Blockade of the TP receptor attenuates the exercise pressor reflex in decerebrated rats with chronic femoral artery occlusion

Cyclooxygenase metabolites stimulate or sensitize group III and IV muscle afferents, which comprise the sensory arm of the exercise pressor reflex. The thromboxane (TP) receptor binds several of these metabolites, whose concentrations in the muscle interstitium are increased by exercise under freely perfused conditions and even more so under ischemic conditions, which occur in peripheral artery disease. We showed that the exercise pressor reflex is greater in rats with simulated peripheral artery disease than in rats with freely perfused limbs. These findings prompted us to test the hypothesis that the TP receptor contributes to the exaggerated exercise pressor reflex occurring in a rat model of peripheral artery disease. We compared the cardiovascular responses to static contraction and stretch before and after femoral arterial injections of daltroban (80 μg), a TP receptor antagonist. We performed these experiments in decerebrate rats whose femoral arteries were ligated 72 h before the experiment (a model of simulated peripheral artery disease) and in control rats whose hindlimbs were freely perfused. Daltroban reduced the pressor response to static contraction in both freely perfused (n = 6; before: Δ12 ± 2 mmHg, after: Δ6 ± 2 mmHg, P = 0.024) and 72-h-ligated rats (n = 10; before: Δ25 ± 3 mmHg, after: Δ7 ± 4 mmHg, P = 0.001). Likewise, daltroban reduced the pressor response to stretch in the freely perfused group (n = 9; before: Δ30 ± 3 mmHg, after: Δ17 ± 3 mmHg, P < 0.0001) and in the ligated group (n = 11; before: Δ37 ± 5 mmHg, after: Δ23 ± 3 mmHg, P = 0.016). Intravenous injections of daltroban had no effect on the pressor response to contraction. We conclude that the TP receptor contributes to the pressor responses evoked by contraction and stretch in both freely perfused rats and rats with simulated peripheral artery disease.     

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.     

Blockade of ATP-sensitive potassium channels prevents the attenuation of the exercise pressor reflex by tempol in rats with ligated femoral arteries

We reported previously that tempol attenuated the exercise pressor and muscle mechanoreceptor reflexes in rats whose femoral arteries were ligated, whereas tempol did not attenuate these reflexes in rats whose femoral arteries were freely perfused. Although the mechanism whereby tempol attenuated these reflexes in rats whose femoral artery was ligated was independent of its ability to scavenge reactive oxygen species, its nature remains unclear. An alternative explanation for the tempol-induced attenuation of these reflexes involves ATP-sensitive potassium channels (K(ATP)) and calcium-activated potassium channels (BK(Ca)), both of which are opened by tempol. We tested the likelihood of this explanation by measuring the effects of either glibenclamide (0.1 mg/kg), which blocks K(ATP) channels, or iberiotoxin (20 or 40 μg/kg), which blocks BK(Ca) channels, on the tempol-induced attenuation of the exercise pressor and muscle mechanoreceptor reflexes in decerebrated rats whose femoral arteries were ligated. We found that glibenclamide prevented the tempol-induced attenuation of both reflexes, whereas iberiotoxin did not. We also found that the amount of protein comprising the pore of the K(ATP) channel in the dorsal root ganglia innervating hindlimbs whose femoral artery was ligated was significantly greater than that in the dorsal root ganglia innervating hindlimbs whose femoral arteries were freely perfused. In contrast, the amounts of protein comprising the BK(Ca) channel in the dorsal root ganglia innervating the ligated and freely perfused hindlimbs were not different. We conclude that tempol attenuated both reflexes by opening K(ATP) channels, an effect that hyperpolarized muscle afferents stimulated by static contraction or tendon stretch.     

Contribution of nerve growth factor to upregulation of P2X₃ expression in DRG neurons of rats with femoral artery occlusion

Femoral artery occlusion augments the sympathetic nerve and pressor responses to muscle contraction and muscle metabolites injected into the arterial blood supply of the hindlimb muscles in rats. The underlying mechanism by which these reflex responses are enhanced after muscle vascular insufficiency is unclear. Purinergic P2X(3) receptor has been reported to contribute to the metabolic component of the exercise pressor reflex. Thus the purpose of this study was to examine if chronic femoral occlusion would alter the expression of P2X(3) in dorsal root ganglion (DRG) neurons of rats. Also, P2X(3)-mediated sympathetic responsiveness was examined after femoral occlusion. In addition, the role played by nerve growth factor (NGF) in regulating the expression and response of P2X(3) was examined. Western blot analysis showed that 24 h of femoral ligation increased the levels of P2X(3) (optical density: 0.93 ± 0.07 in control and 1.37 ± 0.10 after occlusion; P < 0.05 vs. control). The fluorescence immunohistochemistry further demonstrated that the occlusion elevated P2X(3) expression in DRG neurons (percentage of P2X(3)-positive cells: 33 ± 3% in control and 51 ± 3% in occlusion; P < 0.05 vs. control). Furthermore, the results showed that responses of renal sympathetic nerve activity and blood pressure to stimulation of P2X were greater in occluded rats than responses in control rats by injection of α,β-methylene ATP into the arterial blood supply of the hindlimb muscle. Finally, infusion of NGF in the hindlimb muscles of healthy rats increased P2X(3) (optical density: 0.98 ± 0.12 in control and 1.37 ± 0.16 with NGF; P < 0.05 vs. control). The pressor response to injection of α,β-methylene ATP was increased in the rats with NGF infusion. Likewise, blocking NGF attenuated exaggeration of the reflex response induced by α,β-methylene ATP in occluded rats. The findings of this study suggest that the levels of P2X(3) in primary afferent neurons are upregulated as the blood supply to the hindlimb is deficient under ischemic conditions, leading to augmentation of the muscle reflex. NGF is closely related to increases in P2X(3) receptor expression and response.     

Estrogen's attenuating effect on the exercise pressor reflex is more opioid dependent in gonadally intact than in ovariectomized female cats.

Using gonadally intact female cats, we showed previously that estrogen, applied topically to the spinal cord, attenuated the exercise pressor reflex. Although the mechanism by which estrogen exerted its attenuating effect is unknown, this steroid hormone has been shown to influence spinal opioid pathways, which in turn have been implicated in the regulation of the exercise pressor reflex. These findings prompted us to test the hypothesis that opioids mediate the attenuating effect of estrogen on the exercise pressor reflex in both gonadally intact female and ovariectomized cats. We therefore applied 200 microl of 17beta-estradiol (0.01 microg/ml) with and without the addition of 1,000 microg naloxone, a mu- and delta-opioid antagonist, to a spinal well covering the L6-S1 spinal cord in decerebrated female cats that were either gonadally intact or ovariectomized. The exercise pressor reflex was evoked by electrical stimulation of the L7 or S1 ventral root, a maneuver that caused the hindlimb muscles to contract statically. We found that, in gonadally intact cats, the attenuating effect of estrogen was more pronounced than that in ovariectomized cats. We also found that, in gonadally intact female cats, naloxone partly reversed the attenuation of the pressor response to static contraction caused by spinal estrogen application. For example, in intact cats, the pressor response to contraction before estrogen application averaged 39 +/- 4 mmHg (n = 10), whereas the pressor response 60 min afterward averaged only 18 +/- 4 mmHg (P < 0.05). In contrast, the pressor response to contraction before estrogen and naloxone application averaged 33 +/- 5 mmHg (n = 11), whereas afterward it averaged 27 +/- 6 mmHg (P < 0.05). In ovariectomized cats, naloxone was less effective in reversing the attenuating effect of estrogen on the exercise pressor reflex.     

Acute antioxidant supplementation and skeletal muscle vascular conductance in aged rats: role of exercise and fiber type

Age-related increases in oxidative stress contribute to impaired skeletal muscle vascular control. However, recent evidence indicates that antioxidant treatment with tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) attenuates flow-mediated vasodilation in isolated arterioles from the highly oxidative soleus muscle of aged rats. Whether antioxidant treatment with tempol evokes similar responses in vivo at rest and during exercise in senescent individuals and whether this effect varies based on muscle fiber type composition are unknown. We tested the hypothesis that redox modulation via acute systemic tempol administration decreases vascular conductance (VC) primarily in oxidative hindlimb locomotor muscles at rest and during submaximal whole body exercise (treadmill running at 20 m/min, 5% grade) in aged rats. Eighteen old (25-26 mo) male Fischer 344 x Brown Norway rats were assigned to either rest (n = 8) or exercise (n = 10) groups. Regional VC was determined via radiolabeled microspheres before and after intra-arterial administration of tempol (302 μmol/kg). Tempol decreased mean arterial pressure significantly by 9% at rest and 16% during exercise. At rest, similar VC in 26 out of 28 individual hindlimb muscles or muscle parts following tempol administration compared with control resulted in unchanged total hindlimb muscle VC (control: 0.18 ± 0.02; tempol: 0.17 ± 0.05 ml·min(-1)·100 g(-1)·mmHg(-1); P > 0.05). During exercise, all individual hindlimb muscles or muscle parts irrespective of fiber type composition exhibited either an increase or no change in VC with tempol (i.e., ↑11 and ?17 muscles or muscle parts), such that total hindlimb VC increased by 25% (control: 0.93 ± 0.04; tempol: 1.15 ± 0.09 ml·min(-1)·100 g(-1)·mmHg(-1); P ≤ 0.05). These results demonstrate that acute systemic administration of the antioxidant tempol significantly impacts the control of regional vascular tone in vivo presumably via redox modulation and improves skeletal muscle vasodilation independently of fiber type composition during submaximal whole body exercise in aged rats.     

Role played by purinergic receptors on muscle afferents in evoking the exercise pressor reflex.

The exercise pressor reflex is believed to be evoked, in part, by multiple metabolic stimuli that are generated when blood supply to exercising muscles is inadequate to meet metabolic demand. Recently, ATP, which is a P2 receptor agonist, has been suggested to be one of the metabolic stimuli evoking this reflex. We therefore tested the hypothesis that blockade of P2 receptors within contracting skeletal muscle attenuated the exercise pressor reflex in decerebrate cats. We found that popliteal arterial injection of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 10 mg/kg), a P2 receptor antagonist, attenuated the pressor response to static contraction of the triceps surae muscles. Specifically, the pressor response to contraction before PPADS averaged 36 +/- 3 mmHg, whereas afterward it averaged 14 +/- 3 mmHg (P < 0.001; n = 19). In addition, PPADS attenuated the pressor response to postcontraction circulatory occlusion (P < 0.01; n = 11). In contrast, popliteal arterial injection of CGS-15943 (250 micro g/kg), a P1 receptor antagonist, had no effect on the pressor response to static contraction of the triceps surae muscles. In addition, popliteal arterial injection of PPADS but not CGS-15943 attenuated the pressor response to stretch of the calcaneal (Achilles) tendon. We conclude that P2 receptors on the endings of thin fiber muscle afferents play a role in evoking both the metabolic and mechanoreceptor components of the exercise pressor reflex.     

Estrogen attenuates the exercise pressor reflex in female cats.

In humans, the pressor and muscle sympathetic nerve responses to static exercise are less in women than in men. The difference has been attributed to the effect of estrogen on the exercise pressor reflex. Estrogen receptors are abundant in areas of the dorsal horn receiving input from group III and IV muscle afferents, which comprise the sensory limb of the exercise pressor reflex arc. These findings prompted us to investigate the effect of estrogen on the spinal pathway of the exercise pressor reflex arc. Previously, we found that the threshold concentration of 17beta-estradiol needed to attenuate the exercise pressor reflex in male decerebrate cats was 10 microg/ml (Schmitt PM and Kaufman MP. J Appl Physiol 94: 1431-1436, 2003). The threshold concentration for female cats, however, is not known. Consequently, we applied 17beta-estradiol to a well covering the L6-S1 spinal cord in decerebrate female cats. The exercise pressor reflex was evoked by electrical stimulation of the L7 or S1 ventral root, a maneuver that caused the hindlimb muscles to contract statically. We found that the pressor response to contraction averaged 38 +/- 7 mmHg before the application of 17beta-estradiol (0.01 microg/ml) to the spinal cord, whereas it averaged only 23 +/- 4 mmHg 30 min after application (P < 0.05). Recovery of the pressor response to contraction was not obtained for 2 h after application of 17beta-estradiol. Application of 17beta-estradiol in a dose of 0.001 microg/ml had no effect on the exercise pressor reflex (n = 5). We conclude that the concentration of 17beta-estradiol required to attenuate the exercise pressor reflex is 1,000 times more dilute in female cats than that needed to attenuate this reflex in male cats.     

Independent modification of baroreceptor and exercise pressor reflex function by nitric oxide in nucleus tractus solitarius

It has been suggested that nitric oxide (NO) is a key modulator of both baroreceptor and exercise pressor reflex afferent signals processed within the nucleus tractus solitarius (NTS). However, studies investigating the independent effects of NO within the NTS on the function of each reflex have produced inconsistent results. To address these concerns, the effects of microdialyzing 10 mM L-arginine, an NO precursor, and 20 mM N(G)-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, into the NTS on baroreceptor and exercise pressor reflex function were examined in 17 anesthetized cats. Arterial baroreflex regulation of heart rate was quantified using vasoactive drugs to induce acute changes in mean arterial pressure (MAP). To activate the exercise pressor reflex, static hindlimb contractions were induced by electrical stimulation of spinal ventral roots. To isolate the exercise pressor reflex, contractions were repeated after barodenervation. The gain coefficient of the arterial cardiac baroreflex was significantly different from control (-0.24 +/- 0.04 beats.min(-1).mmHg(-1)) after the dialysis of L-arginine (-0.18 +/- 0.02 beats.min(-1).mmHg(-1)) and L-NAME (-0.29 +/- 0.02 beats.min(-1).mmHg(-1)). In barodenervated animals, the peak MAP response to activation of the exercise pressor reflex (change in MAP from baseline, 39 +/- 7 mmHg) was significantly attenuated by the dialysis of L-arginine (change in MAP from baseline, 29 +/- 6 mmHg). The results demonstrate that NO within the NTS can independently modulate both the arterial cardiac baroreflex and the exercise pressor reflex. Collectively, these findings provide a neuroanatomical and chemical basis for the regulation of baroreflex and exercise pressor reflex function within the central nervous system.     

Gadolinium attenuates exercise pressor reflex in cats

The exercise pressor reflex, which arises from the contraction-induced stimulation of group III and IV muscle afferents, is widely believed to be evoked by metabolic stimuli signaling a mismatch between blood/oxygen demand and supply in the working muscles. Nevertheless, mechanical stimuli may also play a role in evoking the exercise pressor reflex. To determine this role, we examined the effect of gadolinium, which blocks mechanosensitive channels, on the exercise pressor reflex in both decerebrate and alpha-chloralose-anesthetized cats. We found that gadolinium (10 mM; 1 ml) injected into the femoral artery significantly attenuated the reflex pressor responses to static contraction of the triceps surae muscles and to stretch of the calcaneal (Achilles) tendon. In contrast, gadolinium had no effect on the reflex pressor response to femoral arterial injection of capsaicin (5 microg). In addition, gadolinium significantly attenuated the responses of group III muscle afferents, many of which are mechanically sensitive, to both static contraction and to tendon stretch. Gadolinium, however, had no effect on the responses of group IV muscle afferents, many of which are metabolically sensitive, to either static contraction or to capsaicin injection. We conclude that mechanical stimuli arising in contracting skeletal muscles contribute to the elicitation of the exercise pressor reflex.     

Severe exercise alters the strength and mechanisms of the muscle metaboreflex

Previous studies have shown that in dogs performing mild to moderate treadmill exercise, partial graded reductions in hindlimb blood flow cause active skeletal muscle to become ischemic and metabolites to accumulate thus evoking the muscle metaboreflex. This leads to a substantial reflex increase in mean arterial pressure (MAP) mediated almost solely via a rise in cardiac output (CO). However, during severe exercise CO is likely near maximal and thus metaboreflex-mediated increases in MAP may be attenuated. We therefore evoked the metaboreflex via partial graded reductions in hindlimb blood flow in seven dogs during mild, moderate, and severe treadmill exercise. During mild and moderate exercise there was a large rise in CO (1.5 +/- 0.2 and 2.2 +/- 0.3 l/min, respectively), whereas during severe exercise no significant increase in CO occurred. The rise in CO caused a marked pressor response that was significantly attenuated during severe exercise (26.3 +/- 7.0, 33.2 +/- 5.6, and 12.2 +/- 4.8 mmHg, respectively). We conclude that during severe exercise the metaboreflex pressor response mechanisms are altered such that the ability of this reflex to increase CO is abolished, and reduced pressor response occurs only via peripheral vasoconstriction. This shift in mechanisms likely limits the effectiveness of the metaboreflex to increase blood flow to ischemic active skeletal muscle. Furthermore, because the metaboreflex is a flow-raising reflex and not a pressure-raising reflex, it may be most appropriate to describe the metaboreflex magnitude based on its ability to evoke a rise in CO and not a rise in MAP.     

IL-17-mediated oxidative stress is an important stimulator of AT1-AA and hypertension during pregnancy

Preeclampsia is associated with autoimmune cells T(H)17, secreting interleukin-17, autoantibodies activating the angiotensin II type I receptor (AT1-AA), and placental oxidative stress (ROS). The objective of our study was to determine whether chronic IL-17 increases blood pressure by stimulating ROS and AT1-AAs during pregnancy. To answer this question four groups of rats were examined: normal pregnant (NP, n = 20), NP+IL-17 (n = 12), NP+tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (n = 7) (a superoxide dismutase mimetic that scavenges ROS), and NP+IL-17+tempol (n = 11). IL-17 (150 pg/day) was infused into NP rats while tempol was administered via the drinking water ad libitum. On day 19 blood pressure (MAP) was recorded, and plasma, urine, and tissue were collected for isolation of ROS detected by chemilluminescent technique. Urinary isoprostane was measured by ELISA. AT1-AAs were determined via cardiomyocyte assay and expressed as beats per minute. MAP increased from 98 ± 3 mmHg in NP to 123 ± 3 mmHg in IL-17-infused NP rats. Urinary isoprostane increased from 1,029 ± 1 in NP to 3,526 ± 2 pg·mg(-1)·day(-1) in IL-17-infused rats (P < 0.05). Placental ROS was 436 ± 4 RLU·ml(-1)·min(-1) (n = 4) in NP and 702 ± 5 (n = 5) RLU·ml(-1)·min(-1) in IL-17-treated rats. Importantly, AT1-AA increased from 0.41 ± 0.05 beats/min in NP rats (n = 8) to 18.4 ± 1 beats/min in IL-17 rats (n = 12). Administration of tempol attenuated the hypertension (101 ± 3 mmHg) ROS (459 ± 5 RLU·ml(-1)·min(-1)) and blunted AT1-AAs (7.3 ± 0.6 beats/min) in NP+IL-17+tempol-treated rats. Additionally, AT1 receptor blockade inhibited IL-17-induced hypertension and placental oxidative stress. MAP was 105 ± 5 mmHg and ROS was 418 ± 5 RLU·ml(-1)·min(-1) in NP+IL 17-treated with losartan. These data indicate that IL-17 causes placental oxidative stress, which serves as stimulus modulating AT1-AAs that may play an important role in mediating IL-17-induced hypertension during pregnancy.     

VR-1 receptor blockade attenuates the pressor response to capsaicin but has no effect on the pressor response to contraction in cats

Vanilloid type 1 (VR-1) receptors are stimulated by capsaicin and hydrogen ions, the latter being a by-product of muscular contraction. We tested the hypothesis that activation of VR-1 receptors during static contraction contributes to the exercise pressor reflex. We established a dose of iodoresinaferatoxin (IRTX), a VR-1 receptor antagonist, that blocked the pressor response to capsaicin injected into the arterial supply of muscle. Specifically, in eight decerebrated cats, we compared pressor responses to capsaicin (10 mug) injected into the right popliteal artery, which was subsequently injected with IRTX (100 mug), with those to capsaicin injected into the left popliteal artery, which was not injected with IRTX. The pressor response to capsaicin injected into the right popliteal artery averaged 49 +/- 9 mmHg before IRTX and 9 +/- 2 mmHg after IRTX (P < 0.05). In contrast, the pressor response to capsaicin injected into the left popliteal artery averaged 46 +/- 10 mmHg "before" and 43 +/- 6 mmHg "after" (P > 0.05). We next determined whether VR-1 receptors mediated the pressor response to contraction of the triceps surae. During contraction without circulatory occlusion, the pressor response before IRTX (100 mug) averaged 26 +/- 3 mmHg, whereas it averaged 22 +/- 3 mmHg (P > 0.05) after IRTX (n = 8). In addition, during contraction with occlusion, the pressor responses averaged 35 +/- 3 mmHg before IRTX injection and 49 +/- 7 mmHg after IRTX injection (n = 7). We conclude that VR-1 receptors play little role in evoking the exercise pressor reflex.     

Skeletal muscle reflex-mediated changes in sympathetic nerve activity are abnormal in spontaneously hypertensive rats

In hypertension, the blood pressure response to exercise is exaggerated. We demonstrated previously that this heightened pressor response to physical activity is mediated by an overactive skeletal muscle exercise pressor reflex (EPR), with important contributions from its metaboreflex and mechanoreflex components. However, the mechanisms driving the abnormal blood pressure response to EPR activation are largely unknown. Recent evidence in humans suggests that the muscle metaboreflex partially mediates the enhanced EPR-induced pressor response via abnormally large changes in sympathetic nerve activity (SNA). Whether the muscle mechanoreflex induces similarly exaggerated alterations in SNA in hypertension remains unknown, as does the role of the mechanoreceptors mediating muscle reflex activity. To address these issues, the EPR was selectively activated by electrically inducing hindlimb muscle contraction in decerebrate normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Stimulation of the EPR evoked significantly larger increases in mean arterial pressure (MAP) and renal SNA (RSNA) in SHR compared with WKY (ΔRSNA from baseline: 140 ± 11 vs. 48 ± 8%). The mechanoreflex was stimulated by stretching hindlimb muscle which likewise elicited significantly greater elevations in MAP and RSNA in SHR than WKY (ΔRSNA from baseline: 105 ± 11 vs. 35 ± 7%). Blockade of mechanoreceptors in muscle with gadolinium significantly attenuated the MAP and RSNA responses to contraction and stretch in SHR. These data suggest that 1) the exaggerated pressor response to activation of the EPR and muscle mechanoreflex in hypertension is mediated by abnormally large reflex-induced augmentations in SNA and 2) this accentuated sympathetic responsiveness is evoked, in part, by stimulation of muscle mechanoreceptors.     

Role of spinal NMDA and non-NMDA receptors in the pressor reflex response to abdominal ischemia

Abdominal ischemia induces a pressor reflex caused mainly by C-fiber afferent stimulation. Because excitatory amino acids, such as glutamate, bind to N-methyl-D-aspartate (NMDA) and non-NMDA [dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)] receptors and serve as important spinal neurotransmitters, we hypothesized that both receptors play a role in the abdominal ischemia pressor reflex. In chloralose-anesthetized cats, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate did not alter the pressor reflex (33 +/- 9 to 33 +/- 7 mmHg, P > 0.05, n = 4), whereas AMPA receptor blockade with 4.0 mM 6-nitro-7-sulfamylbenzo(f)quinoxaline-2,3-dione significantly attenuated the reflex (29 +/- 5 to 16 +/- 4 mmHg, P < 0.05, n = 6). Because several studies suggest that anesthesia masks the effects of glutamatergic receptors, this experiment was repeated on decerebrate cats, and in this group, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate significantly altered the pressor reflex (36 +/- 3 to 25 +/- 4 mmHg, P < 0.05, n = 5). Our combined data suggest that spinal NMDA and AMPA receptors play a role in the abdominal ischemia pressor reflex.     

Central integration of muscle reflex and arterial baroreflex in midbrain periaqueductal gray: roles of GABA and NO

It has been suggested that the midbrain periaqueductal gray (PAG) is a neural integrating site for the interaction between the muscle pressor reflex and the arterial baroreceptor reflex. The underlying mechanisms are poorly understood. The purpose of this study was to examine the roles of GABA and nitric oxide (NO) in modulating the PAG integration of both reflexes. To activate muscle afferents, static contraction of the triceps surae muscle was evoked by electrical stimulation of the L7 and S1 ventral roots of 18 anesthetized cats. In the first group of experiments (n = 6), the pressor response to muscle contraction was attenuated by bilateral microinjection of muscimol (a GABA receptor agonist) into the lateral PAG [change in mean arterial pressure (DeltaMAP) = 24 +/- 5 vs. 46 +/- 8 mmHg in control]. Conversely, the pressor response was significantly augmented by 0.1 mM bicuculline, a GABAA receptor antagonist (DeltaMAP = 65 +/- 10 mmHg). In addition, the effect of GABAA receptor blockade on the reflex response was significantly blunted after sinoaortic denervation and vagotomy (n = 4). In the second group of experiments (n = 8), the pressor response to contraction was significantly attenuated by microinjection of L-arginine into the lateral PAG (DeltaMAP = 26 +/- 4 mmHg after L-arginine injection vs. 45 +/- 7 mmHg in control). The effect of NO attenuation was antagonized by bicuculline and was reduced after denervation. These data demonstrate that GABA and NO within the PAG modulate the pressor response to muscle contraction and that NO attenuation of the muscle pressor reflex is mediated via arterial baroreflex-engaged GABA increase. The results suggest that the PAG plays an important role in modulating cardiovascular responses when muscle afferents are activated.     

Inhibition of aortic chemoreceptor discharge by pressor response to muscular contraction

We have examined the effect of static contraction of the hindlimb muscles on the discharge of aortic chemoreceptors in chloralose-anesthetized cats. The responses of the chemoreceptors to contraction were dependent on the arterial pressure response to this maneuver. When contraction reflexly evoked a pressor response of at least 20 mmHg, the discharge of 26 chemoreceptors was reduced from control levels by 53% (P less than 0.01). The contraction-induced inhibition of chemoreceptor discharge was prevented by phentolamine, an alpha-adrenergic antagonist that also attenuated the contraction-induced pressor response. In addition, the inhibition evoked by contraction was simulated by injection of phenylephrine and inflation of an aortic balloon, both of which evoked pressor responses. However, when contraction failed to significantly change arterial pressure, the discharge of 20 aortic chemoreceptors was not significantly changed from control levels. We conclude that the reflex pressor response to static contraction inhibits the discharge of aortic chemoreceptors. This inhibition of discharge needs to be considered when interpreting the effects of aortic barodenervation on the cardiovascular responses to exercise.     

Attenuating effects of intrathecal clonidine on the exercise pressor reflex

We tested the hypothesis that intrathecal injection of clonidine, an alpha 2-adrenergic agonist, attenuated the reflex cardiovascular and ventilatory responses to static muscular contraction in cats. Before clonidine (1 microgram in 0.2 ml), contraction-induced reflex increases (n = 10) in mean arterial pressure and ventilation averaged 25 +/- 3 mmHg and 359 +/- 105 ml/min, respectively, whereas after clonidine these increases averaged 8 +/- 4 mmHg and 200 +/- 114 ml/min, respectively (P less than 0.05). Clonidine had no effect on the heart rate response to contraction. Intrathecal injection of yohimbine (10 micrograms; n = 5), an alpha 2-adrenergic antagonist, but not prazosin (10 micrograms; n = 3), an alpha 1-adrenergic antagonist, prevented the attenuating effects of clonidine on the reflex pressor and ventilatory responses to contraction. Our findings were not due to the spread of clonidine to the medulla, because the reflex pressor and ventilatory responses to contraction were not attenuated by injection of clonidine (1 microgram) onto the medulla (n = 3). In addition, our findings were not due to a clonidine-induced withdrawal of sympathetic outflow, because intrathecal injection of clonidine (1 microgram) did not attenuate increases in arterial pressure and ventilation evoked by high-intensity electrical stimulation of the cut central end of the sciatic nerve (n = 5). Furthermore, our findings were not due to a local anesthetic action of clonidine, because application of this agent to the dorsal roots had no effect on the discharge of group IV muscle afferents. We conclude that stimulation of alpha 2-adrenergic receptors in the spinal cord attenuates the reflex pressor and ventilatory responses to static contraction.     

Cyclooxygenase blockade attenuates responses of group III and IV muscle afferents to dynamic exercise in cats

Cyclooxygenase products accumulate in statically contracting muscles to stimulate group III and IV afferents. The role played by these products in stimulating thin fiber muscle afferents during dynamic exercise is unknown. Therefore, in decerebrated cats, we recorded the responses of 17 group III and 12 group IV triceps surae muscle afferents to dynamic exercise, evoked by stimulation of the mesencephalic locomotor region. Each afferent was tested while the muscles were freely perfused and while the circulation to the muscles was occluded. The increases in group III and IV afferent activity during dynamic exercise while the circulation to the muscles was occluded were greater than those during exercise while the muscles were freely perfused (P < 0.01). Indomethacin (5 mg/kg iv), a cyclooxygenase blocker, reduced the responses to dynamic exercise of the group III afferents by 42% when the circulation to the triceps surae muscles was occluded (P < 0.001) and by 29% when the circulation was not occluded (P = 0.004). Likewise, indomethacin reduced the responses to dynamic exercise of group IV afferents by 34% when the circulation was occluded (P < 0.001) and by 18% when the circulation was not occluded (P = 0.026). Before indomethacin, the activity of the group IV, but not group III, afferents was significantly higher during postexercise circulatory occlusion than during rest (P < 0.05). After indomethacin, however, group IV activity during postexercise circulatory occlusion was not significantly different from group IV activity during rest. Our data suggest that cyclooxygenase products play a role both in sensitizing group III and IV afferents during exercise and in stimulating group IV afferents during postexercise circulatory occlusion.