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

Sex-dependent attenuating effects of capsaicin administration on the mechanoreflex in healthy rats

Stimulation of mechanically sensitive channels on the sensory endings of group III and IV thin fiber muscle afferents activates the mechanoreflex, which contributes to reflex increases in sympathetic nerve activity (SNA) and blood pressure during exercise. Accumulating evidence suggests that activation of the nonselective cation channel transient receptor potential vanilloid-1 (TRPV1) on the sensory endings of thin fiber afferents with capsaicin may attenuate mechanosensation. However, no study has investigated the effect of capsaicin on the mechanoreflex. We tested the hypothesis that in male and female decerebrate, unanesthetized rats, the injection of capsaicin (0.05 µg) into the arterial supply of the hindlimb reduces the pressor and renal SNA (RSNA) response to 30 s of 1 Hz rhythmic hindlimb muscle stretch (a model of isolated mechanoreflex activation). In male rats (n = 8), capsaicin injection significantly reduced the integrated blood pressure (blood pressure index or BPI: pre, 363 ± 78; post, 211 ± 88 mmHg·s; P = 0.023) and RSNA [∫ΔRSNA; pre, 687 ± 206; post, 216 ± 80 arbitrary units (au), P = 0.049] response to hindlimb muscle stretch. In female rats (n = 8), capsaicin injection had no significant effect on the pressor (BPI; pre: 277 ± 67; post: 207 ± 77 mmHg·s; P = 0.343) or RSNA (∫ΔRSNA: pre, 697 ± 123; post, 440 ± 183 au; P = 0.307) response to hindlimb muscle stretch. The data suggest that the injection of capsaicin into the hindlimb arterial supply to stimulate TRPV1 on the sensory endings of thin fiber muscle afferents attenuates the mechanoreflex in healthy male, but not female, rats. The findings may carry important implications for chronic conditions in which an exaggerated mechanoreflex contributes to aberrant sympathoexcitation during exercise.NEW & NOTEWORTHY Recent evidence in isolated sensory neurons indicates that capsaicin-induced stimulation of TRPV1 attenuates mechanosensitivity. Here we demonstrate for the first time that capsaicin exposure/administration reduces the reflex pressor and renal sympathetic nerve response to mechanoreflex activation in male rats, but not female rats, in vivo. Our data may carry important clinical implications for chronic diseases which have been linked to an exaggerated mechanoreflex, at least in males.

Neurocirculatory regulation and adaptations to exercise in chronic kidney disease

Chronic kidney disease (CKD) is characterized by pronounced exercise intolerance and exaggerated blood pressure reactivity during exercise. Classic mechanisms of exercise intolerance in CKD have been extensively described previously and include uremic myopathy, chronic inflammation, malnutrition, and anemia. We contend that these classic mechanisms only partially explain the exercise intolerance experienced in CKD and that alterations in cardiovascular and autonomic regulation also play a key contributing role. The purpose of this review is to examine the physiological factors that contribute to neurocirculatory dysregulation during exercise and discuss the adaptations that result from regular exercise training in CKD. Key neurocirculatory mechanisms contributing to exercise intolerance in CKD include augmentation of the exercise pressor reflex, aberrations in neurocirculatory control, and increased neurovascular transduction. In addition, we highlight how some contributing factors may be improved through exercise training, with a specific focus on the sympathetic nervous system. Important areas for future work include understanding how the exercise prescription may best be optimized in CKD and how the beneficial effects of exercise training may extend to the brain.

Bradykinin 2 receptors contribute to the exaggerated exercise pressor reflex in a rat model of simulated peripheral artery disease

We investigated the role played by bradykinin 2 (B2) receptors in the exaggerated exercise pressor reflex in rats with a femoral artery ligated for 72 h to induce simulated peripheral artery disease (PAD). We hypothesized that in decerebrate, unanesthetized rats with a ligated femoral artery, hindlimb arterial injection of HOE-140 (100 ng, B2 receptor antagonist) would reduce the pressor response to 30 s of electrically induced 1 Hz hindlimb skeletal muscle contraction, and 30 s of 1 Hz hindlimb skeletal muscle stretch (a model of mechanoreflex activation isolated from contraction-induced metabolite production). We hypothesized no effect of HOE-140 in sham-operated "freely perfused" rats. In both freely perfused (n = 4) and "ligated" (n = 4) rats, we first confirmed efficacious B2 receptor blockade by demonstrating that HOE-140 injection significantly reduced (P < 0.05) the peak increase in mean arterial pressure (peak ΔMAP) in response to hindlimb arterial injection of bradykinin. In subsequent experiments, we found that HOE-140 reduced the peak ΔMAP response to muscle contraction in ligated (n = 14; control: 23 ± 2; HOE-140: 17 ± 2 mmHg; P = 0.03) but not freely perfused rats (n = 7; control: 17 ± 3; HOE-140: 18 ± 4 mmHg; P = 0.65). Furthermore, HOE-140 had no effect on the peak ΔMAP response to stretch in ligated rats (n = 14; control: 37 ± 4; HOE-140: 32 ± 5 mmHg; P = 0.13) but reduced the integrated area under the blood pressure signal over the final ∼20 s of the maneuver. The data suggest that B2 receptors contribute to the exaggerated exercise pressor reflex in rats with simulated PAD, and that contribution includes a modest role in the chronic sensitization of the mechanically activated channels/afferents that underlie mechanoreflex activation.

Sensory neuron inositol 1,4,5-trisphosphate receptors contribute to chronic mechanoreflex sensitization in rats with simulated peripheral artery disease

The mechanoreflex is exaggerated in patients with peripheral artery disease (PAD) and in a rat model of simulated PAD in which a femoral artery is chronically (∼72 h) ligated. We found recently that, in rats with a ligated femoral artery, blockade of thromboxane A2 (TxA2) receptors on the sensory endings of thin fiber muscle afferents reduced the pressor response to 1 Hz repetitive/dynamic hindlimb skeletal muscle stretch (a model of mechanoreflex activation isolated from contraction-induced metabolite production). Conversely, we found no effect of TxA2 receptor blockade in rats with freely perfused femoral arteries. Here, we extended the isolated mechanoreflex findings in "ligated" rats to experiments evoking dynamic hindlimb skeletal muscle contractions. We also investigated the role played by inositol 1,4,5-trisphosphate (IP3) receptors, receptors associated with intracellular signaling linked to TxA2 receptors, in the exaggerated response to dynamic mechanoreflex and exercise pressor reflex activation in ligated rats. Injection of the TxA2 receptor antagonist daltroban into the arterial supply of the hindlimb reduced the pressor response to 1 Hz dynamic contraction in ligated but not "freely perfused" rats. Moreover, injection of the IP3 receptor antagonist xestospongin C into the arterial supply of the hindlimb reduced the pressor response to 1 Hz dynamic stretch and contraction in ligated but not freely perfused rats. These findings demonstrate that, in rats with a ligated femoral artery, sensory neuron TxA2 receptor and IP3 receptor-mediated signaling contributes to a chronic sensitization of the mechanically activated channels associated with the mechanoreflex and the exercise pressor reflex.

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

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

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

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

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

NEW FINDINGS

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

ABSTRACT

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