Mechanical and metabolic signals arising during skeletal muscle contraction reflexly increase sympathetic nerve activity and blood pressure (i.e., the exercise pressor reflex). In a rat model of simulated peripheral artery disease (PAD) in which a femoral artery is chronically (~72 hours) ligated, the mechanically-sensitive component of the exercise pressor reflex during 1 Hz dynamic contraction is exaggerated compared to that found in normal rats. Whether this is due to an enhanced acute sensitization of mechanoreceptors by metabolites produced during contraction or involves a chronic sensitization of mechanoreceptors is unknown. To investigate this issue, in decerebrate, unanesthetized rats we tested the hypothesis that the increases in mean arterial blood pressure (MAP) and renal sympathetic nerve activity (RSNA) during 1 Hz dynamic stretch are larger when evoked from a previously "ligated" hindlimb compared to those evoked from the contralateral "freely perfused" hindlimb. Dynamic stretch provided a mechanical stimulus in the absence of contraction-induced metabolite production that replicated closely the pattern of the mechanical stimulus present during dynamic contraction. We found that the increases in MAP (freely perfused: 14±1, ligated: 23±3 mmHg, p=0.02) and RSNA were significantly greater during dynamic stretch of the ligated hindlimb compared to the increases during dynamic stretch of the freely perfused hindlimb. These findings suggest that the exaggerated mechanically-sensitive component of the exercise pressor reflex found during dynamic muscle contraction in this rat model of simulated PAD involves a chronic sensitizing effect of ligation on muscle mechanoreceptors and cannot be attributed solely to acute contraction-induced metabolite sensitization.iv

Hindlimb skeletal muscle stretch (i.e., selective activation of the muscle mechanoreflex) in decerebrate rats evokes reflex increases in blood pressure and sympathetic nerve activity. Bradykinin has been found to sensitize mechanogated channels through a bradykinin B2 receptor-dependent mechanism. Moreover, bradykinin B2 receptor expression on sensory neurons is increased following chronic femoral artery ligation in the rat (a model of simulated peripheral artery disease). We tested the hypothesis that injection of bradykinin into the arterial supply of a hindlimb in decerebrate, unanesthetized rats would acutely augment (i.e., sensitize) the increase in blood pressure and renal sympathetic nerve activity during hindlimb muscle stretch to a greater extent in rats with a ligated femoral artery than in rats with a freely perfused femoral artery. The pressor response during static hindlimb muscle stretch was compared before and after hindlimb arterial injection of 0.5 µg of bradykinin. Injection of bradykinin increased blood pressure to a greater extent in "ligated" ( = 10) than "freely perfused" ( = 10) rats. The increase in blood pressure during hindlimb muscle stretch, however, was not different before vs. after bradykinin injection in freely perfused (14 ± 2 and 15 ± 2 mmHg for pre- and post-bradykinin, respectively, = 0.62) or ligated (15 ± 3 and 14 ± 2 mmHg for pre- and post-bradykinin, respectively, = 0.80) rats. Likewise, the increase in renal sympathetic nerve activity during stretch was not different before vs. after bradykinin injection in either group of rats. We conclude that bradykinin did not acutely sensitize the pressor response during hindlimb skeletal muscle stretch in freely perfused or ligated decerebrate rats.

Mechanical and metabolic signals associated with skeletal muscle contraction activate a reflex that increases blood pressure during exercise (i.e., the exercise pressor reflex). Bradykinin is one metabolite that has been found to play a role in activating the exercise pressor reflex and also contribute to the reflex's exaggeration in a rat model of simulated peripheral artery disease (PAD) induced by chronic femoral artery ligation. Specifically, bradykinin 2 (B2) receptor expression was found to be greater in rat dorsal root ganglia (DRG) tissue of “ligated” rats compared to the expression found in DRG tissue of rats in which the femoral artery was patent (i.e., “freely perfused”). Moreover, the hindlimb arterial injection of bradykinin reflexly increased blood pressure more in ligated rats than it did in freely perfused rats. We tested the hypotheses that, 1) HOE‐140 (5 μg/kg), a selective B2 receptor antagonist, would reduce the pressor response to bradykinin (0.5 μg/kg) injection in both ligated and freely perfused decerebrate, unanesthetized rats and 2) the magnitude of the reduction would be greater in ligated rats than in freely perfused rats. In five rats in which a femoral artery was ligated 72 hours before the experiment, HOE‐140 significantly reduced the pressor response to bradykinin injection (control: 23±2, post HOE‐140: 5±1 mmHg; p<0.01). In freely perfused rats, bradykinin injection evoked pressor responses in two rats and depressor responses in four rats. HOE‐140 reduced the magnitude of the pressor response (control: 16±7, HOE‐140: 4±2 mmHg, n=2) and the depressor response (pre: −22±5, post: −2±1 mmHg; p=0.02, n=4) in those experiments. We hypothesized that the depressor response in the freely perfused rats was due to the fact that hindlimb blood flow was not arrested during bradykinin injection and, therefore, the vasodilatory effects of bradykinin overwhelmed reflex‐mediated vasoconstriction. In four additional freely perfused rats in which a depressor response to bradykinin injection was initially observed (−31±8 mmHg), we subsequently tightened an iliac artery/vein snare to arrest the hindlimb circulation. Tightening the snare in this manner unmasked bradykinin injection‐induced pressor responses (9±4 mmHg). We conclude that hindlimb arterial bradykinin injection in ligated rats consistently produced a pressor response that was mediated by B2 receptors. In freely perfused rats, the divergent responses to bradykinin injection (i.e., pressor or depressor) were also B2 receptor mediated. B2 receptors may play a role in evoking the exaggerated exercise pressor reflex that is found in the rat model of simulated PAD.Support or Funding InformationSupported by University/Department funds.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The exercise pressor reflex, which is activated by mechanical and metabolic stimuli arising from within contracting skeletal muscles, contributes to the increase in blood pressure that occurs during exercise. GsMTx4, a mechano‐gated channel inhibitor that is partially selective for piezo channels, was recently found to reduce the pressor response during static rat hindlimb muscle stretch, a maneuver that is commonly used to study the mechanical component of the exercise pressor reflex. However, the GsMTx4‐induced reduction of the pressor response was limited to the initial phase (i.e., the first ~5 sec) of the stretch when muscle length was changing. That finding may reflect the rapid deactivation kinetics of piezo2 channels and the possibility that they do not contribute importantly to the later phases of a static stretch when muscle length is not changing. To investigate this possibility, we tested the hypothesis that in decerebrate, unanesthetized rats, GsMTx4 would reduce the pressor response throughout the duration of a 1 Hz dynamic hindlimb muscle stretch protocol that produced repeated changes in muscle length. Young adult male Sprague‐Dawley rats (n=12) had a jugular vein and both common carotid arteries cannulated. The left calcaneus was cut and the triceps surae muscles were linked by string to a force transducer. The pressor response during 30 seconds of 1 Hz dynamic stretch was measured before and after the injection of 10 μg of GsMTx4 into the arterial supply of the left hindlimb. We found that GsMTx4 reduced the peak pressor response during dynamic stretch (control: 15±4, GsMTx4: 5±2 mmHg, p=0.047, n=7). Moreover, the effect of GsMTx4 on the pressor response was evident throughout the duration of the dynamic stretch protocol. GsMTx4 did not, however, reduce the pressor response that resulted from the hindlimb arterial injection of 24 mmol lactic acid (control: 32±5, post‐GsMTx4: 29±4 mmHg, p=0.53) which indicates that GsMTx4 did not block voltage‐gated sodium channels in our experiments. Furthermore, the injection of GsMTx4 into the jugular vein (n=5) had no effect on the pressor response during dynamic stretch (control: 19±6, GsMTx4: 19±4, p=0.98). This finding indicates that when GsMTx4 was injected into the arterial supply of the hindlimb its effect on the pressor response during stretch was due to its actions on the peripheral endings of the sensory neurons. Our present findings suggest that piezo channels, most likely piezo2 channels, contribute to the pressor response throughout the duration of dynamic hindlimb muscle stretch in decerebrate rats.Support or Funding InformationSupported by university/departmental fundsThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.