Attenuation of reflex pressor and ventilatory responses to static muscular contraction by intrathecal opioids

We have tested the hypothesis that intrathecal injections of opioid peptides attenuate the reflex pressor and ventilatory responses to static contraction of the triceps surae muscles of chloralose-anesthetized cats. We found that before intrathecal injections of [D-Ala2]Met-enkephalinamide (100 micrograms in 0.2 ml), static contraction increased mean arterial pressure and ventilation by 32 +/- 5 (SE) mmHg and 227 +/- 61 (SE) ml/min, whereas after injection of this opioid peptide, static contraction increased mean arterial pressure and ventilation by only 15 +/- 5 mmHg and 37 +/- 33 ml/min, respectively. The attenuation of both the pressor and ventilatory responses to static contraction by [D-Ala2]Met-enkephalinamide were statistically significant (P less than 0.05). Moreover, the attenuation was probably not caused by an opioid-induced withdrawal of sympathetic outflow because [D-Ala2]Met-enkephalinamide had no effect on the pressor and ventilatory responses evoked by high-intensity electrical stimulation of the central cut end of the sciatic nerve. In addition, intrathecal injection of peptides that were highly selective agonists for either the opioid mu- or delta-receptor attenuated the reflex responses to static contraction. Naloxone (1,000 micrograms), injected intrathecally, prevented the attenuation of the reflex responses to contraction by opioid peptides. We speculate that the opioid-induced attenuation of the reflex pressor and ventilatory responses to static contraction may have been due to suppression of substance P release from group III and IV muscle afferents.     

Attenuation of reflex pressor and ventilatory responses to static contraction by an NK-1 receptor antagonist.

The chemical messengers released onto second-order dorsal horn neurons from the spinal terminals of contraction-activated group III and IV muscle afferents have not been identified. One candidate is the tachykinin substance P. Related to substance P are two other tachykinins, neurokinin A (NKA) and neurokinin B (NKB), which, like substance P, have been isolated in the dorsal horn of the spinal cord and have receptors there. Whether NKA or NKB plays a transmitter/modulator role in the spinal processing of the exercise pressor reflex is unknown. Therefore, we tested the following hypotheses. After the intrathecal injection of a highly selective NK-1 (substance P) receptor antagonist onto the lumbosacral spinal cord, the reflex pressor and ventilatory responses to static muscular contraction will be attenuated. Likewise, after the intrathecal injection either of an NK-2 (NKA) receptor antagonist or an NK-3 (NKB) receptor antagonist onto the lumbrosacral spinal cord, the reflex pressor and ventilatory responses to static contraction will be attenuated. We found that, 10 min after the intrathecal injection of 100 micrograms of the NK-1 receptor antagonist, the pressor and ventilatory responses to contraction were significantly (P < 0.05) attenuated. Mean arterial pressure was attenuated by 13 +/- 3 mmHg (48%) and minute volume of ventilation by 120 +/- 38 ml/min (34%). The cardiovascular and ventilatory responses to contraction before either 100 micrograms of the NK-2 receptor antagonist or 100 micrograms of the NK-3 receptor antagonist were not different (P > 0.05) from those after the NK-2 or the NK-3 receptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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.     

Spinal vasopressin modulates the reflex cardiovascular response to static contraction.

Recent evidence has demonstrated that arginine vasopressin (AVP) may modulate primary afferent activity of nociceptors in the dorsal horn of the spinal cord. Because nociceptors are group III and IV afferents, spinal AVP also may modulate the activity of group III and IV afferents that cause reflex cardiovascular responses to muscle contraction. Thus, we compared the pressor (mean arterial pressure), myocardial contractile (dP/dt), and heart rate (HR) responses to electrically induced static contraction of the cat hindlimb before and after lumbar intrathecal (IT) injection (L1-L7) of AVP (n = 9), the V1 receptor antagonist d(CH2)5Tyr(Me)AVP (n = 6), the V2 receptor antagonist d(CH2)5[D-Ile2,Ile4,Ala-NH2(9)]AVP (n = 6), and the V2 agonist [Val4,D]AVP (n = 8). After IT injection of AVP (0.1 or 1 nmol) the pressor and contractile responses to static contraction were attenuated by 55 and 44%, respectively. HR was unchanged. Forty-five to 60 min after AVP injection, the contraction-induced pressor and contractile responses were restored to control levels. V1 receptor blockade augmented contraction-induced increases in mean arterial pressure (36%) and dP/dt (49%) but not HR. V2 receptor blockade had no effect on the cardiovascular response to contraction, whereas selective V2 stimulation attenuated the dP/dt (-20%) and HR (-33%) responses but not the pressor response. These results suggest that AVP attenuates the reflex cardiovascular response to contraction by modulating sensory nerve transmission from contracting muscle primarily via a V1 receptor mechanism in the lumbar spinal cord.     

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.     

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'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.     

Spinal P2X receptor modulates reflex pressor response to activation of muscle afferents

Static contraction of skeletal muscle evokes increases in blood pressure and heart rate. Previous studies suggested that the dorsal horn of the spinal cord is the first synaptic site responsible for those cardiovascular responses. In this study, we examined the role of ATP-sensitive P2X receptors in the cardiovascular responses to contraction by microdialyzing the P2X receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) into the L7 level of the dorsal horn of nine anesthetized cats. Contraction was elicited by electrical stimulation of the L7 and S1 ventral roots. Blockade of P2X receptor attenuated the contraction induced-pressor response [change in mean arterial pressure (delta MAP): 16 +/- 4 mmHg after 10 mM PPADS vs. 42 +/- 8 mmHg in control; P < 0.05]. In addition, the pressor response to muscle stretch was also blunted by PPADS (delta MAP: 27 +/- 5 mmHg after PPADS vs. 49 +/- 8 mmHg in control; P < 0.05). Finally, activation of P2X receptor by microdialyzing 0.5 mM alpha,beta-methylene into the dorsal horn significantly augmented the pressor response to contraction. This effect was antagonized by prior PPADS dialysis. These data demonstrate that blockade of P2X receptors in the dorsal horn attenuates the pressor response to activation of muscle afferents and that stimulation of P2X receptors enhances the reflex response, indicating that P2X receptors play a role in mediating the muscle pressor reflex at the first synaptic site of this reflex.     

Effects of alpha 1 and alpha 2 activation of adrenergic receptors on aqueous humor dynamics

Effects of phenylephrine (alpha 1-adrenergic agonist), prazosin (alpha 1-adrenergic antagonist), clonidine (alpha 2-adrenergic agonist), and yohimbine (alpha 2-adrenergic antagonist) on aqueous humor (AH) dynamics were studied with a cat eye model. Phenylephrine (130 microgram/ml) inhibited AH outflow (67% at 90 min. period) more than AH formation (26% at the same period) indicating the intraocular pressure (IOP) might be raised by the administration of phenylephrine. Prazosin (0.1 microgram/ml) produced effects opposite to those of phenylephrine (55% reduction of AH formation and 25% reduction of AH outflow at 3 hr. period) suggesting the alpha 1-adrenergic receptor is responsible for increases rather than decreases of IOP. Both clonidine (10 microgram/ml) and yohimbine (0.1-1.0 microgram/ml) inhibited AH formation (60% inhibition) more than AH outflow (no inhibition for clonidine and 40% inhibition for yohimbine) to lower IOP. The conventional theory of receptor antagonism does not seem to function at alpha 2-receptor sites.     

Nitric oxide modulates sympathoexcitatory cardiac-cardiovascular reflexes elicited by bradykinin

A number of studies have demonstrated an important role for nitric oxide (NO) in central and peripheral neural modulation of sympathetic activity. To assess the interaction and integrative effects of NO release and sympathetic reflex actions, we investigated the influence of inhibition of NO on cardiac-cardiovascular reflexes. In anesthetized, sinoaortic-denervated and vagotomized cats, transient reflex increases in arterial blood pressure (BP) were induced by application of bradykinin (BK, 0.1-10 microg/ml) to the epicardial surface of the heart. The nonspecific NO synthase (NOS) inhibitor NG-monomethyl-L-arginine (L-NMMA, 10 mg/kg iv) was then administered and stimulation was repeated. L-NMMA increased baseline mean arterial pressure (MAP) from 129 +/- 8 to 152 +/- 9 mmHg and enhanced the change in MAP in response to BK from 32 +/- 3 to 39 +/- 5 mmHg (n = 9, P < 0.05). Pulse pressure was significantly enhanced during the reflex response from 6 +/- 4 to 27 +/- 6 mmHg after L-NMMA injection due to relatively greater potentiation of the rise in systolic BP. Both the increase in baseline BP and the enhanced pressor reflex were reversed by L-arginine (30 mg/kg iv). Because L-NMMA can inhibit both brain and endothelial NOS, the effects of 7-nitroindazole (7-NI, 25 mg/kg ip), a selective brain NOS inhibitor, on the BK-induced cardiac-cardiovascular pressor reflex also were examined. In contrast to L-NMMA, we observed significant reduction of the pressor response to BK from 37 +/- 5 to 18 +/- 3 mmHg 30 min after the administration of 7-NI (n = 9, P < 0.05), an effect that was reversed by L-arginine (300 mg/kg iv, n = 7). In a vehicle control group for 7-NI (10 ml of peanut oil ip), the pressor response to BK remained unchanged (n = 6, P > 0.05). In conclusion, neuronal NOS facilitates, whereas endothelial NOS modulates, the excitatory cardiovascular reflex elicited by chemical stimulation of sympathetic cardiac afferents.     

Tempol attenuates the exercise pressor reflex independently of neutralizing reactive oxygen species in femoral artery ligated rats

In decerebrate rats, we reported previously that the exercise pressor reflex arising from a limb whose femoral artery was occluded for 72 h before the experiment was significantly higher than the exercise pressor reflex arising from a contralateral freely perfused limb. These findings prompted us to examine whether reactive oxygen species contributed to the augmented pressor reflex in rats with femoral artery occlusion. We found that the pressor reflex arising from the limb whose femoral artery was occluded for 72 h before the experiment (31 ± 5 mmHg) was attenuated by tempol (10 mg), a superoxide dismutase (SOD) mimetic (18 ± 5 mmHg, n = 9, P < 0.05), that was injected into the arterial supply of the hindlimb. In contrast, the pressor reflex arising from a freely perfused hindlimb (20 ± 3 mmHg) was not attenuated by tempol (17 ± 4 mmHg, n = 10, P = 0.49). Nevertheless, we found no difference in the increase in 8-isoprostaglandin F(2α) levels, an index of reactive oxygen species, in response to contraction between freely perfused (3.76 ± 0.82 pg/ml, n = 19) and 72-h occluded (3.51 ± 0.92 pg/ml, n = 22, P = 0.90) hindlimbs. Moreover, tempol did not reduce the 8-isoprostaglandin F(2α) levels during contraction in either group (P > 0.30). A second SOD mimetic, tiron (200 mg/kg), had no effect on the exercise pressor reflex in either the rats with freely perfused hindlimbs or in those with occluded femoral arteries. These findings suggest that tempol attenuated the exercise pressor reflex in the femoral artery-occluded hindlimb by a mechanism that was independent of its ability to scavenge reactive oxygen species.     

Reflex effect of skeletal muscle mechanoreceptor stimulation on the cardiovascular system

To determine the potential for mechanical stimulation of skeletal muscle to contribute to the reflex cardiovascular response to static contraction (exercise reflex), we examined the cardiovascular effects caused by either passive stretch or external pressure applied to the triceps surae muscles. First, the triceps surae were stretched to an average developed tension of 4.8 +/- 0.3 kg. This resulted in increases in mean arterial pressure (MAP) of 28 +/- 7 mmHg, dP/dt of 1,060 +/- 676 mmHg/s, and heart rate (HR) of 6 +/- 2 beats/min (P less than 0.05). Additionally, increments of 0.3, 0.5, 1.0, 2.0, 4.0, and 8.0 kg of tension produced by passive stretch elicited pressor responses of -6 +/- 1, 7 +/- 1, 16 +/- 3, 21 +/- 8, 28 +/- 6, and 54 +/- 9 mmHg, respectively. External pressure, applied with a cuff to the triceps surae to produce intramuscular pressures (125-300 mmHg) that were similar to those seen during static contraction, also elicited small increases in MAP (4 +/- 1 to 10 +/- 1 mmHg) but did not alter HR. Transection of dorsal roots L5-L7 and S1 abolished the responses to passive stretch and external pressure. Moreover, when the triceps surae were stretched passively to produce a pattern and amount of tension similar to that seen during static hindlimb contraction, a significant reflex cardiovascular response occurred. During this maneuver, the pressor response averaged 51% of that seen during contraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

"Pertussis toxin induces tachycardia and impairs the increase in blood pressure produced by alpha 2-adrenergic agonists"

Administration of purified pertussis toxin to rats induced persistent tachycardia, (observed in conscious rats but not after pithing); as little as 0.05 microgram/100 g produced a significant effect. Pertussis toxin-treatment did not affected the pressor response produced in the pithed rats by the alpha 2-adrenergic agonist methoxamine but markedly diminished the pressor effect of the alpha 2-adrenergic agonists clonidine and azepexole. A role of adenylate cyclase inhibition in the action of postsynaptic vascular alpha 2-adrenergic receptors is suggested.     

Does leukotriene C4 mediate hypoxic vasoconstriction in isolated ferret lungs?

To evaluate leukotriene (LT) C4 as a mediator of hypoxic pulmonary vasoconstriction, we examined the effects of FPL55712, a putative LT antagonist, and indomethacin, a cyclooxygenase inhibitor, on vasopressor responses to LTC4 and hypoxia (inspired O2 tension = 25 Torr) in isolated ferret lungs perfused with a constant flow (50 ml.kg-1.min-1). Pulmonary arterial injections of LTC4 caused dose-related increases in pulmonary arterial pressure during perfusion with physiological salt solution containing Ficoll (4 g/dl). FPL55712 caused concentration-related inhibition of the pressor response to LTC4 (0.6 micrograms). Although 10 micrograms/ml FPL55712 inhibited the LTC4 pressor response by 61%, it did not alter the response to hypoxia. At 100 microgram/ml, FPL55712 inhibited the responses to LTC4 and hypoxia by 73 and 71%, respectively, but also attenuated the vasoconstrictor responses to prostaglandin F2 alpha (78% at 8 micrograms), phenylephrine (68% at 100 micrograms), and KCl (51% at 40 mM). At 0.5 microgram/ml, indomethacin significantly attenuated the pressor response to arachidonic acid but did not alter responses to LTC4 or hypoxia. These results suggest that in isolated ferret lungs 1) the vasoconstrictor response to LTC4 did not depend on release of cyclooxygenase products and 2) LTC4 did not mediate hypoxic vasoconstriction.     

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.     

P2 antagonist PPADS attenuates responses of thin fiber afferents to static contraction and tendon stretch

Injection into the arterial supply of skeletal muscle of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a P2 receptor antagonist, has been shown previously to attenuate the reflex pressor responses to both static contraction and to tendon stretch. In decerebrated cats, we tested the hypothesis that PPADS attenuated the responses of groups III and IV muscle afferents to static contraction as well as to tendon stretch. We found that injection of PPADS (10 mg/kg) into the popliteal artery attenuated the responses of both group III (n = 16 cats) and group IV afferents (n = 14 cats) to static contraction. Specifically, static contraction before PPADS injection increased the discharge rate of the group III afferents from 0.1 +/- 0.05 to 1.6 +/- 0.5 impulses/s, whereas contraction after PPADS injection increased the discharge of the group III afferents from 0.2 +/- 0.1 to only 1.0 +/- 0.5 impulses/s (P < 0.05). Likewise, static contraction before PPADS injection increased the discharge rate of the group IV afferents from 0.3 +/- 0.1 to 1.0 +/- 0.3 impulses/s, whereas contraction after PPADS injection increased the discharge of the group IV afferents from 0.2 +/- 0.1 to only 0.3 +/- 0.1 impulses/s (P < 0.05). In addition, PPADS significantly attenuated the responses of group III afferents to tendon stretch but had no effect on the responses of group IV afferents. Our findings suggest that both groups III and IV afferents are responsible for evoking the purinergic component of the exercise pressor reflex, whereas only group III afferents are responsible for evoking the purinergic component of the muscle mechanoreflex that is evoked by tendon stretch.     

Dorsal horn administration of muscimol abolishes the muscle pressor reflex.

The purpose of this study was to determine the effect of blocking synaptic transmission in the dorsal horn on the cardiovascular responses produced by activation of muscle afferent neurons. Synaptic transmission was blocked by applying the GABA(A) agonist muscimol to the dorsal surface of the spinal cord. Cats were anesthetized with alpha-chloralose and urethane, and a laminectomy was performed. With the exception of the L(7) dorsal root, the dorsal and ventral roots from L(5) to S(2) were sectioned on one side, and static contraction of the ipsilateral triceps surae muscle was evoked by electrically stimulating the peripheral ends of the L(7) and S(1) ventral roots. The dorsal surface of the L(4)--S(3) segments of the spinal cord were enclosed within a "well" created by applying layers of vinyl polysiloxane. Administration of a 1 mM solution of muscimol (based on dose-response data) into this well abolished the reflex pressor response to contraction (change in mean arterial blood pressure before was 47 +/- 7 mmHg and after muscimol was 3 +/- 2 mmHg). Muscle stretch increased mean arterial blood pressure by 30 +/- 8 mmHg before muscimol, but after drug application stretch increased MAP by only 3 +/- 2 mmHg. Limiting muscimol to the L(7) segment attenuated the pressor responses to contraction (37 +/- 7 to 24 +/- 11 mmHg) and stretch (28 +/- 2 to 16 +/- 8 mmHg). These data suggest that the dorsal horn of the spinal cord contains an obligatory synapse for the pressor reflex. Furthermore, these data support the hypothesis that branches of primary afferent neurons, not intraspinal pathways, are responsible for the multisegmental integration of the pressor reflex.     

Differential cardiovascular effects of central clonidine and B-HT 920 in conscious rats

The effect of intracerebroventricular (i.c.v.) injection of the alpha 2-adrenoceptor agonists clonidine and B-HT 920 on mean arterial pressure (MAP), heart rate (HR), and plasma concentrations of noradrenaline and adrenaline was examined in conscious unrestrained rats. The injection of 1.0 microgram clonidine significantly decreased MAP and slightly decreased HR. Plasma noradrenaline and adrenaline levels were slightly but not significantly decreased after the injection of 1 microgram clonidine. In contrast, the injection of 0.1-10.0 micrograms B-HT 920 increased MAP and decreased HR. Plasma noradrenaline and adrenaline levels were slightly increased after the injection of the 1- and 10-micrograms doses. The i.c.v. injection of the alpha 2-antagonist rauwolscine slightly but not significantly increased MAP and plasma noradrenaline and adrenaline levels. The responses to i.c.v. injection of clonidine and B-HT 920 were not changed by prior administration of rauwolscine. Neither the pressor response to B-HT 920 nor the depressor response to clonidine was abolished by rauwolscine, suggesting that neither response was mediated by alpha 2-adrenoceptors.