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

Abstract

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

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

“…However, we did not observe consistent pressor responses, and often observed marked depressor responses, following the injection of either PGE 2 (an EP4‐R agonist) or U‐46619 (a TxA 2 ‐R agonist; Leal et al., 2011) into the arterial supply of the rat hindlimb (unpublished observations). Thus, as indicated above, we must rely on previous findings from our laboratory (Rollins et al., 2020) and others (Leal et al., 2011; Yamauchi et al., 2013) using these same protocols as evidence that EP4‐R and TxA 2 ‐R were likely blocked by their respective antagonists in our experiments. Second, we stimulated the mechanically activated channels on thin fibre muscle afferents by passively stretching rat hindlimb skeletal muscles, whereas during exercise, those channels are stimulated when skeletal muscles contract and shorten and intramuscular pressure increases (Gallagher et al., 2001).…”

“…Third, baseline MAP was significantly reduced following EP4‐R blockade in HF‐rEF rats and there was a trend ( P = 0.057) towards a reduction in SHAM rats. The effect on baseline MAP was surprising given the recent findings that the same EP4‐R blockade protocol had no effect on baseline MAP in control rats or rats with a chronically ligated femoral artery (Rollins et al., 2020; Yamauchi et al., 2013). It is unlikely that the lower baseline MAP impacted the present findings given that the baseline MAP following injection of the EP4‐R antagonist in both SHAM and HF‐rEF rats did not suggest poor physiological status ( i.e ., they were not abnormally low).…”

“…The experimental protocols used in this investigation ( i.e ., the receptor antagonists, the dose of the antagonists and the timing of the post‐injection manoeuvres) have been used recently by our laboratory (Rollins et al., 2020) and others (Leal et al., 2011; Yamauchi et al., 2013) to investigate the role played by EP4‐R and TxA 2 ‐R in the mechanoreflex and EPR in healthy rats and rats with femoral artery ligation‐induced simulated peripheral artery disease. Specifically, injection of 1 µg of the EP4‐R antagonist L‐161,982 into the arterial supply of the hindlimb of rats with simulated peripheral artery disease reduced the pressor response to static hindlimb muscle contraction (Rollins et al., 2020; Yamauchi et al., 2013). Moreover, injection of 80 µg of the TxA 2 ‐R antagonist daltroban into the arterial supply of the hindlimb of rats with ligated femoral arteries reduced the pressor response to static (Leal et al., 2011) and dynamic (Rollins et al., 2020) hindlimb muscle stretch and static hindlimb muscle contraction (Leal et al., 2011).…”

“…In TxA 2 ‐R blockade experiments, the hindlimb was reperfused for 10 min. This dose of daltroban and the timing of the injection protocol have been shown to reduce the pressor response to hindlimb muscle stretch and contraction in rats with simulated peripheral artery disease (Leal et al., 2011; Rollins et al., 2020). Following the 25 or 10 min hindlimb reperfusion period, the static stretch protocol was then repeated exactly as described above.…”

“…We have recently used these exact protocols to investigate the role played by EP4‐R and TxA 2 ‐R in the chronic sensitization of the mechanoreflex present in a rat model of simulated peripheral artery disease (Rollins et al., 2020). Further, we have previously demonstrated that up to 1% DMSO (Rollins et al., 2020) and up to 10 min of hindlimb circulation arrest with an iliac snare followed by a reperfusion period had no effect on the pressor response to rat hindlimb muscle stretch (Sanderson et al., 2019). The tension generated during the post‐drug manoeuvre was matched as closely as possible to that produced during the control stretch.…”