r play an endogenous role in evoking the exercise pressor reflex in rats with ligated femoral arteries but not in rats with freely perfused femoral arteries.

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.

Purinergic 2X receptors play a role in evoking the exercise pressor reflex in rats with peripheral artery insufficiency (1165.6)

Purinergic 2X (P2X) receptors on the endings of thin fiber afferents have been shown to play a role in evoking the exercise pressor reflex in cats. In this study, we attempted to extend this finding to decerebrated, unanesthetized rats whose femoral arteries were either freely perfused or were ligated 72 hours before the start of the experiment. We first established that our dose of PPADS (10 mg/ kg), a P2X receptor antagonist, attenuated the pressor response to α, β methylene ATP (10 µg/ kg), a P2X receptor agonist. We then compared the exercise pressor reflex before and after infusing PPADS into the arterial supply of the hind limb muscles that were statically contracted. In rats with freely perfused femoral arteries, the peak pressor responses to contraction were not significantly attenuated by PPADS (before PPADS: 19±2mmHg, 13 min after PPADS: 17±2mmHg, and 25 min after PPADS: 17±3mmHg). Likewise, the cardioaccelerator and renal sympathetic nerve responses were not significantly attenuated. In contrast, we found that in rats whose femoral arteries were ligated, PPADS significantly attenuated the peak pressor responses to contraction (before PPADS: 37±5mmHg, 13 min after PPADS: 27±6mmHg, 25 min after PPADS: 25±5mmHg; p&lt;0.05). Heart rate was not significantly attenuated, but renal SNA was at certain time points over the 30s contraction period. We conclude that P2X receptors play a role in evoking the exercise pressor reflex in rats whose femoral arteries were ligated, but do not play a role in evoking the reflex in rats whose femoral arteries were freely perfused.Grant Funding Source: Supported by NIH grants HL‐096570 and AR‐059397

High salt intake and aldosterone independently augment the exercise pressor reflex in rats (1165.10)

Evidence suggests the exercise pressor reflex (EPR) is overactive in hypertension. However, the underlying mechanisms are poorly understood. The purpose of this study was to determine whether excessive salt intake and/or chronic administration of aldosterone potentiates EPR function in rats. Changes in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) to stimulation of the EPR were assessed in vehicle and aldosterone treated (4 weeks via osmotic mini‐pump) Sprague‐Dawley rats given either water or saline (elevated salt load) to drink. Compared to vehicle/water‐treated rats, contraction induced stimulation of the EPR evoked significantly greater increases in RSNA in vehicle/saline, aldosterone/water and aldosterone/saline‐treated animals (39±11, 98±19, 110±20, and 154±29%/kg, respectively, P&lt;0.01). A similar MAP response profile was likewise produced. The sympathetic and pressor responses to passive muscle stretch and intra‐arterial capsaicin administration were also augmented in the vehicle/saline group and aldosterone treated rats, suggesting both mechano‐ and metaboreflex sensitization. In conclusion, our data provide the first direct evidence for both high salt intake and aldosterone as major contributors to the neural mechanisms underlying the exaggerated rise in sympathetic activity and blood pressure during exercise in hypertension. Supported by NIH HL‐113738Grant Funding Source: Supported by NIH HL‐113738

Purinergic 2X receptors play a role in evoking the exercise pressor reflex in rats with peripheral artery insufficiency

Purinergic 2X (P2X) receptors on the endings of thin fiber afferents have been shown to play a role in evoking the exercise pressor reflex in cats. In this study, we attempted to extend this finding to decerebrated, unanesthetized rats whose femoral arteries were either freely perfused or were ligated 72 h before the start of the experiment. We first established that our dose of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 10 mg/kg), a P2X receptor antagonist, attenuated the pressor response to α,β-methylene ATP (10 μg/kg), a P2X receptor agonist. We then compared the exercise pressor reflex before and after infusing PPADS into the arterial supply of the hindlimb muscles that were statically contracted. In rats with freely perfused femoral arteries, the peak pressor responses to contraction were not significantly attenuated by PPADS (before PPADS: 19 ± 2 mmHg, 13 min after PPADS: 17 ± 2 mmHg, and 25 min after PPADS: 17 ± 3 mmHg). Likewise, the cardioaccelerator and renal sympathetic nerve responses were not significantly attenuated. In contrast, we found that in rats whose femoral arteries were ligated PPADS significantly attenuated the peak pressor responses to contraction (before PPADS: 37 ± 5 mmHg, 13 min after PPADS: 27 ± 6 mmHg, and 25 min after PPADS: 25 ± 5 mmHg; P < 0.05). Heart rate was not significantly attenuated, but renal SNA was at certain time points over the 30-s contraction period. We conclude that P2X receptors play a substantial role in evoking the exercise pressor reflex in rats whose femoral arteries were ligated but play only a minimal role in evoking the reflex in rats whose femoral arteries were freely perfused.

Endoperoxide 4 receptors play a role in evoking the exercise pressor reflex in rats with simulated peripheral artery disease

Ligating the femoral artery for 72 h in decerebrated rats exaggerates the exercise pressor reflex. The sensory arm of this reflex is comprised of group III and IV afferents, which can be either sensitized or stimulated by PGE2. In vitro studies showed that endoperoxide (EP) 3 and 4 receptors were responsible for the PGE2-induced sensitization of rat dorsal root ganglion cells. This in vitro finding prompted us to test the hypothesis that blockade of EP3 and/or EP4 receptors attenuated the exaggerated exercise pressor reflex in rats with ligated femoral arteries. We measured the cardiovascular responses to static hindlimb contraction or tendon stretch before and after femoral arterial injection of L798106 (an EP3 antagonist) or L161982 (an EP4 antagonist). The pressor and cardioaccelerator responses to either contraction or tendon stretch were not attenuated by L798106 in either the ligated or freely perfused rats. Likewise in five rats whose hindlimb muscles were freely perfused, the pressor and cardioaccelerator responses to either contraction or tendon stretch were not attenuated by L161982. In the six ligated rats, however, the pressor response to contraction was attenuated by L161982, averaging 37 ± 3 mmHg before, 18 ± 2 mmHg afterward (P < 0.05). Western blotting analysis revealed that ligation of the femoral artery for 72 h increased the EP4 receptor protein in the L4 and L5 dorsal root ganglia over their freely perfused counterparts by 24% (P < 0.05). We conclude that EP4 receptors, but not EP3 receptors, play an important role in the exaggerated exercise pressor reflex found in rats with ligated femoral arteries.

Endoperoxide 4 receptors attenuate the exercise pressor reflex in rats with ligated femoral arteries

Ligating the femoral artery for 72 hours in in decerebrated rats exaggerates the exercise pressor response (EPR).The sensory arm of this reflex is comprised of group III and IV afferents, which can be either sensitized or stimulated by PGE2. Endoperoxide (EP) 3 and EP4 (PGE2 subtype) receptors were found to be responsible for the PGE2‐induced sensitization of rat dorsal root ganglion cells. We tested the hypothesis that L798106 (EP3 antagonist) or L161982 (EP4 antagonist) attenuated the exaggerated exercise pressor reflex in rats with ligated femoral arteries. We measured the cardiovascular responses to static hindlimb contraction or tendon stretch before and after iliac arterial injection of L798106 or L161982. The pressor and cardioaccelerator responses to either contraction or tendon stretch were not attenuated by L798106 in either the ligated or freely perfused rats. Likewise in five rats whose hindlimb muscles were freely perfused, the pressor and cardioaccelerator responses to either contraction or tendon stretch were not attenuated by L161982. In the five ligated rats, however, the pressor response to contraction was attenuated by L161982, averaging 37 ± 3 mmHg before, 18 ± 2mmHg afterward (p &lt; 0.05). In addition, western blots revealed that ligation of the femoral artery for 72 hours increased the EP4 receptor protein in the L4 and L5 dorsal root ganglion by 24% (p &lt; 0.05) over their freely perfused counterparts. We conclude that blockade of EP4 receptors, but not blockade of EP3 receptors, attenuated the EPR in rats with ligated femoral arteries.

Glibenclamide prevents the attenuation of the exercise pressor reflex by tempol in the ligated rats

In decerebrated rats, ligation of a femoral artery has been shown to exaggerate the exercise pressor reflex, an effect which is attenuated by tempol. The action of tempol has been attributed to its ability to scavenge superoxide radicals. Tempol, in addition, opens ATP sensitive potassium channels, an effect which could also explain its ability to attenuate the exercise pressor reflex in rats with ligated femoral arteries. To test this alternative explanation, we measured the effect of femoral arterial injection of glibenclamide (0.1 mg/kg), which blocks ATP sensitive potassium channels, on the tempol‐induced attenuation of the exercise pressor reflex in decerebrated rats whose femoral arteries had been occluded 72 hours before the start of the experiment. In the first group of rats (n=6), which were not given glibenclamide, tempol attenuated the exercise pressor reflex from 36 ± 5 to 15 ± 2 mmHg (P&lt; 0.05). In the second group of rats (n=8), which were given glibenclamide just before tempol, the superoxide scavenger had no effect on the exercise pressor reflex, which averaged 28 ± 2 mmHg before tempol and 26 ± 2 mmHg afterwards (P&gt; 0.05). We conclude that tempol attenuated the exercise pressor reflex by opening ATP sensitive potassium channels on the endings of group III and IV muscle afferents. Tempol does not appear to attenuate the exaggerated exercise pressor reflex by scavenging superoxide radicals.

Blockade of acid sensing ion channels attenuates the augmented exercise pressor reflex in rats with chronic femoral artery occlusion

We found previously that static contraction of the hindlimb muscles of rats whose femoral artery was ligated evoked a larger reflex pressor response (i.e. exercise pressor reflex) than did static contraction of the contralateral hindlimb muscles which were freely perfused. Ligating a femoral artery in rats results in blood flow patterns to the muscles that are remarkably similar to those displayed by humans with peripheral artery disease. Using decerebrated rats, we tested the hypothesis that the augmented exercise pressor reflex in rats with a ligated femoral artery is attenuated by blockade of the acid sensing ion channel (ASIC) 3. We found that femoral arterial injection of either amiloride (5 and 50 μg kg(-1)) or APETx2 (100 μg kg(-1)) markedly attenuated the reflex in rats with a ligated femoral artery. In contrast, these ASIC antagonists had only modest effects on the reflex in rats with freely perfused hindlimbs. Tests of specificity of the two antagonists revealed that the low dose of amiloride and APETx2 greatly attenuated the pressor response to lactic acid, an ASIC agonist, but did not attenuate the pressor response to capsaicin, a TRPV1 agonist. In contrast, the high dose of amiloride attenuated the pressor responses to lactic acid, but also attenuated the pressor response to capsaicin. We conclude that ASIC3 on thin fibre muscle afferents plays an important role in evoking the exercise pressor reflex in rats with a compromised arterial blood supply to the working muscles.

Blockade of acid sensing ion channels attenuates the augmented exercise pressor reflex in rats with chronic femoral artery occlusion

In decerebrated rats, we reported previously that the pressor reflex arising from a limb whose femoral artery was occluded for 72 h before contraction was significantly higher than the pressor reflex arising from a contralateral freely perfused limb. Moreover, increased levels of acid sensing ion channel 3 (ASIC3) on muscle afferent neurons have been shown in rats whose femoral artery was occluded for 24 to 72 hours. These findings prompted us to examine whether blockade of acid sensing ion channels, especially ASIC3, contributed to the augmented pressor reflex evoked by static contraction in rats with chronic femoral artery occlusion. We found that the pressor reflex arising from the limb whose femoral artery was occluded for 72 h before contraction (32 ± 5 mmHg, n=9) was significantly attenuated by a non-selective ASIC antagonist amiloride (15 ± 3 mmHg, n=9, P=0.01) injected into abdominal aorta (50 μg/kg). Intravenous injection of this dose of amiloride had no effect on the pressor reflex. Likewise, the selective ASIC3 antagonist, APETx2 (100μg/kg), significantly attenuated the pressor response to static contraction in rats whose femoral artery was occluded for 72h before the start of contraction (before: 37 ± 6 mmHg, after: 18 ± 4 mmHg, n=4, P=0.008). We conclude that ASICs on muscle afferents are responsible for the augmented pressor reflex seen in rats with chronic femoral arterial occlusion.

Dorsal root tetrodotoxin-resistant sodium channels do not contribute to the augmented exercise pressor reflex in rats with chronic femoral artery occlusion

We investigated the contribution of tetrodotoxin (TTX)-resistant sodium channels to the augmented exercise pressor reflex observed in decerebrated rats with femoral artery ligation. The pressor responses to static contraction, to tendon stretch, and to electrical stimulation of the tibial nerve were compared before and after blocking TTX-sensitive sodium channels on the L3-L6 dorsal roots of rats whose hindlimbs were freely perfused and rats whose femoral arteries were ligated 72 h before the start of the experiment. In the freely perfused group (n=9), pressor (Δ22±4 mmHg) and cardioaccelerator (Δ32±6 beats/min) responses to contraction were attenuated by 1 μM TTX (Δ4±1 mmHg, P<0.05 and Δ17±4 beats/min, P<0.05, respectively). In the 72 h ligated group (n=9), the augmented pressor response to contraction (32±4 mmHg) was also attenuated by 1 μM TTX (Δ8±2 mmHg, P<0.05). The cardioaccelerator response to contraction was not significantly attenuated in these rats. In addition, TTX suppressed the pressor response to tendon stretch in both groups of rats. Electrical stimulation of the tibial nerve evoked similar pressor responses between the two groups (freely perfused: Δ74±9 mmHg and 72 h ligated: Δ78±5 mmHg). TTX attenuated the pressor response to the tibial nerve stimulation by about one-half in both groups. Application of the TTX-resistant sodium channel blocker A-803467 (1 μM) with TTX (1 μM) did not block the pressor response to tibial nerve stimulation to any greater extent than did application of TTX (1 μM) alone. Although the contribution of TTX-resistant sodium channels to the augmented exercise pressor reflex may be slightly increased in rats with chronic femoral artery ligation, TTX-resistant sodium channels on dorsal roots do not play a major role in the augmented exercise pressor reflex.

Dorsal Root Tetrodotoxin-resistant Sodium Channels Play Only A Small Role In The Augmented Exercise Pressor Reflex In Rats With Chronic Femoral Artery Ligation

Dorsal Root Tetrodotoxin-resistant Sodium Channels Play Only A Small Role In The Augmented Exercise Pressor Reflex In Rats With Chronic Femoral Artery Ligation