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

Abstract

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.

Keywords: PPADS; autonomic nervous system; renal sympathetic activity; thin fiber muscle afferents; α,β-methylene ATP.

Fig. 1.

Efficacy of the P2X receptor blockade in freely perfused rats. Pyridoxal phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS; 10 mg/kg; A) significantly attenuated the pressor response to α,β-methylene ATP (10 μg/kg), whereas sterile water (B), the vehicle, had no effect. MAP, mean arterial pressure. *P < 0.05, significantly smaller pressor response from the pressor response before PPADS.

Fig. 2.

Peak pressor (A) and cardioaccelerator (B) responses to static contraction in freely perfused rats. C: increases in integrated renal sympathetic nerve activity (RSNA) averaged over the 30-s contraction period before and after infusion of PPADS (10 mg/kg). D: tension-time indexes (TTIs) after PPADS were not significantly different from those before PPADS. PPADS had no significant effect (P > 0.05) on the peak pressor, cardioaccelerator and renal nerve responses to static contraction in freely perfused rats. Likewise, PPADS had no effect on baseline mean arterial pressure or heart rate, the levels of which are shown inside the bars. Each of the increases for the pressor, cardioaccelerator, and integrated renal sympathetic nerve responses to contraction were significantly greater than their corresponding baseline values (P < 0.05).

Fig. 3.

Time courses of the average changes in pressor and renal sympathetic nerve responses to static contraction in freely perfused rats before and 13 min after PPADS infusion (A and B) as well as those before and 25 min after PPADS infusion (C and D). *P < 0.05, significant post hoc difference at 17 s between pressor responses before PPADS and 13 min after PPADS.

Fig. 4.

Efficacy of the P2X receptor blockade in ligated rats. PPADS (A; 10 mg/kg) significantly attenuated the pressor response to α,β-methylene ATP (10 μg/kg), whereas sterile water (B), the vehicle, had no effect. *P < 0.05, significantly smaller pressor response from the pressor response before PPADS.

Fig. 5.

Peak pressor (A) and cardioaccelerator (B) responses to static contraction in ligated rats. C: increases in integrated RSNA averaged over the 30 s contraction period before and after infusion of PPADS (10 mg/kg). D: TTIs after PPADS were not significantly different from those before PPADS. PPADS significantly decreased (P < 0.05) the peak pressor, but not the cardioaccelerator and renal nerve responses, to static contraction in ligated rats. PPADS had no effect on baseline mean arterial pressure or heart rate, the levels of which are shown inside the bars. *P < 0.05, significantly smaller pressor response from the pressor response before PPADS. Each of the increases for the pressor, cardioaccelerator, and integrated renal sympathetic nerve responses to contraction were significantly greater than their corresponding baseline values (P < 0.05).

Fig. 6.

Peak pressor (A) and cardioaccelerator (B) responses to static contraction in ligated rats before and after intravenous infusion of PPADS (10 mg/kg). C: TTIs after PPADS were not significantly different from those before PPADS. PPADS had no effect on baseline mean arterial pressure or heart rate, which are shown inside the bars. Each of the increases for the pressor, cardioaccelerator and integrated renal sympathetic nerve responses to contraction were significantly greater than their corresponding baseline values (P < 0.05).

Fig. 7.

Time courses of the average changes in pressor and renal sympathetic nerve responses to static contraction in ligated rats before and 13 min after PPADS infusion (A and B) as well as those before and 25 min after PPADS infusion (C and D). *P < 0.05, significant post hoc differences between pressor responses before PPADS and 13 min after PPADS.

Fig. 8.

Increase in integrated RSNA during the first five s of static contraction in freely perfused rats (A and C) and in ligated rats (B and D) before and 13 min after PPADS infusion (A and B), as well as before and 25 min after PPADS infusion (C and D). *P < 0.05, significant decrease in activity after PPADS at its corresponding time point.